Slaughterhouse effluent treatment by thermophilic aerobic process

War. Res. Vol. 23, No. 5, pp. 573-579, 1989 Printed in Great Britain

0043-1354/89 $3.00+ 0.00 Pergamon Press plc

SLAUGHTERHOUSE E F F L U E N T TREATMENT BY THERMOPHILIC AEROBIC PROCESS D. COUILLARD, S. GARII~PY and F. T. T R A N Institut national de la recherche scientifique(INRS-Eau), Universit~ du Qu6bec, C.P. 7500, Ste-Foy, Quebec, Canada G I V 4C7

(First received January 1988; accepted in revised form November 1988) Abstract--A pig slaughterhouse effluent was submitted to laboratory runs in order to assess the potentiality of the thermophilic aerobic process for the treatment of meat processing industry's effluents. A mixed aerobic bacterial culture was successfully maintained at 52 and 58°C in a semi-continuously fed bioreactor, without recycling the cells. Operation was conducted at 6, 12, 18, 24 and 30 h of solids retention time. Over 93% of the CODe (Chemical Oxygen Demand) were removed during the treatment at 52°C for all the retention times investigated, as compared to 860 of removal at 58°C, with an exception for the case of 6 h retention time. Reduction of phosphorus in the form of orthophosphate ranged between 72-90°/, with the best efficiency noted at 52°C and 6 h retention time. Extremely high specific utilization rates (qm) were observed and revealed that the process is about 10 times faster than the mesophilic process for slaughterhouse effluent treatment. Biokinetics parameters/~ (maximal specific growth rate) and kd (endogenous respiration coefficient) were evaluated by fitting to appropriate mathematical models. They were found superior to those reported in the literature for mesophilic process, with the exception of the Y (actual yield) coefficient which is of the same magnitude. High kd values had a marked effect on the apparent yield of sludges which showed low values as compared to those found in mesophilic systems. Key words--thermophihc, aerobic, bacteria, activated sludge, slaughterhouse, mixed culture, orthophosphate, kinetics, ammonia, microbial

INTRODUCTION For the treatment of agricultural and food industries' effluents, conventional biological processes do not offer the best solution. They require high investment cost and generate huge amounts o f sludges (Law, 1986; Carberry et aL, 1978). As an alternative of a more efficient and less costly treatment process for treating highly loaded effluents, the aerobic thermophilic process is particularly designed to effluents discharged at high or warm temperature (30--70°C) (Siiriicii et al., 1976). This process would then benefit food industry effluents due to the higher reactor rates, superior efficiency, pathogenes destruction and limited amount of sludge produced. Among these effluents, wastewaters from slaughterhouses and meat processing industries have been classified by EPA as the most harmful to the environment (Walter et al., 1974). Meat processing effluents exhibit high organic and inorganic load, high suspended solids content, dark color and offensive odor, and are of poor bacteriological standards. When non-treated, these wastewaters contribute to greatly degrading the aquatic environment (Sachon, 1986; Weiers and Fisher, 1978). Several authors have succeeded in operating within the range of temperatures viable to thermophilic microorganisms without external energy addition, at relatively high organic load (Vismara, 1985; Jewell and Kabrick, 1980; Kambhu and Andrews, 1969). Experimental trials have shown the efficiency of the

process for effluents such as corrugated box recycling industries (Jackson, 1982), sewer sludges (Vismara, 1985; Wolinski and Bruce, 1984; Deeny et al., 1984; Jewell and Kabrick, 1980; Matsch and Drnevich, 1977), liquid pig and cattle wastes (Pilon, 1984; Ptpel and Ohnmacht, 1972), domestic effluents (Shindala and Parker, 1972; Husmann and Malz, 1960; Gehm, 1956). The present research was undertaken to investigate quantitative and practical aspects related to the utilization of the aerobic thermophilic process for a pig slaughterhouse effluent (Gari~py, 1987). Important biokinetic parameters were assessed. The system performances were compared to those of the mesophilic counterpart. Kinetics o f microbial growth Among mathematical models used for the representation of microbial growth and substrate utilization in systems operating in continuous mode, Monod's equation is by far the most useful one. Although its applicability in transient conditions has been questioned (Andrews and Tien, 1977), its use in steady state is well justified and generally accepted (Gaudy et al., 1977). Elaborated and detailed discussions on several forms of Monod's equation have already been presented by Lawrence and McCarty (1970), Sherrard (1977), Metcalf & Eddy (1979), Benefield and Randall (1980) and Eckenfelder (1982). Only relations dealing directly with the present study will be referred to hereafter.

