Slide 1

(1+R).Q, S, X

Volume = V; Q, So, XoS, X

(RQ+ Qw), Xr, S

Qw, Xr, SRQ, Xr, SMicrobial Kinetics in a Reactor with Recycle(Q-Qw), Xe, SHydraulic Retention Time (q):

Biological Solids Retention Tine (qc):

Sludge Wasted (DX) = Qw.XrX, i.e., Biomass is also known as Mixed Liquor Volatile Suspended Solids (MLVSS)

Inert Solids in a Reactor with Recycle(1+R).Q, C

Volume = V; Q, CoInert Solids: C

(RQ+ Qw), Cr

Qw, CrRQ, Cr, S(Q-Qw), CeInert Solids Balance for the whole system:

Q.Co [(Q - Qw).Ce + Qw.Cr] = 0

Q.Co = Qw.Cr (A)

Retention Time of Inert Solids: (B)

Total Sludge Wasted (DX + DC) = Qw.(Xr+ Cr)(C)X + C, i.e., total sludge is also known as Mixed Liquor Suspended Solids (MLSS)

http://www.youtube.com/watch?v=GTLKoGGo0dM&feature=player_detailpage

Sewage: Before and After Secondary Treatment

Settleability of Activated Sludge Solids

One of the factors essential to the performance of the activated sludge process is effective flocculation of the sludge, with subsequent rapid settling and compaction.

Two types of bacteria are found in ASP sludge: 1) floc-forming, and 2) filamentous bacteria

Normal flocs: A balance between floc-forming and filamentous bacteria results in strong flocs that keeptheir integrity in the aeration basin and settle well in the sedimentation tank. (High BSRT)

Pin-point flocs: In these flocs, filamentous bacteria are absent or occur in low numbers. This results insmall flocs that do not settle well. (Low BSRT)

Filamentous bulking: Filamentous bulking is caused by the predominance of filamentous organisms. The filaments interfere with sludge settling and compaction. (very high BSRT)Sludge Volume Index (SVI): Sludge settleability is determined by measuring the sludge volume index (SVI), which is given by:

where SV = volume of settled sludge after 30 min (mL/L); and MLSS = mixed liquor suspended solids (mg/L).

Low SVI (< 100) means good sludge settleabilityHigh SVI (> 150) means bad settleability

Solids (X)mg/LV, m/hr20003.6025001.8630001.0935000.6945000.3350000.2460000.1465000.1180000.06100000.03120000.02

Interfacial Velocities of Sludge SolidsInterfacial settling velocity v (in m/hr) = [2x1010]. [X in mg/L]-2.9521Empirical Formula:Type III / Hindered / Zone Settling

Interfacial Height TimeIncreasing (X + C)http://www.youtube.com/watch?v=GTLKoGGo0dM&feature=player_detailpageMeasurement of Sludge Settling Characteristics

Concept of Solid Flux

Solid Flux (SF) = Mass of solids passing through unit surface area of the clarifier in unit time, kg/m2/hr

At any depth ,

Gravity Flux (SFG) = (Ci + Xi).vivi is the interfacial velocity

Underflow Flux (SFu)= (Ci + Xi).u

Total Flux (SFT) = SFG + SFu

Under steady state conditions,

SFT is same at all depths

(Ci + Xi) increases with depth

Design of Secondary Sedimentation Tank

Consider a clarifier with solids conc. (C + X) on top calculated as 5760 mg/L and the solids concentration at the bottom (Cr + Xr) being chosen as 16000 mg/L

It is also given that Q = 50 MLD, while the values, R = 0.5 and Qw = 2 MLD were established through design

Assuming no solids escape with the treated effluent, solids input rate to the clarifier must be equal to the sludge output rate from the clarifier, i.e., Q.(1 + R).(C + X) = (RQ + Qw).(Cr + Xr)

If cross sectional area is As, then solid flux (kg/m2/hr) through the tank,

It is also obvious that the SF through the tank cross-section is the same both at the top and bottom of the tank and also at any intermediate height in the tank.

The question is then, what should be the value of SF.

As soon as we decide on the value of SF, the tank surface area can be calculated.

Solid Flux Theory and the Concept of Limiting Solid Flux

Underflow velocity (u) is defined as,

u may be determined by equating the solid flux at the top and bottom of the clarifier,

In the present case, (C + X) = 5760 mg/L;v1 = 0.16 m/hr (Cr + Xr) = 16000 mg/Lv2 = 0.10 m/hr

Thereforeu = 0.077 m/hr;SF = 1.365 kg/m2/hr;is to SF value acceptable?? No!!!

Solid Flux (SF) = (C + X).(v1 + u) = (Cr + Xr).(v2 + u)

SolidsVGravity Fluxu, m/hrUnderflow FluxTotal Fluxmg/Lm/hrkg/m2/hrKg/m2/hrkg/m2/hr20003.607.1960.0770.1547.35025001.864.6550.0770.1934.84730001.093.2610.0770.2313.49235000.692.4140.0770.2702.68345000.331.4780.0770.3471.82450000.241.2030.0770.3851.58860000.140.8430.0770.4621.30565000.110.7210.0770.5011.22180000.060.4810.0770.6161.097100000.030.3110.0770.7701.081120000.020.2180.0770.9241.142140000.010.1610.0771.0781.239160000.010.1240.0771.2321.356180000.010.0990.0771.3861.485

Suggested Flux (1.365)Actual Flux (1.081)Actual Flux through the tank should be 1.081 kg/m2/hr or less for u = 0.077 m/hr

