Preliminary Treatment Units

Dr. Akepati S. ReddySchool of Energy and Environment

Thapar University, Patiala

Bar Screens

Screens• First unit usually encountered in the STP• Important to avoid damage to subsequent process equipment

and increase overall treatment reliability & effectiveness • Provided on channels carrying the sewage and include

– A bar rack– Bar raking (cleaning) facility– A perforated plate– Temporary storage provision for the screenings

• Often housed in a screening chamber (if provided on an under ground sewer)– Access to the screen– Length of the chamber should be sufficient

• to accommodate the screen and the perforated plate • to facilitate cleaning of the approach channel

– Width of the chamber should include• landing and working space for the worker• Space for temporary storage of the screenings

ScreensApproach channel:

– Should have a straight approach– Grit and heavy materials should not accumulate either

ahead of or following the screen during use Bar screen/bar rack (a screen with parallel bars/rods)

– Bars: thickness, depth and length – Spacing between bars and spacer bars

• 25-50 mm (for manual screens) and 15-75 mm (for mechanical screens)

– Spacer bar: located behind (and perpendicular to bars) out of the way of the lines of the rake and welded to the bars

– Angle of inclination of the screen: 30-45 from the vertical for manual screens and 0-30 for mechanical screens

Perforated drainage plate is provided at the top of bar rack for temporary storage and drainage of rakings

Even when flooded the sewage should pass to the other side only through the bar screen

Front view

Top view

Bar Screen and Perforated Draining Plate

Side view

Spacing bar

Parallel bar

Perforated draining plate

Screens: Classification• Coarse screens (6-150 mm):

– Used to protect pumps, valves, pipelines and other appurtenances from damage or clogging

– Bar screens - two types - hand cleaned screens and mechanically cleaned screens– Chain driven mechanical screens: Front clean - front return type;

Front clean - back return type and Back clean - back return type– Reciprocating rake type bar screens; Catenary screen

• Fine screens (<6 mm): – usually used after the coarse screens to protect the process

equipment– Static wedge wire, drum or step types; Continuous belt screen

• Micro screens (<50 microns) – used principally for the removal of fine solids from the treated

effluent

Screens: Design Considerations• Location - installed ahead of grit chambers and pumps• Approach velocity:

– 0.3 to 0.6 m/sec for manual screens – 0.3 to 1.0 m/sec for mechanical screens

• Velocity through the bar screen: – should be <0.9 m/sec. during peak flow

• Bar width: can be 5 to 15 mm • Bar depth: can be 25-38 mm• Rectangular bars (also teardrop and trapezoidal bars) are used

- Wider width dimension is on the upstream side• Clear openings or spacing between bars

– 25-50 mm for manual screens– 15-75 mm for mechanical screens

• Slope of the screen from the vertical:– 30-45 for manual screens and 0-30 for mechanical screens– Flatter angle increases screen submerged area

Screens: Design Considerations • Head loss through the screens: 150 mm for manual screens

and 150-600 mm for mechanical screens• Screenings handling, processing and disposal

• Screen raking mechanism• Drainage plate• Temporary storage (hoppers or containers)• Removal, transport and disposal - Conveyors/Pneumatic

ejectors may be used to transport screenings• Screenings grinder may be used to grind and shred the

screenings and return to the wastewater• Screenings are discharged directly into a hopper or container or

into a screening press• Mechanical screens: 2 or more screens are provided to

facilitate screen isolation & maintenance• Manual screen: Provided in smaller STPs

– Often provided as standby units - used as an overflow device whenever the mechanical screens are in-operational

Approach channelFind width and slope of the approach channel

Assume depth of flow during peak flow as 2/3rd of the width Find velocity during average flow and check whether it is >0.3 m/sec.

Provide >0.2 m freeboard for the channel

21

32

..1 SRn

V V is velocity (m/sec.): 0.6 m/sec. for design peak flown is Mannings coefficient (0.015 for concrete channels and 0.013 PVC channelsR is hydraulic radius (m) (ratio of flow cross section area to wetted perimeter)S is slope W is width of the channel (m)D is depth of flow (m)Q is flow rate (m3/sec.)Dw

Qv

DwDwR

.

2.

