SANITARYENGINEERINGREVISIONQUESTIONS&ANSWERS

ByAlbertKenyaniInima

16April2015QUESTIONONE[24marks]

a. Afactoryispoweredusingadieselpowergenerator.Thegeneratorisplacedatthebottomofacornerofaroommeasuring12m[long],6m[wide]and5m[high].Thenoiseabsorptioncoefficientsfortheroomare0.9fortheceiling,0.2forthefloorand0.5forthewalls.Thenoiseabsorptionofthedoorandwindowisnegligible.TheNoisePowerLevel[SWL]ofthegeneratoris100decibels[Ascale].

Listofformulae:

Lp=Lw+10log(Q/(4r2)+4/Rc) Rc=ST/(1m)(i)ExplainthemeaningofthenoisedirectivityfactorQ[2marks]====Modelanswer===Q=Areaofasphere/ActualAreaofNoiseRadiation(ii)WhatisthevalueoftheQfactorforthegenerator,assumingitcanberegardedto

beapointnoisesource[2marks]====Modelanswer====Q=8becausetheareaofnoiseradiationis1/8ofasphereattheroomcorner.(iii)CalculatethetotalroomsurfaceareaST[1mark]====Modelanswer====

Calculationoftotalsurfacearea,ST:=6x12[floor]+6x12[ceiling]+2x[5x12+6x5][walls]=72[floor]+72[ceiling]+180[walls]=324m2

(iv)Calculatetheaverageroomnoiseabsorptioncoefficientm[4marks].====Modelanswer====

STx=72x0.2+72x0.9+180X0.5324x=14.4+64.8+90=169.2/324=0.52v)CalculatetheroomconstantRc[4marks]====Modelanswer====

Rc=STm/(1m)=[324x0.52]/[10.52]

=351Sabines[m2] (Vi)Calculatethenoiselevelexperiencedatthefloorofthefarcorner[oppositecorner]fromthegenerator[4marks]===Modelanswer===Distancetofarcorner=r=SQRT[62+122]=SQRT[180]=13.4m

Lp=Lw+10log(Q/(4r2)+4/Rc)

Lp=100+10log(8/(4X13.42)+4/351)=100+10log(8/(4X13.42)+4/351)=100+10log{0.00354+0.01140}=1001.83=98.17dB b.Twopowergeneratormachineshaveanoisepowerrating[SWL]of90dB(A)and100dB(A).Whatwouldbethetotalnoiselevelifbothmachinesareplacedsidebyside?[3marks]Notethethresholdnoisepowerlevelis1.0x1012W/M2andSWL=10log[W/W0]===Modelanswer===SWL=10log[W/W0]W=W010SWL/10

Totalpower=W1+W2=W010SWL1/10+W010SWL2/10=1012[109+1010]=0.011WattsConvertbacktodecibels,SWLT=10log[0.011/1012]=100.4dBc.Thenoisepressurelevelexperienced20mawayfromafactoryalarmsirenis90dBA.Calculatethenoisepressurelevelexperienced50mawayfromthesireninthesamedirection.Youmayusethefollowingformulaesheet:LP=LW10logA

Areaofsphere=4r2Q=Areaofsphere/Actualareaofnoisepropagation[A][4marks]===Modelanswer==

Actualareaofnoisepropagation=halfasphere=A=2r2LP=LW10logA

LP=LW10log2r290=LW10log[2(20)2]Eqn1LP=LW10log[2(50)2]Eqn2Eqn1Eqn2,

90LP=10log[2(50)2]10log[2(20)2]=10log(2.5)2=7.9dBLP=82.1dBQUESTIONTWO[23marks]

a. Alaboratorytesthasestablishedthatawastewatersamplehasthefollowingparameters:

TotalDissolvedSolids[TDS]=100mg/l TotalVolatileSolids[TVS]=170mg/l TotalSolids[TS]=350mg/l VolatileSuspendedSolids[VSS]=120mg/l

Calculatethefollowingparameters:

TotalFixedSolids[TFS] TotalSuspendedSolids[TSS] FixedSuspendedSolids[FSS] VolatileDissolvedSolids[VDS] FixedDissolvedSolids[FDS]

