96 Chem. Eng. Technol. I 2 (1989) 96- 102

References

[ I ] Schiller, W. , Forsch. Geh. Ingenieurwes. 4 (1933) pp. 128- 137. 121 Michalke, A. , 1ng:Arc.h. 57(1987) pp. 377-392. [3] Friedrich, H., Vetter, G . , Energie 14 (1962) No. I , pp. 3-9 . [4] Henry, E., Nucl. Sci. Eng 41 (1970) pp. 336-342. [S] Fauske, H.K., Chem. EngProg. Symp. Ser. 61 (196S)pp. 210-216. 161 Ogasawara. H . . Bull. JSME 12 (1969) No. 52. pp. 837-846. [7] Sozzi, G.L. , Sutherland, W.A. , Critical Flow ofSuturuted t i r id Suh-

cooled Wuter ut High Pressure; Proc. of the ASME Sytnp. o t i Noti- Equilibrium Two-Phase Flow; Winter Annual Meeting. Houston, Texas, Nov. 30-Dec. 5, 1975.

[Sl Uchida, H. . Nariai. H., Discharge ofSuturuted Wuter Through Pipes und Orifices: Proc. ofthe 3rdInt. Heut Trurisfer Cot$, Chicago. Vol. 5, pp. 1 - 12, 1966.

1171 1181

Fletcher, B. , Chem. Eng Prog. (1984) pp. 76 - 81. Pdsqua, P.E.. Rejrigeruting Progre.s.s 61 (1953) pp. 1084- 1088. Winter, E.R.F. , Petry, G. , Adiabatic Ent~pciiinurigs~~erc/atnpfu,,fi 1'011

R 12 in Kupilluren wid Blenden unter kritischen Striit~rutigsbediii,~u,l- Ken; Thermodyi~umik Kolloquium des VDI, Bad Kissingen. Sept. 1987.

Seynhaeve, J .M., Criticul Fl(iw Through Orifices; Europecm TNW Phase Flow Group Meeting; Grenoble. 6 - 8 June, 1977. Deich, M.E. . HiXh Temperuture (USSR) 7 (1969) pp. 294 - 299. Starkman, E.S. , Truns. ASME 86 (1964) No. 2, pp. 247-256. Henry, R.E.. Fauske, H . , Truris. ASME 93 (1971) No. 5. pp.

Bonnet, F.W., Adv. Cnog. Gig 12 (1966) pp. 427-437. Leung, J .C . , AlChE J . 32 (1986) No. 10, pp. 1743 - 1746.

OgaYdWara, H., Bull. J S M E 12 (1969) No. 52, pp. 847-856.

179- 187.

Effects of Pretreatments on Semi-continuous Anaerobic Digestion of Wheat Straw

Ayse Tosun, Nevin Selquk and Selquk Soyupak*

The effects of alkali treatment, nitrogen supplement and hydraulic retention time on methane pro- duction rate from semi-continuous anaerobic digestion of 5 % wheat straw-water mixtures were investigated. The experiments were carried out in laboratory scale fermenters, fed with 1 I of basic, alkali treated and nitrogen supplemented 5 % wheat straw-water mixtures, respectively, and maintained at 55 "C. Digestion experiments were performed for hydraulic retention times of 8, 10 and 15 days. The amount and composition of produced gas were measured until steady state was attained in each run. The steady-state methane production rates were found to increase with hydraulic retention time and with the type of slurry in the following order; basic, nitrogen supplemented and alkali treated slurry. Data obtained from the experiments were employed to determine the kinetics of methane production from anaerobic digestion of wheat straw, for the assessment of pretreatment effects on process kinetics. The predicted methane production rates were found to be in a reasonably good agreement with the measurements.

