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Importance of cell growth rate and stoichiometry to the removal of phosphorus from the activated sludge process
Water Research Pergamon Press 1972. Vol. 6, pp. 1051-1057. Printed in Great Britain
IMPORTANCE OF CELL GROWTH RATE A N D STOICHIOMETRY TO THE REMOVAL OF PHOSPHORUS FROM THE ACTIVATED SLUDGE PROCESS JOSEPH H. S n ~
Department of Civil and Environmental Engin~ring, Cornell University, U.S.A. and EDWARD D. SCHROEDF.R Department of Civil Engineering Universityof California, Davis, U.S.A. (Received 7 March 1972)
INTRODUCTION THE REMOVALof phosphorus from wastewaters is important because this essential element has been identified as the limiting nutrient for algal growth in many receiving waters. Excessive growth of aquatic plants due to disposal of increased quantities of phosphorus has resulted in accelerated rates of aging of lakes (eutrophication). Recent studies have reported on the removal of phosphorus in the activated sludge process (BARTH and Errr~GEg, 1967; BAgTH et al., 1968; Wrrm~ow, 1969; MENAg and JENga'NS, 1969; GEINOPOLOSand Vtt.EN, 1971 and MILBtntY et al., 1971). These studies were, for the most part, stimulated as the result of a report from San Antonio, Texas, in which observations of approximately 90 per cent phosphorus removal was recorded at the Rilling Plant and approximately 50 per cent removal was obtained at the San Antonio East and West Plants (VACI~R et al., 1967). Increased phosphorus removal has been speculated to be attributed to two separate and distinct theories. Some researchers have concluded that increased removals are due to "luxury uptake", a condition whereby the biological organisms incorporate excessive quantities of this nutrient into cell material. Other researchers, however, have concluded that removal of phosphorus in excess quantities is a chemical phenomena and occurs as a result of precipitation. The purpose of this paper is to: (a) briefly review the proposed theories of phosphorus removal and (b) illustrate that in certain instances high removals may be attributed to the chemical stoichiometry of the process, cell growth rate, and the net mass of sludge produced at specific cell growth rates. "LUXURY UPTAKE" THEORY The storage of phosphorus in volutin granules within cellular tissue has been well documented in the microbial literature (MANDLESTAM and McQUILLEN, 1968). Furthermore, phosphorus is an essential nutrient for the energy metabolism of all living systems and has the property of being rapidly extracted from solution for use by living systems (ODUM, 1963). As a result, several researchers have proposed that under the proper conditions phosphorus can be extracted from solution and stored in cellular tissue in quantities in excess of that found under normal conditions. After performing extensive laboratory studies LEVlN and SHAPmO(1965) concluded that excess quantities of phosphorus could be incorporated into activated sludge. 1051
10 52
JOSEPHH. SH~tRARDand EDWARDD. SCHROEDER
These workers stated that the concentration level of dissolved oxygen (D.O.) exerted a strong influence over the rate of phosphorus uptake. High levels of D.O. were found to increase the rate of uptake and low levels of D.O. were found to effect a leakage of phosphorus from the cells. Hydrogen ion concentration (pH) was also noticed to affect the results and a maximum removal was obtained in pH ranges of 7.0-8.0 BORCHAS~T and AZAD (1968) studies the removal of phosphorus in algae and extended their conclusions to bacterial mediated reactions. Their conclusion was that "luxury uptake" in biological systems only occurred after periods of phosphate starvation. Phosphorus removal in San Antonio was reported by VACKERet al. (1967). They utilized the "luxury uptake" concept in explaining high phosphorus removals. Their data was taken on activated sludge plants treating San Antonio sewage and led to the following important conclusions: (a) up to 96 per cent removal of phosphorus occurred when digester supernatant was not returned to the plant, (b) up to 7 per cent elemental phosphorus was contained in the sludge on a dry weight basis, (c) maximum phosphorus removal was observed when nitrification was minimal, and (d) high removals of phosphorus occurred at a daily biochemical oxygen demand, (BED), loading rate of 50 lb B e D per 100 lb aeration solids and D.O. concentration was 2 mg 1-1 at the midpoint of the aeration basin and 5 mg 1- x at the treated effluent end of the basin. Despite the findings of these and other studies no conclusive data have been presented to demonstrate that higher than normal levels of phosphorus exist within cellular protoplasm found in an activated sludge process, as opposed to adsorption of phosphate on floc surfaces. CHEMICAL PRECIPITATION THEORY To date the best and most convincing explanation of the fate of phosphorus in the activated sludge process has been given by M ~ x g and J~Kt~s (1969). These workers, utilizing the works of SAW3'Ea (1944), SeKIKAWA et aL (1966), HALL and ENGELBR1~4T (1967) and JeNrdlqS and M_~AK (1967) as