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An GhnIomhaireacht urn ChaornhnUComhshaoil
WATER TREATMENT
MANUALS
FILTRATION
This document
contains 80 pages
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ENVIRONMENTAL PROTECTIONAGENCY
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©Environmental Protection Agency 1995
Parts of hispublication maybereproduced without urtherpermission, provided the
source isacknowledged.
WATER TREATMENTMANUALS
FILTRATION
Publishedbythe Environmental Protection Agency, Ireland.
The Agency personnel involved in the production and preparation of this manual
were Mr. Noel Bourke,Mr. Gerry Carty, Dr. MattCrowe and Ms. MarionLambert
(word processing).
IFC.
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WATER TREATMENTMANUALS
FILTRATION
Environmental ProtectionAgencyArdcavan, Wexford, Ireland.
Telephone:+353-53-47120 Fax : +353-53-47119
Co 3S—5'3
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CONTENTS 1
CONTENTS
CONTENTS 1
LISTOFFIGURES 4
LIST OF TABLES 4
PREFACE 5
ACKNOWLEDGEMENTS 6
LIST OFABBREVIATIONS 7
1 INTRODUCTION 9
1.1 PROCESS DESCRIPTION 9
2 FILTRATIONMECHAN1MS 11
N3 SLOWSANDFILTRATION 13
3.1 SLOWSAND FILTER MEDIA 13
3.2 FILTER LAYOUTANDOPERATION 13
3.3 SLOWSAND FILTER CONTROL 15
3.4 SLOWSAND FILTER CLEANING ANDRESANDING 15
3.4.1 WORKER SAFETY AND HYGIENE PRACTICE 15
3.4.2RECOMMENDEDPROCEDURE OR CLEANING ASLOW SAND FILTER 16
3.4.3 RECOMMENDEDPROCEDUREFOR RESANDINGASLOW SANDFILTER 16
4 RAPIDGRAVITY ANDPRESSURE FILTRATION 19
4.1 RAPIDGRAVITY FILTRATION 19
4.2 PRESSURE FILTRATION 19
5. PROCESS CONSIDERATIONS 23
5.1 RAPID GRAVITY FILTER MEDIA 23
5.2OPERATIONAL CRITERIA 24
5.2.1FILTER LAYOUT 24
5.2.2 FILTER PRODUCTIONAND FILTRATION RATE 24
5.2.3FILTRATION EFFICIENCY 24
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2 FILTRATION
5.3FILTER OPERATION 25
5.3.1 RAPID GRAVITY FILTRATION 25
5.3.2BACKWASHING25
5.4 FILTER CONTROL SYSTEMS 26
6 ACTIVATED CARBONFILTERS 29
7 INTERACTION WITH OTHER TREATMENT PROCESSES 31
7.1 PREFILTRATION TREATMENT 31
7.2 IN-LINE FILTRATION 31
8 PROCESS MONITORING ANDCONTROL 35
9 OPERATING PROCEDURESASSOCIATED WITHNORMAL PROCESS CONDITIONS 37
9.1 INDICATORS OFNORMAL OPERATING CONDITIONS 37
9.2 PROCESS ACTIONS 37
9.3 PROCESS CALCULATIONS 40
9.3.1 FILTRATION RATE 40
9.4 RECORDKEEPINGANDQUALITY CONTROL40
9.5 FILTER MONITORING INSTRUMENTATION 41
10 OPERATING PROCEDURES ASSOCIATED WITH ABNORMALPROCESS CONDITIONS 43
10.1 INDICATORS OF ABNORMAL CONDITIONS 43
10.2 PROCESS ACTIONS 43
10.3AIRBINDING 44
10.4 EXCESSIVE HEAD LOSS44
11 STARTUP ANDSHUTDOWN PROCEDURES 47
11.1 CONDITIONS REQUIRING IMPLEMENTATIONOFSTARTUP ANDSHUTDOWN PROCEDURES 47
11.2 IMPLEMENTATION OF STARTUP/SHUTDOWN PROCEDURES 47
11.3 FILTER CHECKING PROCEDURES 47
11.4 BACKWASH PROCEDURES 47
11.5 FILTER STARTUP PROCEDURES48
11.6 FILTER SHUTDOWN PROCEDURES 48
12 PROCESS ANDSUPPORT EQUIPMENT OPERATION ANDMAINTENANCE
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CONTENTS 3
12.1TYPESOFEQUIPMENT 51
12.2EQUIPMENT OPERATION 51
12.3PREVENTIVE MAINTENANCE PROCEDURES 51
12.4 SAFETY CONSIDERATIONS 52
GLOSSARY 53
APPENDIX A: TYPICAL FILTERS OPERATING RECORD SHEET 57
APPENDIX B: CONVERTING TO SI (METRIC) UNITS 62
APPENDIX C: MUDBALLEVALUATION PROCEDURE 65
APPENDIX D: HYDRAULIC CALCULATIONS 67
REFERENCESAND READING LIST 71
USER COMMENT FORM 73
RECENT ENVIRONMENTAL PROTECTIONAGENCYPUBLICATIONS 75
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4 FILTRATION
List of Figures
FIGURE]:TYPICAL WATER TREATME)VTPROCESSES 10
FIGURE 2:COMBINATION OF TWO TYPES OF STOCKSAND 14
FIGURE 3: SLOWSANDFILTERAND PREFILTRATIONCONTROL CHAMBER 15
FIGURE 4: RESANDINGA SLOW SAND FILTER USINGTHE TRENCHINGMETHOD 18
FIGURE 5: CONVENTIONALANDDIRECTFILTRATION 20
FIGURE 6: RAPIDGRAvifYFiLTER 20
FIGURE 7: GRAVifY FiLTERMEDiACONFIGURATIONS 21
FIGURE 8:PRESSURE FILTER 22
FIGURE 9:MEASUREMENTOFHEADLOSS 39
FIGURE 10: TYPICAL O[TTFLOWTURBIDifY DATA 39
FIGURE 11: FILTRATION PROCESSMONiTORING -PARAMETERS ND LOCATiONS 41
FIGURE 12: FILTRATIONMODE OF OPERATION 50
FIGURE 13:BACKWASHMODE OFOPERATION 50
FIGURE 14:MUDBALL SAMPLER 66
List of Tables
TABLE 1: TYPICAL MEDIA FILTERCHARACTERISTICS 24
TABLE2:SUMMARYOFROUTiNE ILTRATION PROCESS ACTIONS 32
TABLE3:FILTRATIONPROCESSTROUBLESHOOTING 33
TABLE 4:READINGATURBIDITYROD 37
TABLE 5: PROCEDUREFOR BACKwASHINGA FILTERUNDER MANUALCONTROL 49
TABLE6: POTENTIALHAZARDS NFILTRATION 52
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PREFACE 5
PREFACE
The EnvironmentalProtectionAgencywasestablished in 1993 to licence, regulate and controlactivities forthe purposesof environmental protection. In the Environmental ProtectionAgency Act, 1992, it is statedthat "theAgency may, andshall ifso directedbytheMinister, specify andpublishcriteria andprocedures,which in the opinion of the Agency are reasonable and desirablefor the purposes of environmentalprotection". Thesecriteriaandprocedures n respectofwater reatment arebeingpublished by the Agencyinanumberofmanuals under hegeneral headingofWaterTreatmentManuals.
This manual on Filtrationsets out thegeneralprinciples and practices which shouldbe followedby thoseinvolved in the production of drinking water. Next year, the Agency intends to prepare and publishadditional manuals on Disinfection, Coagulation, Flocculation, Clarification and Fluoridation. Wherecriteria and procedures are published by theAgency,a sanitary authorityshall, in the performance of itsfunctions, haveregardtosuch criteriaandprocedures.
This manual includes nformation on many aspects of the filtration process. The Agencyhopes that it willprovide practical guidance to those involved in plant operation, use, management, maintenance andsupervision. TheAgency welcomes anysuggestions which usersof the manualwishtomake. Theseshouldbe returnedto theEnvironmental Management and PlanningDivision at the Agency headquarters on theenclosedUserComment Form. -
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6 FILTRATION
ACKNOWLEDGEMENTS
The Agencywishes oacknowledge thosewhocontributed toand reviewed this manual. A review panelwas
established by the Agency to assist in the finalisation of the manual and we acknowledge below those
persons who took the time to offer valuable information, advice and in many cases comments and
constructive criticism on the draft manual. We gratefully acknowledge the assistance offered by the
following persons:
John Anderson, Department of he Environment
MartinBeirne, Environmental HealthOfficers Association
AideenBurke, Jones Environmental (Ireland) Ltd.
ProfessorTomCasey, University College,Dublin.
Ned Eeming,DublinCorporation
TomLoftus,DublinCorporation
Eamon Mansfield, Waterford County Council
DavidMcBratney,M.C. O'Sullivan& Co. Ltd.
ColumMcGaughey, Dublin Corporation
JohnO'Flynn,Waterford County Council(representingtheCounty and CityEngineers Association)
M.C.O'Sullivan,M.C. O'Sullivan& Co.Ltd.
Paul Ridge, Galway County Council
John Walsh, E.G. Pettit&CompanyLtd.
The Agency also wishes to acknowledge the assistance of the Sanitary Services sub-committee of the
Regional Laboratory, Kilkenny, whocommented onthedraftmanual.
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LIST OF ABBREVIATIONS
• EBCT: Empty Bed ContactTime
ES: Effective Size
GAC: GranularActivated Carbon
LOH: LossofHead
rn/h: m3 perm2 perhour
mm: millimetres
NTU: Nephelometric TurbidityUnit
p.s.i.: pounds persquare inch
THMs: Trihalomethanes
TON: Threshold OdourNumber
TWL: TopWaterLevel
UC: Uniformity Coefficient
UFRV: UnitFilter RunVolume
VOC: Volatile Organic Carbon
LISTOFABBREVIATIONS 7
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8 FILTRATION
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1. INTRODUCTION
1.1 PROCESS DESCRIPTION
Filtration is theprocess ofpassingwater throughmaterial to remove particulate and other
impurities, including floc, from the water beingtreated. These impurities consist of suspendedparticles (fine silts and clays), biological matter
(bacteria, plankton, spores, cystsor othermatter)and floc. The material used in filters for publicwater supply is normallya bed of sand, coal,orothergranular ubstance. Filtration processes can
generally be classified as being either slow or
rapid.
Slow sand filters are the original form offiltration. Thefirst one was builtin 1804 byJohnGibb of Paisley, Scotland to treat water for his
bleachery, with the surplus treated water sold to
thepublic.2 Slow sand filters were first used inLondon in 1820 to treat water from the River
Thames. From about the 1930s water treatment
by coagulationand rapid gravity filtration or
pressure filtration tended to replace slow sand
filtrationin new plantsand, in some cases, slow
sand filters were replaced by rapid gravity iltersfollowing ntroduction ofacoagulation stage.The
slow sand filtration process has come back intofavour in recentyearsdueto its superior ability,
compared to rapid gravity filtration, to remove
pathogenic micro-organisms such as Giardialamblia andCryptosporidium.
Typical water treatment processes are shown
schematically in Figure i"4, with the relative
position of iltraticfn llustrated.
-4 9
I INTRODUCTION
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TREATEDWATER
EFFECTON WATER
Excludes fishand emus es leases, sticks and other
largedebris.
Breaks down colloidal stability. Adjusts pH for optimum
coagulation.
Mixes chemicalso th raw water,containing ine particles thato ll not readilysettleor filter oat of he water.
Gathers together ine, lightparticles to form arger clumps flocto aid the sedimentationiflotation nd iltrationprrcesses.
Sedimentation settlesour large suspended particles.Plotation floats oat the particles with dissolvedair.
Rapidgravits filtration iltersor remoses
remaining uspended particles.
Slow sand filtrarioralso involvesbiologicalaction.
Kills / nactisares disease-causing organisms. Pros des chlorine
residual fordistribution system. where chlorine s used.
Helpscontrol corrosivepropertiesof water.
Helpscontrol dental caries n children andyoungadults.
FIGURE1: TYPICAL WATERTREATMENT PROCESSES
10 FILTRATION
RAW TREATMENT
0
0
z
DISINFECTION
]_STERILIZATION
pH CORRECTION
Stores waterprior o discharge to service esers ems.
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2 FILTRATION MECHANISMS
2 FILTRATIONMECHANISMS
Filtration is essentiallya physical and chemical The relative importance of these removal
process and, in the case of slow sand filtration, mechanisms will depend largelyon thenature ofbiological as well. The actual removal the water being treated, choice of filtrationmechanisms are interrelated and rather complex, system, degree of pre-treatment, and filterbut removal of colour and turbidity is based on characteristics.thefollowing factors:
• chemical characteristics of the water beingtreated(particularly source water quality);
• nature of suspension (physical and chemical
characteristics ofparticulates suspended in thewater);
• typeanddegreeofpre-treatment (coagulation,flocculation, andclarification); and
• filtertypeandoperation.
A popular misconception is that particles areremoved in the filtration process mainly byphysical straining. Straining is a term used to
describe the removal of particles from a liquid(water) by passing the liquid through a filter orfabric sieve whose pores are smaller than the
particles to be removed. While the strainingmechanism doesplayarolein theoverall emoval
process, especially in the removal of largeparticles, it is important to realize that mostof the
particles removed during filtration are
considerably smaller than thepore spaces in the
media. This is particularly true at thebeginningof the filtrationcycle when the pore spaces areclean (that is, not clogged by particulatesremovedduring filtration).
Thus, a number of interrelated removalmechanisms within the filter media itself arerelied uponto achieve high removal efficiencies.Theseremoval mechanisms include the following
processes:
• sedimentation on media(sieveeffect);
• adsorption;
• absorption;
• biological action; and
• straining.
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12 FILTRATION
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31 SLOW SAND FILTRATION
3 SLOW SAND FILTRATION
3.1 SLOW SAND FILTERMEDIA
Allen Hazen was one of the pioneers in the
scientific investigation ofslowsand filtration and
introduced, as long ago as 1892, the concept of
effective size (ordiameter).
