4084WWT Primary Treatment

7/31/2019 4084WWT Primary Treatment

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1

Dr. Abdel Fattah Hasan

An-Najah National University

Masters Program of

Water and Environmental Engineering

461652, Spring 2011

WWT

2- Primary Treatment

Primary Sedimentation

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Primary Sedimentation

Objective: To remove settleable organic solids in large basinsunder relatively quiescent conditions

Removal efficiencyBOD5: 30~40%TSS: 50~70%

Settled solids: collected by mechanical scrapers into a hopper,from which they are pumped to a sludge-processing area.

Oil, grease, and other floating materials: skimmed from thesurface

Effluent: discharged over weirs into a collection trough

Types of primary sedimentation tanks

Horizontal flow Solids contact Inclined surface

Stacked or two-tray Proprietary

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Horizontal Flow Rectangular

Sedimentation Tank

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Primary Sedimentation Tank

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Horizontal Flow

Advantages Occupy less land area when multiple units are used Provide economy by using common walls for multiple units Easier to cover the units for odor control Provide longer travel distance for settling to occur Less short-circuiting Lower inlet-outlet losses Less power consumption for sludge collection and removal

mechanisms

Disadvantages Possible dead spaces

Sensitive to flow surges Restricted in width by collection equipment Require multiple weirs to maintain low weir loading rates High upkeep and maintenance costs of sprockets, chain, and

flights used for sludge removal

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V-notched overflow weirs


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Manual scum

removal system


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Horizontal Flow RectangularSedimentation Tank

Longitudinal section with skimmer

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Cross section

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Horizontal

FlowSedimentatio

nTanks

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V-notched

overflow weirs


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Horizontal Flow- continued

Horizontal flow

circular clarifier

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Primary Sedimentation Basin

Odor Control Pipe

Covered Overflow Weir

Scum Removal Weir

Scrubber for Odor Control

Scum Collection Arm

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Floating Inorganic Materials

in Primary Sedimentation Tank

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Cover

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Inside of Covered Primary

Sedimentation Tank

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Potential maintenance

problem during cleaning

Too low


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Solids Contact

Incoming solids rise and come in contact with thesolids in the sludge layer. This layer acts as ablanket, and the incoming solids agglomerate andremain enmeshed within this blanket. The liquidrises upward while a distinct interface retains thesolids below.

Better hydraulic performance and shorter detentiontime for equivalent solids removal in horizontal flowclarifiers.

Either circular or rectangular Not suitable for biological sludges because long

sludge-holding times may create undesirable septicconditions.

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Solids-ContactClarifier

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Inclined Surface

Utilize inclined trays to divide the depth intoshallower sections, which in turn results insignificantly short settling time.

Frequently used to upgrade the existing overloadedprimary and secondary clarifiers.

Tube settlers: use thin-wall tubes in circular, square,hexagonal, or any other geometric shape

Parallel plate separators: provide a large surfacearea, thereby reducing the clarifier size. Little windeffect, laminar flow, good for upgrading overloadedhorizontal flow clarifiers

Disadvantages: septic condition, sludge sloughingoff, and clogging of inner tubes and channels.

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Inclined Plate Settlers

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Lamella

Settlers

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Design Factors

Design Objective: provide sufficient time under quiescentconditions for maximum settling to occur.

Conditions causing decrease in solids removal efficiency Eddy currents induced by incoming fluid Surface currents provided by wind action Vertical currents induced by outlet structure Vertical convection currents induced by the

temperature difference between the influent and thetank contents

Density currents causing cold or heavy water tounderrun a basin, and warm or light water to flowacross its surface

Currents induced due to the sludge scraper and sludgeremoval system

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Stacked or Two-Tray Sedimentation Basin

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Design overflow rates for sedimentation tanks (m3/m2day)

Condition Range Typical

Primary sedimentation prior to secondary treatmentAverage flow 30~50 40Peak flow 70~130 100

Primary sedimentation with WAS returnAverage flow 25~35 30Peak flow 45~80 60

Detention Times for Various Overflow Rates and Tank Depth

Overflow rate Detention period (hrs)(m3/m2day) 2-m 2.5-m 3-m 3.5-m 4-m 4.5-m

depth depth depth depth depth depth

30 1.6 2.0 2.4 2.8 3.2 3.640 1.2 1.5 1.8 2.1 2.4 2.750 1.0 1.2 1.4 1.7 1.9 2.260 0.8 1.0 1.2 1.4 1.6 1.870 0.7 0.9 1.0 1.2 1.4 1.580 0.6 0.8 0.9 1.1 1.2 1.4