573

574

D. COUILLARDet al. Table 1. Characteristics of screened and de-fatted effluent Concentration Parameter Abbreviation (mg I- i) Total chemical oxygen COD~ 3015 demand Biochemical oxygen BOD5 1905 demand Oils and greases O&G 4.9 Nitrites and nitrates NO~ + NO~ 0.03 Ammonia-nitrogen NH4+ 14.5 Orthophosphate PO~ 5.2 Suspended solids SS 283 Volatile suspended solids VSS 276 pH 7.3

In a c o n t i n u o u s system without biomass recycle the solid retention time is equal to the hydraulic retention time. The c o n c e n t r a t i o n o f organic matters present in the effluent (S) is given by:

gs(l + kd 0c) s = 0c(Um - - k d ) -- l"

(1)

The e q u a t i o n for biomass c o n c e n t r a t i o n may be written as: Y (So - S ) x

=

-

(2)

-

1 + k d Oc

MATERIALS AND METHODS

in which:

Equipment set-up and experimental procedure For this study, a semi-continuous system without recycle has been selected. The semi-continuous feeding simulates full-scale operation, while the non-recycling of solids is to render the study easier to control and of wider application. Effluent samples were obtained from an industrial slaughterhouse (2500 pigs processed per day), after solids screening and fat and grease removal. Samples were stored at 4°C in containers of 201. each and were used as feed medium for bacterial growth. The characteristics are listed in Table 1. As a statute growth of thermophilic microorganisms, inoculum culture from a municipal wastewater treatment plant was added. The culture was initiated at 30°C with a retention time of 48 h and was progressively increased. The fermenter is Bioflow Model C30 bioreactor of New Brunswick Co. with an operational volume of 1500 ml. An air compressor delivered an average of 0.71. of air per minute. Agitation speed was maintained at 200 rpm. Feeding to and withdrawal from culture medium were performed by peristaltic pumps controlled by an electronic programmable timer. The influent was pre-heated by a temperature controlled water bath before entering the bioreactor. Tygon tubings were cleaned with a dilute HC1 solution and rinsed with water when microbial growth was visually clogging the inner walls

K, = s a t u r a t i o n c o n s t a n t /~m = m a x i m u m g r o w t h rate of m i c r o o r g a n i s m s kd = e n d o g e n o u s respiration coefficient 0 c = cell retention time So = i n p u t substrate c o n c e n t r a t i o n S = o u t p u t a n d bioreactor's substrate concentration Y = actual yield X = m i c r o o r g a n i s m s c o n c e n t r a t i o n in the bioreactor. In a strict sense, these relations should only be applied to pure cultures growing o n simple substrates with a single limiting n u t r i e n t (Lawrence a n d M c C a r t y , 1970). Nevertheless, application of these equations has successfully been extended to complex substrates a n d mixed cultures. Jackson (1982), Sfiriic/i (1975) a n d Matsh6 a n d A n d r e w s (1973) have s h o w n t h a t the model c a n be adjusted to the growth b e h a v i o r o f mixed thermophilic bacteria.

,~)InfLuent pump

~ SampLingport Heatingbath Medium storage tank Heating element ~"

®

h ~

EffLuent pump SampLing

port

EffLuent storage tank

Air

Gas

i II,

~ Iltr i

I I llt.l

$.1

lloreactor

Fig. 1. Schematic diagram of experimental set-up.

l Foam controLLing unit

Thermophilic aerobic process

575

Table 2. Treatment efficiency

COD (mgl -~) T

P-PO~- (mgl -I)

(°C) 52

(h) C* C'[" E (%) 6 2210 137 93.8 12 2310 120 94.8 18 2150 106 95.1 24 2220 101 95.5 30 2090 ll2 94.6 58 6 2390 740 69.0 12 2620 320 87.8 18 2700 363 86.6 24 2775 286 89.7 30 2250 147 93.5 *Initial concentration of the effluent. ?Final concentration of the effluent.