The EFFECTIVE SIZE (ES) is defined as thesize ofa sieve opening through which 10percent
(by weight)of the particles (sand)will just passand is given thesymbol d10. Inasimilarway,the
size ofasieve opening through which60percent
(by weight)of the particles (sand) will just passis given the symbol d60. The UNIFORMITY
COEFFICIENT, (UC) which is ameasureof the
gradingof the material, is the ratiod d10
It is normal to employ ungraded sand as
excavated from natural deposits in slow sand
filters. The sand should have a uniformitycoefficient of less than 3, but there is little
advantage in having a U.C. of less than 1.5 ifadditional cost is thereby incurred. Pit-run sand
maybe washed to remove admixedclay , loam ororganic matter and this will remove the finest
grains thus lowering the U.C. and raising the
average particlediameter.
Ideally the effective size should be just small
enough to ensure a good quality outflow and
prevent penetration of clogging matter to such
depth that it cannot be removed by surface
scraping. This is usually in the range 0.15 - 0.35
mm and is determined byexperiment. Both finer
and coarser materials have been found to work
satisfactorily in practice, and the final selectionwillusually depend on the available materials. It
is possible tocombine wo ormoretypesofstocksand to bring the effective size of the mixture
closer to the ideal, as shown in Figure 22. The
broken line in the Figure indicates the filteringmaterial obtainedby mixing 3 parts of sand A
with one part of sand B to give an effective
diameterofabout 0.25mm.
Desirable characteristics forall filter media are as
follows:
goodhydraulic characteristics (permeable);
• does not react with substances in the water(inertandeasy toclean);
hard anddurable;
• freeof mpurities; and
• insoluble inwater.
Gravel is used to support the filter sand and
shouldhave similarcharacteristics.
3.2 FILTER LAYOUTAND OPERATION
The ilter media is usually containedin concrete
filter tanks which are all the same size, but the
size will vary widely from works to works. In
general, the minimumnumberof iltersis three o
allow for one filter to be out of service and yethave sufficient capacity available to meet the
average demand. Atypicalsectionofaslowsand
filter and prefiltration control chamberis shown
inFigure3.
Water, admitted to a slow sand filter, properly"conditioned", flows downward through the
media. In the slow sand filtration process,
particles are removed primarily by straining,
adsorption, and microbiological action. Themajor
partof heparticulate material isremoved ator inthe top layer of sand, which becomes covered
with a thin slimy layer, called the
"schmutzdecke", built up by micro-organisms as
the filter 'ripens'.
Slow sand filtration does not materially reducethe true colour of a water. True colour is the
colour attributable to light-absorbing organicmolecules which are dissolved in the water as
distinct from that attributable to light-scatteringcolloidalparticles such as clay. Truecolour 5,13 is
measured ollowing the removal ofsuchcolloidal
matter by passing a sample of water through a
membrane filter of aperture approximately 0.5
tm. With a standard of 20 mg/i platinum/cobaltfor colour as the maximum allowable
concentration in Ireland, slow sand filtration is
unlikely to be suitable forraw waterwith colourinexcessofabout25 mg/i platinum/cobalt, unless
colour soxidised in the filtratebyozonisation.
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14 FILTRATION
99
90
80
70
60
50
40
30
20
0.!
Grain size,d, mm
0.2 0.3 0.4 0.5 0.7 I .0
FIGURE2:C0MBINATI0N F TwoTYPES OF STOCK SAND
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SLOW SAND FILTRATION
Filtration rates areextremely low, typically 0.1 to0.2 mperhour,depending on whether owland orupland water is being filtered and the treatment
regimeapplied. The higherfiltration ratesapplyto upland waters, with which the classicschmutzdecke does not form, or to lowlandwaters which are pretreated by microstraining orroughing filters, as in the case of the Belfast
supply rom CastorBay, Lough Neagh, where thewater is first treated by rapid gravity filtration.Chemicals such as chlorine arenot addedbeforeslow sand filtration as this would inhibit
microbiological activity.
The entire top layer of the filter, including theschmutzdecke, must be physically removed whenthe filter becomes clogged, or else the filtercleaned in-situ. No works within-situmechanical
cleaning exists in Ireland as far as is known.
Disadvantages toslowsand
filtration are the argeland area required and the manpower, orinvestment in plant, required for cleaning thefilters.
3.3 SLOWSANDFILTER CONTROL
Slow sand ilters are normally supplied throughan inlet chamberhousinga control valve and ameasuring weir (see Figure 3). A single inletchamberusually feedsa groupof filtersor even
an entire small works and controls the level ofwater in the filters. The head on the filters insmaller works is normally controlled by anadjustable bellmouth in the outlet chamber ofeach filter,sometimes fitted withascale o enablethe flow over the lip to be gauged. Whenafilterhas been cleaned and put into service, the
headloss through it is only a few cm and thebellmouth is set near the top of its travel. Theheadloss increases as the filter ages and thebellmouth is lowered to maintain thedesired flow
through the filter. When the belimouth hasreached the bottom of its travel,the filter hasreached theendof tsrun and needs to be cleaned
again.The
adjustableoutlet bellmouth
ensuresthat negative pressures cannot occur within thebed. Alternative forms of adjustable outlets tothat describedaboveareavailable. Controlof hefiltration rate on larger works may be achieved
automatically by measuring the outflow fromeach filter usinga flow rate controllerand usingthis signal to raise and lower the bellmouth orotherdevice.
3.4 SLOWSANDFILTER CLEANING ANDRESANDING
3.4.1 WORKER SAFETY AND HYGIENEPRACTICE
During filter cleaning orresanding operations allworkers should wear rubber boots, which aredisinfected in a tray of bleaching solution before
entering the filters. The highest standards ofpersonal hygiene should be observed by allworkers and no worker with symptoms that might
be attributable to waterborne or parasitic diseaseshouldbepermitted tocome nto director indirectcontact with the filtermedium.
FIGuI3: SLOWSANDFILTERAND PREFILTRATIONCONTROL CHAMBER
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16 FILTRATION
3.4.2 RECOMMENDED PROCEDURE FOR
CLEANING A SLOW SAND FILTER
1. Draining he filter andpreparing tor
cleaning
Whena filter has reached the end of its run and
arrangements for itscleaning have beenmade, the
raw water inlet valve is closed, to initiate the
cleaning operation, thus allowing the filter to
continue to discharge at a reducing rate to the
clear water tankfor as long as possible (usually
overnight).
The outflow valve is then closed (e.g. next
morning) and the remaining water abovethe sandisrun to waste.
The water withinthe bed is then lowered about
100mmbelow the surface of the bed by openingthe drainvalve.
2. Cleaning the filter
Cleaning should start as soon as the
schmutzdecke is dry enough to handle. If the
filter is left too long, it is likely to attractscavenging birds that will not only pollute the
filter surface but disturb the sand to a greater
depth than will be removed by scraping. If, as is
normal, mechanised methods are not available,
workers using squarebladed shovels should stripoff the schmutzdecke and the surface sand
adhering to it and stack it in ridgesor heaps for
removal.
Great care should be taken to minimise
disturbance of the upperlayersof filter media so
that the biomass is protected. For this reason,dumpers or other machines used for removal
shouldbe designedto operate with low pressure
tyres and barrows or handcarts should always be
run on protective planks.
Cleaning is a simple matter when the
schmutzdecke consists largely of filamentous
algaeformingan interwovenmat. The knackof
curlingback this mat in reasonably largesections
is quickly acquired, provided that the operation is
timed so that the material is neitherwaterlogged
nor so driedout that it isbrittle.
Cleaningwill be
less easy if the schmutzdecke consists largelyof
non-filamentous algae and greater care will be
necessary to control the depth ofscraping, which
shouldbe between 15 and 30mm.
Afterremoval of the scrapings the bed should be
smoothed to a level surface. The quicker the
filter-bed is cleaned the less will be the
disturbance to the biomass and consequently the
shorter the period of re-ripening.The micro-
organisms immediately below the surface will
quickly recover, provided they have not been
completely dried out, and will adjust to their
position relative to the new bed level. A day or
two should be sufficient for re-ripening in this
event.
Before refilling the filter, hewallsbelow normal
top water level should be swabbed down to
discourage the growth of slimesandalgae.
3. Refilling the filter
The filter is refilled by closing the drain valve,
reopening the outflow valve and allowing the
filtered waterto backflow through the underdrain
system until thereis sufficient depth on the sand
surface to prevent disturbance of the bed by
inflowing raw water. When the filter is
sufficiently chargedthe outflow valve is closed.
The raw water inlet valve is then gradually
opened and the filter is filled to normal operatinglevel.
The bellmouth ofthe telescopic outletis raisedto
near its maximum and the filter outflow is run to
wasteata gradually increasing rate, (or if there is
a suitable pump, recycled to theworks inlet), for
a day or two until the filter is re-ripened and
analysis shows that the outflow satisfies the
requiredquality tandards.
4. Disposalofscrapings
The materialremoved rom the filter, dependingon the size and available equipment at the works.
may be washed for reuse or disposed of on land
by burial or used in agriculture. The workforce
whoscrape the filters or wash the sand should be
instructed innecessary hygiene practice.
3.4.3 RECOMMENDEDPROCEDURE FOR
RESANDINGA SLOW SAND FILTER
Each cleaning of the filter removes between 10
and 15 mm of filter sand, so that after twenty orthirty scrapings the thicknessof the sand bedwill
have been reduced to its minimum designthickness, usually about 300mm and resanding is
then necessary. The following procedure is
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SLOW SAND FILTRATION
17
recommended for re-sanding a slow sand filter.The filter can either be re-sanded by the'trenching' method which makes use of theresidual
sand,or
by refilling with newsand.
Generally, re-ripening of the filter is quicker ifthe renching method isemployed.
1. Preparation
The bed is cleaned, as described n section3.4.2above, and the water level lowered to thebottomofthe sand layer.
2a.Resanding - he 'trenching' method
Mostof theresidualsand is removed from a stripofthe filter along one wall thus forming a trench,taking care not to disturb the underlying gravellayer by leaving 100 to 150 mm of residual sandaboveit. The sandis placed adjacent to the filterfor laterre-use.
Fresh sand (either new or washed sand fromfilter
cleaning) is placed in the trench to a thicknesswhich, withtheresidual sand, equals thedepthof
sand in thefilterpriortore-sanding seeFigure 4)or as determined in consultation with thedesigner.
Residual sand from theadjacent strip is "thrownover" on topofthe freshlyplaced sand inthe first
strip, fresh sand is placed in the second trench
justformed andsand from the nextadjoining stripis"thrownover" as shown inFigure 4.
Thisprocess is continued untiltheentirefilter hasbeenresandedand theresidualsand fromthefirststrip is placed on top of the last strip filled. Inthis way, the residualsand forms the upper layerof he re-sanded filter and thenew sand the lowerlayer.
and replaced during the resanding operations, asthe carbon will have become saturated with theimpurities and/or nactivated.
2b. Resanding - cleansandmethod
Analternative to the trenching method describedaboveis to remove all theold sand from the beddown to the support gravel and refill with cleansand.
3. Refilling he filter
The bed should be smoothed to a level surfaceand refilled with filtered water through the
underdrainage system. The raw water inlet isthenopened as described in section 3.4.2.
4. Re-ripening the filter
The ripeningof a filter with clean sand will take
longer than the time for re-ripening a filterresandedusingthe trenching method. Caremustbe taken to ensure that a satisfactory quality
outflow is being produced beforethe filteris putback into service.
The filter-bed mayhavea layerofcrushed shells
incorporated near the bottom, to correct the pH,where theraw water is naturallyaggressive. Theresidue of the shell layer must normally beremoved and replaced during re-sandingoperations. Occasionally, a layer of activated
carbon about 100mm deep is placed near thebottom of the filter-bedto absorb traces of tasteand odour-producing substances that havepassedthrough hefilters. This layer shouldbe removed
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18 FILTRATION
FIGURE4: REASANDING ASLOWSANDFILTERUSINGTHE TRENCHINGMETHOD
level th ifiter
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4RAPIDGRAVITYANDPRESSURE FILTRATION
4 RAPID GRAVITY AND PRESSURE FILTRATION
Filtrationpreceded by coagulation, flocculationand clarification is commonly referred to asconventional filtration. In the direct filtration
process, although coagulation and flocculation
may be used, the clarification step is omitted.Typical treatment processes for these twofiltration methods are shown in Figure 5. Theconventional filtration (treatment) process is usedin mostmunicipal reatment plants. Thisprocessincludes "complete" pretreatment (coagulation,
flocculation,and
clarification/flotation). Thissystemprovidesflexibility and reliability in plantoperation, especially when source waterqualityisvariable or the water is high in colour and
suspended solids.
Directfiltrationis an alternative to conventionalfiltration, particularly whensource watersare owin turbidity, colour, plankton, and coliform
organisms. Direct filtration can be definedas atreatment system in which filtration is notprecededbyclarification orflotation.
4.1 RAPIDGRAVITYFILTRATION
In all gravity filtration systems thewater evel or
pressure (head) abovethemediaforces the water
through the filtermediaasshown inFigure6.Therate at which water passes through the granularfiltermedia(the filtration rate) may vary widely,dependingon the purposefor which the water isrequired. However, for public water supply in
Ireland, 5 m/hourmaybe regarded as the standard
rate and most authorities limit the maximumfiltration ate to between5 and7.5 rn/hour. Ratesin excess of this may be used where specialtechnology is employed. The rate of water flow
through the filter is referredto as the hydraulicloading or the filtration rate. The filtration rate
depends on the raw water quality and the type offilter media. Various filter media configurationsused are illustrated nFigure7.
They are:
• multi or mixed media (sand, anthracite,garnet).
Activatedcarbon (in granular form)can also beused in association with theseconfigurations forthe removal of tastes, odours, and organicsubstances.
In rapid gravity filtration the particulateimpurities are removed in or on the media, thus
causingthe filterto clogafter aperiod. Cloggedfilters are cleaned by backwashing. Gravityfiltration is the most widely used form of watertreatment nthiscountry.
4.2 PRESSURE FILTRATION
Apressurefilteris similarto a gravity sandfilterexcept that the filter is completely enclosedin apressure vessel such as a steel tank, and isoperatedunder pressure, as shown in Figure 8.Pressure filters have been used in public water
supplies, and have limited applicability, forinstance, in the removal of iron and manganesefromgroundwaters. Their use in Irish practice smainly confined to the treatment of water forindustrial purposes, but they have been installed
byanumberof local authorities.