Detention time at average design flow Primary sedimentation tanks - 1~2 hrs Secondary clarifiers - 2~4 hrs

1 m3/m2day =

24.5424 gal/ft2day 28


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BOD5 and TSS Removal

Efficiency with Respectto Overflow Rate andDetention Time

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Design Factors - continued

Weir loading rate (< 370 m3/m2day) (Ten-States Standards)124 m3/m2day for plants designed for average design flow of 44 L/sec186 m3/m2day for plants designed for average design flow of > 44 L/sec

DimensionsType Range Typical

RectangularLength, m 10~100 25~60Length-to-width ratio 1~7.5 4Length-to-depth ratio 4.2~25 7~18Sidewater depth, m 2.5~5 3.5Width, m 3~24 6~10

Clarifier

Diameter, m 3~60 10~40Side depth, m 3~6 4

Solids LoadingNot an important deciding factor for primary sedimentation tank designPrimary sedimentation tanks: 1.5~34 kg/m2daySecondary clarifiers: 49~98 kg/m2day

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Influent structure Dissipate energy in incoming flow by means of baffles or stilling

basin Distribute flow equally along the width Prevent short circuiting by disturbing thermal and density

stratification Provide small head loss

Provision for flow control, scum removal, and maintenance

Velocity at inlet pipe: 0.3 m/sec

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Influent Structures

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Effluent structureProvide a uniform distribution of flow over a large areaMinimize the lifting of the particles and their escape into the effluentReduce the escape of

floating matter to theeffluent

Weir loading for plants 44 L/sec 124 m3/mday> 44 L/sec - 186 m3/mday

Straight (1~2 mm head overweirs capillary clingingeffects slime accumulationor V-notches either one side

or both sides of troughBaffle in front of the weir

to stop the floatingmatter from escaping intothe effluent

33Recommended


Sludge collection Bottom slope: to facilitate draining of the tank and to remove the

sludge toward the hopper. Rectangular tanks: 1~2%; circularclarifiers: 40~100 mm/m diameter

EquipmentRectangular tanksA pair of endless conveyor chains running over sprockets

attached to the shafts or moving-bridge sludge collectorshaving a scraper to push the sludge into the hopper

Suction-type arrangement to withdraw the sludge from basinsCircular tanksScraping mechanism with radial arms having plows set at an

angle supported on center pier ( 10 m diameter) or on a beamspanning the tank (< 10 m diameter) (flight travel speed -0.02~0.06 revolution/min)

Suction-type units for handling light sludge.

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Conveyor Chain

One endless chain is connected to a shaft and a drive unit. Linear conveyor speed is 0.3~1 m/min for primary and 0.3

m/min for secondary clarifier Cross-wood (flights) (5 cm thick and 15~20 cm deep) are

attached to the chain at 3-m intervals and are up to 6 m inlength.

For tanks greater than 6 m in width, multiple pairs of chainsare used.

The floating material is pushed in opposite direction ofsludge and is collected in a scum collection box.

Advantages: simple to install, low power consumption,

efficient scum collection, and suitable for heavier sludge Disadvantages: high maintenance cost of chain and flightremoval mechanism, dewatering of tanks for gear and chainrepair, and potential resuspension of light sludge

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Chain-and-Flight Sludge Collector

A. ChainB. SprocketsC. Shear pinD. Chain tightener bracketsE. Wall bearings

F. Pillow block bearingsG. Sewage take-up bearingH. Set collarsI. Screw conveyerJ. Shafting

K. Return rail systemL. Floor rail wear stripM. CouplingsN. Wear shoesO. Filler blocks

P. Gear boxQ. BafflesR. Flights (fiberglass or

wood)S. WeirsT. Scum pipe

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Bridge Drive Scraper

Standard traveling beam bridges for spans up to 13 m (40 ft)and truss bridge for spans over 13 span are used.