a basis, concluded that the biological removal of phosphorus may be explained as follows: (a) the per cent by weight (volatile mass basis) of incorporated phosphorus in cellular protoplasm is between 2 and 3 per cent regardless of the physiological state of the microorganisms, (b) the amount of incorporated phosphorus is not affected by the growth rate of the microorganisms or by process operating parameters such as organic loading, mixed liquor suspended solids (MLSS) concentration, aeration rate, or mixed liquor D.O. concentration and (c) the removal of phosphorus is proportional to net microbial growth. Menar and Jenkins proposed that any additional removal of phosphorus in excess of the 20 to 30 per cent normally obtained for typical domestic sewages could be attributed to a precipitation phenomenon. The chemical precipitation theory may be explained on the basis that calcium phosphate is precipitated from solution (solubility is a function of pH), becomes enmeshed in the sludge floc, and is subsequently removed from the process in the waste sludge. The proposed mechanism of precipitation for a conventional plug flow process is explained as follows: (a) at the head end of the aeration basin where return sludge and primary effluent enter, the production of CO2 is high as the result of biological oxidation and the pH is low, (b) farther down the aeration basin C02 production decreases due to the depletion of organic material available for oxidation and a pH increase occurs due to the reduced
Removal of Phosphorus from the Activated Sludge Process
1053
production of CO, and stripping of COt from solution by aeration and (c) the increased pH results in environmental conditions favorable for the precipitation of calcium phosphate. Menar and Jenkins were able to qualitatively demonstrate that phosphorus removal was a precipitation phenomena by performing pilot plant studies • on a wastewater similar in characteristics to that found at the Rilling plant. In a later study, FntousoN and MCCAXTY (1969) proposed an explanation for the reason that data of Menar and Jenkins did not fully complement data reported at the Rilling plant. Ferguson and McCarty concluded that the amount of 15hosphorus precipitated was dependent upon the molar ratio between magnesium and calcium, and that (a) the calcium to magnesium ratio was higher in the waste at Rilling than in the waste studied by Menar and Jenkins and (b) the wastewater carbonate carbon concentration was lower at Rifling than for the wastewatcr studied by Menar and Jenkins. The higher concentration of carbonate carbon was speculated to increase the phosphate residual because, of complex formation and competitive precipitation of calcium carbonate. UPTAKE DEPENDENT UPON NET BIOLOGICAL GROWTH AND PROCESS STOICHIOMETRY
In recent studies StmRRARD and SCHROED~t (1972) have demonstrated that net production of biological solids is related to cell growth rate. The relationship between these two variables was established much earlier in the works of SAWYER(1940) and WUHRMAN(1955), although it was not stated in explicit terms. Net cell growth rate is the reciprocal of mean cell residence for a process operating at steady-state conditions. Mean cell residence time is the average length of time a bacterial cell remains in the aeration basin before being wasted from the process. The importance of cell growth rate and mean cell residence time in biological wastewater treatment processes has been recently demonstrated by SCHgOEDER(1971), LAWRENCE and MCCARTY (1970) and JENKr~S and GARglSON (1968). Production (wastage) of cellular material obtained for a steady-state laboratory activated sludge treatment process treating a soluble wastewater is shown in FIG. 1 (SHERg~D and SCHROEDER, 1972). As shown in FIG. 1 net solids produced per day during steady-state operation varies from a maximum at low cell residence times to minimum values at long cell residence times. Based upon the results shown in FIG. 1, a 63 per cent difference in sludge production is apparent when data at 2 and 18 day mean cell residence times are compared. Despite the fact that cell production varied, hydraulic residence time was maintained constant, and influent and effluent organic concentrations were constant for the range of conditions studied. Hydraulic residence time was maintained between 6.14 and 6.52 h; influent chemical oxygen demand (COD) was maintained at 269 mg 1-1 and, effluent COD was observed to be essentially a constant and average 40 mg 1-1 COD between the 2 and 18 day cell residence times studied. Data were recorded from an experimental reactor (see FIG. 2) during steady-state operation. This reactor was described earlier by SHERRARDand SCHROEDER(1972). The data obtained from study cited above were found to be described by an equation of the form: ¥obs -- 0.406 exp (--0.067 0c) w.R. 6/9--i
(1)
1054
JOSEPH H. SHERRARD and EDWARD D . SCHROEDER 40
I
I
I
30 C3
o t) _= 2o
o
,0
£= o
-=
1
o
I
5
I
ICI
Mean cell residence
20
15
rime ,
days
FIG. 1. Pound cells produc~l per lb COD removed between cell residence times of 2 and 18 days.