Pressure filters have been found to offer lowerinstallationand operation costsin small filtration
plants. However, they are generally somewhatless
reliable than gravity filters. Maximumfiltration rates forpressurefilters are in the5 to7.5 rn/hrrange.
• singlemedia(sand);
• dual media(sandandanthracite); and
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c..—t,
Fl1t,Uou
FIGURE5: CONVENTIONALAND DIRECTFILTRATION
FIGURE6:RAPID GRAvITY FILTER
20 FILTRATION
PtocoitionAid
Co.gd.LloAFioccul.tio Clrlflc.to Filton
Coiiveaboia1Filtratloil
IAiiorI—
Direct Filtretlon
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(14 Mono Media
toflow
TWLV7
RAPIDGRAVITYAND PRESSURE FILTRATION
coal
FIGuRE 7: GRAVITY FILTERMEDIACONFIGURATIONS
1
FUteeOutflow
Inflow
Inflow
IWL T
coal
sand
I. grav&
1500 o 2000 m
800mm
t11terOutflow
Underdnino
IC)Multi-Media
brmixed)
I sand
1500 o2000mm
tt800mm
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22 FILTRATION
FIGURE8: PRESSUREFILTER
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5. PROCESS CONSIDERATIONS
5 PROCESS CONSIDERATIONS
23
5.1 RAPIDGRAVITY FILTER MEDIA
The most common filtering material in rapidgravity (and pressure) filters is sand3. The dualmedia filter is a refinement of the rapid gravitymonosand filter, in which an upper layer ofanthracite or similar material above the sand
provides ncreased void space to store impuritiesremoved from the incoming water. The mixed
media filter is a further refinement, in which a
layerof garnetorsimilardensematerial isplacedbelow the sand. Other nert materials mayalso be
used including activated carbon. Gravel iscommonly used to support the filter media.
Desirable haracteristics for all filter mediaareasfollows
• goodhydraulic characteristics (permeable);
• does not react with substances in the water
(inertandeasytoclean);
. hardanddurable;
• freeof impurities; and
• insoluble in water.
Gravel is used to support the filter sand and
shouldalso have he abovecharacteristics.
Ithas been thecustom to designate sandforrapid
gravity filters in terms of its effective size and
uniformity coefficient. Othermaterials for rapid
gravity filters are referenced by means of the
supplierscode reference
(e.g. No 2Anthracite
)5A U.K. standard for the specification, approvalandtestingofmaterials for rapid gravity filtrationhas recently been published by the British
Effluentand WaterAssociation6. Thestandard isintended to cover all granular materials used in
rapid gravity iltration, in terms of theparameters
necessary for the successful operation of thefiltration system including backwash. Effectivesize and uniformity coefficient are omitted and anew parameter, hydraulic size, is defined. This
parameter can be used in calculating the
fluidization hreshold, the point in backwashingwhere the hydraulic (pressure) loss through thefilter sandequalsthe dead(submerged) weightofthe material. Other important parameters in this
calculation are thedensity of thematerial and its
voidage, or porosity as it is more commonly
known. The porosity is dependant on theshapeof the grains. Rounded grains tend to have alower porosity and can be washed at a lowerbackwash rate; however, as a filtering material ithas less space within it to hold the flocs andsolids removed fromthewater and alsoproducesa higher headloss. Given the correct backwashrate in the first place, material with higher
porosity has much to commend it. Anthracite,whichis acrushed material, is agoodexample ofahighporosity medium which has been inuse for
many years.
Two factors are very important in making
judgementsabout media selection:
• the time required for turbidity to break
through hefilterbed;and
• the time required for the filter to reach
limitinghead oss.
With a properly selected media, these times are
aboutthesame.
If the limitinghead loss is frequently a problemand turbiditybreakthrough rarely occurs, then a
largermedia sizemaybe considered. If turbidity
breakthrough is frequentlyaproblemand limitinghead loss is rarely encountered, then a smallermediasizemaybeconsidered.
Ifboth head loss andturbiditybreakthrough area
problem, while the filter is operating within itsrated capacity, a deeper filter bed with a largersand size maybe required. The optimumdepthneededtoobtain agivenqualityand length ofrunvaries with the size of the sand. However,
increasing the media depth is notalwayspossiblewithout modification of the filter. Adequateclearance mustbe allowedbetween he topof themedia and theweir cill ofthe washwater channel.
Otherwise, filter media will be carried over into
the washwater channel during backwash, whenthebedexpands.
The relationship between turbiditybreakthroughand
limitinghead ossis also
stronglyaffected
bythe efficiency of chemical pre-treatment. Poorchemical pre-treatment can often result in earlyturbiditybreakthrough, rapidhead lossbuildupor
bindingof he filtersand.
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24 FILTRATION
It must also be remembered that backwashing is
possibly a more critical part of the filtration
processthan the forward filtration phase. If thebackwash rate is
slightlytoo
high.filter material
can be lost rapidly. If it is slightly too low, the
cleaning efficiency will fall abruptly. the filterwill cease to be cleaned properly and its
performance will quickly deteriorate. Thebackwash sequence must be selected takingaccount of the material used and the temperatureof the backwash water. Existing backwash
procedures will require adjustment if a new
filtering material has a differentsize, density orvoidsratio.
Selection of anappropriate
media forrapidgravity filtration depends on the source water
quality, filter design and anticipated filtrationrate. Generally the more uniform the media theslower the head loss buildup. Media with
uniformity coefficients ofless than 1.5 are readilyavailable. Mediawith uniformity coefficients ofless than 1.3 are only available at a high cost.
Typical filter media characteristics are given in
Table1.
Table 1: Typical Media FilterCharacteristics
Material SizeRange(mm)
Specific
GravityConventional 0.5-0.6 2.6
5.2.1 FILTER LAYOUT
In rapid gravity filtration, the filter media isusually contained in a numberof concrete filter
tanks(or cells)which arc all the same size. Steel
or aluminium tanks are sometimes used.
However, the size will varywidely fromplant to
plant. In general, the minimum numberoffilters
is three. Thisal!oss for one filter to be taken outof service leaving sufficient filter capacityaailable to meet the demand. Typical filtersections are shown in
Figure7.
5.2.2 FILTER PRODUCTION AND
JLTRATJONPATE
For public watersupply in this country. thestandard filtration rate for rapid gravity filters is
usually 5 rn/hour and most authorities limit the
maximum filtration rate to between 5 and 7.5rn/hour. The minimum number of filters should
be three for the smallest works as noted above.but the number and size should be
keptreasonable in relation to the total works
throughput with the size based on a 20 hour
working day. In larger water treatment plants.filter capacities range up to about 25.000 m3/dayfor the largest filter units. Filtration plants
operatedat rates in excess of thosequoted above
generally require additional supervision.
5.2.3 FILTRATION EFFICIENCY
Rapidgravity filtration efficiency is roughly
measured by overall plant reduction in turbidity.although it should be noted that up to 90% of thereduction may take place in the pretreatment
stages. Overall reductions of over 99.5 percentcan be achievedunderoptimum conditions, while
a poorly operated filter and inadequate pre-treatment (coagulation. flocculation, and
clarification) can result in turbidity removals ofless than 50 percent. The bestwayto assure highfiltration efficiency is to select an outflow
turbidity target and stay below the target value
[such as 0.5 NTU (Nephelometric TurbidityUnits)1.
Solids removal efficiency depends largelyon: the
quality of the water being treated: theeffectiveness of the pre-treatment processes in
conditioning the suspended particles for removal
by clarification and filtration: and, filter
operation.
Filter unit design and filter media typeand depthalso play a role in determining solids removal
efficiency. but are less important than water
quality and pre-treatment considerations. Rapidgravity sand filters usually produce a filtered
water urbidity comparable to thatofadual-media
filter if the applied water quality is similar.
However, the operational differences betweensand and dual-media filters are significant.
Sand
CoarseSand 0.7-3.0 2.6
Anthracite/Coal 1.0-3.0 1.5-1.8
Gravel 1.0-50 2.6
5.2 OPERATIONAL CRITERIA
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r 25
0 PROCESS CONSIDERATIONS
Because of their smaller media grain size,
typically 0.8 to 1.0 mm,sand filters tendto clogwithsuspended matterand floc morequickly than
dual-media filters. This means hatmore frequent
backwashing will be required to keep the sandfilteroperating efficiently. Sand ilters have fine,
light grains on the top which retain all floc and
particulates at or near the surface of the filter.
Dual-media filters have lighter, larger diameter
grainsin the top layerof the media which retain
the larger particles; the smaller particles are
usually retained deeperin the filter. The larger
grainsizeof theanthracite layer (upto 1.5 mm) n
the top portion of a dual-media filter permits
greater depth penetration of solids into the
anthracite
layerand
largersolidsstorage volume
in the filter. The sand layerbelow the anthracite
is used as a protective barrier against
breakthrough. These characteristics generally
producefilter runs which are longer than those
achievedbysand filters. Tapsatvarious depths in
the filter may be used to observe the depth ofsolids penetration.
Multi-media filters (sand, activated carbon and
anthracite) are alsoused to extend filter run times,and these filters generally perform in a manner
similar o sandfilters,except that the filtermediais enclosed ina pressure vessel. With the filter
media fullyenclosed, t is impossible to assess themedia condition by simple visualobservation. Inaddition, excessive pressure in the vessel will
force solids as well as water through the filtermedia. Obviously, this will result in the
deterioration of filtered water quality. Pressureshould therefore be maintained withinthe range
provided orbythedesigner.
Particular care should be taken when a filter is
being backwashed to ensure that the remainingfilters are notoverloaded. Backwashing, togetherwith routine and emergency maintenance, should
be completed withoutoverloading the remainingfilters.
5.3 FILTEROPERATION
5.3.1 RAPID GRAVITY FILTRATION
In the filtration mode of operation, watercontaining suspended solids is applied to the
surface of the filter media. Depending on theamount of suspended solids in the water beingtreated and the filtration rate, the filter will
exhibit head loss and "clog" after a given time
period (varies fromseveral hours toseveral days).
Clogging may be defined as a buildupof head
loss (pressure drop)across the filter media until itreaches some predetermined design limit. Total
design head loss ingravity filters generally rangesfrom about 1.8 to 3.0 m depending on the depthof the water over the media. Cloggingof the
filter leads to breakthrough, a condition in which
solids are no longer removed bythe already over-
loaded filter. The solids pass into the filtered
waterwhere hey appearasincreased turbidity.
A filter is usually operated until just before
clogging or breakthrough occurs, or a specifiedtimeperiodhas passed, generally24 to 40 hours
andis related to the efficiency of the clarification
process. In order to save money, energy and
water by maximizing production before
backwashing, filters are sometimes run until
clogging orbreakthrough occurs. This is a poor
practice and should be discouraged. When
breakthrough occurs, therewill be an increase in
filtered water turbidity and hence a decrease in
water quality with the risk that pathogenic
organismsmay pass through thefilter.
5.3.2 BACKWASHING
Afterafilterclogs (reaches maximum head loss),or break-through occurs, or a specified time
period haspassed, the filtration process is stoppedand the filter is taken out ofservicefor cleaningor backwashing. Detailed procedures for
backwashing tend to be particular o each plantand specific instructions are provided by plantcontractors.
Backwashing is theprocess of reversing the flow
of water through the filter media to remove theentrapped solids. Backwashing may comprisethe application of water alone, air and water
seperately and sequentially, or air and water
simultaneously. The latterprocedure is generally
acknowledged tobe the most efficient, but filters
must, in general, be designed for an air/waterbackwash as changing existingfilters to use this
systemis fraughtwith difficulty. With all three
typesofwash, the process conditions are related
to the (minimum) fluidization threshold as areference point,even withcombined air andwater
washes. This threshold is the point where thehydraulic (pressure) loss through the filter bed
equals the dead (submerged) weight of thematerial. The maximum backwash water flowrate shouldnot exceed 20 rn/hr. as higher flow
rates will result in excessive media loss. Duringthebackwash cycle the bed should be expanded
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26 FILTRATION
bya minimum of 10% and amaximum of20% to
ensure adequate leaning.
As mentioned previously, backwashing is
possibly a more critical part of the filtration
process than the forward filtration phase. If the
backwash rate is slightly toohigh, material can be
rapidly lost. If it is slightly too low, the cleaning
efficiency will fall abruptly, the filter will cease
to be cleaned properly and its performance will
quickly deteriorate. The backwash sequence andduration must be selected taking account of the
filtering materials in use and the temperature ofthe backwash water. It may be necessary in
some cases to adjust the backwash rates
seasonally in order to maintain optimum
conditions without osingmedia.
Filtered water is almost invariably used for
backwashing and is usually supplied by a
backwash pump. Anelevated storage tank mayalso be used to store water to backwash filters.The backwashing process will use about two to
four percent of the filter output, the lower
percentages being associated with combined
air/waterwashing.
It is very important to observe the surface of the
filter during the wholebackwash operation. Anynon-uniform surface turbulence during air scour
mayindicate a problem with the air distribution
arrangements or the occurrence of "mudballs" in
themedia. Mudballs occur whenbackwashing ofafilter is inadequate. The surface 'crust', formed
by the filtered solids and sand at the top of the
filter bed becomes cemented into a compact crust.
This cracks and pieces sink into the expandedsand bed during backwashing, without beingbroken up. These pieces of crust graduallyincrease in size during the subsequent operation
of the filter. If mudballing is suspected themudball evaluation procedure detailed in
Appendix C should be used. The simplestmethod of improvement of a filter affected by
mud-balling is to agitate the expandedbed with a
"drag"or long-tined rake.
The used washwater should be settled and thesettled washwater either discharged to waste or
recycled through the treatment process.Recycling of washwater (without balancing)
directlyto the head of theworks has been found
to cause operational difficulties with chemical
dosingequipmentand control of chemical dosingplantandhence it s notrecommended.