Assume D=2/3 width for design peak flow and solve through iterations

Approach ChannelApproach channel length can be >4 times the channel widthIf the screen is to be provided in a screen chamber then length

of the screening chamber can be

• If perforated plate is planned over the channel behind the screen, its foot print can be >2 times the channel width

• Width of the screen chamber can be (3 times its width) or (channel width +1.0 m), whichever is larger

• Access to the screen should be ensured• Provision for temporary storage of screenings should be made• For mechanically cleaned screens additional space for the facility

and associated machinery should be provided

sewertheofdiameterisDchanneltheofwidthisW

horizontalfromscreenofangleninclinatiois

DWWL channelApproach

2tan3.032

Bar Screen SpecificationsDecide on the bars thickness and depth and on the spacing

between the bars and find the number of bars needed

Decide the inclination angle and find out length of the bars– D/sin(ө) + grouting requirements

(ө is inclination angle from horizontal)

Decide on the spacer bars (number, size and length)Decide on the bar rack grouting requirementsDecide on the perforated drain plate to be used behind the

screen– Width can be 0.2m plus width of the channel– Length can be > 2 times the width

1

barsbetweenbar SpacingThickness

channeltheofWidthBarsofNumber

Head loss across the screenHead loss through a coarse screen can be estimated by

For 50% clogged screens velocity through the screen can be taken as 0.9 m/sec.

Drop provided downstream to screen may be greater than the head loss across it when clogged for peak flow conditions>150 mm for manually cleaned screens

gvV

ChL 2

1 22

C is discharge coefficient (taken as 0.7 for clean screen and 0.6 for

clogged screen)V is velocity through the screenv is approach velocityg is acceleration due to gravity

ScreensControls• Raking mechanisms

– Based on differential head loss through the screen (measuring water level both before and after the screen)

– Based on a time clock (cleaning at predetermined time intervals)

• Flow control/regulation– Slide gates or recesses are provided in the channel, both

ahead of and behind to facilitate dewatering of the screen– Flow is diverted through a bypass channels in larger

installations with the help of slide or sluice gates

http://members.aol.com/ErikSchiff/barscr1.gif

ScreensReciprocating rake type bar screens• Rake moves to the base of the screen, engages the bars

and pulls screenings to the top of the screen for removal• All parts requiring maintenance are above the water line

and can be easily inspected and maintained• Front clean and front return feature minimizes solids carry

over• Have only one rake and hence have limited capacity to

handle heavy screenings loads• High overhead clearance is required to accommodate the

rake mechanism• Grit accumulation in front of the bar can impede rake

movement

ScreensCatenary screen• A type of front cleaned and front return chain driven

screen, but has no submerged sprockets• Rake is held against the bar rack by weight of the chain

(less sensitive to bar jamming by heavy objects)• Multiple cleaning elements, shorter cleaning cycles enable

handling of large objects (very little screenings carryover)• Chains are very heavy, inclination angle of the screen is

higher (43 to 75) and hence large footprint• Jammed racks can cause misalignment and warpage• Open design can cause odors problem

ScreensContinuous belt screen• It is a continuous self-cleaning screening belt that

removes fine and coarse solids• Overhauling or replacement of the screening

elements is time consuming and expensive.• Screen openings may range from 0.5 to 30 mm and

hence be used as either as a coarse screen or as a fine screen

• Have no submerged sprockets

Grit Separators

Grit removal facilitiesLocated after bar screens, ahead of wastewater sump and

pumps – provided to – Protect moving mechanical equipment from abrasion and

abnormal wear (pumps, valves, etc.)– Reduce formation of heavy deposits in tanks, basins, pipelines,

channels, etc.– Reduce frequency of cleaning of digesters

Designed for the removal of grit particles of size 0.15 (100 mesh) or 0.21 mm (65 mesh) and specific gravity >2.65– Sand, gravel, cinders, and other heavy solid particles having high

specific gravity and settling velocities (> organic solids)

Types of grit chambers• Horizontal flow type

– Long grit channels with influent distribution gate at the inlet end and weir at the effluent end – mostly manual grit removal – preferred for low flows (<1 MLD)

– Horizontal grit chambers (square shaped - facilitates proper functioning of raking mechanism – mechanical grit removal - rectangular chambers!

• Aerated type – spiral flow aeration tank is used – preferred for larger flows (>2 MLD)

• Vortex type – Cylindrical tank with tangential entry of flow creating vortex

flow pattern – Centrifugal and gravitational forces cause the grit to separate – Grit cyclones (and hydroclones!)– Preferred for moderate flows (1 – 2 MLD)