where Yob, is an observed cell yield coefficient corresponding to a value of mean cell residence time, 0c. Because the observed cell yield coefficient decreases at increased values of 0c microbial solids production is larger at low values of 0c and smaller at high values of 0~ (see FIG. 1). As a result the amount of waste solids can be varied by controlling mean cell residence time and the uptake of phosphorus and other nutrients which are incorporated into cellular material should depend upon solids production. Therefore, at low cell residence times maximum phosphorus removal would be expected and at long cell residence times minimal phosphorus removals should occur. Varying phosphorus removals dependent upon cell residence time can be demonstrated more Waste pump timer P rQduated cylinder
Feed pump Mercury rthel hermo-
I
/--Effluent
removal tube
I
met~~.~ffluent
sampler (removable) Vacuum source
(aspirator) Calibrated feed tanks
cU:re rPre•uS
Reacto ~---
Air flow meter and regulator
Effluent ¢ollect~n tank diffuser stones
Cotton air filter
FIG. 2. Experimental activated sludge unit.
Removal of Phosphorus from the Activated Sludge Process
] 055
clearly with the aid of FIG. 1 and stoich/ometric equations descriptive of process reactions. As shown in FIG. 1, 34 lb of cells are produced per 100 lb of COD utilized at a 2 day cell residence time. At a value of a 14 day cell residence time only 17 lb of cells are produced per 100 Ib COD utilized. A 50 per cent difference in solids production is therefore obtained when comparing production for these two conditions. The following stoichiometric equations are written to describe the biological growth reaction assuming that cellular composition is constant regardless of growth rate and can be described by the formula C6oHsTO2~NI2P. A stoichiometric equation written for sludge production corresponding to a 2 day cell residence time when nutrients are present in exact proportions indicates that 100 per cent phosphorus removal is possible. 21 C6H1206 + 63.5 02 + 12 NH3 + H~PO, -> CeoHsTO23N12P + 66 CO2 + 102 H 2 0
(2)
However, for the same amount of organic material and nutrients a stoichiometric equation at a 14 day cell residence time would be 21 C6H12Oe + 94.75 02 + 12 NH3 + H~PO4 -> ½ C6oHsTO2sNt2P + 96 CO2 + 114 H 2 0 + 6 NH3 + ½ H3PO,
(3)
I00
- C O D : P -130:1 - o - CO0:P "65:1
#
- C O O : P , 4:3:1 80
60 o
40-
o.
~ ' ~ . ~ (c)
" - - .....
20-Q. "''
0
I
I
5
I0 0©,
-,....,. ,..,..._
I
15
:'0
days
RG. 3. Phosphorus removal efficiency vs mean cell residence time for wastewater with different stoichiometric ratios of nutrient.
As shown in equation (3) only one-half as many cells are produced as in equation (2) and therefore one-half of the phosphorus content of the influent is present in the effluent. Varying phosphorus removal efficiencies corresponding to different cell residence times and different initial ratios of phosphorus in the waste are shown in RG. 3. As shown in Fro. 3 a 21:1 ratio of C6H120~ to H3PO, (curve a) produces
1056
Josep~t H. SHERRARDand EDWARDD. SCHROEDER
a relationship as described above. Ratios of C6H~206 to H3PO, of 21:2 and 21:3 are shown in curves b and c. These ratios correspond respectively to C O D : P ratios of 130:1, 65:1 and 43:1. DISCUSSION Current theories of mechanisms of phosphorus removal from wastewater have been discussed. The importance of cell growth rate, inet solids production, and wastewater stoichiometry have been presented to clarify the manner in which phosphorus is removed biologically. The above analysis was based upon a premise that cellular ratios of carbon, hydrogen, oxygen, phosphorus and nitrogen were constant. Based upon the preceding presentation and given that cellular composition is constant over the range of growth rates maintained in the activated sludge process, it is possible to conclude that high phosphorus removals reported for scwages are most likely attributed to a precipitation phenomena. However, it is shown that phosphorus removal efficiency can be varied by controlling cell growth rate (mean cell residence time) and that the effectiveness of removal is dependent upon wastewater stoichiometry. A presentation of laboratory or field data to verify the above hypothesis is necessary. Unfortunately, those studies currently being performed, and those performed in the past, have not been reported with sufficient data to allow a complete mass balance on phosphorus to be made. This has occurred because it has been generally assumed that the amount of microbial solids produced per unit time is constant regardless of process operating conditions. In reality, however, a large variation in solids production is possible. CONCLUSIONS Based upon the preceding analysis the following conclusions can be made: (a) Increased amounts of inorganic nutrients can be extracted from wastewater by operating an activated sludge process at low cell residence times. (b) The chemical precipitation theory is most likely descriptive of high removals in sewages because of wastewater stiochiometry. However, removal is dependent upon net solids production, a parameter that can be regulated by controlling cell growth rate. (c) Contrary to the statement of MENA~ and JENKINS (1969) in certain cases cell growth rate can significantly affect nutrient removal rates. (d) Increased oxygen consumption is required to satisfy stoichiometry at long cell residence times. This is illustrated by comparing equations (2) and (3). (e) There is an inherent danger in writing stoichiometric equations such as equations (2) and (3) unless net solids production data and cellular composition are .known. critique and suggestions offered by Dr. ALONZOWM. LAWRENCEof the Department of Civil and Environmental Engineering, Cornell University, are acknowledged gratefully.
