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The activated sludge process: A proposed method to predict process parameters
War. Res. Vol. 24, No. II, pp. 1361-1363, 1990 Printed in Great Britain. All rights reserved
0043-1354/90 $3.00 + 0.00 Copyright ~ 1990 Pergamon Press pk
THE ACTIVATED SLUDGE PROCESS: A PROPOSED M E T H O D TO PREDICT PROCESS P A R A M E T E R S W. H. CP.AW4AN@ 8 Belgrave Road, Weston-super-Mare, Avon B522 8AJ, England
(First receivedApril 1989;accepted in revisedform May 1990) Abstract--This paper is based on the author's observations whilst running various sewage treatment plants in the Middle East. These observations are used to develop a theory on activated sludge and to illustrate how the theory can be used to aid both operation and design of activated sludge plants.
Key words--activated sludge, respiration rate, sewage treatment, design, modelling, sludge, aeration, oxygen requirement
INTRODUCTION
DESCRIPTION OF WORKS "A"
Sewage treatment plants in the Middle East operate on a wide temperature range and this necessitates a revised approach to process control on these plants the paper covers the effect of the higher temperatures on an extended aeration plant and a conventional plant. A theoretical explanation of these observations is offered and this is used to suggest methods of operation and design.
The works received a domestic sewage pumped into the works. The design dry weather flow is 40,000 m3/d and a peak flow of 3 DWF. After screening and aerated grit tanks the sewage flow was split between 4 aeration tanks. Each aeration tank has a capacity of 11,400 m3/d containing 5 aerators driven by 75 kW motors. The aerators are 2.1 m dia and rotate at 54.5 rpm. There are 4 clarifers each 38 m diameter fitted with double sided weirs. Sludge could be returned at 200% of DWF. Surplus sludge is removed continuously and pumped to drying beds after thickening.
MIDDLE EAST OPERATING CONDITIONS
The ambient temperatures vary from place to place within the Middle East but are generally around 50°C at maximum in summer and can fall to a few degrees above freezing point in winter. The mixed liquor temperatures are likely to vary from about 10°C to over 30°C and have been observed by the author to be as high as 37°C. These wide fluctuations in temperature cause wide differences in sludge production and in power required Vonder Emder (1989). As the temperature increases the sludge production decreases and the power required increases. If a plant is run at a constant BOD load per K of mixed liquor solids (f/m ratio) there are wide variations in the sludge age and this can lead to the plant becoming de-stabilised. On some extended aeration plants particularly, this can lead to the rapid development of fungal growths appearing as a foam. This foam is most common in spring and autumn when the plant is heating up and cooling down. This foaming is similar to the fungus described by Sezin eta!. (1988). This paper is based on observations of two works: works "A" is an extended aeration works, and works "B" a conventional activated sludge plant.
Process data (averagefor 12 months) Average daily flow 61,700 m3/d Sewage: BOD 123, SS 148, COD 243, Ammoniacal N 15.3, TON 12 Effluent: BOD 4.3, SS 8.7, COD 43, Ammoniacal N 0.83, Nitrate N 8.5 (all results in rag/l). Typical figures obtained on works " A " Summer MCRT 15f/m 0.I1 S/B 0.56, temp 32:C Winter M C R T 15 f/m 0.053 S/B 1.25, temp 15°C. Where MCRT is mean cell residence time, f]m is feed to mass ratio, and S/B is sludge produced per unit weight of BOD removed. The oxygen required for carbonaceous oxidation in winter was 1.75 rising to 2.25 kg/kg BOD in summer.