of tubings. Foam generation was observed about 2 min after fresh feed addition. A pump system allowed high speed foam pumping and returned it to the bioreactor in the collapsed liquid form. The entire experimental set-up is shown in Fig. 1. Analysis Every sample drawn from the effluent was acidified to pH 2 in order to keep the characteristics unchanged. Suspended solids content was measured after centrifuging a duplicate sample of 35 rnl of treated sample at 17,500g, for 15 rain at 22°C. Cells were then separated from the supernatant, rinsed with demineralized water and dried at 103°C in aluminum containers until a constant weight was reached. Supernatant was filtered through Whatmann filters 934 AH, and used for the determination of soluble COD, PO 3and NH +. COD was measured according to dichromate methods as described in Standard Methods (APHA, 1985). Determination of N-NH 3 and orthophosphate was performed with Technicon auto-analysers (Technicon, 1973, 1978). RESULTS AND DISCUSSION Treatment efficiency Values of soluble C O D , PO 3- and N H 2 concentrations at the entrance and exit of the bioreactor and those o f the treatment efficiency are given in Table 2. Figure 2 shows the evolution of C O D concentration in the treated effluent and the efficiency o f C O D removal as a function of solids retention time. C O D values revealed a removal percentage over 93% during the treatment at 52°C for all retention times studied, as compared to 86% reached at 58°C, except for the case of 6 h retention time. Comparable results were obtained by Heddle (1979) and Lovett et al. lOO

8OO "

700 ¢~ 6 0 0 E 500 0

0 400 (J ~., 3 0 0

9o /

~

0 52"C

-

°

° ~ *

I I I I 12 18 24 30 SoLids retention time ( h I

~

so ~ 0

7o

200 ~. 100 Z tU 0 I 6

N-NH+ (mgl -=)

0o

6o

E

p o

o

50

Fig. 2. Steady-state COD in the reactor and COD removal efficiency as function of solids retention time.

CO 15.5 15.0 14.8 13.4 13.5 15.5 15.2 15.7 13.4 15.3

C 1.5 2.5 3.4 3.0 3.2 3.9 3.1 3.3 3.8 3.7

E (%) 90.3 83.5 77.3 77.3 76.4 74.8 79.4 79.2 72.0 83.2

C0 91 121 122 132 106 183 172 168 182 138

C 144 118 109 87 74 160 117 95 80 66

E (%) -58.5 2.1 10.7 34.0 30.5 12.8 32.1 43.5 56.3 52.2

(1984) for slaughterhouse treated with a conventional mesophilic process. These authors used a much lower F / M ratio 0.2-2.0 g C O D g-~ SS d -m in comparison to 14.8 and l l . 2 g C O D g -] SS d -~ for the thermophilic process, respectively, obtained for 6 and 12 h at 52 and 58°C. This great removal efficiency contrasts with the value of 0.63 for the ratio B O D J C O D in the gross effluent (Table 1), which shows a biodegradability well below 90%. Since removal efficiency was based on soluble C O D , one can deduce that an important part o f total C O D was removed as a particulate form during filtration prior to C O D analysis, thus giving prominence to solid-liquid separation for an optimal removal. Lovett et al. (1984), who made the same observation, have suggested the possible absorption o f nonbiodegradable C O D into microbial flocs with subsequent removal in the waste activated sludge, and the direct adsorption of partially oxidized proteins into microbial cells for cells synthesis. Phosphorus abatement is usually expressed in terms o f Pt or total phosphorus. Knowing that orthophosphate constitutes about 60% o f total phosphorus content in slaughterhouse effluents (Sachon, 1986) and considering that from 50 to 80% of condensed phosphorus forms is hydrolyzed to PO~4in the treatment (Benefield and Randall, 1980), one may assume that removal rates for P O ] - and P, species are proportional. F o r waste activated sludge systems operating with a high F / M ratio (1.8-2.0 g C O D g - l SS d - l ) , phosphorus removal as reported by Heddle (1979) may reach the range o f 72-90% as obtained for this work. At low F / M ratio (0.2--0.4 g C O D g-] SS d - l ) , values reported by the same author vary between 33 and 47%. Lovett et ai. (1984) recorded a better Pt removal efficiency at short retention times with reduction values of 89, 60 and 60% at retention times of 5, 10 and 20 days, respectively. This variation of phosphorus removal, in the opposite direction o f C O D , was also observed in the present study for thermophilic microorganisms, at 52°C (see Fig. 3). According to Sherrard and Schroeder (1973), this phenomenon would be attributable to the high requirements of cells for phosphorus during the cell synthesis period. Lovett et al.