The settled solids should be treated with the
sludge from the clarification stage. It should be
noted that recycling of washwater can result in a
concentration of viruses, cysts and other
undesirable particles, as well as polyelectrolyes,on the filters which, in turn, increases the
potential of breakthrough into the water supply.Undernormal operating conditions where the raw
water is uncontaminated this should not pose a
problem. However, it is important that plants
develop contingency plans which avoid the
recycling ofwashwater when a pollution incidentis known to have contaminated the raw water
supply. Under normal conditions, the settled
washwater should be recycled, if possible, to the
balancing tank or reservoirsupplying theworks.or mixed with the raw water, ahead of the
flashmixer.
Any polyelectrolytes used in the solids handlingphasesshouldbe suitable for use in potable water,ifsupernatant /otherliquorsare to berecycled.
5.4 FILTERCONTROL SYSTEMS
The filter control system regulates the flow rate
through the filter by maintaining an adequatehead above the media surface. This head
(submergence) forces water through a gravity
filter. The flow through afiltermust be as stableas possible and any changes in flow rate,whenever operating conditions at the plant
change. should be controlled inorder for thefilter
to yield the optimum outflow quality. The bestcontrolsystem herefore is one with simple, safe
and reliable controllers that controls filtrationwithout hunting, and includes sensors that
monitor the, largest possible water surface areasso that changes in set-point values are
representative.
Without aneffective filter control system, sudden
flow increases or surges could dislodge solids
trapped on the filter media. If these solids were
discharged, they would seriously degrade water
quality. An adequate depth of water above themedia surface is essential to ensure that the
inflow does not disturb(scour) the media. In this
way, the energy of the inflow is absorbed before
it reaches the media, thuspreventing scouring.
An essential element in the control systemfor
rapid gravity filtration is a slow start controller,which restricts the output from a filter for a
period after backwashing while the filter is
ripening'. In new works, slow start controls are
generally incorporated in the plant design, whilein older or smaller works hydraulic/mechanicalcontrols are used. The latter depend on the flow
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PROCESS CONSIDERATIONS
27
ofwater into a ank, through a submerged orifice
with increasing depth of submergence, to opentheoutletvalvefromthefilter.
Rapid gravity filter control systems can beclassified into three ypes
constant rate,withacontroller;
constant rate,variable head ype; and
declining rate (orvariable flow rate).
The constant rate typewith acontroller, maybe
operatedeither on the ConstantLevel system orwith Flow Measurement. With ConstantLevel
control the inflow to the plant is distributed
equallybetween he filters,each receiving a flow
equal to the incoming flow rate divided by the
number of operating filters. Each filter is
equipped with a controller which detects the
upstream level, which it keeps constant by
adjusting the outflow controller. Because the
upstream level is kept constant, the outflow is
equal to the inflow and clogging is compensated
for until it reaches a limit, whichdepends on theavailable head. When a filter is shut down for
backwashing or maintenance, the inflow is
automatically distributed over the filters that arestillin servicewiththeexception offilters usingasurface flush of settled water. Equal distribution
of inflow is achieved simply andreliablyby staticdevices ( orifice plates,weirs, etc ). Useof this
control system also eliminates the discrepanciesbetween total filtered flow and incoming flow
thatcanoccur with control systemsbased onflow
ratemeasurements.
In the Constant Rate control with Flow
Measurement system each filter outlet has aflowmeter linked toa controller whichcomparesthemetered flow from the filter to the flow rate
setpointand adjuststhe outflow valve until theycoincide. This system has no means of
maintaining a specific water level above he filter
media, so an additional central controller isneeded. Normally the nflow rate to the filters ismeasured and the central controller
adjuststhe
individual set-point rateof the filters accordingly.If the nflow rate increases, the level upstream ofthe filters rises and thecentralcontroller adjuststhe set-point rate for the filters until the upstreamlevel stabilizes and plant inflow and outfloware
in balance. The central controller may
alternatively adjust he ndividual set-point rate ofthe filters by reference to the water level in theclearwater tank. Another central controller
detects the water level in the inflow channel and
adjusts the inflow control valve to provide the
filters with a flow to correspond with their set-
point rate. Thechange n waterlevel in the filters
canbeasmuch as 300 mm with his system.
Constant ate, variable head ilters operate with afiltered water outlet control structure (weir) to
control theminimum water level just above thatof the media in the filters. The total inflow to the
plant is distributed equally between all the
operating filters. When the filter is clean the
media is just coveredby water and at maximum
clogging thewaterreaches the level in the inflow
channel.
The self-backwash(orStreicher design) system, isa variation of the constant rate, variable head
filter. The inflow to the plant is distributed
equallybetweenalltheoperating filters,byaweir
arrangement. The water surface level in each
filter variesaccording to head loss, but the flow
rate remains constant or each filter. This systemreduces the amount of mechanical equipment
required for operation and backwashing, such as
washwater pumps, and also requires an outflow
control structure and adeeperfilter. Thesefiltershave a use where neither electricity nor
compressed air is available and the water to be
treated has low to moderate levels of suspendedsolids.
In declining-rate filters, flow rate varies with
head loss. Each filter operates at the same, but
variable, water surface level. This system is
relatively simple, but again requires an outflow
control structure (weir) to provide adequate media
submergence.
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28 FILTRATION
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6 ACTIVATED CARBON FILTERS7
ACTIVATED CARBON FILTERS
29
Theprimary purpose
of filtration is to remove
suspended particles and floc from the water beingtreated. Another dimension is added to thefiltration process by the use of activated carbon
(granular form) as a filter media. The high
adsorptive capacity ofactivated carbonenables ittoremove taste andodour-causing compounds, aswell as other trace organics from the water.
However, not all organic compounds areremoved
withthesame degree ofefficiency.
While activatedcarbon filtration is very effective
in removing taste and odour-causing compounds,the construction, carbonhandling equipment and
operating costs are generally quite high.Activated carbon can be addedto existing filters
provided here is sufficient depthavailable orcanbe incorporated as a separate process. Provision
shouldbemade for regeneration orreactivation of
"spent" carbon (carbon which has lost its
adsorptive capacity) eitheron oroff-site.
Theeffectiveness ofactivated carbon inremovingtaste and odourproducing organic compounds is
well known. The chlorination of water
containing certain natural organic substances,such as humic or fulvic acids or substancesderived from algae may give rise to the
production of trihalomethanes (THMs) orhaloforms. The concern regarding these
substances is reflected in the Drinking Water
Regulations8which state that "Haloformconcentrations must be kept as low as possible".The use of granulated activated carbon (GAC)filters to remove theorganic substances that arethe 'precursors' of THMs is becoming more
widespread.
GAC filters employed for organics removal are
similar to sand filters of the rapid gravity or
pressure type with generally similar design,filtration rates and backwashing arrangements,with the flow rate adjusted to suit the lower
density ofGAC. The GAC is placed in the filter
over the support gravel. Media depth is afunctionof theempty bed contact time (EBCT).GAC characteristics will vary according to thematerial from which it is made whether wood,coconutshell orpeat.
EBCT is normally in the range 5 to3O minutesand will vary for differentmicropollutants. For
pesticides removal an EBCTof 15 to 30 minutes
is used while 10 minutesis considered
adequatefor THMs and VOCs (volatile organic
compounds).
Although GAC removes most micropollutants
efficiently, its adsorption capacity towards someis low, so that frequent regeneration becomes
necessary, withconsequent costs. Breakthroughwill not normally occur for 2 to 3 years, when
usingan EBCTofabout 10 minutes, ifonly taste
and odour removal is required. Most pesticidesmayshow breakthrough in6 to 24 months usingan EBCTof 10 to 30 minutes; THMs in 6 to 12
monthsandVOCs in3 to 9months.
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30 FILTRATION
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32 FILTRATION
TABLE2: SUlMARYOF ROUTINE FILTRATION PROCESS ACTIONS
Actions Location Frequency Possible OperatorActionsMonitorProcessPefoi-nianceandEvaluateWaterQua//tv Conditions
MgaavteTurbidits Outfiou/lnfioss Intloss at least onceper
• increase sampling frequency shen
day ssaterqualitysvariable. PerformjarOutflow every 2hours tests f equired
• Makenecessary process changesMeasure Colour Infloss/Outflow At eastonce eseo 2hours a) Adjustcoagulanllcoagulant id
dosage
b) Adjust flash mixer/flocculator
mixing intensity
c Change filtration rateDetermineHeadLoss Filter At east twiceper da d) Backwash filter
• Verify esponses to process changes atMeasure Metal ion residuals Outflow Daily -
appropriate imes
OperateFiltersandBackss'ash
Put filter nto ervice Filter Depends on process • See operating procedures in Section
conditions 11.5
Change filtration rate
Remove filter from service
Backwash filter
Change backwash rate
CheckFilterMediaConditionMediaDepthevaluation Filter At east monthly • Replace losi iltermedia
Media cleanliness • Change backwash procedure
Cracksorshnnkage • Send sampleofmedia or esting
Make Visual ObservationsofBacksi'ash OperationCheckformediaboils Filter Ai eastonce per dayor • Change backash rate
wheneserbackwashing
Observe mediaexpansion occurs when ess frequent • Change backuash cycle ime
Check for media carryover into • Adjusi wash rate, air scour
washwater channel orcycle time
Obserse clanty ofwashwater • inspect iltermedia for isturbance
Check Filtration ProcessandBacksi'ash Equipment Condition
Noise/Vibration/Leakage Various Onceperday • Correctminorproblems
Overheating• Notif maintenance/supervisors
personnel ofmajor problems
Accuracyof losss Infloss/outfioss DailyAirscour Monthl • Recalibrate meters orauesBackwash Monthly
Inspect Facilities
Checkphysical facilities Various Once per day • Report abnormal conditions o
Daily maintenance/supervisory personnel
Checkfor lgae buildupon filter
walls and channels • Remove debris rom filtermedia
surface
• Perform outinecleaning/maintenance
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INTERACTION WITH OTHER TREATMENT PROCESSES
TABLE3: FILTRATIONPROCESSTROUBLESHOOTING
33
Trigger ibjatorctionsSourceWaterQualityChanges
Possible Process Changes
Changes in:• Turbidity• Colour• pH• Temperature• Alkalinity• Chlorine demand
• Perform necessary analysis todetermine • Adjust coagulant osageextentofproblem • Adjustflashmixer/flocculatormixing• Assessoverallprocessperformance. intensity
• Performjar ests if ndicated • Change frequency of ludge emoval• Make appropriate process changes (see (increase ordecrease)
nght-hand olumn, 'Possible rocess • Change filtration rate(add ordeleteChanges') filters)
• Increase frequencyofprocess monitoring • Adjustbackwashcycle(rate,duration)• Verify response toprocess changes at
appropriate time(besure toallowsufficient timeforchange o akeeffect)
• Add ime orcausticsoda if lkalinity is
low
ClarificationProcessQuality Changes- • .• .• .- - - - • :
Changes in: • Assessoverallprocess performance Samesuggestions as orsourcewater- • Turbidity or loc carryover • Performjar estsif ndicated quality changes• Metal on residual arryover • Check hatselected chemical dosesare
- being applied ---------—-- - - - -- -
• Makeappropriate processchanges• Verify response toprocess changes at
appropriate time
FiltrationProcesschanges/Problems
Changes resulting in: • Assess overallprocessperformance • Adjust oagulant dosage• Head oss increase - • Performjar estsif ndicated • Adjust lashmixer/flocculator mixing• Short filter uns • Make appropriateprocess changes intensity• Media surface sealing • Verify responsetoprocess hanges at • Change requency ofsludge removal• Mudballs appropriate time • Decrease filtration rate• Filter media racks, shrinkage • Manually remove mudballs • Adjustbackwash cycle(rate,duration)• Filterwill notclean • Replenish lost media• Mediaboils • Clearunderdrain openingsofmedia,• Media oss corrosion orchemical deposits when• Excessive head loss filter is outofactive service
• Checkhead loss indicator for orrect
operation
FilterOutflow_Quality_Changes
Changes resulting in: - • Assessoverallprocess performance • Adjust oagulant dosage• Turbidity breakthrough - • Performjar estsif ndicated • Adjust lashmixer/flocculator mixing• Colour • Verifyprocessperformance: intensity• pH (a)Coagulation-flocculation process • Change frequency of ludge removal.• Chlonnedemand (b)Clarificationprocess • Decrease filtrationrate (add more(c)Filtration process filters)
• Makeappropriate process changes. • Changechlorine dosage• Verify response toprocess changes at
appropriate time
-
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34 FILTRATION
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835
PROCESS MONITORINGANDCONTROL
8 PROCESS MONITORING AND CONTROL
The quality of treated water for human
consumption must,afterdisinfection, meet legallydefined standards as set out in the EuropeanCommunities (Quality of Water Intended forHuman Consumption) Regulations, 1988g.
Parameters tobe monitored under he regulationsare divided into five groups -
organoleptic,physico-chemical, undesirable in excessive
amounts, toxicandmicrobiological.
It has been found in practice that control ofturbidity is, except in rare instances, perfectlyadequate formonitoring process performance. In
avery shortperiod, thecorrelation between ilterinflow turbidity or colour, filter performance(head loss builduprate and filter run time) andfilter outflow turbidity or colour becomes
apparentso that any departure from the normcanbe detected by the use of a turbidityrod andalmost by eye. It is of course necessary toconfirm the visual observation by instrument
readings for recordpurposes. Wheremetals fromtheprimarycoagulant have been detectedin thefinal water,atsignificant evels in relationto thestandards (e.g. aluminium), the level ofdissolvedmetals in the filter outflow shouldbe monitoredat intervals, particularly if any change in thechemical dosing regimehas occurred. In some
plants it may be possible to establish a
relationship between turbidity (or colour) andmetal ion residuals nd this maybe used toreducethefrequencyofmetal ionanalysis.
The recyclingof washwaters has been found tocauseprocesscontrol problems, where therecyclerate is variableand forms a significant proportionof the plant inflow. The optimum means ofhandling washwaters from a process control
viewpoint is to recycle the supernatent waterto abalancing tank or reservoirsupplying the worksto even out theeffects, both in terms of qualityand quantity, on theworks inflow. Where this isnot possible,it shouldbe fed to the inflow, at asuniformarateas possible, upstream of the flashmixer.