1361
1362
W.H. CHAPMAN If equation (1) is divided by MESS then it becomes:
DESCRIPTION OF WORKS "B"
Sewage is pumped into works "B" at a constant rate of 500 l/s to each of three stages. When the flow drops below this figure, effluent is recirculated to give a constant flow. At present only one stage is in use. In each stage the pumped flow receives preliminary treatment of screening and grit removal in aerated grit tanks, and then is divided between 2 primary settlement tanks each 29.1 m dia. The effluent from the two primary tanks receives secondary treatment in 6 aeration tanks with 4 aerators per tank. The total capacity of the 6 tanks is 8555 m 3. The mixed liquor is settled in clarifiers each 32.3 m dia and activated sludge is returned at a constant rate of 2001/s. Surplus activated sludge is removed during the day shift. Primary and secondary sludge is digested and dried on drying beds. Process data (30 day trials)
The average daily flow of sewage was 33,675 m3/d, recirculated effluent increased this to 39,040mZ/d. Average analytical data
Z , U = I/MCRT + Rt
(2)
where U = B/MESS which is feed to mass ratio/ plant efficiency MCRT = mean cell residence time. This means Z • U cannot be less than Rt or the M C R T would be negative. Work carried out by the author indicates that the respiration rate is 0.128 kg/d/kg MESS when the temperature was 28°C, the M C R T 15 and U is 0.066. Substituting these values into equation (2), Z is found to have a value of 2 kg sludge/kg BOD. Equation (2) can be rewritten as: Rt = 2 * U - I/MCRT.
(3)
Equation (2) indicates that Rt is not a constant because plotting U against I / M C R T would be a straight line and indicates that at low sludge loadings I / M C R T would be negative. If equation (1) is divided by B the result is: Z = S/B + Rt/U
For average analytical data see Table 1.
rearranging this gives: ACTIVATED SLUDGE THEORY
Rt = U , ( Z - S / B ) .
If the mass balance for an activated sludge plant is considered, and the system is in equilibrium, the material entering the plant is:
Expressing this in a mathematical form gives an equation: = S + MLSS,Rt
(1)
where B=
Z= S=
MLSS = Rt =
BOD removed in the plant (kg/d) kg of sludge produced per kg of BOD surplus sludge removed in the waste activated sludge and in the effluent suspended solids (kg/d) weight of mixed liquor solids in the aeration tanks (kg) respiration rate at temperature t (kg/d/kg MESS).
This equation is based on the Eckenfelder (1980) equation and was used by Johnstone (1984).
Sewage lYrE Effluent
Flow
Table I BOD SS
(m J/d)
(nag/I)
(rag/I)
33,675 39,040 33,675
179 81 6.3
346 80.5 9.3
NH3-N NO3-N (mg/I) 35.8
33.I 21.7
R t = ( Z - S / B ) / ( S / B • MCRT).
(4)
Experimental evidence indicates that the expression: ( Z - S / B ) / S / B is a logarithmic relationship to time. The following empirical relationship appears to give a good working model:
Discharged in the effluent Removed as surplus sludge Destroyed by respiration in the sludge.
Z,B
Substituting I / ( S / B * MCRT) for U gives
(nag/I)
( Z - S / B )/S / B = k * (In MCRT) * 0 ~r-'°~
where 0 is the temperature coefficient. Substituting this into (4) gives: R t = (k * 0 (r- 20), In MCRT)/MCRT.
(5)
Using the values MCRT = 15, R t = 0.0745, t = 20, k is found to be 0.413. Thus Rt = (0.413 * In MCRT) * 0 tr- 2°~/MCRT
(6)
U = (Rt + I/MCRT)/2 S / B = i / M C R T * U. Oxygen requirements
The oxygen requirement (Johnston, 1984) for carbonaceous oxidation in an activated sludge plant (COR) had been given as COR = 0.75 • B + MESS • Rt. Dividing by B
Trace
(COR/B) = 0.75 + R t / U .
(7)
Activated sludge process prediction Thus at 20°C if the MCRT is 2, the calculated values for Rt and U are: 0.143 and 0.32. Substituting these values into equation (7) the oxygen required is I. 19 kg/kg BOD. Similarly if the MCRT is 15, the Rt is 0.074 and U is 0.0706 so oxygen required is 1.79 kg/kg BOD. The temperature coefficient 0 is taken as 1.07 (Johnston, 1984). The total oxygen requirement for an activated sludge plant is B • (0.75 + Rt/U) + 4.43 • N - 2.85 • N. The figures for nitrogenous demand are those quoted by Johnstone (1984). To test the theory which was developed on works " A " the following figures from a conventional activated sludge plant were used from a 30 day trial period and compared with the predictions. Actual figures obtained on works " B "
Temperature is 27°C, MCRT 6.26, f / m 0.12 and S / B 1.43. Predicted figures using MCRT 6.26 and 27°C are f / m 0.11 and S / B 1.45. Both works "A" and works "B" are treating domestic sewage and the theory advanced seems to be able to facilitate reasonable predictions for works treating domestic sewage.