D. COUILLAII, D et al.

576 100o ~

i

o 52"C z~ 5 8 " C

90

~ " ~ - -

8o o t-

70

0 " t~ 0

6O

8

The substrate specific maximum rate utilization allows comparison among several treatment systems as to their rapidity of organics removal. This rate was calculated from the equation:

Temperoture z~

-

--~ A

5o

q~ = - Y

I

I

I

I

I

6

12

18

24

$0

SoLids retention time (h) Fig. 3. Phosphorus removal (orthophosphat¢). .

(1984) related this event to the setting-free of organically bound phosphorus during the endogenous respiration, which caused a proportional increase of phosphorus concentration in the treated effluent. Tables 1 and 2 reveal that ammonification did actually occur during the storage period, under psychrophilic anaerobic conditions. The result was a 10-fold increase of the initial concentration of NH2 (14.5mgl-l). According to Moore and Moore (1976), the ammonia generated under anaerobic conditions was due to the activity of microorganisms which oxidized the amines remaining in the animal wastes and freed the ammonium hydroxydes. Consequently, the level of ammoniacal nitrogen concentration introduced in the bioreactor did not correspond to the level which prevailed in real operational conditions. Notwithstanding this fact, obtained results showed a removal efficiency of NH2 varying from 2 to 30% at 52°C, and from 12 to 52% at 58°C. Moreover, the efficiency increased with retention time. At 52°C, for a retention time of 6 h, an increase of NH2 concentration was observed. This was an indication that ammonification process might have occurred in aerobic conditions and contributed to increase the nitrogen load. Although ammonia stripping might have occurred due to high pH and temperature conditions, nitrogen microbial requirements based on a B O D : N : P ratio of 100:5:1 indicate that reduction in NH 2 essentially resulted from biological causes.

Kinetic parameters The modeling of kinetic parameters (#m, K,, Y and kd) was effected via linearized forms of equations (1) and (2) where the substrate concentration was expressed in COD and biomass concentration in terms of SS. The actual yield (Y) and endogenous respiration coefficient (kd) were determined by a least squares fitting method to the equation:

S o - S =ka 0 1 X Y ¢ + -Y"

(3)

For determination of the K, and #m coefficients, the linear equation proposed by Muck and Grady (1974) was used: 1 Um 0¢ 1

S=K,I+Oo4 K~

(4)

(5)

where qm is the substrate maximum specific rate of utilization. Prior to equation (4) application, substrate concentrations were corrected to take into account the nonbiodegradable portion of Sn. This amount is estimated in order to meet the requirement that the substrate specific utilization rate should be approaching zero when COD is zero. A straight-line representation of COD vs specific rate of substrate utilization can be extrapolated to zero on the specific rate axis to yield the value of Sn. Values of S, thus obtained were 93 and - 34 mg 1- ~, respectively, at 52 and 58°C, with coefficients of determination of 0.88 and 0.83. At 58°C, no correction for COD concentrations was attempted because of the lack of physical meaning of a negative COD value. At 52°C, data collected for 30 h of retention time were not used for the calculation of the/Q and #m coefficients; this results from the irregular behavior of COD concentrations which should have been lower than that registered at 24 h retention time. This behavior can be explained by the fact that a steady-state situation had not been reached before the recording of biological kinetic parameters. Instead, a transient state prevailed with subsequent species fluctuations (Parkes, 1982). Bacteria sticking to the bioreactor walls and tubings and the complexity of the culture medium may also add to the error margin. All these have to be taken into account in interpretation of the results. Biokinetic parameter values are given in Table 3. Coefficients of determinations obtained by equations (3) and (4) curve fittings were, respectively, 0.68 and 0.92 at 52°C and 0.90 and 0.71 at 58°C. Values of biokinetic coefficients were also reported and found for similar substrates in mesophilic and thermophilic processes. Specific substrate utilization rates in this study confirm the great velocity of the thermophilic process compared to the mesophilic waste activated sludge process. In fact, values obtained for the qm coefficient at 52 and 58°C are, respectively, 8 and 12 times superior to the best rate reported for slaughterhouse effluent treated by the mesophilic process (2.6day-l). Appreciable difference observed for #m and kd for these two types reflect the difference of behavior of these two different microorganism populations. It is obvious from the higher values of maximum growth rate (#m) and endogenous respiration coefficients (kd) obtained for thermophilic microorganisms, that these last ones multiply and decay faster than their mesophilic counterparts. However, actual yield coefficients are hardly different. It is admitted that actual yield is a relatively stable parameter towards temperature changes (Matsh~ and