Stoke’s Law and Terminal settling velocity of particles

Forces acting on a suspended particle are• Gravity force

• Buoyant force

• Drag force – Increases with increasing speed – zero for zero speed

gpparticle

gpfluid

2

2pfluidpd vAC

ppdv3For laminar flow conditions

Vp is volume of the partcicle

2

3

24

234

pp

pp

dA

dV

Stoke’s Law and terminal settling velocity of particle

Net force of the particle (ma) makes the particle to accelerateWhen drag force becomes equal to the net of gravity force and

buoyant force, acceleration of the particle becomes zero and the particle settles at constant velocity (terminal settling velocity)

pfluid

fluidparticle

dp d

Cgv

34

34.0324

RRd NN

C

pp

R

dvN

Where

Where

18

2p

w

wp

p

dgv

For laminar flow

pw

wpp dgv

33.3

For turbulent flow

is 1.003 x 10-6

Discrete Particle SettlingSettling tanks are designed for a selected design terminal

settling velocity (vt)A particle is considered as removed if it touches the bottom of

the tankFor 100% settling removal, particles with vt terminal settling

velocity have to be surface loading or overflow rate

In a settling basin design settling velocity, detention time (HRT) and depth are related

Actual design takes into account the effect of inlet and outlet turbulence, short circuiting, sludge storage, and velocity gradients due to operation of sludge removal equipment

AQvt

timeDetentiondepthTankvt

Q is flow rateA is surface area

AQHv

WHQ

LHv

WHQv

WHvQLHvv

vL

vH

t

t

h

h

ht

ht

AQvt

Indicates grit removal efficiency is independent of depth and detention time of the channel/chamberDepth can be reduced – scouring problem is a limitation – horizontal flow velocity should be <0.4 m/sec.Increase of depth or width of channel reduces horizontal flow velocity and results in settling of organics

p

s fKgdV 8

‘k’ depends on the material being scoured (0.04 for unigranular particles and 0.06 for sticky interlocked matter)‘ρp and ρ’ are densities of particles and liquid‘d’ is particle diameterf is Darcy-Weisbach friction factor (influenced by surface roughness and Reynolds number, etc. – typical values 0.02-0.03)

Camp-Shields equation for scour velocity

Discrete Particle Settling

Only a fraction of the particles with the terminal settling velocity < design settling velocity are removed in the settling tank

Discrete Particle Settling

td

tp

vv

removedFraction

n

ii

n

ii

td

tpi

td

tptd

n

nvv

dXvv

XremovedfractionTotal

1

1

vtp is terminal settling velocityvtd is design terminal settling velocity

Xtd is fraction with terminal settling velocity >vtd

dX is the fraction of grit with vtp

ni is the fraction of grit falling in the ith category

Horizontal flow grit channels• Representative design data

– Horizontal flow velocity: 0.25 – 0.4 m/sec. (0.3)– Head loss: 30-40% of channel depth (36%)– Added length for inlet and outlet turbulence allowance: 25-50%

of actual length (30%)• At the design horizontal flow velocity heavier grit particles

settle while the organic particles tend to get resuspended• Flow control section at the outlet

– Parabolic channel cross section is ideal if parshall flume is used– Rectangular channel cross section is ideal if proportional weir

(sutro weir) is used• Has isolating gate valves at the inlet• Has provisions for draining out the wastewater and manually

removing the accumulated grit • Floating oil separation is often integrated

Grit channels

Horizontal flow grit channels• Find design terminal settling velocity • Find grit channel surface area for the peak flow condition

• Find flow cross sectional area (for the horizontal flow velocity of 0.3 m/sec. for average flow), and, find the grit channel width (take width to depth ratio of 2:1)

• Find length of the grit channel (A/W) and make adjustments for inlet and outlet disturbances

• Find channel slope for the average flow and the drop needed • Find overall channel dimensions: depth at peak flow for

horizontal flow at 0.3 m/sec. + grit storage depth (0.15 m) + freeboard

• Provide proportional weir (sutro weir) at the control section for maintaining flow depth proportional flow rate

AQvt max Qmax is peak flow rate

A is surface area

322

3297.4

5.0

5.0

ahgabCQ

ahbaQ

d

5.01tan21aybx

.max.max

.

5.0

.max

.5.1.max

.max

262.0

HQQ

a

QQ

gH

Qb

avg

avg

Cd value is 0.6 to 0.65b is taken as ‘channel width – 150 mm’!