1363
The oxygen required and the mixed liquor solids levels required under any operational conditions can be predicted and enable the plant to be operated to produce a good effluent whilst minimalising the power consumed and sludge produced. W h e n a plant is operated at constant mixed liquor solids, the power required varies to a greater degree than if the M C R T is kept constant. In the author's experience this stillenables a good quality effluentto be produced and reduces the riskof forming excessive fungal growths. Allowing the mixed liquor solids to increase in winter within reasonable limits allows better use to be made of sludge treatment facilities. CONCLUSIONS The rate of destruction of sludge is important to the operation of an activated sludge plant and for design an empirical formula has been derived: Rt = k * In MCRT * & - 2°~/MCRT
where k is a constant (0.413). From this equation other parameters can be predicted U -~ Rt + I/MCRT S / B = I/U • MCRT.
APPLICATIONOF THE THEORY The equations derived can be used to predict the operation of an activated sludge plant under varying site conditions. When an activated sludge plant is being designed it is necessary to know the MCRT that the plant will operate at then, the respiration rate and f / m ratio can be calculated at maximum and minimum temperatures. These figures can be used to predict the surplus sludge produced and power required under these conditions. Predictions of the variation in oxygen required and thus the degree of adjustment that is required on the aeration system can be made. Predicting the sludge produced under winter conditions, when it is maximum quantity and most difficult to treat will facilitate provisions being made to allow for this at the design stage.
Oxygen required for carbonaceous in kg/kg of BOD is:
0.7 + Rt/U. These equations can be used to optimise plant operation and design. REFERENCES Eekenfielder W. W. (1980) Principles of Water Quality Monitoring. CBI, Boston, Mass. Johnstone D. W. M. (1984) Oxygen requirements, energy consumption and sludge production in extended aeration plants. War. Pollut. Control 83, 100-113. Sezgin M., Lechevalier M. P. and Karr P. R. 0958) Isolation and identification of actinomycetes present in activated sludge scum. Wat. Sci. TechnoL 20(11/12), 257-263. Vonder Emder W. (1989) IAWRPC Specialist Group on Design and Operation of Large Waste Water Plants.
0043-1354/90 $3.00 + 0.00 Copyright ~ 1990 Pergamon Press pk
THE ACTIVATED SLUDGE PROCESS: A PROPOSED M E T H O D TO PREDICT PROCESS P A R A M E T E R S W. H. CP.AW4AN@ 8 Belgrave Road, Weston-super-Mare, Avon B522 8AJ, England
(First receivedApril 1989;accepted in revisedform May 1990) Abstract--This paper is based on the author's observations whilst running various sewage treatment plants in the Middle East. These observations are used to develop a theory on activated sludge and to illustrate how the theory can be used to aid both operation and design of activated sludge plants.
Key words--activated sludge, respiration rate, sewage treatment, design, modelling, sludge, aeration, oxygen requirement
INTRODUCTION
DESCRIPTION OF WORKS "A"
Sewage treatment plants in the Middle East operate on a wide temperature range and this necessitates a revised approach to process control on these plants the paper covers the effect of the higher temperatures on an extended aeration plant and a conventional plant. A theoretical explanation of these observations is offered and this is used to suggest methods of operation and design.
The works received a domestic sewage pumped into the works. The design dry weather flow is 40,000 m3/d and a peak flow of 3 DWF. After screening and aerated grit tanks the sewage flow was split between 4 aeration tanks. Each aeration tank has a capacity of 11,400 m3/d containing 5 aerators driven by 75 kW motors. The aerators are 2.1 m dia and rotate at 54.5 rpm. There are 4 clarifers each 38 m diameter fitted with double sided weirs. Sludge could be returned at 200% of DWF. Surplus sludge is removed continuously and pumped to drying beds after thickening.