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36 FILTRATION
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OPERATING PROCEDURES ASSOCIATED WITH NORMAL PROCESS CONDITIONS
9 OPERATING PROCEDURES ASSOCIATED WITHNORMAL PROCESS CONDITIONS
37
.9.1 INDICATORS OF NORMALOPERATING CONDITIONS
Filtration is the final step in the solids removal
process. From a water quality standpoint, filteroutflow turbidity will give a good indication ofoverall process performance. However, theoperatormust also monitor the performance ofeach of the preceding treatment processes(coagulation, flocculation, and clarification
/flotation), in which up to 90% of the solidsremoval occurs as well as filter outflow water
quality, in order to anticipate water quality or
process performance changes which mightnecessitate changesin the treatment process suchasadjustmentofthechemical dosage.
Filter inflow turbidity levels can be checked on a
periodic basisby securing agrab sample eitheratthe filter or from the laboratory sample tap (ifsuch facilities are provided). Filter outflow
turbidity maybe monitored
andrecorded
on acoiitinuous basis by an on-line turbidimeter. Ifthe turbidimeter is provided with an alarmfeature, virtually instantaneous response to
process failures can be achieved. As noted
previously the level of dissolved metals in thefilter outflow shouldbe monitored ifmetals fromthe primary coagulanthave been detectedin thefinal water at more than trace levels. In largerworks, an on-line detector for dissolvedaluminium is desirable for the purpose ofdemonstrating thattheresidual s keptatall times
withinthe standard set in the Regulations. It isessential to have the services of an instrumenttechnician available on demand, where on-linemonitors areinstalled.
Forsmallerworks a turbidity rod, madebyfixingtwobrightplatinumwires, one 1mmdiameterandtheother 1.5mm diameter, at right angles to thebottomofarodmarked incentimetres, isauseful
practical guide to turbidity. The depth ofimmersion atwhichone wire disappears while theother remainsvisible whenviewed from above isrelatedto urbidity3asshown inTable4.
Other indicators that can be monitored todetermine if the filter is performing normally
include head loss build up and filter outflowcolour.
DEPTH OFIMMERSION
*(cm)
TURBIDITY
(mgfl SiOScale)
NOTES
210
1000100
Filterclogs
quickly
15 65Filters
operate with
difficulty
30
45
3018
Special careinoperationrequired
80 10Maximum
desirablelimit
A written set of process guidelines should be
developedto assistin evaluating normal processconditions and in recognizing abnormalconditions. These guidelines should be
developed based on water quality, designconsiderations, water qualitystandards and, most
importantly, experience inoperation of heplant.
9.2 PROCESS ACTIONS
In the ormal operation of the filtration process,a
varietyof functions areperformed with emphasison maintaining ahigh qualityfiltered water. Forall practical purposes, the quality of the filteroutflow constitutes the final productquality thatwill be distributed to consumers (subject to
disinfection).
The recommendations of the plant designer andsupplier in regard to process actions should befollowed in all cases. Typicalfunctionsto be
performed inthenormal operation of thefiltration
process include thefollowing:
TABLE4: READINGATURBIDITY ROD
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38 FILTRATION
monitorprocess performance;
evaluate water quality conditions (colour and
turbidity) and make appropriate process
changes;
check and adjust process equipment (checkoutputsof chemical dosingplants);
backwash filters;
evaluate filter media condition (media loss,mudballs, cracking); and
visually nspect facilities.
Monitoring process performance is an ongoingactivity. It is essential hat seasonal water qualityvariations and the necessary chemical dose rate
adjustments are planned and actions taken toensure that the finished water quality ismaintained.
Measurement of head loss buildup (Figure9) inthe filter media will give a good indication ofhow well theprefiltration solids removal processis performing. The total design head loss fromthefilterinflow to theoutflow inagravity filter is
usually less than 3 metres at maximum. Theactual head loss from a point above the filtermediatoa reference point in the outflow can bemonitored as "loss-in-head". For example,suppose that a gravity filter isdesigned ora otal
potential head loss of2.5m. Ifatthebeginning ofthe filtrationcyclethe actual measured head lossdue to clean media and other hydraulic losses isO.5m; hiswould permitan additional head ossof2.Om due to solids accumulation in the filter. Inthis example, a working limit might beestablished at an additional l.8m of head loss
(totalof2.3m forbackwashing purposes).
The rateofhead oss buildup is also an importantindicator of process performance. Suddenincreases in head loss might be an indication ofsurface sealing of the filter media(lackofdepthpenetration). Early detection of this condition
may permit the making of appropriate processchanges such as adjustment of the chemical feedrateor the filtration rate.
Monitoring of filter outflowturbidity
on acontinuous basis withanon-line urbidimeter will
provide continuous feedback on the performanceof the filtration process. In most instances it isdesirable to cut off (terminate) filteroperation atapredetermined outflow turbidity evel. Preset the
filter cutoff control at a point where experienceand tests show breakthrough will soon occur
(Figure 10). Results of metal ion residualdeterminations should be checked for correlation
with turbiditybreakthrough. Residual metal iondeterminations should be carried out on a dailybasisifanaccuratecorrelation isnotestablished.
In the normal operation of the filter process, it isbest to calculate when the filtration cyclewill be
completed on the basis of the followingguidelines:
head oss;
outflow turbidity level; and
elapsed run time.
A predetermined value is established for each
guideline as a cutoff point for filter operation.Whenanyone of these levels isreached, hefilterisremoved rom service andbackwashed.
Filter performance from season to season, filterbed to filter bed and from plantto plant maybe
compared by reference to the filter run length in
hours. The reason for this is that as thefilter run
gets shorter, the amount of water used inbackwash becomes increasingly important when
compared tothe amount ofwaterproduced duringthe filter run. Percent backwash water statisticsshouldbe collected.
Although of some use, filter run length is not a
satisfactory basis for comparing filter runswithout considering the filtration rate as well.For example, at a filtration rateof7.5 rn/hour , a27 hour filter run is quiteadequate, whereas, at afiltration rateof4 rn/hour a 27 hour filterrun is
notsatisfactory.
The best way to compare filter runs is by usingthe Unit Filter Run Volume (UFRV) technique.The UFRV is the volume of waterproducedbythe filter during the course of the filter rundivided by the surface area of the filter. This isusually expressed in m3 per m2. UFRVsof200m3 per m2 orgreaterare satisfactory, and UFRVs
greaterthan 300 m3 per m2 are desirable. In the
examples in theparagraph above the UFRV forthe filter operating at 7.5 m/hour would be 202.5
m3/m2, and for the filter operating at 4 rn/hourwould be 108m3/m2.
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1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
OPERATING PROCEDURES ASSOCIATED WITI-I NORMAL PROCESS CONDITIONS
FIGURE10: TYPICAL OUTFLOWTURBIDITY DATA
39
FIGuRE9: MEASUREMENT OF HEADLOSS
E0
—Ripe ingPeriod
0
- ITuzbfdlty
akthrou
0 4
0
-I0
n__________00
—
0:
I0 0
T
0
calPerfo
0 0
unce
0 00 0 0 0
0
0
0
0
,— — deal Performane
•. kI I I I • • • • • I I I I I • I
0 4 8 12 16 20
FILTRATION TIME (Bonn)
24 28 32
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40 FILTRATION
Water quality indicators used to assess process
performance includeturbidity and colour (Figure11). Basedon an assessment ofoverall processperformance, changes in the coagulation-
flocculation process or inthe clarification processmayberequired.
At least onceayear:
examine the filter media and evaluate itsoverall condition;
measure the filter media depth for anindication of media loss during the
backwashing process; if noticeable sendmedia sample for analysis and/or adjust
backwash rates;and
measure mudball accumulation in the filter
media, as detailedin Appendix C, to evaluatethe effectiveness of the overall backwashing
operation.
In dailyoperations:
observe the backwash process to qualitativelyassess process performance. Watch for mediaboils
(unevenflow
distribution) duringbackwashing, media carryover into the
washwater channel, and clarity of thewashwater near the end of the backwash
cycle;
upon completion of the backwash cycle,observe thecondition ofthe media surface and
check for filter sidewall or media surface
cracks; and
inspect physical facilities and equipment as
part of good housekeeping and maintenancepractice. Correct or report abnormal
equipment conditions to the appropriatesupervisor ormaintenancepersonnel.
9.3 PROCESS CALCULATIONS
In the routine operation ofthe filtration process, a
variety of process calculations have to be
performed, related to filter operation (flow rate,filtration rate), backwashing (backwash rate,
surface wash rate), water production, and percentof water production used to backwash filters.
Typicalcalculations are shown inAppendix D.
9.3.1 FILTRATION RATE
Filtrationrates aremeasured inm3/m2/hourorper
day, abbreviated to rn/hr or rn/day. For public
water supply in this country, the standardfiltration rate for rapid gravity filters is usually 5
rn/hr and most authorities limit the maximum
filtration rate to between 5 and 7.5 rn/hr.
Problems candevelopifdesignfiltration ratesare
exceeded.
9.4 RECORDKEEPINGAND QUALITYCONTROL
Accurate records ofthefollowing
items shouldbemaintained:
process water quality (turbidity,
temperature, conductivity andcolour);
pH,
process operation ( filters in service, filtration
rates, loss of head, length of filter runs,
frequency of backwash, backwash rates and
UFRV);
process water production (water processed,
amount of backwash water used, andchemicals used);
percent ofwaterproduction used to backwash
filters;
process equipment performance (types of
equipment in operation, equipment
adjustments, maintenance proceduresperformed, andequipment calibration); and
media condition(e.g. congealing
of sand
grains which maybe caused by carry-over of
polyelectrolyte).
Entries in logs should be neat and legible, should
reflect the date and timeof an event, and shouldbe initialled bythe operator making theentry.
An example ofa typical dailyoperatingrecordforwater treatment plant filters is presented in
Appendix A. Irrespective ofthe size of plantornumberoffilters,asimilar ecordsheet should be
kept.
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OPERATING PROCEDURES ASSOCIATED WITH NORMAL PROCESS CONDITIONS
41
9.5 FILTERMONITORINGINSTRUMENTATION
To evaluate filtration process efficiency,familiarity with the measurement of turbidity,colour, pH, temperature and conductivity is
essential. Turbidity may be quickly estimated
using the turbidity rod described in section 9.1
and moreexactly, for recordpurposes usinga
turbidimeter. In addition, on-line or continuouswater quality monitors, such as turbidirneters,
soluble aluminium and pH monitors will giveearly warning of process failure and will aid in
making a rapid assessment of process
performance.
Familiarity with methods used to measure filter
media loss and to determine the presence ofmudballs in thefiltermedia will alsobeneeded.
FIGURE 11: FILTRATION PROCESSMONITORING-PARAMETERS AND LOCATIONS
FilterInflow
Metalion esiduals,pH, Temperature,
Turbidity COlOUr
Filter Outflow
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1 0 OPERATING PROCEDURES ASSOCIATED WITHABNORMAL PROCESS CONDITIONS
10 OPERATING PROCEDURES ASSOCIATEDWITHABNORMAL PROCESS CONDITIONS
43
10.1 INDICATORS OFABNORMALCONDITIONS
Abrupt changes in water quality indicators suchas turbidity, pH, alkalinity, threshold odour
number (TON), temperature, chlorine demand
(sourcewater), chlorimi residual (in-process), orcolour are signals that the performance of thefiltration process, as well as pretreatmentprocesses (coagulation, flocculation, and
clarification/flotation) should be immediately
reviewed.
During a normal filter run, watch for both rapidchangesin turbidity and head loss buildupin thefilter. Significant changes in either of these
parametersmayindicateanupsetorfailureinthefiltration processorpretreatment rocesses.
Other indicators of abnormal conditions are asfollows:
• mudballs infiltermedia;
• mediacrackingorshrinkage;
• mediaboils during backwash;
• excessivemedia oss orvisibledisturbance;
• shortfilterruns;
• filters that will not come clean duringbackwash;
• algaeonwalls andmedia;and
• congealing of sand grainspolyelectrolyte carryover).
10.2 PROCESS ACTIONS
(indicates
A summary ofroutine iltration process actions is
presented in Table 2. Significant changes insource water turbidity levels, either increases ordecreases, require immediate verification of theeffectiveness of hefiltrationprocess in removingsuspended solids and floc. Aquick determination
of filtrationremoval efficiency can be made bycomparing filter inflow and outflow turbiditylevels with thoseofrecentrecord.
In theevent hat filterturbidity emovalefficiencyis decreasing, look firstattheperformance ofthe
coagulation and flocculation processes todetermineif the coagulant dosage is correct forcurrent conditions. This may require the
performance of jar tests in the laboratory toproperlyassess treatment conditions.
Increasesin source water turbidityand resultantincreases in coagulant feed rates may impose a
greater load on the filters if the majority of
suspended solids and floc are notremoved in theclarification tanks. This condition may requireadecrease in filtration rates (put additional filtersinto service) or more frequent backwashing offilters.
Changesin sourcewaterqualitysuchasalkalinityand pH may also affect filtration performance
throughdecreased
coagulation-flocculationprocessperformance. This isparticularly evident
when source water quality changes result from
precipitationand runoff, or algal blooms in asource water reservoir. It would appear to beAmerican practice to use filter aid chemicals,such as nonionic polymers,, to improve filter
performance nsuch cases. Current Irishpracticewould avoid the use of polymers, or other filteraid chemicals, in such circumstances and wouldconcentrateon the adjustment of the prefiltrationtreatmentprocesses.
Theneed for addition of polyelectrolytes duringperiods when water temperatures areabove12°Cshould be examined. Above this temperature,coagulants work more effectively and at some
plants it may be possible to reduce or omit
polyelectrolytes during theperiod from April to
early October, if the raw water quality is of areasonable tandard. The upflow ratesadopted nthe clarification stageshouldnot exceed1.0 m/hr,unless specialised technologies are employed, ifproblemswith filtration and floc carryover are to
be avoided. Coagulant and polyelectrolyte doserate adjustments shouldonlybe carried out by orunderthe control of a competent personand theeffectsofalladjustments shouldbemonitored.