Bridge travel is accomplished by the use of a gear motor. The wheels run on rails which are attached to the footing

wall along each side wall of the basin. Mechanical scrapers or rakes are hung from the top carriage

that push the sludge to the hopper. Separate blades are provided on top to move the scum. Advantages: all moving mechanisms above water, scraper

repair or replacement without dewatering tanks, no widthrestrictions, longer operation life, and lower maintenance

cost in low-span bridges. Disadvantages: high power requirement, not suitable forcold weather (ice formation in tanks), and frequent break-down due to wheel climbing over rails in long-span bridges.

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Traveling Bridge Sludge Collector

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Bridge Drive Sludge Suction

The bridge design is similar to the bridge drive scraper. The sludge removal mechanisms are attached to the bridge

and provide continuous removal of sludge along the lengthof travel.

Pump, siphon, or airlift arrangements are used to suck andremove the sludge.

Advantages: Better pickup of light sludge, all movingmechanisms above water, scraper repair or replacementwithout dewatering tanks, no width restrictions, longeroperation life, and lower maintenance cost in low-spanbridges, and suitable for biological and chemical sludges.

Disadvantages: high power requirement, not suitable forcold weather (ice formation in tanks), and frequent break-down due to wheel climbing over rails in long-span bridges,and not used in primary sedimentation tanks.

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Vacuum Sludge Removal

Rectangular sedimentation basinsSouthern States and warm climate areasNone freezing environmentsSurface ice is difficult to design for. 40


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Vacuum

SludgeRemoval

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Sludge Removal Removed by means of a pump.

Design considerationsProvision of continuous sludge pumping is desirable.Each sludge hopper should have individual sludge withdrawal

line at least 15 cm in diameter. In rectangular tanks, cross-collectors are preferred over multiple

hoppers.Screw conveyors for sludge removal are also used.An automatic control of sludge pump or siphon pipes using a

photocell-type or sonic-type sludge blanket detector isdesirable, especially for secondary clarifiers.

The sludge pump used are self-priming centrifugal andnormally discharge into a common manifold. One sludgepumping station can serve two rectangular sedimentation tanks.The circular clarifiers are normally arranged in groups of two orfour.

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Scum removal Generally pushed off the surface to a collection sump. In

rectangular tanks, the scum is normally pushed in the oppositedirection by the flights of the sludge mechanism in its returntravel. In circular clarifiers, the scum is removed by a radial armwhich rotates on the surface with the sludge removal equipment.Sometimes, removed by water sprays.

Scraped manually or mechanically up an inclined apron. All effluent weirs have baffles to stop the loss of scum into the

effluent. The scum has a specific gravity of 0.95. Solids content may vary

from 25 to 60%. The quantity of scum varies from 2 to 13 kg/103 m3 (17~110

lb/million gallon). Pipings are often glass-lined and kept reasonably warm to

minimize blockage. Scum has been digested in aerobic and anaerobic digesters.

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ScumCollection andRemoval

Arrangements

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Information Checklist

Average and peak design flows including the returned flowsfrom other treatment units

All sidestreams from thickener, digester, and dewateringfacility

Treatment plant design criteria prepared by the concernedregulatory agencies

Equipment manufacturers and equipment selection guide Information on the existing facility if the plant is being

expandedAvailable space and topographic map of the plant siteShape of the tank (rectangular, square, or circular)

Influent pipe data, to include diameter, flow characteristics,and approximate water surface elevation or hydraulic gradeline

Headloss constrains for sedimentation facility

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Design Calculations

Design criteria1. Two rectangular units shall be designed for independent operation.

Allow for a bypass to secondary treatment process when one unitis out of service.

2. Overflow rate and detention times shall be based on an averagedesign flow of 0.44 m3/sec (10 MGD).

3. The overflow rate shall be < 36 m3/m2day at average design flow.4. The detention time shall be > 1.5 hrs at average design flow.5. The influent structure shall be designed to prevent short circuiting

and reduce turbulence. The influent channel shall have a velocity< 0.35 m/sec at design peak flow (0.661 m3/sec through eachbasin).

6. All sidestreams shall be returned to aeration basins.

7. The weir loading shall be < 186 m3/mday at average design flowand < 372 m3/mday at design peak flow.

8. The launder and outlet channels shall be designed at the peakdesign flow of 1.321 m3/sec (0.661 m3/sec through each basin).