MIDDLE EAST OPERATING CONDITIONS
The ambient temperatures vary from place to place within the Middle East but are generally around 50°C at maximum in summer and can fall to a few degrees above freezing point in winter. The mixed liquor temperatures are likely to vary from about 10°C to over 30°C and have been observed by the author to be as high as 37°C. These wide fluctuations in temperature cause wide differences in sludge production and in power required Vonder Emder (1989). As the temperature increases the sludge production decreases and the power required increases. If a plant is run at a constant BOD load per K of mixed liquor solids (f/m ratio) there are wide variations in the sludge age and this can lead to the plant becoming de-stabilised. On some extended aeration plants particularly, this can lead to the rapid development of fungal growths appearing as a foam. This foam is most common in spring and autumn when the plant is heating up and cooling down. This foaming is similar to the fungus described by Sezin eta!. (1988). This paper is based on observations of two works: works "A" is an extended aeration works, and works "B" a conventional activated sludge plant.
Process data (averagefor 12 months) Average daily flow 61,700 m3/d Sewage: BOD 123, SS 148, COD 243, Ammoniacal N 15.3, TON 12 Effluent: BOD 4.3, SS 8.7, COD 43, Ammoniacal N 0.83, Nitrate N 8.5 (all results in rag/l). Typical figures obtained on works " A " Summer MCRT 15f/m 0.I1 S/B 0.56, temp 32:C Winter M C R T 15 f/m 0.053 S/B 1.25, temp 15°C. Where MCRT is mean cell residence time, f]m is feed to mass ratio, and S/B is sludge produced per unit weight of BOD removed. The oxygen required for carbonaceous oxidation in winter was 1.75 rising to 2.25 kg/kg BOD in summer.
1361
1362
W.H. CHAPMAN If equation (1) is divided by MESS then it becomes:
DESCRIPTION OF WORKS "B"
Sewage is pumped into works "B" at a constant rate of 500 l/s to each of three stages. When the flow drops below this figure, effluent is recirculated to give a constant flow. At present only one stage is in use. In each stage the pumped flow receives preliminary treatment of screening and grit removal in aerated grit tanks, and then is divided between 2 primary settlement tanks each 29.1 m dia. The effluent from the two primary tanks receives secondary treatment in 6 aeration tanks with 4 aerators per tank. The total capacity of the 6 tanks is 8555 m 3. The mixed liquor is settled in clarifiers each 32.3 m dia and activated sludge is returned at a constant rate of 2001/s. Surplus activated sludge is removed during the day shift. Primary and secondary sludge is digested and dried on drying beds. Process data (30 day trials)
The average daily flow of sewage was 33,675 m3/d, recirculated effluent increased this to 39,040mZ/d. Average analytical data
Z , U = I/MCRT + Rt
(2)
where U = B/MESS which is feed to mass ratio/ plant efficiency MCRT = mean cell residence time. This means Z • U cannot be less than Rt or the M C R T would be negative. Work carried out by the author indicates that the respiration rate is 0.128 kg/d/kg MESS when the temperature was 28°C, the M C R T 15 and U is 0.066. Substituting these values into equation (2), Z is found to have a value of 2 kg sludge/kg BOD. Equation (2) can be rewritten as: Rt = 2 * U - I/MCRT.
(3)
Equation (2) indicates that Rt is not a constant because plotting U against I / M C R T would be a straight line and indicates that at low sludge loadings I / M C R T would be negative. If equation (1) is divided by B the result is: Z = S/B + Rt/U
For average analytical data see Table 1.
rearranging this gives: ACTIVATED SLUDGE THEORY
Rt = U , ( Z - S / B ) .
If the mass balance for an activated sludge plant is considered, and the system is in equilibrium, the material entering the plant is:
Expressing this in a mathematical form gives an equation: = S + MLSS,Rt
(1)
where B=
Z= S=
MLSS = Rt =
BOD removed in the plant (kg/d) kg of sludge produced per kg of BOD surplus sludge removed in the waste activated sludge and in the effluent suspended solids (kg/d) weight of mixed liquor solids in the aeration tanks (kg) respiration rate at temperature t (kg/d/kg MESS).
This equation is based on the Eckenfelder (1980) equation and was used by Johnstone (1984).
Sewage lYrE Effluent
Flow
Table I BOD SS
(m J/d)
(nag/I)
(rag/I)
33,675 39,040 33,675
179 81 6.3
346 80.5 9.3
NH3-N NO3-N (mg/I) 35.8
33.I 21.7
R t = ( Z - S / B ) / ( S / B • MCRT).