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44 FILTRATION
Increases in filter outflow turbidity may alsoresult from floc carryover from theclarification/flotation process. The optimum flocsize developed in the flocculation process rangesfrom about 0.1 to 3.0 mm. In conventional
filtration, the optimum floc size is closer to 3.0mmforsettling purposes. However, in thedirectfiltration process (no clarification stage) the
optimum floe size is closer to 0.1 mm to permitdepth penetration of the filter media. Whenflotation is not used and larger floe is notremoved in clarification (too light), it will becarried over into the filters, rapidly clogging themedia surface. Hydraulic forces in the filter willshear weakfloes, furthercontributing to turbiditybreakthrough. Re-evaluation of coagulation-flocculation and clarification performance may
be required if floe carryover into the filtersreducesfiltration efficiency. The size of floe canbe estimated by observation, as it is seldom
necessary to make an accurate measurement offloe size.
If backwash problems such as mediaboils,media
loss, or failureof the filter to come cleanduringthe backwash process are encountered, correctiveactions should be taken immediately. Generally,these problems can be solved by adjusting airscourand backwash flow rates, surface washflow
rateorduration, or adjusting the sequence and/orduration of the backwash cycle. In filters with
nozzle-type underdrains, boils are often the resultof nozzle failure. In this situation the filtershouldbe taken out of service and the nozzles
replaced. Possible corrective actions aresummarized inTable3whichgivesa summary offiltration process problems, how to identify thecauses of problems, and also how to correct the
problems.
Problems within the filter itself, such as mudball
formation or filter cracks and shrinkage, resultfrom ineffective or improper filter backwashing.Correction of these conditions will requireevaluation and modification of the backwash
procedures.
If filters are not thoroughly washed, materialfiltered from thewater is retained on the surfaceofthe filter. Thismaterial is sufficiently adhesiveto form minute balls. In time these balls ofmaterial come together in clumps to formmudballs. Usually as time goes on, filter media
becomes mixed in to give it additional weight.When the mass becomes greatenough, it causesthe mudballs to sink into the filter bed. These
mudballs, if allowed o remain, will clogareas in
the filter. Generally. regular and controlled
backwashing willprevent mudball formation.
10.3AIR BINDING
Shortened filter runs can occur because of aitbound filters. This is caused by the release ofdissolved air in saturated water due to a decreasein pressureor air trapped in the filter media oncompletion ofthebackwash cycle. Air isreleasedfrom the water when passing through the filterbed by differences in pressure produced byfriction through the media. Subsequently thereleased air is entrapped in the filter media. Air
binding will occur more frequently when largehead losses are allowed to developin the filter.Whenevera filterisoperated oahead loss whichexceeds the headof water on the media, air willbe released. Air bound filters are objectionablebecause the air prevents water from passingthrough the filter and causes shortened filter runs.When an air bound filter is backwashed, thereleased air can damage the filter media. Whenair is released during backwashing, the mediabecomes suspended in the washwater and iscarriedoutof the filter. Significant media loss
can occur,particularlywhenlightermediasuch asactivatedcarbonisused.
10.4 EXCESSIVE HEAD LOSS
Shortfilter runs mayresult from increased solids
loading, excessively high filtration rates,excessivemudball formation in the filter media,or clogging of the filter underdrain system.Possible corrective actions are summarized inTable3.
If excessive head loss persists in a filter after
backwashing, a epresentative sample of the filtermedia should be sent for analysis, to include thedetermination of:
hydraulic sizeofmedia;
organic content;
particle size distribution;
uniformity co-efficient;
grain shape actor;
grainsphericity; and
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45
OPERATING PROCEDURES ASSOCIATED WITHABNORMAL PROCESS CONDITIONS
grain porosity.
The degradation of thefilter media over timecan
be determined by calculating the 'resistancetofiltration'and the 'efficiency of iltration'. If he
natureof thefilter media does not account for the
headloss, the filter underdrain system and the
head loss measurement equipment should bechecked for malfunctioning. High head lossescanbecausedby reduction in thesize andnumber
ofunderdrain openings. The underdrain openingscan be reduced in size or clogged by media,corrosion orchemical deposits. Pipeworkshould
also be checked for blockages; deposits, for
example,have been asociated with the use of
magnetic flowmeters and sodaash.
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46 FILTRATION
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11 STARTUPANDSHUTDOWN PROCEDURES
1.1 STARTUP AND SHUTDOWN PROCEDURES
47
11.1 CONDITIONS REQUIRING
IMPLEMENTATION OF STARTUP ANDSHUTDOWN PROCEDURES
Startup and shutdown of filtration is a routine
procedure in most water treatment plants, unlikethe coagulation, flocculation and clarification
processes. This is true even if the treatment plantis operated on a continuous basis, since it isconmon practice for a filter to be brought intoservice or taken off line for backwashing. Aclean filter may be put into service whena dirty
filter is removed for backwashing, when it isnecessary to increase filtration rates, or when
plantproduction needsto be increased as aresultof increased demand for water. However, most
plants keep all filters on line except for
backwashing and in service except formaintenance. Filtersare routinely taken off linefor backwashing when the media becomes
cloggedwith particulates, turbidity breakthroughoccurs,ordemands forwater arereduced.
11.2 IMPLEMENTATION OFSTARTUP/SHUTDOWN PROCEDURES
Typical actions performed in the startup andshutdown of the gravity filtration process areoutlined below. Theseprocedures also generallyapplytopressure ilters.
Figures 12 and 13 illustrate sectional views oftypical gravity filters, Thefigures show the valve
position and flow patterns in the filtration andbackwash mode of ilteroperation.
11.3FILTERCHECKING PROCEDURES
The followingactions should be taken to checktheoperational tatus ofafilter:
• check that filter flow control equipment isoperational;
• check that filter media and washwaterchannels areclearofall debris such as leaves,twigsand tools;
• ensure allaccess covers andwalkway gratings
are inplace;
• ensureprocess monitoring equipment such ashead loss and turbidity systems are
operational; and
• check source of backwash water to ensurethere is sufficient volume available. Thiscould be an elevated reservoir, washwater
storage tankorothersource.
11.4BACKWASHPROCEDURES
Filters shouldbewashed priortoplacingthem inservice. There is avarietyofdifferentbackwash
arrangements for filtration plants and these will
vary with individual plant designs. Usually filter
washing is divided into three distinct operations
Operation DetailsAir Scouring consisting of low pressure /
high volume compressed airONLYappliedup through thefilterbed.
Washing(low rate)
consisting ofa ow rate of low
pressure washwater ONLY
applied up through the filter
bed, with wash water drainedtowaste.
Washing(high ate)
consisting of a high rate oflow pressure washwaterONLY applied up through thefilter bed, with washwaterdrained owaste.
asfollows:
A furthercombination maybe the nclusion ofanair scourand low rate wash together for aperiodoftime. Thismaysupercede the necessity for alow rate wash.
Generally wash cycles are automaticallycontrolled,withpre-set wash rates andadjustabletime durations. The duration ofeach part of thewashcycle can therefore be adjustedin order toensure efficient ilter washing.
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48 ALTRATION
If filters are to be washed automatically, checkthat the length of cycle times set for air scour,backwash and surface wash cycles are "correct".
"Correct" imesvaryfromplanttoplant andwith
timeofyear. These settingsshould be based on
physical observations of actual time required to
clean hefilter.
Iffilters are usually washed automatically, it is
good practiceto occasionally use a manual wash
procedure to ensure efficient cleaning of themedia during thewashcycle. This is carriedout
asfollows.
Each filter to be washed is drained down to the
level required to commence washing. This isachieved by closure of the filter inlet, whilst
maintaining thefilter outlet flow. Thisallows thefilter to drain down evenly, without any unduestress to, or compaction of, the filter bedprior to
washing.
When the water level has drained down to the
required level,a fewcentimetres above hemedia,the backwash cycleis started. The surface of themedia shouldshowanevenspread ofburstingair
bubbles coming through. Any unevenness in thedistribution shouldberegarded as an ndication of
potential difficulty, itscauseinvestigated asfaras
possible andtheobservation logged.
If ufficient time has been allowed orcleaning ofthe filter media the backwash water coming upthrough themediabecomes clear. This generallytakes from threeto eightminutes. If looding ofwashwater channels orcarryover of ilter media sa problem, the backwash rate must be reduced.Thismaybe accomplished by adjusting thebackwash control valve, thereby throttling heamount
ofwashwaterused.
Inmanywatertreatment plantsbackwash water isallowed to settle in a tank, and then the
supematant (clear, top portion of water) is
pumped back, if possible to the reservoir or
balancing tankfeedingthe works, to be recycledthrough the plant. Usually it isbest to graduallyaddthebackwash water to theheadworks of thetreatment plant(ahead of theflash mixer). This
is becausea sudden return requires changes inchemical dosagesdue to the additional flow andincreased turbidity.
Procedures forbackwashing afilter undermanualcontrol are providedoverleaf: (Referto Figures12 and 13).
11.5 FILTERSTARTUPPROCEDURES
The initial few hours after a filter is placed in
service isa time whenturbiditybreakthrough canpose aproblem. For hisreason,filters shouldbeeased into service o avoid hydraulic shock loads.
Afterwashing, filters shouldbe broughtback online gradually. With automatic equipmenthis is
generally donebyagradual openingof hefilter'soutflow valve (usually an actuated butterflyvalve). Manual operations require a gradualincrease of the amount of water treated by the
filter,usually achievedby anautomatic slowstartcontrol device. Many plants haveprovisions to
waste some of the initial filtered water (byopening V-6). This provision can bevery helpfulif turbidity breakthrough occurs. Turbidity
analyses of filtered water should be carriedoutand processadjustments madeasnecessary.
11.6FILTERSHUTDOWN PROCEDURES
Remove thefilter fromservice by:
•closing
inflow valve(V-l);
• allowing filterto drainto correctbackwashinglevel; and
• closing outflow valve (V-5).
Backwash the filter, as describedin precedingsection. If the filter is to be out ofservice for a
prolonged period, drain water from filterto avoid
algae growth. Note status of the filter in
operating record. Backwash againbeforeplacingfilter on-line',asnotedpreviously.
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STARTUP ANDSHUTDOWN PROCEDURES
TABLE5:PROCEDURE FOR BACKWASHINGA FILTERUNDERMANUALCONTROL
Step ActionLog length offilterrun since lastbackwash
2 Logfilterloss ofhead(L.O.H.)
3 Closefilterinflow valve(V-1)
4 Allow filter to drain tocorrectbackwashing level
5 Closefilteroutflow valve (V-5)
6 Opendrainvalve(V-4)
7 Open air scourvalve(V-7)
8 Start air blower(s)
9 Runairscour orpre-determined period
10 Open backwash valve(V-3)
11 Startwash water pump(s) at owrate
12 Run low rate wash forpre-determined period
13 Stopair blower(s)
14 Closeair scourvalve(V-7)
15 Opensurface washvalve(V-2) (optional)
16 Increase wash rate toHighRateWash
17 RunHighRateand surface wash (optional) forpre-determined period
18 When washwater going o drain becomes clear:• Closedrain valve (V-4)• Allow filtertorefill
19 Once filter is refilled, put nto service by:• Stopping wash waterpump(s)• Closing backwash valve(V-3)• Closing surface wash valve (V-2) (optional)• Opening ilter inletvalve(V-i)
• Opening ilteroutletvalve (V-5)
20 Logdurationofeach partofwash and watervolumes used
21 Llossofheadatcommencement ofservice run
49
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50 FILTRATION
FIGu1u13: BACKWASHMODEOF OPERATION
FIGuRE 12: FILTRATIONMODEOF OPERATION
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PROCESSANDSUPPORT EQUIPMENT OPERATION ANDMAINTENANCE
12 PROCESS AND SUPPORTEQUIPMENT OPERATIONAND MAINTENANCE
51
12.1 TYPES OF EQUIPMENT
To runa iltration process he operating personnelmust be competent and familiar with the
operation and minor(preventive) maintenance of
a varietyof mechanical, electrical, andelectronic
equipment including:
• filter control valves;
• backwash and surface washpumps;
• air (dryer)compressor ndblowers;
• flowmeters and leveL/pressuregauges;
• water qualitymonitorssuch as colorimeters,
thermometers, pH and conductivity meters,turbidimeters and dissolved aluminium
detector/recorders;
•process monitors(head
loss and waterlevel);and
• electrical filtercontrol systems.
Since awide variety ofmechanical, electrical, and
electronic equipment is used in the filtration
process, the operating personnel should be
broadly familiar with the operation and
maintenance instructions for each specific
equipment temorcontrol system intheplant.
12.2EQUIPMENT OPERATION
Beforestartinga piece ofmechanical equipment,such as a backwash pump, be sure that the unithas been serviced on schedule and itsoperationalstatus spositively known.
After startup, always check for excessive noiseand vibration, overheating, and leakage (water,
lubricants). When in doubt about the
performance ofa pieceofequipment, refer tothe
manufacturer's nstructions.
Much of the equipmentused in the filtration
process may be automated and only requireslimited attentionby operating personnel during
normal operation. However, periodic calibration
and maintenance of this equipment is necessary,and this usually involves special procedures.Detailed operating, repair, and calibration
procedures are usually described in the
manufacturer's orsuppliers literature.
12.3PREVENTIVE MAINTENANCEPROCEDURES
Preventive maintenanceprogrammes aredesignedto ensure the continued satisfactory operation oftreatment plant facilities by reducing the
frequencyof breakdown. This is accomplished
by performing scheduled or routinemaintenance
on valves, pumps, and other electrical and
mechanical equipmenttems.
In the normal operation of the filtration process,routinemaintenance functions mustbe performedas part of an overall preventive maintenance
programme. Typical unctions include:
•keepingelectric motors free ofdirt, moisture,and pests (spiders, flies, larvae, rodents and
birds);
• ensuring good ventilation (aircirculation) n
equipmentworkareas;
• checkingpumps and motorfor leaks, unusual
noise,vibrations oroverheating;
•maintaining proper lubrication and oil levels;
• checking bearings for overheating and proper
lubrication;
• checkingfor proper valve operation, leakageor amming;
• checkingautomatic control systems orproper
operation;
• checking air/vacuum relief systems for proper
functioning, dirt andmoisture;
• checking chemical delivery lines for leakage
[chemical delivery lines should be colour
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52 FILTRATION
coded, placed in ducts and lengths
minimisedi;
verifying correct operation of filtration and
backwashcycles byobservation; and
inspecting filter media condition (look for
algae and mudballs and examine gravel and
media forpropergradation).
