1
Introduction
Many physical and chemical separation processes are used in waste water treatment industry. Clarification is a salience physical separation process for remove solid particles or suspended solids from liquid. Concentrated solid particles which discharged from the bottom of the tank are known as sludge and the particles which float to the surface of liquid are called scum. Generally clarifiers can be divided in to some categories considering operation or shape of it. When considering the operation, Clarifier can divided in to two types.
Primary Clarifier Secondary Clarifier
As well as when consider the shape of clarifier, there are mainly three types.
Rectangular Clarifier Circular Clarifier Inclined plate/ Lamella
In this design project consider the secondary clarifier for latex waste water treatment process. 1.1
Theory
The design of the clarifier tank is depend on the concentration, size and behavior of the solid suspension. Normally there are four types of sedimentation. Type i sedimentation This type of sedimentation is called as discrete sedimentation. That means low concentrations of particles that settle as individual entities. Type ii sedimentation This is known as flocculent settling which sedimentation of high concentrations of solids that agglomerate as they settle. Type iii sedimentation Type iii sedimentation is called as hindered settling or zone settling. That means solids particles are suspended with solid concentration sufficiently high to cause the particles to settle as a mass. Type iv Sedimentation This is known as compression settling, means sedimentation of suspensions with solid concentration so high that the particles are in contact one another and further sedimentation can occur only by compression of the mass.
Clarifier tank An ideal clarifier is divided into four zones. There are inlet zone, settling zone, sludge zone, outlet zone. In inlet zone flow is uniformly distributed across the tank cross section. In settling zone water is quiescent and gradually flows horizontally toward the basin inlet as well as sedimentation occurs. The settled sludge accumulates in sludge zone. The treated water is collected evenly across section of the basin in outlet zone.
1.2
Factors that affect clarifier performance Hydraulic & Load factors Wastewater flow(ADWF, PDWF, PWWF) [ADWF-average dry weather flow; PDWF-peak dry weather flow; PWWF-peak wet weather flow]
Surface overflow rate Solid loading rate Hydraulic retention time Underflow recycle ratio External physical features Tank configuration Surface area Depth Flow distribution Turbulence in conveyance structures Internal physical features Presence of flocculation zone Sludge collection mechanism Inlet arrangement Weir type, length, position Baffling Hydraulic flow patterns & turbulence Density & convection currents Site condition Wind & wave action Water temperature variation Sludge characteristics MLSS concentration Sludge age Flocculation, settling & thickening characteristics Type of biological process [1]
1.3 1.3.1
Types of Clarifiers Primary Clarifier
Primary treatment has applied a clarification process to separate the readily precipitated and floatable solids from waste water. That is used as preliminary step in the further processing of waste water. The suspended solid and BOD are decreased between 50% to 70% and 25 to 40% respectively by Well-designed and operated primary clarifier. [2] 1.3.2
Secondary Clarifier
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The main difference between primary and secondary clarifiers is density of the sludge handled. Secondary sludge are generally less dense than primary sludge as well as effluent of secondary clarifier is usually clearer than primary effluent. 1.3.3
Rectangular Clarifier
Rectangular clarifier is a common type of clarifier in waste water treatment process. Either chain & flight solids collectors or traveling bridge type collectors are used in this clarifier .The construction of these types of clarifiers are cheap due to construct multiple units with common wall. Length, width, depth of the tank, diffusers or ports, internal baffles, flocculation zones, skimmers, scrapers or suction mechanism, sludge flights, type & location of weirs or submerged outlets, location of sludge hoppers, tank covers and materials of construction are included in design criteria of rectangular clarifiers.
Figure . Rectangular clarifier design features
1.3.4
Circular Clarifier
Circular clarifiers are most suitable than other types when considering mechanism of sludge collection. There are Square, hexagonal, octagonal shape of clarifier tanks are also established which closely similar to the circular shape. But these types are not more popular in practically. There are nearly all the advantages of circular clarifiers are given by these alternative shapes. The disadvantage of circular clarifiers is to allocate more floor space than rectangular clarifier tanks in equivalent capacity due to common wall construction. Although the range of diameters of the tank is 3m to greater than 100m, in practically diameter of tank are kept in less than 50m to avoid the adverse effects of wind on the surface.