(4)
Experimental evidence indicates that the expression: ( Z - S / B ) / S / B is a logarithmic relationship to time. The following empirical relationship appears to give a good working model:
Discharged in the effluent Removed as surplus sludge Destroyed by respiration in the sludge.
Z,B
Substituting I / ( S / B * MCRT) for U gives
(nag/I)
( Z - S / B )/S / B = k * (In MCRT) * 0 ~r-'°~
where 0 is the temperature coefficient. Substituting this into (4) gives: R t = (k * 0 (r- 20), In MCRT)/MCRT.
(5)
Using the values MCRT = 15, R t = 0.0745, t = 20, k is found to be 0.413. Thus Rt = (0.413 * In MCRT) * 0 tr- 2°~/MCRT
(6)
U = (Rt + I/MCRT)/2 S / B = i / M C R T * U. Oxygen requirements
The oxygen requirement (Johnston, 1984) for carbonaceous oxidation in an activated sludge plant (COR) had been given as COR = 0.75 • B + MESS • Rt. Dividing by B
Trace
(COR/B) = 0.75 + R t / U .
(7)
Activated sludge process prediction Thus at 20°C if the MCRT is 2, the calculated values for Rt and U are: 0.143 and 0.32. Substituting these values into equation (7) the oxygen required is I. 19 kg/kg BOD. Similarly if the MCRT is 15, the Rt is 0.074 and U is 0.0706 so oxygen required is 1.79 kg/kg BOD. The temperature coefficient 0 is taken as 1.07 (Johnston, 1984). The total oxygen requirement for an activated sludge plant is B • (0.75 + Rt/U) + 4.43 • N - 2.85 • N. The figures for nitrogenous demand are those quoted by Johnstone (1984). To test the theory which was developed on works " A " the following figures from a conventional activated sludge plant were used from a 30 day trial period and compared with the predictions. Actual figures obtained on works " B "
Temperature is 27°C, MCRT 6.26, f / m 0.12 and S / B 1.43. Predicted figures using MCRT 6.26 and 27°C are f / m 0.11 and S / B 1.45. Both works "A" and works "B" are treating domestic sewage and the theory advanced seems to be able to facilitate reasonable predictions for works treating domestic sewage.
1363
The oxygen required and the mixed liquor solids levels required under any operational conditions can be predicted and enable the plant to be operated to produce a good effluent whilst minimalising the power consumed and sludge produced. W h e n a plant is operated at constant mixed liquor solids, the power required varies to a greater degree than if the M C R T is kept constant. In the author's experience this stillenables a good quality effluentto be produced and reduces the riskof forming excessive fungal growths. Allowing the mixed liquor solids to increase in winter within reasonable limits allows better use to be made of sludge treatment facilities. CONCLUSIONS The rate of destruction of sludge is important to the operation of an activated sludge plant and for design an empirical formula has been derived: Rt = k * In MCRT * & - 2°~/MCRT
where k is a constant (0.413). From this equation other parameters can be predicted U -~ Rt + I/MCRT S / B = I/U • MCRT.
APPLICATIONOF THE THEORY The equations derived can be used to predict the operation of an activated sludge plant under varying site conditions. When an activated sludge plant is being designed it is necessary to know the MCRT that the plant will operate at then, the respiration rate and f / m ratio can be calculated at maximum and minimum temperatures. These figures can be used to predict the surplus sludge produced and power required under these conditions. Predictions of the variation in oxygen required and thus the degree of adjustment that is required on the aeration system can be made. Predicting the sludge produced under winter conditions, when it is maximum quantity and most difficult to treat will facilitate provisions being made to allow for this at the design stage.
Oxygen required for carbonaceous in kg/kg of BOD is:
0.7 + Rt/U. These equations can be used to optimise plant operation and design. REFERENCES Eekenfielder W. W. (1980) Principles of Water Quality Monitoring. CBI, Boston, Mass. Johnstone D. W. M. (1984) Oxygen requirements, energy consumption and sludge production in extended aeration plants. War. Pollut. Control 83, 100-113. Sezgin M., Lechevalier M. P. and Karr P. R. 0958) Isolation and identification of actinomycetes present in activated sludge scum. Wat. Sci. TechnoL 20(11/12), 257-263. Vonder Emder W. (1989) IAWRPC Specialist Group on Design and Operation of Large Waste Water Plants.