1 Introduction

Rapid industrialization and population growth have increased the global energy requirement over the recent decades. Due to the growing demand for energy, combined with dwindling reserves of fossil fuels, there is an increased interest in the pro- duction of fuels from renewable sources. In the investigation of

* Graduate Student A. Tosun, Prof. Dr. N . Selquk, Department of Chemical Engineering, and Assoc. Prof. Dr. S. Soyupak. Department of Environmental Engineering, Middle East Technical University, Ankara 0653 I , Turkey.

alternative energy sources, attention has been paid in recent years to the microbial conversion of biomass into energy pro- ducts such as methane and alcohol. Anaerobic digestion is a method of converting agricultural wastes to methane as an alter- native source of natural gas. This process occurs in three stages. First, agricultural wastes are dissolved enzymatically. The solu- ble organic compounds formed are subsequently metabolized by bacteria to aliphatic acids and alcohols. Methane producing bacteria then convert these fatty acids and alcohols to methane and carbon dioxide.

Crop residues constitute an appreciable amount of biomass. Although the production of biogas from animal manures has

0 VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1989 0930-7516/89/0204-0096 $02.5010

Chem. Eng. Technol. I 2 (1989) 96- 102 97

been extensively investigated [ 1 - 51, there is only a small number of studies on actual crop residues [6 - 91, and no data are available on anaerobic digestion of wheat straw.

Anaerobic digestion can be carried out in either batch or con- tinuous reactors. When the objective is to use methane as a fuel gas supply to agricultural industry, large scale continuous reac- tors offer advantages such as lower labour requirements, con- stant gas output, smaller reactors etc. However, only a few studies are available on the anaerobic digestion of crop residues in semi-continuous reactors [8 - 91.

Digestion of crop residues is relatively more difficult than that of animal wastes since they are nitrogen deficient and most of the carbohydrate material consists of cellulose. The above limitations may be alleviated by supplying nitrogen and hydrolyzing before feeding, respectively.

Therefore, in this paper, the effects of alkali treatment, nitrogen supplement and hydraulic retention time on semi-continuous anaerobic digestion of 5% wheat straw-water mixtures at 55 "C are investigated. Experiments were carried out for hydraulic retention times of 8, 10 and 15 days. The amount and composi- tion of gas produced were measured until steady state was achieved in each run.

2 Experimental

2.1 Experimental Set-up

Digestion experiments were carried out in the set-up shown in Fig. 1. The set-up consists of two main units, the first being a 1 .5 litre perspex digester with one litre of liquid volume and the second, a 2.5 litre gas collection column, operating on gas- liquid displacement principle. The digester was placed in a ther- mostatic water bath at 55 "C. Three such set-ups were con-

Feed Injection

Fig. 1. Diagram of experimental set-up.

t

Gas Collection

Displaced Water

structed so that digestion experiments could be carried out simultaneously.

2.2 Experimental Procedure

2.2.1 Chemical Composition of Wheat Straw

For the determination of chemical composition of wheat straw, ultimate, proximate and cellulose analyses were carried out, the results being shown in Table I ,

Table 1. Physical and chemical characteristics of wheat straw

Ultimate analysis [dry basis, mass %]:

C 44.49 H 5.85 N 0.55 P 0.06 0 41.98 Ash 7.07

Proximate analysis [mass %]:

Moisture Volatile Matter Ash Fixed Carbon

Organic components [mass %]:

Crude Protein Cellulose

7.24 70.64 7.07

15.05

3.44 28.71

C/N ratio: 80.90

2.2.2 Slurry Preparation and Inoculum

The wheat straw was first ground to pass through a 0.5 mm sieve and mixed with water for the preparation of a basic slurry containing 5 % total solids.

It was necessary to procure a suitable culture qf bacteria as in- oculum, in order to initiate rapid digestion of wheat straw. This culture was a fermented slurry of cow manure obtained from a primary anaerobic digester operating at 37 "C. One litre of this culture was heated gradually to 55 "C and, then, 100 ml was replaced by 5 % total solids of wheat straw-water mixture every day, so that a slurry, conditioned to the experimental operating conditions, was obtained. This slurry was used as inoculum in the digestion tests.