Accurate recordkeeping is the most importantelementofany successful preventive maintenance
programme. Theserecords provide operation and
maintenance personnel with clues fordeterminingthe causes ofequipment failures. Theyfrequently
can be usedto forecast
impendingfailures thus
avoiding costly repairs.
12.4 SAFETY CONSIDERATIONS9'10'11
The filtration process does not normally involve
the useof chemicals. There are howeverhazards
involved for theplantoperators, which shouldbeidentified in the Safety Statement prepared for
each treatment works, as required by the Safety,Health and Welfare at Work Act, 1989 and
Regulations made under the Act. Reference
should be made to this Safety Statemenl by all
persons involved in the operation and
maintenance of he works.
Some of the potential hazards, which might be
encountered in differentareas and operations in
the filtration process are listed in Table6.
TABLE 6:POTENTIALHAZARDS N FILTRATION
Area Uperation
Electical Equipment
Mechanical Equipment
OpenWater Surfaces
Confined Space
PotentialHazard
• earthing of ools• locking out and tagging of switches and panels when
servicing equipment• electric shock due to lying waterorgrounding onpipes
• removal ofguards from rotating equipment• locking out and tagging of switches and panels when
servicing pumps, automatic valves andotherequipment• slippery surfaces due to ubricant spills• wearing of ooseclothing in the vicinity of rotating
equipment
• damage tohandrails or ailure toclose safety chains• slippery surfaces on stairways or adders due to spillages or
use ofunsuitable footwearS
• Hazardous atmospheres (toxic or explosive gases, lack of
oxygen)• Presence ofdust
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GLOSSARY 53
GLOSSARY
ABSORPTIONThe taking inorsoaking up ofonesubstance intothebodyofanotherbymolecular orchemical action.
ACTIVATEDCARBON
Adsorptive particles or granules ofcarbonusually obtainedby heatingcarbon. Theseparticles orgranules
haveahighcapacity oselectively remove certain trace and solublematerials from water. -
ADSORPTIONThegathering ofagas, liquid,ordissolved substanceon the surface or nterface zoneofanother material.
AIRBINDING(LOCKING)
The clogging ofafilter,pipe or pumpdue to the presence ofair releasedfrom water. Air contained in the
filter media is harmful to the filtrationprocess. Air can preventthe passage of water during the filtration
process and can cause the oss offilter mediaduring thebackwash process.
ALGAE
Algaeare primitive organisms whichareusually classified as plants. There are hundreds ofdifferent ypes,
many of themmicroscopic, whichmaybecome visibleby multiplication. Whenpresent to excess theycause
troubleby blocking ilters. Outbreaks varywith theregionand theseason.
BACKWASHINGThe processof reversing the flow of water, either aloneor in association with air, back through the filter
mediatoremove theentrapped solids.
BASE METALAmetal (suchas iron) whichreacts with dilutehydrochloric acidtoformhydrogen.
BREAKTHROUGHA crackor break in a filter bedallowing thepassageofflocor particulatematter hrough afilter. This will
cause an ncrease in filteroutflow turbidity. Abreakthroughcanoccurwhen a filterisfirst placed inservice,
when theoutflow valve suddenly opensor closes, and during periodsofexcessive head loss through the
filter.
COLLOIDS
Verysmall, finely divided solids (particles that do notdissolve) hat remain dispersed ina liquidfor a long
time due to their small size and electrical charge. When most of the particles in water have a negativeelectrical charge, theytend to repeleachother. Thisrepulsionprevents theparticles from becoming heavier
and settlingout.
COLOUR
Manywaters havea distinct colour, normally due to the presence of complex organic molecules derived
fromvegetablematter(suchas peat,leaves,branchesetc.,), evenafterall turbidity has been removed. This
is expressed in erms of theplatinum-cobalt scale Hazen units). Exceptionally, naturalcolourmaybe due to
thepresenceofcolloidal ironand/or manganese inawater.
CONVENTIONALFILTRATIONAmethod of treating water which consistsof headditionofcoagulantchemicals, flashmixing, coagulation-
flocculation, clarification bysedimentation/ lotation and filtration.
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54 FILTRATION
CRYPTOSPORJDIUM
The general descriptive term for the parasite Crvptosporidiuin Parvuni (C. Parvuni). C. Parvum is the only
species of cryptosporidium knowm to cause disease in man. The environmentally resistant transmittable
form ofcryptosporidium excreted in the faeces ofan infected host is calledanOocyte.
DIRECT FILTRATIONA method of treatingwaterwhich consists ofthe addition ofcoagulant chemicals, flash mixing, coagulation.minimal flocculation, and filtration. The flocculation facilities are occasionally omitted, but the physical-chemical reactions will occur to some extent. Theclarification/flotation process stage is omitted.
EFFECTIVE SIZE (ES)The diamter of the particles in a granular sample (filtermedia) for which 10 percent of the total grainsare
smallerand 90 percent larger on a weight basis. Effective size is obtained by passing granularmaterial
through sieves withvaryingdimensions ofmeshandweighing the material retained by each sieve.
FLASHMIXERA chamber in which
coagulantsare stirred into the raw water with considerable turbulence, inducedeither
hydraulically ormechanically.
FLOCFloe is the fine cloud of spongey particles that form in water to which a coagulant has been added. The
particles are basically hydroxides. commonly of aluminium or iron. They accelerate the settlement of
suspended particles byadhering tothe particles and neutralizing such negative charges asmaybe present.
FLOCCULATIONFlocculation is the practiceof gently stirring water in which floc has formed to induce the particles to
coalesce and 8row and thus settle more rapidly.
FLUIDIZATIONTHRESHOLDThe fluidization thresholdis the point during backwashing where the hydraulic pressure loss through the
filterbedequals the dead (submerged) weight of the filtermaterial.
FLUIDIZEDA massof solid particles that is made toflow like liquid by injection of water or gas is said to have been
fluidized. In water treatment, a bed of filter media is fluidizedby pumping backwash water andlor air
through thefilter.
GARNETA group of hard, reddish, glassy,mineral sands made up of silicates ofbase metals (calcium, magnesium,ironand manganese). Garnet hasahigherdensity thansand.
GIARDIA JNTESTINALJS (Giardialamblia)Aprotozoan parasite capable of infectingmanand causingdiarrhoea.
HEADLOSS
The head, pressureor energy lost by waterflowing in a pipeorchannel as a result of turbulence caused bythe velocity of the flowing water and the roughness of the pipe, channel walls or restrictions caused byfittings. Waterflowing in a pipeloseshead,pressure or energy as a esult offrictionlosses. The head loss
througha filter isdue to friction losses caused by material building up on the surface or in the interstices ofthe filtermedia.
HYRAULICSIZE
The hydraulic size of filter material is that grain size which gives the same surface area, and therefore thesame hydraulic behaviour,as the mixture ofsizes in the filtermaterial.
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GLOSSARY 55
IN-LINE FILTRATIONThe addition ofchemical coagulants directly o the filter inlet pipe. Thechemicals aremixed bytheflowingwater. Flocculation and sedimentation facilities are eliminated. This pre-treatment method is commonlyused in pressure filterinstallations.
INTERFACEThe common boundary layerbetween two substances such as water and a solid (metal); or between twofluids such as waterand gas(air);orbetweena iquid (water) an another liquid (oil).
NTU
Nephelometric Turbidity Unit, numerically equivalent toJacksonTurbidity Unit.
PERMEABILITYThe property of a material or soil that permits considerable movement of water through it when it issaturated.
POREAverysmallopenspace in a rockorgranularmaterial.
POROSITYThe ratio (normally expressed as a percentage ) of thevolume of the space between grains (voids) to theoverall volume of the granular material. This will vary depending on howthematerial has beenhandled,whether tipped, backwashed orpacked. Alternatively calledvoidage.
SLURRYA watery mixture or suspension of insoluble (notdissolved) matter; a thin watery mudor any substances
resemblingit(such as a
gritslurryora ime
slurry)
SPECIFIC GRAVITY
(1) Weight of a particle, substance, or chemical solution in relation to theweightof an equal volume ofwater. Water hasa specific gravity of 1.000at4°C (or39°F). Particulates in raw water mayhaveaspecificgravity of 1.005 to2.5.
(2) Weightofaparticulargas inrelation o an equal volume ofair at thesame emperature andpressure airhasaspecific gravity of 1.0). Chlorine forexample hasaspecific gravity of2.5 as agas.
SUBMERGENCEThedistance between thewatersurface and the mediasurface inafilter.
TURBIDIMETERAn instrument for measuring and comparing the turbidityof liquids by passing light through them and
determining howmuch light is reflected bytheparticles in the liquid. The normal measuring range is 0 to100andisexpressed as Nephelometric Turbidity Units(NTUs).
UNIFORMITYCOEFFICIENT(U.C.)The ratio ofthediameter ofa grain ofa size that is barelytoo largeto pass through a sieve that allows60
percent ofthematerial (byweight) to pass through,to thediameter of a grain of a size that is barely too
large to pass through a sieve that allows 10 percent of the material (by weight) to pass through. The
resulting ratio is ameasure of thedegreeofuniformity inagranularmaterial.
Uniformity Coefficient = ParticleDiameter 0% /ParticleDiameter 0%
VISCOSITYThe property ofa substance to offer internal resistance to flow. Specifically, the ratioof the shear stress tothe shearstrain.
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56 FILTRATION
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APPENDIX A 57
APPENDIX A: TYPICAL FILTERS OPERATING RECORD
SHEET
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58 FILTRATION
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APPENDIX A 59
Sheet No.: 3/F
NAMID AIJFHOR1TY
'a€4de9 WATFR TRFATMRT PLANT
FILTERS
Daily Operating Record Date: 7/11/94
Filter Tine&Date Hours Operated Head Loss Backwash
No. Stail Stop TodajPrevious Total Start(m) Stop(m) Minutes OutityjpNotesoncondition of ilters
&problems inope_
1 04/11 07/?! 11:30
0S30 /1.30
2 05/11 '4:00
17:00
63:30 75:00 0.15 F.15 6 10! owJtatdt12
amalt'caeemed 2.30
31 55 0.155
3 03/!! 24:00 54 71 0.165
/100 :
4 03/11 07/1/ /1:00
14:00 /1.00
12:00 100:00 0.15 1,1 5 14 Stt F130.
a(eemed 9.00
5 05/!! 24:00
09:15
31:45 62:45 0.145
6 04/11 24:00
17:20
30:40 54:40 0.16
Shift I Operator
No.of ilters washed 2 Average Filtration Rate -ni/hr 4.97
'I9€31
AverageRun-Hours -TotalWash Water-m3
Percent ofWaterFiltered
17.5 MaximumFilter Rate -m3fhr 720
115 TotalWaterFiltered-n
1.01 No.ofFiltersOperating
/7160
6
AV.TiIIr ofWash -Minutes 5.5 Filters OutperWash -Mins 30
WaterQualitySampling Integrator Readings •
Location Turbidity Colour
Inflow
Temp (C) pH Timu Location Units Read'g/Tin Difference from Previous
Outflow I
Inflow Production Report Percentage &Reniirks
Outflow Total Inflow Oj.m 100%
Inflow To SupplyOutflow To Site Storage
To Filter Backwash
Other --
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60 FILTRATION
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APPENDIX A 61
NAMED AUTHORiTY
WATFR TRFATMENT PLANT
FLLTFRS
Sheet No.: Daily Operating Record Date:
Filter Tirre&Date Hours Operated Head Loss Backwash Notesoncondition of ilters
Start Stop Today Previous Tota1Start(m) Stop (rnj_ Minutes[Quantity m3 &problems in operation
1
2'I - . -
— -.--.. ——--....— I -- —5
6
Inflow
Outflow- Production_Report
Totalthflow'
Cu.mPercentage &Remarks
100%
Inflow ToSupply
9 tfiow To SiteStorage[ToFilterBackwashj
.
______ ______ _______ _______ ______ __________ Shift I Operator
[No.of ilters washed - ____ AveragçiltrationRate -mhr
AverageRun-Hours -. - MamarnFilter Rate -m3/hr ________
TotalWash Water- n3 TotalWaterFiltered m3
Percent ofWaterFiltered _______- No.ofFiltersQperating _______
TiofWas_TNinutes. [Filters OutperWash -Mins ________
WaterQualitySampling ____ _____ IntegratorReadings -
Lthca1L Tuthidy Colour_Temp C) pH Tint Location Units Readg/Tme Difference fromPrevious
Inflow
Outflow [
- - __________ _______________________
Other - . _______ ____
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62 FILTRATION
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APPENDIX B 63
APPENDIX B: CONVERTING TO SI (METRIC) UNITS
CONVERSION TO SI(METRIC)
UNITS FROM IMPERIAL
(FPS )UNITS'4"5
The SI ( orMetric)base units ofmost interest forwatertreatment are thoseof time, massand length. Theunitoftime is the second(symbol 's'),whichis the same in bothImperial and SI systems. The unitofmassis thekilogram symbol 'kg') (1 kg = 2.20462 lbs), which for allpracticalpurposes is themassof 1 litre of
pure waterat20°C. Theunit of length is themetre (symbol 'm') (1 m= 39.37008 inches). The other unitsused in water treatment are derived from these base units. The prefixkilo (k) means 1000 times the unit
following - kilogram (kg) is 1000 grams, kilometre km) is 1000 metres and the prefix mega (M) means
1,000,000 times theunit following but megagram is customarily referred to as a metric tonne. The term
megalitre (Ml) is frequently used for 1000 m3. Conversely, theprefixmilli(m) means one thousandth partof
the unit following - milligram (mg) is 1 / 1000of a gram,millimetre (mm) is 1/1000 ofa metre, millilitre(ml) is 1/1000 ofa litre; the prefixmicro (t - he Greek letter mu ) meansone millionth part of theunit
following - microgram(.tg) is 1/1 000 000 of a gram, while the prefix nano (n) means one thousandmillionth partof the unit following - nanogram (ng)is 1/1 000 000 000 ofagram.
