4082wwt grit removal

8/13/2019 4082WWT Grit Removal

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Grit Removal

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Location of Grit Removal Facility

Location Advantages DisadvantagesAhead of lift Max. protection of pumping Frequently deep in the

station equipment ground, high constructioncost, not easily accessible,and difficult to raise the gritto ground level

After pumping Ground level structure - Some abnormal wear tostation easy to access and pumps

operateDegritter in Usually low capital and Pumping equipment not

conjunction operation and maintenance adequately protectedwith primary costs, cleaner and drier

sludge grit

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Velocity-Controlled Grit Channel

A long narrow sedimentation basin with better control offlow through velocity - used for small plants

Design FactorsDetention time: 60 sec.Horizontal velocity: 0.3 m/secSettling velocity for a 65-mesh material: 1.15 m/minHeadloss: 30~40% of the max. water depth in the channelGrit removal: manual or mechanical

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Grit Channel

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Recommended for small to medium size plants. This requires highmaintenance and grit removal is not easy. In this plant, a crane is

used by an operator, which is an labor-intensive operation.

Grit Removal System

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Grit removal by grab bucket crane.


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Grit Removal System

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Grab bucket crane

Grit Hopper

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Grit Loading System

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Pista Grit Removal SystemOperates on the vortex

principle.Headloss: max. 0.25 inchRemoval efficiency95% of the 50 mesh size grit83% of the 80 mesh size grit73% of the 140 mesh size

gritCapacity: 1~70 MGDCan be installed above or

below groundLower power usageSupplied in steel for easy

installation and/or attachmentto a concrete channelInstalled in multiples

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Removal

System

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Pista Grit Removal System


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Grit Removal System

Impeller mixer

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Grit King Dynamic Separator Has no moving parts Requires no external power source Virtually maintenance free Highly efficient w/ min. headloss Recovers clean grit Self-cleansing

Designed to operate over a widerange of flows

Compact, requiring minimal space Simple to install and operate Easily linked with new or existing

plant Economical, reduces long term

expenditure

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Grit Washing/SludgeDegritting System

Degritting dilute primary orsecondary sewage sludges

The SCS uses a very strongfree vortex and an acceleratedboundary layer to separateabrasives as small as 50 msand from organic solids andwater and concentrate theseabrasives in a slurry stream.Sand is then separated fromthe slurry stream anddewatered by the total particlecapture GRIT SNAIL.

Washing (classifying) and dewatering grit abrasives removed by aheadworks grit chamber

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Grit Separation and Washing Unit

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Grit Separation and Washing Unit

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Aerated Grit ChamberWidely used for selective removal of gritsCreate a spiral current within the basin using diffusedcompressed airDesigned to removegrit particles having aspecific gravity of 2.5and retained over a65-mesh (0.21-mm )screenUsed for medium tolarge treatment plants

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Aerated Grit Chamber

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Aerated Grit Chamber- continued

AdvantagesCan be used for chemical addition, mixing, andflocculation ahead of primary treatmentFresh wastewater, thus reduce odors and remove BOD 5Minimal headlossGrease removal by providing a skimming deviceRemove low putrescible organic matter by air supplyRemove any desired size by controlling the air supply

Volatile organic compound (VOC) and odor emissionDue to a health risk, covers may be required ornonaerated type grit chambers may be used.

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Aerated Grit Chamber - continuedDesign Factors

Depth: 2~5 m; length: 7.5~20 m; width: 2.5~7 m;width/depth ratio: 1:1~5:1; length/width ratio: 2.5:1~5:1Transverse velocity at surface: 0.6~0.8 m/secDetention time at peak flow: 2~5 minAir supply: 4.6~12.4 L/secm of tank length (3~8 cfm/ft) -Higher air rate should be used for wider and deeper tanks;provision for air flow controlInlet structure: Inlet to the chamber should introduce theinfluent into circulation pattern. > 0.3 m/sec under allflow conditionsOutlet structure: Outlet should be at a right angle to theinlet. > 0.3 m/sec under all flow conditionsBaffles: longitudinal and transverse bafflesChamber geometry: consider location of air diffusers,sloping tank bottom, grit hopper, and accommodation ofgrit collection and removal equipment

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Aerated Grit ChamberDesign Checklist