Thermophilic aerobic process

577

Table 3. Comparison of biokinetic coefficients* Reference

Substrate

This study This study Loven et al. (1984) Loven et al. (1983) Heddle (1977) Jackson (1982) Sfirficfi (1975) Matsh6 and Andrews (1973)

Slaughterhouse Slaughterhouse Slaughterhouse Slaughterhouse Slaughterhouse Industrial effiuent~/ Glucose

Glucose

T (°C)

Y~'

kd (d-')

/am (d-l)

52 58 22 19-23 22 53 58 45.5 52 56

0.30 0.32 0.34 0.41 0.42 0.60 0.34 0.43 0.41 0.42

0.32 0.78 0.03 0.04 0.10 0.52 0.48 0.31 0.72 0.0l

6.00 10.1 0.58 0.27 1.10 3.36 5.23 ----

*The coefficients were calculatedfrom measured values of COD and SS, except were used. "f'Expressed in mg SS (VSS) rag-) COD. ~:Corrugated boxes recycling plant.

Andrews, 1973; Topiwala and Sinclair, 1974), and its value generally depends on the nature of substrate used (Litchfield, 1985). The saturation constant (Ks) does not follow any specific tendency whether the microbial population is mesophilic or thermophilic. Regarding microbial kinetics, Parkes (1982) defined the saturation constant Ks to express the affinity of the microbial population for the substrate. The smaller the value the greater the affinity. The Ks value, obtained at 52°C for the present series of runs, shows a great affinity of bacterial population at this temperature for slaughterhouse effluent, while at 58°C the high Ks values demonstrate that the population may be ill-adapted. Table 3 shows that the biokinetic constants obtained from this study are comparable to those determined by Jackson (1982) for an industrial waste effluent and by Siiriicii (1975) and Matsh6 and Andrews (1973) for glucose solutions. The saturation constant of 30 mg 1-1 reached under thermophilic conditions for slaughterhouse effluent is clearly lower than those reported by these authors. Apparent yields The apparent yield is a parameter of great importance in pollution control: it controls the amount of sludge which is generated during the treatment and must be subsequently eliminated at a high cost. The apparent yield is related to Y, kd and 0¢ by the equation: Y

Lovett et al. (1984) where COD and VSS

AT = 3.0 x ACOD

(7)

in which AT = increase of temperature in °C and A COD = COD removal in g l- 1. According to this equation, an initial concentration in the order of 10,000 mg 1-1 with an efficient removal of 90% level would be necessary to bring the slaughterhouse effluent from its average initial temperature of 20-45°C which is the lower limit for the growth of E o

¢O

tO 0.3

-

19.8 31.1 1.7 0.67 2.6 5.6 15.4 ----

362 150 150 890 740 ----

The exact temperature prediction for a specific effluent will be given by a mass and energy balance determination around the reactor. Such calculations have been described in detail by Vismara (1985), Stirficii et al. (1976) and Kambhu and Andrews (1969). Jewell and Kabrick (1980) presented a simplified equation which can be used for estimation of an autothermic thermophilic system. In the case of a bioreactor adequately isolated and provided with a heat recovery system, the predicted temperature increase is given by the equation:

in which the apparent yield is expressed in mg of biomass per mg of substrate. This equation also shows that the apparent yield is strongly related to the endogenous respiration coefficient and to solids retention time. At short retention times, the effect of kd is small and Y, essentially behaves like Y. At longer retention times, the influence of ko is more accentuated. Figure 4 shows that the thermophilic process operated at a sludge retention times from 6 to 12 h at 52°C presents a slightly lower yield compared to the mesophilic waste activated sludge system operated at

-

30

992

Thermal autonomy and oxygen requirement of the process

T 0.4

=

qm (d-j)

10 days' retention time, while maintaining an equal COD removal efficiency. At 58°C, the apparent yields is even lower, but the system should then be operated with a sludge retention time of 24 h to reach a comparable efficiency.