9. The average liquid depth in the basin shall be > 3 m.10.The slope of the tank bottom shall be 1.35%.

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Design Calculations - continued

Basin dimensions1. Select basin geometry and provide two rectangular basins with

common wallAverage design flow through each basin = 0.44/2 = 0.22 m3/secOverflow rate at average design flow = 36 m3/m2daySurface area = 0.22 m3/sec 86,400 sec/day 36 m3/m2day

= 528 m2

Use length-to-width ratio (L:W) = 4:1 ~ OK4WW = 528 m2; W = 11.5 m; use W = 11.58 m (38 ft) due to 2

ft increments for sludge collectors.Thus, length = 46.33 m (152 ft).Depth at mid-length of the tank = 4 m (13.1 ft)Length-to-depth ratio = 46.33 m/4 m = 11.6 ~ OKFreeboard = 0.6 m (2 ft)Average depth of the basin = 4.6 m (15 ft)

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2. Check overflow rateOverflow rate at average design flow = 0.22 m3/sec 86,400

sec/day (11.58 m 46.33 m) = 35.4 m3/m2dayOverflow rate at peak design flow = 0.661 m3/sec 86,400

sec/day (11.58 m 46.33 m) = 106.4 m3/m2day3. Check detention time

Average volume of the basin = 4 m 11.58 m 46.33 m= 2,146.0 m3

Detention time at average design flow = 2,146.0 m3 (0.22 m3/sec 3,600 sec/hr) = 2.7 hrs

Detention time at peak design flow = 2,146.0 m3 (0.661 m3/sec 3,600 sec/hr) = 0.9 hr

Influent structure1. Select the arrangement of the influent structure

The influent structure includes a 1-m wide influent channel thatruns across the width of the tank. Eight submerged orifices 34 cmsquare each, are provided in the inside wall of the channel.

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A submerged influent baffle is provided 0.8 m in front, 1 m deep,and 5 cm below the liquid surface.

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Why?

To prevent settling

Potential constructability issue


2. Compute the headloss in the influent pipe connecting the junctionbox located downstream of the grit chamber and the influentstructure of the sedimentation basinThe elevation of the water surface in the influent channel of thebasin is lower than that in the junction box downstream from thegrit chamber. H is the sum of the headloss in the connectingpipe due to the entrance, friction, bends, and fittings and exit lossinto the influent channel of the sedimentation basin.

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3. Compute the headloss at the influent structureThe horizontal velocity in the sedimentation basin (v2) is small andis ignored. The average velocity in the influent channel (v1) iscalculated at peak design flow. Half of the flow divides on eachside of the basin.

The depth of water into the influent channel is fixed by thedesigner. Assume the depth of water at the entrance of the influentchannel is 1 m and the width of the influent channel is 1 m.

z = hL; Q = CdA

/secm0.3312

/secm0.661

2

basinperflowdesignPeak

channeleachin

Discharge3

3

===

m/sec0.331m1m1

/secm0.331

flowdesignpeakat

channelin theVelocity3

==

L2gh

m0.07m/sec9.812m)(0.340.6

4/secm0.331hz

2

22

3

L=

==

< 0.35 m/sec OK

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Effluent structure

1. Select the arrangement of the effluent structureThe effluent structure consists of weirs, launder, an outlet box, andan outlet pipe. Use V-notched weir.

2. Compute the length of the weirWeir loading = 372 m3/mday at peak design flowPeak design flow per basin = 0.661 m3/sec 86,400 sec/day

= 57,110 m3/dayWeir length = 57,110 m3/day 372 m3/mday = 153.3 mProvide weir notches on both sides of the launder. Thus,Total length of the weir plate = 2(29.5 + 10.38) m + 2(28.3+9.18)

m - 1 m = 153.72 mActual weir loading = 57,110 m3/day 153.72 m

= 371.6 m3/mday 372 m3/mday at peak flow OK3. Compute the number of V-notches

Provide 90 standard V-notches at a rate of 20 cm center to centeron both sides of the launders.

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Weir Arrangement

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Total number of notches = 5 notches per m 153.72 m = 769In order to leave sufficient space on the ends of the weir plate,provide a total of 765 notches.

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4. Compute the head over the V-notches at the average design flowThe average discharge per notch at average design flow

= 0.22 m3/sec 765 notches = 2.88 10-4 m3/sec per notchThe discharge through a V-notch is calculated using the eq. below.