Design average, peak, and low initial flowsInformation on existing facility in case of expansion,site plan, and topographic mapsType of grit removal facility to be providedInfluent pipe data, and static head, force main, hydraulicgrade line if grit removal is preceded by a pumpingstationHeadloss constraints for grit removal facilityTreatment plant design criteriaEquipment manufacturers and equipment selectionguides

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Design Criteria Used in Example

Two grit chambers with spiral circulationTypically designed for max. flow delivered by thepumping station: 1.56 m 3 /sec ( 24,700 gpm)Design peak flow: 1.321 m 3 /sec; due to friction headlossand installation of variable-drive pumps, use designpeak flowDetention time: 4 min at Q maxAir supply rate: 7.8 L/secm of tank lengthProvide nozzle diffusers with coarse bubbles.Provisions for 150% air capacity for peaking purposes

Inlet and outlet min. velocity: 0.3 m/secChamber width: 3.5 mScrew conveyer to move the grit to the hopper and grabbuckets for grit removal

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Design ExampleA. Geometry of Grit Chamber1. Q max through each chamber: 0.661 m 3 /sec

Volume: 0.661 m 3 /sec 4 min 60 sec/min = 158.6 m 3Average water depth at midwidth: 3.65 mFreeboard: 0.8 mTotal depth: 3.65 m + 0.8 m = 4.45 mSurface area: 158.6 m 3 /3.65 m = 43.5 m 2Length: 43.5 m 2 /3.5 m = 12.5 m 13 mDesign surface area: 3.5 m 13 m = 45.5 m 2

2. Diffuser arrangement: along the length of the chamberon one side and place them 0.6 m above the bottom

3. Actual detention time at Q max = (3.5 m 13 m 3.65m) (0.661 m 3 /sec 60 sec/min) = 4.2 minWhen only one chamber is in operation, HRT = 2.1 min

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Design Example - continuedB. Design of Air Supply System1. Air requirements

Air required = 7.8 L/secm 13 m = 101.4 L/secTotal capacity of diffusers: 1.5 101.4 L/sec = 152.1L/sec per chamberBlower capacity: 2 152.1 L/sec = 18.3 standard m 3 /minProvide two 20 m 3 /min blowers (one standby unit) at theoperating pressure of 27.6 kN/m 2 (4 psig) at the outlet.Air piping shall deliver a min. of 0.15 m 3 /sec air to eachchamber. Provide control valves and flow meters on allbranch lines to balance the air flow.

2. Diffusers and blowersProvide coarse diffusers with air pipe headers and hangerfeed pipes having swing joint assembly.

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Design Example - continued

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Design Example - continuedC. Surface Rise Rate1. Overflow rate when both chambers are in operation

Overflow rate: (0.661 m 3 /sec 86,400 sec/day) (3.5 m 13 m) = 1,255.2 m 3 /m2day (30,805 gpd/ft 2)

2. Overflow rate when one chamber is in out of serviceOverflow rate: 2 1,255.2 = 2,510.4 m 3 /m 2day

D. Influent Structure1. Arrangement of influent structure

Provide 1-m wide submerged influent channel with two1 m 1 m orifices. Provide a baffle at the influent todivert the flow transversally to follow the circulationpattern. Provide sluice gates to remove one chamberfrom service for maintenance purposes.

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Design Example - continued2. Headloss calculation through the influent structure

z = z 1 - z2 = difference in elevation of free watersurface into the channel and the chamber (m).

L

21

22 h

2gv

2gv

z +=

m/sec0.33channel)in thedepthwater(assumedm4.06m1

/secm1.321v

3

1 =

=

m/sec0.10chamber)in thedepthwater(assumedm82.3m3.5

/secm1.321v

3

2 =

=

e)(neglegiblm0.0052gv

2gv

21

22 =

hL = hL (into influent channel) + h L (exit loss thru port)negligible0

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where A = orifice area, m 2 andCd = discharge coef. = 0.61 - square-edged entrance

Ld 2ghACQ =

m0.24m/sec9.812m1m10.61

/secm1.321h z

2

2

3

L =

=

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Design Example - continuedE. Effluent Structure1. Arrangement of effluent structure

Provide a 2.5-m long rectangular weir, an effluent trough(2.5 m long 1.5 m wide), an effluent box (2.3 m 1.5m), and an outlet pipe. Provide removable gates at theeffluent box to drain the effluent trough when onechamber is removed from service.

2. Head over the effluent weir

where Q= flow over weir, m 3 /sec;H = head over weir, m;Cd = discharge coef. = 0.624; andL = L - 0.2 H (L = length of weir).