[5marks]===MODELANSWER===

DissolvedSolids SuspendedSolids TotalSolids

VolatileSolids VDS=50mg/l VSS=120mg/l TVS=170mg/l

FixedSolids FDS=50mg/l FSS=130mg/l TFS=180mg/l

TotalSolids TDS=100mg/l TSS=250mg/l TS=350mg/l

b)Compareandcontrastslowsandfilters[SSF]withrapidsandfilters[RSF][6marks]===MODELANSWER[7marksfor4points]====

Parameter SSF RSF

Spacerequirements Occupieslargearea Occupieslittlespace

Effectiveparticlesize 0.20.3mm 0.40.7mm

Requiredpretreatment Plainsedimentationtank Chemicalcoagulationandflocculation

Filtrationrate Slow Morethan100fasterthanRSF

Maintenance Scrapoffcakelayers Backwashing

Headlossbeforemaintenance

HalfofRSF High

Removalofturbidity Good Good

Removalofpathogens Good[Upto98%] Excellent[Upto99.99%]

Operation Doesnotrequireskilledlabour

Requiresskilledlabour

Initialcosts High Low

Runningcosts Low Highc)Describethepurposeofeachofthefollowingwatertreatmentunitoperations:

Prechlorination Rechlorination Postchlorination

[6marks]==Modelanswer===

Prechlorinationistheadditionofchlorinetorawwater

Thepurposeofprechlorinationis:

Disinfectionagainstmicrobesthatcancausediseasetothoseworkingatthewatertreatmentplant

Preventionofalgalbloomwithinthewatertreatmentplant Odourcontrol Helpsindecolourizationofrawwater

Postchlorinationistheadditionofchlorinetowaterthathasalreadyundergonetheprimaryunitoperationsofsoftening,coagulation,flocculation,sedimentationandfiltration.

Themainpurposeofpostchlorinationis:

Todisinfectthewaterfromdiseasecausingmicrobes[pathogens] Tocreatearesidualeffectsothatthisdisinfectionprocesswillcontinueto

occurinthechlorinatedfor2448hours.

Rechlorinationistheposttreatmentadditionofchlorinetowater.Themainpurposeofrechlorinationis:

Todisinfectthewaterthathasoveragedinthewaterdistributionnetwork[egstagnatedwaterindeadendnetworks]

Toboostthechlorinationresidualeffect Tooffersomedisinfectiontothewateragainstpointofusepollution

d)ExplainusingasketchdiagramthephenomenoncalledBreakpointChlorination[6marks]

====MODELANSWER===Explanation[4marks]Addingsufficientfreeavailablechlorinetocompletelyoxidizeallorganicmatterandammonia[nitrogencompoundsinrawwater].Allchlorineaddedafterthatpointisfreeavailable

chlorine.Breakpointchlorinationistheminimumchlorinationconcentrationthatwillcauselastingresidualeffectinthetreatedwater.Diagram[3marks]

QUESTIONTHREE[23marks]UsingasketchdiagramoftheNgethuWaterTreatmentplantanswerthefollowingquestions[5marks]:

a. Whatisthepurposeofthewatersofteningunitoperation?[3marks]b. Whatisthepurposeofthecoagulationflocculationunitoperation?[3marks]c. Whatisthepurposeofthesedimentationunitoperation?[3marks]d. Whatisthepurposeofthefiltrationunitoperation?[3marks]e. Whatisthepurposeofdisinfectionunitoperation?[3marks]f. Whatisthepurposeofwaterneutralization[pHcorrection]unitoperation?[3marks]

===Modelanswer===

[5marks]

a. Purposeofsofteningduringwatertreatment[3marks]

Thepurposeofwatersofteningistoremovecompoundsofcalciumandmagnesiumfromwater[egsulphates,bicarbonatesandchlorides]

Thesecompoundspreventtheactionofsoap[lathering]andalsoprecipitateinwater

pipesleadingtopipecloggingb.Purposeofcoagulationduringwatertreatment[1.5marks]

Coagulationmainlytargetsorganicandsoilparticleslessthan0.002mmdiameterwhichremaininsolutionduetosurfacenegativecharge

Thepurposeofcoagulationistointroduceparticleswithstrongpositivesurfacechargetoneutralizethenegativeparticlessoastomakethemcoalesce

Purposeofflocculationduringwatertreatment[1.5marks]

Thepurposeofflocculationistocontinuetheworkbeganbycoagulationofcoalescingsmallparticlesintobiggerparticlessoastoremovewaterturbidityandcolour.