Q = 8/15 Cd 2g tan(/2) H5/2

where Cd = 0.6, H = head over notch, m, and = angle of the V-notch = 90.

2.88 10-4 m3/sec = 8/15 0.6 2 9.81 m/sec2tan (90/2) H5/2

H = 0.03 m = 3 cm5. Compute head over V-notches at peak design flow

Discharge per notch at peak design flow= 0.661 m3/sec 765 notches = 8.64 10-4 m3/sec per notch

8.64

10

-4

m

3

/sec = 8/15

0.584

2

9.81 m/sec

2

tan (90/2) H

5/2

H = 0.06 m = 5 cm6. Check the depth of the notch

The total depth of the notch is 8 cm. Max. liquid head over thenotch at peak design flow is 5 cm (safe allowance of 3 cm).

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7. Compute the dimensions of the effluent launderWidth of the launder b = 0.6 mWidth of the effluent box = 1.0 mDiameter of the outlet pipe = 0.92 mThe depth of water in the effluent box = 1.0 m (fixed by designer)

Provide the invert of the effluent launder 0.46 m above the invertof the effluent boxDepth of water in the effluent launder at exit point y2= 1.0 m - 0.46 m = 0.54 m

Critical depth of flow in the launder = 0.5 m

Since 0.54 m > 0.5 m, the outfall is submerged.Half of the flow divides on each side of the launderFlow on each side of the launder = 0.661 m3/sec 2 = 0.33 m3/sec

3/2

cccdgbgdAQ ==

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Weir

Trough

Section AA

Section CC57

Effluent Launder Water Surface

Profile

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ut et

Channel

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Generally, an allowance for losses due to friction, turbulence, andbends is 10~30%. In this case, provide a 25% allowance, and add0.6 m for freefall.Water depth at the far end of trough = 0.64 m 1.25 = 0.80 mTotal depth of the effluent launder = 0.80 m + 0.60 m = 1.40 m

Headloss through sedimentation basinHeadloss at the influent structure (calculated)Headloss at the effluent structure (calculated)Headloss in the basin (small - ignored)Headloss at the influent and effluent baffles (small - ignored).

( )m0.64

0.540.69.81

.33020.54

ygb

NLq'2yy

2

22

2

2

2

2

21 =

+=

+=

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Hydraulic profile through the basinA total headloss of 0.99 m (3.25 ft) at the peak design flow.

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Sludge Quantities

1. Establish sludge characteristicsPrimary sludge: specific gravity of 1.03 and a solids content of3~6%. Assume a typical solids content of 4.5%.

2. Compute average quantity of sludge produced per dayAmount of solids produced per basin per day at a removal rate of63% = 260 g/m3 0.63 0.22 m3/sec 86,400 sec/day

kg/1,000 g = 3,113.5 kg/dayAverage quantity of sludge produced per day from both basins

= 2 3,113.5 kg/day = 6,227 kg/day3. Compute the volume of sludge produced per minute per basin

Volume of sludge at specific gravity of 1.03 and 4.5% solids= 3,113.5 kg/day (1.03 1 g/m3 1 kg/1,000 g 0.045

106 cm3/m3 1,440 min/day) = 0.0467 m3/min per basin4. Determine sludge pump size and pumping cycle

Provide separate sludge pumps for each basin. Arrange such thateach pump will serve both basins in case one pump is out of service.

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Operate each pump on a time cycle, at 16.5-min intervals with a1.5-min pumping cycle per basin (total time 18 min per cycle)The desired pumping capacity of the pump = (0.0467 m3/min per

basin 18 min per cycle) 1.5 min pumping per cycle= 0.56 m3/min per basin (150 gpm)

When one pump is used to remove the sludge from two basins, thecycle time will be reduced.Cycle interval in min for two basins

= (16.5 + 1.5) min 2= 9 min per cycle

Effluent quality from primarysedimentation basin1. Establish BOD5 and TSS removal

At overflow rate of 35.4 m3/m2day,BOD5 removal = 34%SS removal = 63%

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2. Compute BOD5 and TSS in the effluentAssume the sidestreams from the thickeners, digesters, anddewatering facilities are returned to the aeration basin.BOD5 in the primary effluent = 250 g/m