3d 2gH'LC

3

2Q =

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Design Example - continuedAt peak design flow when both chambers are in operation,Q = 0.661 m 3 /sec. Calculate H by trial & errorAssume L = 2.47 m H = 0.28 m L = 2.5 - 0.20.28 =2.47 m (same - ok); thus, H = 0.28 m

3. Height of the weir crest above the bottom of the chamberHeight of weir crest = 3.65 m - 0.28 m = 3.37 m

4. Head over the effluent weir at Q max when one chamber isout of serviceAssume L = 2.41 m H = 0.45 m L = 2.5 - 0.20.45 =2.41 m (same - ok); thus, H = 0.45 m

5. Water depth in the chamber at Q max when one chamber is

out of service

= 3.37 m + 0.45 m = 3.82 m

over weirheadchambertheof bottomthe

abovecrestweirof HeightdepthWater +

=

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Design Example - continued6. Depth of the effluent trough

Flow varies in a free falling weir discharge. For uniformvelocity distribution, the drop in the water surfaceelevation between two sections is expressed as follows:

where y = drop in water surface elevation betweensections 1 and 2, m;

x = horizontal distance between sections 1 & 2, m;y1 and y 2 = depth of flow at sections 1 and 2, m;

q1 and q 2 = discharge at sections 1 and 2, m;v1 and v 2 = velocity at sections 1 and 2, m; and(SE)ave = average slope of the energy line, m/m.

x)(S qqv

vqgvq

y' aveE1

2

ave

ave1 +

+=

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Rave = (R 1 + R 2)/2

where n = roughness coefficient andR = hydraulic mean radius, m.

( )( )4/3ave

2ave

2

aveER

vn)(S =

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Design Example - continuedDepth of flow in the trough at the upstream section

where y 1 = water depth at the upstream end, m;y2 = water depth in the trough at a distance L from

the upstream end, m;q = discharge per unit length of the weir, m 3 /secm;b = width of the trough, m; andN = number of sides the weir receives flow (1 or 2).

( )2

2

2221 gb

NLq'2y

y y

+=

m /secm0.5284m2.5 /secm1.321

weirof LengthQ

q' 33

===

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Design Example - continuedAssume water depth in the effluent box at the exit point(center of the effluent pipe) is 1.5 m; thus, the water depthin the trough at the effluent box, y 2, is also 1.5 m.

Allow 12% additional depth to account for friction losses,and add 15 cm to ensure a free fall. Thus,Total depth of trough = 1.54 m 1.12 + 0.15 m = 1.88 m

F. Headloss through the Grit ChamberTotal headloss = h L at the effluent structure + h L at theinfluent structure + hL in the basin + h L due to baffles

( )m1.54

m1.5m)(1.5m/sec9.811m2.5m /secm0.52842

m)(1.5y 22

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1 =

+=

0

outfallsubmergedm0.43m/sec9.81m1.5

/secm1.321d

2/3

2

3

c =

=

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Design Example - continuedHeadloss due to influent and effluent baffles

where v 2 = velocity through the chamber;Ab = vertical projection of the area of the baffle; and

CD = drag coef. = 1.9 for flat plates.v2 = 1.321 m 3 /sec [(3.5 m width) (3.82 m water depth)]

= 0.099 m/sec

The headloss is small; so it can be neglected. Similarly,

the headloss due to effluent baffle can also be ignored.G. Quantity of GritGrit produced = 30 m 3 /10 6 m3 0.44 m 3 /sec 86400 sec/day

= 1.14 m 3 /day

AA

2gv

Ch b22

DL =

m0.00051

0.5m/sec9.812

m/sec)(0.0991.9h 2

2

L =

=

Combined system: 10~30 ft 3 /Mil gal; Separate system: 2~10 ft 3 /Mil gal236

Operation and MaintenanceRequires well-trained operatorsMaximize grit removal efficiencyAdjust the air flow to allow grit to settle but preventorganic material from settlingUse swing type diffusers for easy maintenance

Trouble Shooting GuideRotten-egg odor, corrosion or wear on equipment:increase air supply and inspect the walls, channels, andthe chamber for debrisLow recovery of grit: reduce air supplyGrit chamber overflow: adjust pump controlsReduced surface turbulence: clean diffusersHigh volatile matter content in grit: reduce air supplyGrey in color, smelly, greasy: increase air supply

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4082wwt grit removal

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