1.3.5
Figure . Circular clarifier design features Design Criteria Advantages
Rectangular Clarifiers Less land & construction cost in a
Circular Clarifiers Short detention time for
multiple unit design Longer flow path & less chance
settled sludge Better effect of dynamic
feed/ peripheral overflow circular
filtration Simple & more reliable
clarifiers More even distribution of sludge
sludge-collecting system Low maintenance
loads on collectors Can be shallower Low head loss for flow
distribution Can be easily covered for odor
for short-circuiting than center-
requirements
control More effective foam/ scum
trapping & positive removal Not proprietary
Longer detention time for settled
sludge Possibly less effective for high
solid loading Increased maintenance of
short-circuiting Lower limits for effluent
collectors
weir loading Generally proprietary More susceptible to wind
effects High head loss for flow
Disadvantages
Center feed/ peripheral units have higher potential for
distribution
1.4
Purpose of circular clarifier and parts
Influent control
Stops or throttles the flow to clarifier
gate Influent channel or
Transports wastewater to the clarifier
pipe
Influent well
Receives the wastewater from influent channel or pipe and decreases flow velocity & regularly distributes across the upper portion of the clarifier
Effluent weir
Ensures similar flow over all weirs.
Effluent trough
Receives treated wastewater from the clarifier
Scum skimmer arm
Collects floating particles or skims from the surface of
Scum trough
wastewater. Moves it to the scum trough Receives floating particles which scraped by scum skimmer arm.
Scum pipe
Flows collected scum from skimmer box to a scum tank
Drive unit
Put to the collector to rotate
Vertical drive cage
Transmits power from drive unit to the sludge collector mechanism
Sludge collector
Rotates around the bottom of the clarifier and drags settled solids
mechanism
across clarifier bottom to a sludge collection pit.
Blades and scraper
Scrape sludge form bottom of clarifier to sump
squeegees
Sump
Collects the sludge before withdrawal
Sludge withdrawal
Removes the sludge from the clarifier. [3]
pipe
1.5 1.5.1
Calculation Material balance
Removal=
[Influent concentration−Effluent concentration]× 100 Influent concentration
Literature value for 4hr to 8hr Retention time for rectangular clarifier Settleable solids 95% to 99% Total solids 10% to 15% BOD 20% to 50% COD 60% to 80% [49] Assumption Retention time= 4hr Weight of Water is 15times of solid weight in sludge.
Removal efficiencies Suspended solids = 80% Total solids = 12% BOD = 40% COD =70% Suspended solid in Effluent= 89.13 ×20 =17.83 kg /day Total solid in Effluent= 895.53 ×88 =824.2 kg /day BOD in Effluent= 32.25× 60 =19.35 kg /day COD in Effluent= 597.45× 30 =172.7 kg /day Solid in Effluent = 89.13−17.83=71.3 kg /day Water in Effluent= 71.3× 15=1069.5 kg /day
1.5.2
Energy Balance
1.5.3
Design calculation
Average flow rate=10000 l/hr=10 m3 /hr Peak flow rate=25000 l/hr=25 m3 / hr Retention time=3 hr Average flow rate=10 ×24 m3 /d=240 m3 /d
Circular Clarifier ¿ Flow rate=17 m3 /m2 . d Area=
240 m3 /d =14.12 m2 3 2 17 m /m . d
600 m3 / d 3 2 OFR For peak flow condition= =42.5 m /m . d 2 14.12 m Diameter (D)=
√
14.12 × 4 m=3.57 m≈ 3.6 m π
Assume detention time = 3hrs Height of clarifier =
3 hr ×24 m3 /m2 . d =3 m 24 hr
[4]
When use circular clarifier with diameter of 3.6m, higher potential for short-circuiting and settling effect is very due to small size. Rectangular Clarifier Detention time =4hr 3
2
¿ Flow rate=18 m /m . d Area=
240 m3 /d 2 =13.33 m 3 2 18 m /m . d 3
OFR For peak flow condition= Select width to length 1:3 Area=W × 3 W =13.33 m 2 W = 1.83m , L= 7.36 m
600 m / d =45.01 m3 /m2 .d 2 13.33 m
3
Depth=Detention time ×overflow rate=
2
4 hr ×18 m /m =3 m 24 hr
3
Take Weir loading rate=25 m /m. d Weir length=
240 m 3 /day =9.6 m 25 m 3 /m. day
Available Weir length=πD=
22 × 6.5=20.4 m>9.6 m 7
Flow 240 m3 /d Available Weir loading rate= = =11.76 m3 /m. d< 25 m3 /m . d Weir Length 20.4 m
2
REFERENCES
[1] C. D. F. o. t. W. E. Federation, "FUNCTIONS OF A FINAL CLARIFIER," in
CLARIFIER DESIGN, McGraw-Hil, 2005, p. 146. [2] George Tchbanoglous, Franklin L. Burton, H. David Stensel, "Primary Sedimentation," in Waste water Engineering Treatment and Reuse, Mc Graw Hill, pp. 396-397. [3] C. s. university, in Operation of waste water treatment plants.