Acknowledgement--The
REFERENCES B~TI-I E. F., BRENNERR. C. and LEwis R. F. (1968) Chemical-Biologicalcontrol of nitrogen and phosphorus in wastcwater effincnt.J. War. Pollut. Control Fed. 40, 2040. B~TH E. F. and ETnNO~ M. B. (1967) Mineral controlled phosphorus removal in the activated sludge process. J. War. Pollut. Control Fed. 39, 1362.
Removal of Phosphorus from the Activated Sludge Process
1057
BOReHAP.DTJ. A. and AzAv H. S. (1968) Biological extraction of nutrients. Y. Wat. Pollut. Control Fed. 40, 1739. FEROUSONJ. F. and McCARTYP. L. (1969) The precipitation of phosphates from fresh waters and waste waters. Tech. Rep. No. 120, Department of Civil Engineering, Stanford University. Gr~NOPOLOS A. and VIL~N F. I. (1971) Process evaluation-phosphorus removal. Y. War. Pollut. Control Fed. 43, 1975. HALL M. W. and ENOELBRECHTR. (1967) Uptake of soluble phosphate by activated sludge: parameters of influence. Proceedings 7th Industrial Water and Wuste Conference, University of Texas, Austin, Texas, p. H-8. JENKINS D. and G~d~mSONW. E. (1968) Control of activated sludge by mean cell residence time. J. Wat. Pollut. Control Fed. 40, 1905. JENKINS D. and M~NARA. (1967) A study of the fate of phosphorus during primary sedimentation and activated sludge treatment. SERL Rep. 67-6. Berkeley: Sanit. Eng. Research Lab., University of Califonia. L A w R ~ A. W. and McC~dtr~' P. L. (1970) Unified basis for biological treatment design and operation. J. Sanit. Engrs. Div., Proc. ASCE, SA 3, 757. LL~rN G, V. and SHAPmOJ. (1965) Metabolic uptake of phosphorus by wastewater organisms. Y. War. Pollut. Control Fed. 37, 800. MANDVJ.~r~, J. and McQuIL~N K. eds., (1968) Biochemistry of Bacterial Growth. Wiley, New York, 1968. MENAR A. B. and JENKINSD. (1969) The fate of phosphorus in waste treatment processes: The enhanced removal of phosphate by activated sludge. Proc. 24th Industrial Waste Conference, Purdue University Extension Series 135, p. 655. MILBURYW. F., MCCAUL~ D. and HAWTHORN~C. H. (1971) Operation of conventional activated sludge for Maximum Phosphorus Removal. Y. Wat. PoUut. Control Fed. 43, 1890. ODUM E. P. (1963) Ecology. Holt, Rinehart & Winston, New York, SA~ C. N. (1940) Activated sludge oxidations VI. Results of feeding experiments to determine the effect of the variables temperature and sludge concentration. Sewage Wks Y. 12, 244. S^wYeR C. N. (1944) Biological engineering in sewage treatment. Sewage WksY. 16, September 1944, p. 925. ScI-IRO~m~ E. D. (1971) The effect of cell recycle on activated sludge process operation. Water Research 5, 22. SEKIKAW^Y., NICmKAWAS., Og.AZA~ M. and KATOK. (1966) Release of soluble phosphate in the activated sludge process. 3rd Int. Conf. Water Pollut. Res. Munich, Germany. S H E R ~ J. H. and ScmtOED~ E. D. (1972) Relationship between the observed ceil yield coefficient and mean cell residence time in the completely mixed activated sludge proce*.s. Water Research 6, 1039-1049. VACKeg D., CO~,~EL C. H. and WELLSW. N. (1967) Phosphate removal through municipal wastewater treatment at San Antonio, Texas. Y. Water Pollut. Control Fed. 39, 750. WITHEgOW J. L. (1969) Phosphate removal by activated sludge. Proc. 24th Industrial Waste Conf. Purdue University Extension Series 135, p. 1169. WUHPa~ANNK. (1955) Factors affecting efficiency and solids production in the activated sludge process. In Biological Treatment of Sewage and Industrial Wastes, Vol. I, Aerobic Oxidation, p. 49 Edited by McC~E J. and ECg.EN~LDERW. W. Reinhold Publishing Corp.