The conversion actors mostneeded inrelation owatertreatment are:
1 gallon (g) =4.54609 itres
1 m3 (1000 1)=219.9736 (220)gallons
1 ft2=0.0929m2
1 m2= 10.76392 ft2
14.7p.s.i.= 1.0 1325bar= 1 atm
The term "parts per million" (ppm) wasformerly widely usedinrelationto thedosingofchemicals to water.This termimplies a elationship ofeithervolume with volumeorweight withweight, butsince thedensityofwater is almostexactly unity, this term is in effect synonymous with "milligrams per litre" (mgfl) and is
widely regardedasbeing so. However thepreferred usageismgll,which is anexpression ofmassorweightofchemical perunit volumeofwater, ratherthanppm.
Clarification tanks are rated in terms of flow per unit area per hour (gallons per ft2 per hour,in Imperialunits)with alternatively an upflowrate in feetperhour. In SI units,flow perunit areaper hour is m3 /m2 /hour or, cancelling m2, m/hour,whichisfrequently expressed as anupflow ate in mmIsec.
A standard rateofslow sand filtration is50gallons persquare footperday.
Converting toSI: 50/(220 x24x0.0929)=0.1019m3/m2 /hour=0.10m hr.
A standard rateof apidgravity iltration is 100gallons persquare footperhour.
Converting toSI: 100/ 220xO.0929)=4.8928 m3Im2 Ihour =4.9 mIhr.
5m/hr = 102.2 gallons per square footlhr.
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64 FILTRATION
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APPENDIX C 65
APPENDIX C: MUDBALL EVALUATION PROCEDURE1'4
FREQUENCYOFMUDBALL EVALUATION
Use this procedure onamonthlybasis,if mudballs inthe topof he filtermaterialareaproblem. An annualcheck is sufficient ifmudballsarenotnormally aproblem.
PROCEDUREFOR EVALUATION1. Backwash the filter to besampled and drain the filtertoat east 300 mm (12 inches) below the surface of
thetop media layer(or layerof nterest).
2. Push themudball sampler some 150 mm(6 inches) into thesand near one cornerof the filter. Tilt thehandle until itisnearly evel then lift the sampler ullofmedia andempty its contents intoabucket.
3. Takesamples neartheother cornersand at thecentre ofthe filter andplace
contents nbucket.
4. Hold a 10 mesh (9.5mm/ 8") sievein a bucket or tub ofwaterso that it is nearlysubmerged. Take ahandfulof thematerial removed fromthefilter andplaceitin thesieve. Gently raiseand lower he sieveabout 12 mm ('/2") at a timeuntil thefilter sand is separated from the mudballs. Shift themudballs toone sideby tipping thesubmerged sieve andshaking itgently.
5. Repeat the process until all the material taken from the filter has been washed in the sieve and themudballs separated. If he volume ofmudballs impedes thewashingprocess, move someof them o the
measuring cylinder used inthe nextstep.
6. Place 500 ml ofwater ina 1000ml graduated cylinder. Usea largeror smallercylinder depending on the
volume ofmudballs. When water hasceased to dripfromthemudballs on thesieve,transfer hem o thegraduated cylinder and record the new water level in it. The volume ofmudballs in ml is obtainedbysubtracting500 from the new water evel.
The total volume ofmaterial removed fromthe filter,if the 'mudballsampler' was full each time, would be3,540mJ. This can bechecked by drainingandmeasuring, by displacement, the volume ofsand washed fromthe mudballs. Thepercentage mudball volume iscalculated as
Mudball Volume ml)x 100
3,540
Mudball Volume (%) ConditionofFilteringMaterial
0.0- 0.1 Excellent0.1 -0.2 Vervood0.2-0.5 Good0.5-1.0 Fair1.0-2.5 FairlyBad2.5-5.0 Badflyer5.0 VeryBad
Thecondition of hefiltermaterial isevaluatedby reference to theprecedingTable.
Mudballs sinkmore readilyinanthracite thansand. Thereforemodify he aboveprocedure tocollectsamples fromthebottom 150 mm(6" )ofanthraciteby temporarily removing the upper ayers.
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66 FILTRATION
FIGURE 14: MUDRALLSAMPLER
75 mm diametercopper tubing
150 flfll 1— 200
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APPENDIXD 67
APPENDIX D: HYDRAULIC CALCULATIONS
HYDRAULIC CALCULATIONS5'6"1'12
FILTRATION RATE
In ordertocalculate hefiltration rate, the surface areaof the filter media(not he areawithin the filter walls)must firstbedetermined. The length of hefiltermedia surface maybemeasured between ilter wallsbut he
width, exclusive ofwashwater channels, must be measured when the filter isdrawndown forbackwashing,unless reliable drawings, from which measurements maybe taken, areavailable. Theoutputfromthe filter
for a specific period, say one hour, after it has fullymatured, maybe read from the flow meter, if one is
provided. Thefiltration rate is found by dividing heoutput nm3by thesurface areainm2.
Example:Length of ilter betweenwalls =3.5 mWidth offiltermedia =2.0m (fromwall towashwater channel)
Surface areaoffilter media = 7m2.
Outputoffilterin 1 hour(metered) =33.6m3
FiltrationRate =33.6m3perhr/7m2
=4.8m/hr.
The verage filtration ate forafilter runmaybe calculated similarly f he length of run (say 50 hours)andthe output say 1680m3)are known.Theaverage filtration ate=1680m3/50hours/7m2=4.8mIhour.
Theoutputof hefilter,ifameter s notprovided, maybeestimatedbyclosing the nlet valve andmeasuringthedrop in the water level in the filter in, say, 5 minutes. The estimated hourly filtration rate would betwelve imes the dropin level measured. In theexample abovethedrop in water level in 5 minuteswas 400
mm,so thedrop in an hourwould be 4.8 m, i.e., the filtration rate. The outputof he filter is the filterrate
bythe areaof hemedia:
4.8x7=33.6m3/hr.
The filtration rate for abankof filters all of the same size can be calculated by dividing themeteredoutputfromthebankby the otal filter area(the numberof iltersmultipliedby the ndividual filterarea).
UNITFILTERRUNVOLUME
The Unit Filter RunVolume (UFRV) canbeavaluable indicatorof ncipientor futureproblems inafilter,aswellasausefulmeasure ofcomparison between individual filters. The UFRV or thefilter runquoted in he
example above[output 1680m3 fromafilterofareaof 7m2I s 240m3 /m2.
In a case where the output from an individual filter is not metered, the UFRV can be calculated,byestimating the filter ratea number of timesduring the course of the run from themeasured drop in filter
level, andmultiplying theaverage filter ratebythe length of un. The filterrate, in theexample givenaboveis4.8 m/hr(or 4.8 m3/m2 /hr). If he length offilter run were 52 hoursand30minutes he UFRV for the
filterwouldbe4.8x52.5=252m3/m2.
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68 FILTRATION
FILTER BACKWASHING
Backwashing is the reversal of the flow of water through a filter, possibly preceded or accompanied by air
scour, toremove accumulated matter emoved from themedia.
The hydraulics of filter operation have beenderived from a dimensional analysis of the factors influencingfriction losses, based on initial work by Darcy in France on friction losses in pipes. This work was
developed by Weisbach, Kozeny andCarman.
The Kozeny equation describes the pressure gradient. or laminar low, inabedofgranularmaterial,as
= 5(1—
h e3
where
Apis the pressure difference across a bed ofdepth h.e is the porosity of themedia,S is the specific surfaceperunit volume ofgrain (=6/d for spheres diameter d),
/1 is theabsoluteor dynamic viscosity ofwater,and
V is theapproach velocity of he water at the bed surface.
During backwashing the particles of filter media are suspended in the flow of water. Fluidization of the
filter bed occurs when the drag force or the head loss thrOughthe bed equals the submerged weightof the
media:
(pç-p)(1—e)pgwhere
g is the gravitational constant,
p. is thedensity of hemedia grains, and
p is the density ofwater
A p/h cannot exceed the above limit because the porosity 'e' automatically increases as a result of bed
expansion whichcorrects hehead oss. Theimportant parameters in theaboveequations andthe importantcharacteristicsof iltering materials) are thegrainsize (d) orsurface area(S),thedensity difference (Ps -
Pw)
and the porosity e'.
Backwash water is normally pumped, by fixed rate pumps, through the filters at the rate chosen by the
designer to suit themedia being used. The rate is chosento expand the bed by the requiredpercentage for
efficientcleaning without oss ofmedia over the weir walls. Losses ofmediamayoccur ifair scour pumpsare startedwhile high ratebackwash is inprogress, although interlocks to prevent this are normally provided.
While the density affects the fluidization threshold linearly, the grain size occurs as a square, so thata 10%
increase in grain size requiresa 20% increase in backwash rate. Sieves, as used in analysis of filtering
materials, are spaced at 20% intervals (4J2) and hence a material which is one sieve size coarser requires abackwash rate 44% higher. A specific grainsize must be used in makiug calculations with these equations,
although themediamaynotbe monosize. The parameter needed is the size which givesthe same hydraulicbehaviour asthemixture ofsizes in the media(same surface areaperunitvolume) and is called the hydraulic
size. Thismaybe calculated from the results ofa sieve analysis of the media. The method and an example
aregiveninthe standard published by the British Effluent &WaterAssociation6.
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APPENDIX D 69
Calculation ofHydraulic Size:
1.Divide he %retainedon eachsuccessive sievebythesizeof the sieveaperture.2.Add
upthe
figureshus obtainedand divide
by100.
3.Obtain thereciprocal of heabovesum.4.Add10%tothereciprocal tocorrect he "retained" size tothe centre sizebetween adjacent sieves(seepreceding paragraph).
Example
Sieve Size RetainedonSieve Calculation
2.8 0.1 0.042.36 0.5 0.21
2.0 10.2 5.1
1.7 22.1 13.01.4 42.3 30.25
1.18 22.5 19.1
1.0 1.41.41
0.850 0.35 0.350.71 0.5 0.7
Divide totalcalculated by 100 =0.70Obtain reciprocal of0.70 = 1.43
Add10% =0.14HydraulicSize =1.57mm
Porosity depends on the shape of thegrains. A 1%change n porosity (e.g. from40% to41%)willproducea 9.5% increase inthebackwash fluidization threshold. The temperature of thewater affectsthe requiredbackwash rate as theviscosity ji) s reduced byhalf from 1.781 at0°C to 0.798 at30°C. Thisrangeofwater
temperatures wouldbe extremely unlikely to be encountered in Ireland, but the temperature effect mightcombine witha oss offines fromthemedia, over time, to reduce the effectiveness of thebackwashing incleaning themedia horoughly.
Where flow velocities are abovethe laminarregion, the Carman modification of theKozeny equation isappropriate:
R= 5Re1+O.4Re°'
iV2
where
('11 V't1_e)PV2 and Re
pV2 e3 S(1—e)ji
where R1 is theshearstress at he surface of hegrains
Re is theparticle Reynolds Number
These equations, although presenting problems for manual computation, are readily managed in simplecomputer programs.
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70 FILTRATION
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REFERENCESANDREADING LIST 71
REFERENCES AND READING LIST
1. US EPA(1994) WaterTreatmentPlantOperationVolume1:AFieldStudy Training Program.ThirdEdition.
2. Huisman, L. & Wood,W.E.(1974)SlowSandFiltration, Geneva, WorldHealthOrganisation.
3. Smethurst, George. (1990)Basic Water Treatment. Second Edition,ThomasTelford Ltd.,London.
4. American Waterworks Association. Water QualityandTreatment AHandbookofCommunityWater
Supplies. FourthEdition. A.W.W.A., 6666 W.QuincyAve.,Denver,CO 80235.
5. Stevenson, D.G.,(1994)TheSpecification ofFilteringMaterials orRapidGravityFiltration inJournal
of nstitution ofWater andEnvironmentalManagement, Volume8,October 1994.
6. British Effluent and WaterAssociation (1993)Standard or theSpecflcation,ApprovalandTesting ofGranularFilteringMaterials.BEWA:P.18.93
7. Twort, A.C., Law, F. M., Crowley,F.W.,andRatanayaka, D.D.,(1994) Water Supply. 4thEdition.
EdwardArnold, London.
8. EuropeanCommunities(QualityofWater Intended orHumanConsumption)Regulations 1988. (S.!.
No 84of1988) Stationary Office, Dublin.
9. Safety,HealthandWelfareatWork Act,1989 (No.7of1989). Stationery Office,Dublin.
10. Safety,Health and WelfareatWork(GeneralApplication )Regulations,1993 (S.I. No. 44of1993)
Stationery Office, Dublin.
11.CodeofPractice ortheSafety,Health and WelfareatWork( ChemicalAgents )Regulations,1994 (S.!.
No. 445of1994). Stationery Office, Dublin.
12.Peavey,Howard, Rowe,Donald andTchobanoglous (1986) Environmental Engineering McGraw -
Hill,NewYork.
13. Engineering School, UCD(1972). Course Notes- Water QualityManagement.Department ofCivil
Engineering, University College,Dublin.
14.Flanagan,P.J., (1992)ParametersofWater Quality - Interpretationand Standards.SecondEdition.
Environmental Research Unit, Dublin.
15.Mc Evatt(1973) MetricGuide 1. ThirdEdition. An ForasForbartha, Dublin.
16.Department of heEnvironment (1990)Cryptosporidium inWaterSupplies: ReportofheGroupof
Experts. Chairman: SirJohnBadenoch. HMSO,London.
17.Department oftheEnvironment (1994)ProceedingsofWorkshopon Cryptosporidium inWaterSupplies.
Editedby A. Dawson andALloyd. HMSO, London.
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72 FILTRATION
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USER COMMENT FORM 73
USER COMMENT FORM
NOTE: Completed omments tobeforwarded to: The EnvironmentalManagement and Planning Division,Environmental Protection Agency, Ardcavan, Wexford
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Environmental Protection AgencyESTABLISHED
The EnvironmentalProtectionAgencyAct,1992,was enactedon 23
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