(6)

r,

for

Ks (mgl-')

1 + k d 0c

1oo

~<~'................ ... ~

t~¢~ ~ E 0.2 \ ~

,.':_.. "-

\- ~ "-,,~_ ~ " ~

~

<

o

. . . . . . . . . . ."" ....

so

Temperoture"'"--" 60 52"c 5a*c _

....... z 2 " c

z 4 s a SoLids retention time (days]

~'-

40

,>o

o u

Fig. 4. Apparent yield and COD removal efficiency for mesopbi]ic (22°C) and thermophi]ic (52 and 58°C) treatment.

578

D. COU1LLARDet al.

thermophilic bacteria. The effluent used in this study is thus too diluted to allow the maintenance of energetic self-sufficiency of the process. However, the wastewater characteristics of slaughterhouse plants vary widely according to the different stages of the process flow sheet (Sachon, 1986; Stanley & Dearborn, 1981). These facts suggest the implementation of an in-plant selective collection system for the treatment of the most highly loaded wastewater. Less loaded effluents will be discharged directly into the municipal sewer system. The oxygen uptake rate is defined by: So- S

OUR = a - 0c

+ bX

(8)

in which OUR = oxygen uptake rate a = oxygen utilization coefficient for substrate oxidation b = oxygen utilization for endogenous respiration. Siiriicii (1975) obtained values of 0.54 and 0.31 kg O2 kg-1 SS d-i, respectively, for a and b in the case of a thermophilic population grown at 52°C in a solution of glucose. Assuming an initial COD of 12,000 mg 1-~ undergoing treatment at 52°C with 90% of final COD removal, equation (2) will yield a biomass concentration in the bioreactor of 3000 mg 1-1. Through the application of coefficients obtained by Siiriicfi (1975), equation (8) allows estimation of an oxygen uptake rate of 25 kg O5 m -~ d -~. The transfer of such an amount of oxygen would be efficiently assumed by the use of an air-lift type bioreactor (Tran and Gannon, 1981; Hatch, 1975). CONCLUSIONS According to the results obtained from the study of the aerobic thermophilic process applied to slaughterhouse effluents, the following conclusions can be drawn: (I) Slaughterhouse wastewaters can efficiently be treated by a thermophilic process. More than 90% of soluble COD and PO~- were removed at an operating temperature of 52°C and a retention time of 6 h. At 58°C, the efficiency decreased. (2) Phosphorus uptake increased as an inverse function of sludge retention time for a mixed thermophilic population studied at 52°C. Removal efficiency of ammonia increased with retention time at 52 and 58°C. (3) The aerobic thermophilic process was about I0 times faster than the most rapid mesophilic equivalent process for the treatment of slaughterhouse effluents as proven by the examination of the specific utilization rate of the substrate (qm). (4) Biokinetic parameters such as #= and kd were much larger for the thermophilic than for the mesophilic process. This fact showed that thermo-

philic microorganisms multiply and decay faster than mesophilic ones. (5) Thermophilic Y coefficient was of the same order of magnitude as the value found in mesophilic processes. The Ks coefficient changed much with temperature, which was a sign of fluctuating affinity of bacterial population for the substrate utilized. (6) The effluent used in the present study did not exhibit a high enough organic loading to allow the autothermal sufficiency of the system. Some in-plant modifications to the network of wastewater collection might be necessary. (7) Oxygen requirements in the thermophilic aerobic processes were very high. Improved technological design such as an air-lift type bioreactor would be necessary to ensure an adequate oxygen transfer. Acknowledgements--The authors would like to extend their

thanks to the Natural Sciences and Engineering Research Council of Canada (NSERC grant No. A-3711), to the Government of Qutbec (FCAR 87 grant No. AS-246) and to the Programme de dtveloppement acadtmique du rtseau de l'Universit6 du Qutbec. REFERENCES

Andrews G. F. and Tien C. (1977) New approach to bacterial kinetics in wastewater. J. envir. Engng Div. Am. Soc. civ. Engrs 103, 1057-1074. APHA (1985) Standard Methods for Examination of Water and Wastewater, 16th edition. American Public Health Association, Washington, D.C. Benefield L. D. and Randall C. W. (1980) Biological Process Design in Wastewater Treatment. Prentice-Hall, Englewood Cliffs, N.J. Carberry J. B., McCaffery III T. F. and Robert M. C. (1978) Protein recovery from waste activated sludge. In New Processes of Waste Water Treatment and Recovery

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