Proportional weir (Sutro weir)

Proportional weir for grit channels

32 5.0 ahgabCQ d Cd value is taken as 0.6 to 0.65

5.01tan21aybx

radiansinis

ayhere

5.01tan

Find ‘a’ and ‘b’ values for the proportional weir

.max.max

.5.0

.max

.5.1.max

.max

262.0

HQQ

a

QQ

gH

Qb avg

avg

Find ‘x’ values for different ‘y’ values

Adjust ‘a’ & ‘b’ values to satisfy Q=Qavg for h=0 in this equation

Square (or rectangular) horizontal flow grit chambers– Square tanks with grouted corners– Influent is distributed across the section of the tank by a

series of vanes (adjustable deflector plates) and gates– Wastewater flows in straight lines across the tank and

overflow a weir into the outlet– Mostly designed to remove 0.15 mm grit with 95% efficiency– Rotating, center driven rake mechanism mounted on a

bridge (spanning the basin) rakes grit into a side sump/ collection hopper

– Rake arms have outward raking blades– From sump reciprocating rake mechanism resuspends

organic particles, and concentrates and moves up grit on the incline

– Concentrated grit is washed in a classifier (submerged reciprocating rake or an inclined screw conveyor)

Horizontal flow grit chambers

Grit separator-classifier• Reciprocating rake mechanism is set in an inclined

concrete channel– All drive components and bearings are located above the

liquid level and protected from corrosion– Putrifiable organics are liberated and washed and a recycle

pump returns the organic matter into the degritted sewage• Grit screw classifier: a tubular sheet shaft and helical flight

assembly set in a semicircular inclined concrete trough/channel

• Cyclone separator with a reciprocating rake classifier: grit from the collection basin is pumped through the cyclone separator for concentrating and delivering the solids into the hopper of the classifier for washing and discharge

Grit Collector and Classifier

Cyclone Grit Separator and Classifier

Aerated grit chambers• Rectangular tank with aeration by coarse bubble diffusers

along one side creats spiral flow perpendicular to the flow– Velocity of roll governs size & SG of the particles setting – Strategically positioned longitudinal circulation baffle directs

the rotational flow and a vertical baffle at the far end prevents flow short-circuiting

– Wastewater is introduced in the direction of the roll– Wastewater makes 2-3 turns at peak flow

• Grit hopper: Located along one side of the tank, 0.9 m deep, has steep sloping sides

• Design information– Designed for 0.21 mm grit particles removal– HRT is 2 to 5 minutes at peak hourly flow– Air diffusers: Located 0.45-0.6 m above the normal plane– Width to depth ratio: 1:1 to 5:1 (1.5:1)– Length to width ratio: 3:1 to 5:1 (4:1)– Air supply: 0.2 to 0.5 m3/m.min.

• Expansion by air should be considered in head loss estimation

2000

2500

800 m

Slope 1 in 10

1200

300600m

400

300

300250250

1300

7500

700

Aerated grit chambers

550

1750

250

2501500m

Slope 1 in10

1750

250250

500

Tubular diffuser

Overflow weir

Grit trench

Inlet

250

Drain for degritting

Drain for draining out the sewage

Vortex type grit chambers and Cyclone grit separators

• Type-1– Wastewater enters and exits tangentially– Rotating turbine maintains constant flow velocity and

promotes separation of organics from grit– HRT: 20-30 (30) seconds for average flow

• Type-2– Vortex is generated by the flow entering tangentially at the

top of the unit– Effluent exits the center of the top of the unit– Sized to handle peak flow rates upto 0.3 m3/sec. /unit

• Cyclone grit separators– Usually used in inclined position and deliver grit to classifier– Used mostly for the separation/classification of grit from grit

collection basins or from primary sludge

Grit and its removal and disposal Removal of the accumulated grit• Removal can be simultaneous to operation

– Grab buckets traveling on monorails – Chain and bucket conveyors running full length of grit hopper – Screw conveyors, tubular conveyors, jet pumps and airlifts – Traveling bridge grit collectors

• Removal can be after taking off the grit chamber and involve – Emptying of the grit chamber– Manual removal of the accumulated grit

Grit characteristics and disposal– Predominantly inert and relatively dry – Volatile content: 1 to 56%– Moisture content: 13 to 65%– Specific gravity: 1.3 and Bulk density: 1600 kg/m3

• If no grit chambers provided and if grit accumulates in the primary sludge then for removal – diluted primary sludge is passed through cyclone degritter

• Grit separators and grit washers are used to remove organics– Water sprays in both cases help in grit washing – Inclined submerged rake - necessary agitation is provided for

organics removal while the grit is raised on incline to discharge point

– Inclined screw – lifts the grit up the ramp• Grit disposal

– Transport to landfill and stabilize with lime prior to landfilling (!)– Incinerate the grit with other solids– Pneumatic conveyors for conveying grit over short distances

Grit and its removal and disposal

Equalization

Flow EqualizationDamping of variations in wastewater flow rates and

strength (BOD, etc.) - may be needed to• overcome operational problems• improve performance of the down stream processes –

reduced peak flows and shock loads, and diluted inhibitory substances – for biological treatment

• reduce size and cost of downstream treatment facilitiesDisadvantages of equalization:• Increased capital costs and land requirement• Additional operation and maintenance requirement• Odours problemEqualization types:• Inline equalization: All flow passes through equalization tank