IMPORTANCE OF CELL GROWTH RATE A N D STOICHIOMETRY TO THE REMOVAL OF PHOSPHORUS FROM THE ACTIVATED SLUDGE PROCESS JOSEPH H. S n ~
Department of Civil and Environmental Engin~ring, Cornell University, U.S.A. and EDWARD D. SCHROEDF.R Department of Civil Engineering Universityof California, Davis, U.S.A. (Received 7 March 1972)
INTRODUCTION THE REMOVALof phosphorus from wastewaters is important because this essential element has been identified as the limiting nutrient for algal growth in many receiving waters. Excessive growth of aquatic plants due to disposal of increased quantities of phosphorus has resulted in accelerated rates of aging of lakes (eutrophication). Recent studies have reported on the removal of phosphorus in the activated sludge process (BARTH and Errr~GEg, 1967; BAgTH et al., 1968; Wrrm~ow, 1969; MENAg and JENga'NS, 1969; GEINOPOLOSand Vtt.EN, 1971 and MILBtntY et al., 1971). These studies were, for the most part, stimulated as the result of a report from San Antonio, Texas, in which observations of approximately 90 per cent phosphorus removal was recorded at the Rilling Plant and approximately 50 per cent removal was obtained at the San Antonio East and West Plants (VACI~R et al., 1967). Increased phosphorus removal has been speculated to be attributed to two separate and distinct theories. Some researchers have concluded that increased removals are due to "luxury uptake", a condition whereby the biological organisms incorporate excessive quantities of this nutrient into cell material. Other researchers, however, have concluded that removal of phosphorus in excess quantities is a chemical phenomena and occurs as a result of precipitation. The purpose of this paper is to: (a) briefly review the proposed theories of phosphorus removal and (b) illustrate that in certain instances high removals may be attributed to the chemical stoichiometry of the process, cell growth rate, and the net mass of sludge produced at specific cell growth rates. "LUXURY UPTAKE" THEORY The storage of phosphorus in volutin granules within cellular tissue has been well documented in the microbial literature (MANDLESTAM and McQUILLEN, 1968). Furthermore, phosphorus is an essential nutrient for the energy metabolism of all living systems and has the property of being rapidly extracted from solution for use by living systems (ODUM, 1963). As a result, several researchers have proposed that under the proper conditions phosphorus can be extracted from solution and stored in cellular tissue in quantities in excess of that found under normal conditions. After performing extensive laboratory studies LEVlN and SHAPmO(1965) concluded that excess quantities of phosphorus could be incorporated into activated sludge. 1051
10 52
JOSEPHH. SH~tRARDand EDWARDD. SCHROEDER
These workers stated that the concentration level of dissolved oxygen (D.O.) exerted a strong influence over the rate of phosphorus uptake. High levels of D.O. were found to increase the rate of uptake and low levels of D.O. were found to effect a leakage of phosphorus from the cells. Hydrogen ion concentration (pH) was also noticed to affect the results and a maximum removal was obtained in pH ranges of 7.0-8.0 BORCHAS~T and AZAD (1968) studies the removal of phosphorus in algae and extended their conclusions to bacterial mediated reactions. Their conclusion was that "luxury uptake" in biological systems only occurred after periods of phosphate starvation. Phosphorus removal in San Antonio was reported by VACKERet al. (1967). They utilized the "luxury uptake" concept in explaining high phosphorus removals. Their data was taken on activated sludge plants treating San Antonio sewage and led to the following important conclusions: (a) up to 96 per cent removal of phosphorus occurred when digester supernatant was not returned to the plant, (b) up to 7 per cent elemental phosphorus was contained in the sludge on a dry weight basis, (c) maximum phosphorus removal was observed when nitrification was minimal, and (d) high removals of phosphorus occurred at a daily biochemical oxygen demand, (BED), loading rate of 50 lb B e D per 100 lb aeration solids and D.O. concentration was 2 mg 1-1 at the midpoint of the aeration basin and 5 mg 1- x at the treated effluent end of the basin. Despite the findings of these and other studies no conclusive data have been presented to demonstrate that higher than normal levels of phosphorus exist within cellular protoplasm found in an activated sludge process, as opposed to adsorption of phosphate on floc surfaces. CHEMICAL PRECIPITATION THEORY To date the best and most convincing explanation of the fate of phosphorus in the activated sludge process has been given by M ~ x g and J~Kt~s (1969). These workers, utilizing the works of SAW3'Ea (1944), SeKIKAWA et aL (1966), HALL and ENGELBR1~4T (1967) and JeNrdlqS and M_~AK (1967) as a basis, concluded that the biological removal of phosphorus may be explained as follows: (a) the per cent by weight (volatile mass basis) of incorporated phosphorus in cellular protoplasm is between 2 and 3 per cent regardless of the physiological state of the microorganisms, (b) the amount of incorporated phosphorus is not affected by the growth rate of the microorganisms or by process operating parameters such as organic loading, mixed liquor suspended solids (MLSS) concentration, aeration rate, or mixed liquor D.O. concentration and (c) the removal of phosphorus is proportional to net microbial growth. Menar and Jenkins proposed that any additional removal of phosphorus in excess of the 20 to 30 per cent normally obtained for typical domestic sewages could be attributed to a precipitation phenomenon. The chemical precipitation theory may be explained on the basis that calcium phosphate is precipitated from solution (solubility is a function of pH), becomes enmeshed in the sludge floc, and is subsequently removed from the process in the waste sludge. The proposed mechanism of precipitation for a conventional plug flow process is explained as follows: (a) at the head end of the aeration basin where return sludge and primary effluent enter, the production of CO2 is high as the result of biological oxidation and the pH is low, (b) farther down the aeration basin C02 production decreases due to the depletion of organic material available for oxidation and a pH increase occurs due to the reduced
Removal of Phosphorus from the Activated Sludge Process
1053
production of CO, and stripping of COt from solution by aeration and (c) the increased pH results in environmental conditions favorable for the precipitation of calcium phosphate. Menar and Jenkins were able to qualitatively demonstrate that phosphorus removal was a precipitation phenomena by performing pilot plant studies • on a wastewater similar in characteristics to that found at the Rilling plant. In a later study, FntousoN and MCCAXTY (1969) proposed an explanation for the reason that data of Menar and Jenkins did not fully complement data reported at the Rilling plant. Ferguson and McCarty concluded that the amount of 15hosphorus precipitated was dependent upon the molar ratio between magnesium and calcium, and that (a) the calcium to magnesium ratio was higher in the waste at Rilling than in the waste studied by Menar and Jenkins and (b) the wastewater carbonate carbon concentration was lower at Rifling than for the wastewatcr studied by Menar and Jenkins. The higher concentration of carbonate carbon was speculated to increase the phosphate residual because, of complex formation and competitive precipitation of calcium carbonate. UPTAKE DEPENDENT UPON NET BIOLOGICAL GROWTH AND PROCESS STOICHIOMETRY
In recent studies StmRRARD and SCHROED~t (1972) have demonstrated that net production of biological solids is related to cell growth rate. The relationship between these two variables was established much earlier in the works of SAWYER(1940) and WUHRMAN(1955), although it was not stated in explicit terms. Net cell growth rate is the reciprocal of mean cell residence for a process operating at steady-state conditions. Mean cell residence time is the average length of time a bacterial cell remains in the aeration basin before being wasted from the process. The importance of cell growth rate and mean cell residence time in biological wastewater treatment processes has been recently demonstrated by SCHgOEDER(1971), LAWRENCE and MCCARTY (1970) and JENKr~S and GARglSON (1968). Production (wastage) of cellular material obtained for a steady-state laboratory activated sludge treatment process treating a soluble wastewater is shown in FIG. 1 (SHERg~D and SCHROEDER, 1972). As shown in FIG. 1 net solids produced per day during steady-state operation varies from a maximum at low cell residence times to minimum values at long cell residence times. Based upon the results shown in FIG. 1, a 63 per cent difference in sludge production is apparent when data at 2 and 18 day mean cell residence times are compared. Despite the fact that cell production varied, hydraulic residence time was maintained constant, and influent and effluent organic concentrations were constant for the range of conditions studied. Hydraulic residence time was maintained between 6.14 and 6.52 h; influent chemical oxygen demand (COD) was maintained at 269 mg 1-1 and, effluent COD was observed to be essentially a constant and average 40 mg 1-1 COD between the 2 and 18 day cell residence times studied. Data were recorded from an experimental reactor (see FIG. 2) during steady-state operation. This reactor was described earlier by SHERRARDand SCHROEDER(1972). The data obtained from study cited above were found to be described by an equation of the form: ¥obs -- 0.406 exp (--0.067 0c) w.R. 6/9--i
(1)
1054
JOSEPH H. SHERRARD and EDWARD D . SCHROEDER 40
I
I
I
30 C3
o t) _= 2o
o
,0
£= o
-=
1
o
I
5
I
ICI
Mean cell residence
20
15
rime ,
days
FIG. 1. Pound cells produc~l per lb COD removed between cell residence times of 2 and 18 days.