Flocculationcreatesfavourableconditionsforsmallparticlestocoalesceintobigger

particles,calledflocs.c.Purposeofsedimentationduringwatertreatment[3marks]

Thepurposeofsedimentationistocreatethesettlementoflargeflocs[macroflocs]bygravityontothebottomofthetankwheretheyformsludgethatiscontinuouslyremovedanddisposed.

d.Purposeoffiltrationduringwatertreatment[3marks]

Thepurposeoffiltrationistomechanicallyandbiologicallyremovemicroflocsthatdonotsettleinthesedimentationtank.Biochemicalreactionsinthefiltermediaalsohelpremovesomemicrobes.

e.Purposeofdisinfectionduringwatertreatment[3marks]

Thepurposeofdisinfectionistokilloffdiseasecausingmicrobesthatmaystillbepresentinthewater.

f.ThepurposeofpHCorrectioninwatertreatment[3marks]

ThepurposeofpHcorrectionistoensurethewatertransportedtowaterdistributionnetworksisnotcorrosivetothepipes.

pHmustalsobestrictlycontrolledinordertopreventtheproductionofDisinfectionBy

Products[DPBs],somewhichcancausecancer.DBPsformationincreasesaspHislowered,andespeciallyataroundpH=3.

QUESTIONFOUR[23marks]a)UsingagraphicalsketchdifferentiatebetweenthetermsBOD(remaining)andBODexerted(consumed)[3marks]==Modelanswer====

b)Usingasketchdiagram,brieflygivetheexplanationsforthefollowingphenomenaasobservedduringtheoxygensagcurve:

Increasingoxygendeficitphase Thecriticalpointphase Thestreamrecoveryphase

[6marks]===Modelanswer====

InitiallytheoxygendeficitincreasesbecausetheoxygendemandbyBODconsumingmicrobesexceedsthenaturalstreamaerationability,givinganetnegativeoxygenconcentrationchangeinthestreamwater.

AtthecriticalpointthestreamnaturalaerationabilitybecomesequaltotherateofoxygendemandbymicrobesconsumingBOD.Thenetoxygenconcentrationinthestreambecomeszero.TheDOconcentrationinthestreamislowestduringthisphase

Afterthecriticalpoint,theoxygenconcentrationofthestreamwatercontinuallyincreasestowardssaturationbecausetherateofoxygenremovalbymicrobesbecomeslessthantherateofoxygenadditionbynaturalstreamaerationprocess.

c)TheBOD(remaining)at3dayswasfoundtobe350mg/l.TheBOD(remaining)at5dayswasfoundtobe100mg/l.Calculatethefollowingparameters:

UBOD BODdecayconstant BOD(remaining)at8days. BOD(consumedat8days)

[8marks]===Modelanswer==

BODt=UBODeKt350=UBODe3K....Equation1100=UBODe5K.....Equation2Eqn1dividedbyEqn2,3.5=e2KLn3.5=2K=1.25k=0.625UsingEqn1,350=UBODe3K....Equation1350=UBODe[3xo.625]=UBODe1.875=UBOD/6.5UBOD=350x6.5=2282mg/lThefirstorderkineticsmodel,BODt=2282e0.625tt=8BOD8=2282e0.625x8=15.4mg/lBOD8[Consumed]=UBODBOD8[Remaining]=228215.4=2266.6mg/ld)TheGaussianPlumeModel[givenbelow]iswidelyusedtoestimateairpollutionduetoindustrialfluegasemissions:

Windpowerlaw,

Explainhowyouwouldobtainthefollowingparameters:

Thestandarddeviationparameters Thewindspeed Effectivestackheight Groundlevelpollutantgasconcentrations

[6marks]==Modelanswer===(i)Standarddeviationparametersareobtainedfromlocallydevelopedgraphs[orequations]forvariousatmosphericstabilityclassesandvariousgases,eg

(ii)Windspeediscalculatedfromwindpowerlaw,

(iii)TheeffectiveheightcanbefoundusingPlumeriseformulaegBriggssorHollands,

[iv)Groundlevelcenterlinepollutantgasconcentrationsoccurwhen,Z=0Y=0Substitutingthisintheformulaweobtain,