– Better for achieving constituent concentration damping• Off line equalization: only flow above the predetermined limit

is diverted into equalization tank– Pumping requirements are minimized

Design ConsiderationsWhere to locate an equalization facility? – Optimum

location varies with– Characteristics of the collection system – Wastewater to be handled– Land requirements and availability– Type of treatment required – some may not require

• In-line or off-line equalization which should be used?• Equalization ahead of the primary settling: demands

mixing (and aeration) to prevent deposition of solids and odour problems

• Equalization between the primary treatment and the secondary treatment: can cause fewer problems with solids deposition and scum accumulation

• Better grit removal facilities must precede the equalization tanks

Design ConsiderationsEqualization tank volume• Cumulative inflow diagram is used to find the volume

– Plot cumulative inflow against time of the day – Connect starting and ending points by a straight line to obtain

cumulative average outflow plot– Draw two tangents to the cumulative inflow plot (one above and

the other below), parallel to cumulative average outflow plot and take vertical distance between the 2 tangents as the equalization tank volume

– At the low point of the tangent, the basin will be empty, and at the high point of the tangent, the tank will be full

• Cumulative net flow diagram can also be used– Subtract average hourly outflow from the hourly inflow to get the

net flow– Plot the cumulative net flow against time – Take the vertical distance between the lowest and the high

points as the equalization tank volume

Equalization volume

Actual volume of the equalization tank is larger (by 10-20%)• Continuous operation of aeration and mixing equipment do

not allow complete emptying of the basin• Basin may also receive plant recycle streams• Contingency is needed for taking care of unforeseen changes

in flow rates and flow variations – may require freeboardGeometry of the equalization tank• Depends on the available land area and on whether the

equalization is in-line or off-line– Elongated designs are avoided

• Depth depends on land availability, groundwater level and topography

• Free board depends on tank size and wind conditions• Inlet and outlet are arranged to minimize short circuiting –

often multiple inlets and outlets are used• For better access for cleaning, front end loaders and multiple

compartments are preferred

Design Considerations

Equalization tank/basin

Design considerationsEarthen basins, or concrete or steel tanks can be usedAccess, cleaning and safety should be considered in the

design of equalization tanksOperational appurtenances

– Facilities for flushing any solids and grease that may tend to accumulate on basin walls

– High water takeoff for removing floating material and foam– Water sprays to prevent accumulation and aid foam removal– Separate odour control facilities (covered equalization basins!)

may also be neededPump and pump control system

– Pumping facilities are usually required either before or after the equalization tank • before is preferred for greater reliability

– Flow measurement devices and flow regulation devices may be required on the outlet to monitor the equalized flow

Mixing and aeration requirements• Required to blend the contents and prevent solids

deposition• Floating aerators, if used, may need a minimum operating

level of 1.5 to 2 m of the tank– A concrete pad below to minimize erosion may be needed in

earthen basins• Power requirement for mixing may be in the range of 0.004 to 0.008

kW/m3 (when TSS is <210 mg/L)– In case of mechanical aerators vortex formation should be

avoided (provide baffles in circular tanks!) – The aerators may require low level shut off controls– Aerators can be equipped with legs or draft tubes for resting on

• Aeration is required to prevent septic and odorous conditions– Air requirements for aerobic conditions is 0.01-0.015 m3/m3.min– Diffused aeration systems can be used for mixing and aeration

Design considerations

Impact of Flow Equalization on Strength

Start at the point of time when the tank is at residual volumeCompute tank liquid volume at the end of each time period

– Volume already present in the tank at the beginning– In-flowing during the period– Out-flowing during the period

Compute average concentration in the tank– Divide pollutant load (already present + added during the

period) with effluent volume (already present + added during the period) - Instantaneous mixing is assumed here

Compute mass load of the pollutant during the periodShow the effect of equalization on pollution load to the ETP

– Graphically; or by ratios, such as, peak to average or to minimum, minimum to average or to peak; or Stand. dev.

Impact of flow equalization on strength

Residual Volume and Strength Equalization

• Residual volume is usually provided to protect the aeration/mixing equipment provided – If it increases strength equalization also increases

• Equalization tanks can also be designed for strength equalization– Find flow equalization volume – Take different residual volumes and compute the level of

strength equalization achieved– Choose the minimum residual volume at which the

required strength equalization is achieved• Equalization basins can be designed for strength

equalization only but no flow equalization– Liquid volume in the equalization basin is maintained

constant – The inflow is almost balanced by the outflow - all the inflow

fluctuations are reflected in the outflows