where Yob, is an observed cell yield coefficient corresponding to a value of mean cell residence time, 0c. Because the observed cell yield coefficient decreases at increased values of 0c microbial solids production is larger at low values of 0c and smaller at high values of 0~ (see FIG. 1). As a result the amount of waste solids can be varied by controlling mean cell residence time and the uptake of phosphorus and other nutrients which are incorporated into cellular material should depend upon solids production. Therefore, at low cell residence times maximum phosphorus removal would be expected and at long cell residence times minimal phosphorus removals should occur. Varying phosphorus removals dependent upon cell residence time can be demonstrated more Waste pump timer P rQduated cylinder
Feed pump Mercury rthel hermo-
I
/--Effluent
removal tube
I
met~~.~ffluent
sampler (removable) Vacuum source
(aspirator) Calibrated feed tanks
cU:re rPre•uS
Reacto ~---
Air flow meter and regulator
Effluent ¢ollect~n tank diffuser stones
Cotton air filter
FIG. 2. Experimental activated sludge unit.
Removal of Phosphorus from the Activated Sludge Process
] 055
clearly with the aid of FIG. 1 and stoich/ometric equations descriptive of process reactions. As shown in FIG. 1, 34 lb of cells are produced per 100 lb of COD utilized at a 2 day cell residence time. At a value of a 14 day cell residence time only 17 lb of cells are produced per 100 Ib COD utilized. A 50 per cent difference in solids production is therefore obtained when comparing production for these two conditions. The following stoichiometric equations are written to describe the biological growth reaction assuming that cellular composition is constant regardless of growth rate and can be described by the formula C6oHsTO2~NI2P. A stoichiometric equation written for sludge production corresponding to a 2 day cell residence time when nutrients are present in exact proportions indicates that 100 per cent phosphorus removal is possible. 21 C6H1206 + 63.5 02 + 12 NH3 + H~PO, -> CeoHsTO23N12P + 66 CO2 + 102 H 2 0
(2)
However, for the same amount of organic material and nutrients a stoichiometric equation at a 14 day cell residence time would be 21 C6H12Oe + 94.75 02 + 12 NH3 + H~PO4 -> ½ C6oHsTO2sNt2P + 96 CO2 + 114 H 2 0 + 6 NH3 + ½ H3PO,
(3)
I00
- C O D : P -130:1 - o - CO0:P "65:1
#
- C O O : P , 4:3:1 80
60 o
40-
o.
~ ' ~ . ~ (c)
" - - .....
20-Q. "''
0
I
I
5
I0 0©,
-,....,. ,..,..._
I
15
:'0
days
RG. 3. Phosphorus removal efficiency vs mean cell residence time for wastewater with different stoichiometric ratios of nutrient.
As shown in equation (3) only one-half as many cells are produced as in equation (2) and therefore one-half of the phosphorus content of the influent is present in the effluent. Varying phosphorus removal efficiencies corresponding to different cell residence times and different initial ratios of phosphorus in the waste are shown in RG. 3. As shown in Fro. 3 a 21:1 ratio of C6H120~ to H3PO, (curve a) produces
1056
Josep~t H. SHERRARDand EDWARDD. SCHROEDER
a relationship as described above. Ratios of C6H~206 to H3PO, of 21:2 and 21:3 are shown in curves b and c. These ratios correspond respectively to C O D : P ratios of 130:1, 65:1 and 43:1. DISCUSSION Current theories of mechanisms of phosphorus removal from wastewater have been discussed. The importance of cell growth rate, inet solids production, and wastewater stoichiometry have been presented to clarify the manner in which phosphorus is removed biologically. The above analysis was based upon a premise that cellular ratios of carbon, hydrogen, oxygen, phosphorus and nitrogen were constant. Based upon the preceding presentation and given that cellular composition is constant over the range of growth rates maintained in the activated sludge process, it is possible to conclude that high phosphorus removals reported for scwages are most likely attributed to a precipitation phenomena. However, it is shown that phosphorus removal efficiency can be varied by controlling cell growth rate (mean cell residence time) and that the effectiveness of removal is dependent upon wastewater stoichiometry. A presentation of laboratory or field data to verify the above hypothesis is necessary. Unfortunately, those studies currently being performed, and those performed in the past, have not been reported with sufficient data to allow a complete mass balance on phosphorus to be made. This has occurred because it has been generally assumed that the amount of microbial solids produced per unit time is constant regardless of process operating conditions. In reality, however, a large variation in solids production is possible. CONCLUSIONS Based upon the preceding analysis the following conclusions can be made: (a) Increased amounts of inorganic nutrients can be extracted from wastewater by operating an activated sludge process at low cell residence times. (b) The chemical precipitation theory is most likely descriptive of high removals in sewages because of wastewater stiochiometry. However, removal is dependent upon net solids production, a parameter that can be regulated by controlling cell growth rate. (c) Contrary to the statement of MENA~ and JENKINS (1969) in certain cases cell growth rate can significantly affect nutrient removal rates. (d) Increased oxygen consumption is required to satisfy stoichiometry at long cell residence times. This is illustrated by comparing equations (2) and (3). (e) There is an inherent danger in writing stoichiometric equations such as equations (2) and (3) unless net solids production data and cellular composition are .known. critique and suggestions offered by Dr. ALONZOWM. LAWRENCEof the Department of Civil and Environmental Engineering, Cornell University, are acknowledged gratefully.