3 (1 - 0.34) 0.44m3/sec 86,400 sec/day kg/1,000 g = 6,273 kg/day

TSS in the primary effluent = 260 g/m3 (1 - 0.63) 0.44m3/sec 86,400 sec/day kg/1,000 g = 3,657 kg/day

Volume of primary effluent = Average flow to primary - Sludgewithdrawal = 0.44 m3/sec 86,400 sec/day - (6,227 kg/day 1,000 g/kg) (0.045 g/g 1.03 g/cm3 106 cm3/m3)

= 38,016 m3/day - 134 m3/day = 37,882 m3/dayBOD5 conc. in effluent = 6,273 kg/day 37,882 m

3/day 1,000

g/kg = 165.6 g/m3 = 165.6 mg/LTSS conc. in effluent = 3,657 kg/day 37,882 m3/day 1,000

g/kg = 165.6 g/m3 = 96.5 mg/LScum quantity: ave. quantity of scum (S.G.=0.95) =

8 kg/1,000 m3 38,016 m3/day = 304 kg/day = 0.32 m3/day

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Layout of the Primary Sedimentation Basins

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Common Operating Problems

1. Black and odorous septic wastewater due to decomposing wastewaterin the collection system, recycle of excessively strong digestersupernatant, or inadequate pretreatment of organic discharges fromthe industries preaeration, chlorination, or hydrogen peroxide/potassium permanganate oxidation, control of digester supernatant,and strict enforcement of industrial pretreatment regulations

2. Scum overflow due to inadequate frequency of scum removal,excessive industrial contribution, worn or damaged scum wiperblades, or improper alignment of the skimmer

3. Sludge that is hard to remove from the sludge hopper due toexcessive grit accumulation check the grit removal facility

4. Low solids in the sludge due to excessive sludge withdrawal, shortcircuiting, or surging flow

5. Excessive corrosion of metals due to H2S gas6. Frequent broken scraper chain and shear pin failures due to improper

shear pin sizing and flight alignment, ice formation, or excessiveloading on the sludge scraper

7. A noise chain drive or a loose or stiff chain due to misalignment67

Operation and Maintenance

1. Remove accumulations from the influent baffles, effluent weirs,scum baffles, and scum box each day.

2. Inspect all mechanical equipment at least once each shift.3. Hose down and remove wastewater sludge and spills ASAP.4. Determine sludge level and underflow concentration, and adjust

primary sludge pumping rate accordingly.4. Observe operation of scum pump and provide hosing as

necessary.5. Check daily electrical motors for overall operation, bearing

temperature, and overload detector.6. Check oil levels in gear reducers and bearings on a regular basis.

7. Drain each primary basin annually and inspect the underwaterportion of the concrete structure and all mechanical parts.

8. Inspect all mechanical parts for wear, corrosion, and set properclearance for flights at tank walls.

9. Clean and paint the exposed metal surfaces as necessary.68


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Specification

sGeneral

Include a complete assembly of a sludge

collector mechanism with drive and collector

chains with flights, access bridge and walkway,

influent and effluent structures, pumping

facilities, and overload alarm system.

Dimensions: # of basins (identical basins with

common wall), Length, width, side wall depth

(SWD) at effluent end, influent end, and mid

length, bottom slope, free board, etc.69

Specifications - continued

Materials and Fabrication

Conform to proper ASTM standards. The min.thickness of all submerged metal shall not be< 0.64 mm (1/5 in) and of all-above watermetal 4.8 mm (3/16 in). Conform to allAmerican Institute of Steel Construction (AISC)standards for structural steel buildings.

Collectors

Provide two longitudinal collectors and onecross-collector in each basin

Dimensions, numbers, and materials of flights,sprocket wheels, scrapers, chains, etc.

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Specifications - continuedDrive Unit Consists of a gear motor, gear reducer, drive base, shear pin

coupling, overload alarm device, and drive sprocket andchain

Motor and gear detailed specificationsSludge Pump Self-priming, centrifugal, and nonclogging, pumping rate, etc.Effluent Weir V-notch weir and effluent box dimensions, scum removal, etc.Skimmer Hand-operated adjustable scum troughPainting

Primed and painted with approved paint or epoxy

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Documents

4084WWT Primary Treatment