Higher value of flux will result in solids build up in the tank and ultimately solids overflow with the treated effluent

Sludge

SludgeSludgeSludge

Activated Sludge Process: Secondary Sedimentation

SludgeInfluent Water

Inlet Weir

EffluentLaunder

Scraper

Circular Sedimentation Tank

d

D

Bottom Slope

Motor

Rotating Arm

Design of Activated Sludge Process: Part I

Given Q, assume q (4-12 hrs) and calculate volume of aeration tank (V)

Given S and microbial kinetic constants (YT, kd, Ks and qmax), calculate q

Calculate m and hence qc

Given So, calculate biomass concentration in aeration tank (X) and sludge wasting rate, DX = Qw.Xr

Given Co, calculate inert solids concentration in aeration tank (C) and hence calculate (C+X)

Given (Cr+Xr), calculate Cr and hence Xr. Then using DX and Xr values calculate Qw

Calculate D(C+X) and R

Calculate solids input to the clarifier and also solids output from the clarifier and verify that these values are identical

Assume a value for the underflow velocity (u) from the clarifier and hence calculate As

Calculate the limiting solids flux (SFL) and hence calculate Areq. Verify that As > AReq

Chart13.59798497541.86196406931.08697696230.68958327860.32838350480.24060278270.14045903790.11089921590.06007835670.03109066390.0181501007

Solids Conc. mg/LInterfacial Velocity, m/hInterfacial Settling Velocityy = (2E+10).x-2.9521

Sheet 1Process Design: Assuming that Influent BOD may Vary in 100-400 mg/L RangeSolids, mg/LV, m/hr20003.6025001.8630001.09Flow, Q MLD (Given)50.0050.0050.0050.0050.0050.0050.0035000.69Influent BOD5, mg/L (Given)400.00350.00300.00250.00200.00150.00100.0045000.33Hydraulic Detention Time, hrs (Choose)6.006.006.006.006.006.006.0050000.24Aeration Tank Volume, m3 (Calculate)12500.0012500.0012500.0012500.0012500.0012500.0012500.0060000.14No. of Tanks Provided5.005.005.005.005.005.005.0065000.11Volume/ Tank, m32500.002500.002500.002500.002500.002500.002500.0080000.06Surface Area/Tank, m21250.001250.001250.001250.001250.001250.001250.00100000.03Length/Tank, m40.0040.0040.0040.0040.0040.0040.00120000.02Width/Tank, m31.2531.2531.2531.2531.2531.2531.25Depth, m2.002.002.002.002.002.002.00Minimum BSRT for BOD5, dEffluent BOD5, mg/L (Specify)4.004.004.004.004.004.004.00Ks (mg/L) (Given)25.0025.0025.0025.0025.0025.0025.00qm(/d) (Given)4.004.004.004.004.004.004.00q (/d) (Calculated)0.550.550.550.550.550.550.55YT (Given)0.500.500.500.500.500.500.50Kd (/d) (Given)0.050.050.050.050.050.050.05mu (/d) (Calculate)0.2260.2260.2260.2260.2260.2260.226BSRT, days (Calculate))4.434.434.434.434.434.434.43X, mg/L (Calculate)2871.002508.502146.001783.501421.001058.50696.00Del X, kg/d (Calculate)8105.637082.196058.755035.314011.882988.441965.00Influent FSS concentration, Co mg/L (Given)50.0050.0050.0050.0050.0050.0050.00FSS Conc. in aeration tank, C mg/l (Calculate)885.50885.50885.50885.50885.50885.50885.50Del (C+X), kg/d (Calculate)10605.639582.198558.757535.316511.885488.444465.00Solids conc. to settling tank, (C+X), mg/L (Calculate)3756.503394.003031.502669.002306.501944.001581.50Xr, mg/L (Calculate)137571330412742120281109098017922Qw, MLD (Calculate)0.5890.5320.4750.4190.3620.3050.248R (Calculate)0.2490.2190.1910.1640.1390.1140.091Secondary Settling Tank DesignSolids conc. to settling tank,(C+X), mg/L3756339430312669230619441581interfacial velocity (Corresponding to C+X), v1 m/hr0.5600.7551.0541.5352.3623.9137.195Solids conc. in thickened sludge,(Xr+Cr), mg/L18000180001800018000180001800018000interfacial velocity (Corresponding to Cr+Xr), v2 m/hr0.0050.0050.0050.0050.0050.0050.005underflow velocity, u m/hr (Calculate)0.1410.1240.1080.0930.0790.0650.052Cr, mg/L (Calculate)42434696525859726910819910078Total Flux, (g/m2/hr) (Calculate)26312984352343465629773211461(RQ+Qw) (MLD) (Calculate)13.0311.4910.038.637.306.024.79Surface Area Provided, m2 (Calculate)3860386038603860386038603860Surface Area Required for Clarification, m2 (Must be < calculated Area)3715288921351490972584314Actual Surface Area Provided, m23860386038603860386038603860No. of Settling Tanks Provided (Calculate)10101010101010Area per settling tank, m2 (Calculate)386386386386386386386Diameter (Circular Tank Provided), m (Choose)22222222222222Depth, m (Choose)2.02.02.02.02.02.02.0Detention Time, Hours (Calculate)3.73.73.73.73.73.73.729461.24574829930.0439914062

Sheet 1

Solids Conc. mg/LInterfacial Velocity, m/hInterfacial Settling Velocityy = (2E+10).x-2.9521