Acknowledgement--The
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Removal of Phosphorus from the Activated Sludge Process
1057
BOReHAP.DTJ. A. and AzAv H. S. (1968) Biological extraction of nutrients. Y. Wat. Pollut. Control Fed. 40, 1739. FEROUSONJ. F. and McCARTYP. L. (1969) The precipitation of phosphates from fresh waters and waste waters. Tech. Rep. No. 120, Department of Civil Engineering, Stanford University. Gr~NOPOLOS A. and VIL~N F. I. (1971) Process evaluation-phosphorus removal. Y. War. Pollut. Control Fed. 43, 1975. HALL M. W. and ENOELBRECHTR. (1967) Uptake of soluble phosphate by activated sludge: parameters of influence. Proceedings 7th Industrial Water and Wuste Conference, University of Texas, Austin, Texas, p. H-8. JENKINS D. and G~d~mSONW. E. (1968) Control of activated sludge by mean cell residence time. J. Wat. Pollut. Control Fed. 40, 1905. JENKINS D. and M~NARA. (1967) A study of the fate of phosphorus during primary sedimentation and activated sludge treatment. SERL Rep. 67-6. Berkeley: Sanit. Eng. Research Lab., University of Califonia. L A w R ~ A. W. and McC~dtr~' P. L. (1970) Unified basis for biological treatment design and operation. J. Sanit. Engrs. Div., Proc. ASCE, SA 3, 757. LL~rN G, V. and SHAPmOJ. (1965) Metabolic uptake of phosphorus by wastewater organisms. Y. War. Pollut. Control Fed. 37, 800. MANDVJ.~r~, J. and McQuIL~N K. eds., (1968) Biochemistry of Bacterial Growth. Wiley, New York, 1968. MENAR A. B. and JENKINSD. (1969) The fate of phosphorus in waste treatment processes: The enhanced removal of phosphate by activated sludge. Proc. 24th Industrial Waste Conference, Purdue University Extension Series 135, p. 655. MILBURYW. F., MCCAUL~ D. and HAWTHORN~C. H. (1971) Operation of conventional activated sludge for Maximum Phosphorus Removal. Y. Wat. PoUut. Control Fed. 43, 1890. ODUM E. P. (1963) Ecology. Holt, Rinehart & Winston, New York, SA~ C. N. (1940) Activated sludge oxidations VI. Results of feeding experiments to determine the effect of the variables temperature and sludge concentration. Sewage Wks Y. 12, 244. S^wYeR C. N. (1944) Biological engineering in sewage treatment. Sewage WksY. 16, September 1944, p. 925. ScI-IRO~m~ E. D. (1971) The effect of cell recycle on activated sludge process operation. Water Research 5, 22. SEKIKAW^Y., NICmKAWAS., Og.AZA~ M. and KATOK. (1966) Release of soluble phosphate in the activated sludge process. 3rd Int. Conf. Water Pollut. Res. Munich, Germany. S H E R ~ J. H. and ScmtOED~ E. D. (1972) Relationship between the observed ceil yield coefficient and mean cell residence time in the completely mixed activated sludge proce*.s. Water Research 6, 1039-1049. VACKeg D., CO~,~EL C. H. and WELLSW. N. (1967) Phosphate removal through municipal wastewater treatment at San Antonio, Texas. Y. Water Pollut. Control Fed. 39, 750. WITHEgOW J. L. (1969) Phosphate removal by activated sludge. Proc. 24th Industrial Waste Conf. Purdue University Extension Series 135, p. 1169. WUHPa~ANNK. (1955) Factors affecting efficiency and solids production in the activated sludge process. In Biological Treatment of Sewage and Industrial Wastes, Vol. I, Aerobic Oxidation, p. 49 Edited by McC~E J. and ECg.EN~LDERW. W. Reinhold Publishing Corp.