2.2.3 Procedure

Three sets of experiments were carried out in semi-continuous mode at 55 "C using three different hydraulic retention times (8, 10, 15 days) in each set.

Table 2 illustrates the experiments carried out on the basic slurry with and without pretreatment, for various hydraulic retention times. The first set of experiments was carried out with the basic slurry whose properties are shown in Table 3.

98 Chem. Eng. Technol. 12 (1989) 96- 102

Table 2. Experiments with basic slurry.

Experimental Pretreatment set

Runs

I None 8 days HRT 10 days HRT 15 days HRT

I1 Alkali treated 8 days HRT 10 days HRT 15 days HRT

(2g NaOH/100 g TS)

I11 Nitrogen supplemented (0.6 g/l NH,CI s o h ) (CIN) = 25

8 days HRT I0 days HRT 15 days HRT

Table 3. Properties of 5 % wheat straw-water mixture fed to the digesters

Total solids Ikgim’l

Kjeldahl-N W m ’ l Alkalinity IWm’I PH Carbon lkgIm31

Volatile solids [kgim’j COD [kgIm3]

C/N

50.0 35.32 59.33

0.275 0.71 7.2

22.5 80

The second set corresponds to the digestion of basic slurry pretreated with alkali. The pretreatment consisted in keeping the basic slurry with 2 g NaOH/100 g TS for 24 hours at am- bient temperature and pressure.

The third set corresponds to the digestion of nitrogen sup- plemented basic slurry. The nitrogen supplementation was car- ried out by adding 0.6 g NH,CI to 1 1 of the basic slurry so that the C/N ratio became adjusted to 25.

Each experiment was started by loading 700 ml of 5 % total solids feed slurry and 300 ml of inoculum and de-aerating the digester. Constant volume semi-continuous digestion ex- periments were performed by feeding and withdrawing the same amounts of slurry so that the hydraulic retention times (HRT) would be 8, 10 or 15 days for each set. Since steady-state was expected after at least threefold HRT, the feeding and withdrawal processes were continued until the three volume turnovers were achieved. During this time, the digesters were stirred twice a day for one minute and the total daily gas yields were recorded before feeding. A sample of the gas was taken, by using a 1 ml syringe, to be analyzed in a gas chromatograph. Adjustment of pH was occasionally required when it fell below 6.5, and this was effected with sodium bicarbonate solution, especially for the first and third sets. pH adjustment was not found necessary for the second set as it remained within the range 6.9 to 7.2 throughout the digestion.

The gas production and gas composition were measured every day until steady state was achieved in each run. Once this condi- tion was fulfilled, a sample of effluent was taken from each digester and analyzed for total solids (TS), nutrient concentra- tion, crude protein, cellulose, alkalinity and chemical oxygen

demand (COD). Details of the methods employed for the analyses can be found elsewhere [ 101.

3 Results and Discussion

3. I Biogas Yield and Methane Percentage

Total gas production and methane percentage in the gas were measured and recorded every day of each digestion run. Using these data, the methane production rates or methane yields were calculated. Figs 2 to 4 show the variation of methane production rate with time for digesters fed with basic, alkali treated and nitrogen supplemented slurries, respectively. It can be observed that methane production rate passes through a maximum and decreases continuously towards the steady state values after the threefold hydraulic retention time for each set. As can be seen from the respective figures, the methane production rate for 10 days HRT shows a steeper increase in the early days of digestion than those for 8 and 15 days HRT. This is attributed to the fact that the 10 day HRT test was the first digestion experiment started with fresh inoculum. Digestion experiments for 8 and 15 days HRT were performed on the slurry of the 10 day HRT ex- periment.

Tables 4 to 6 show the steady-state effluent and gas data from the digesters fed with basic, alkali treated and nitrogen sup- plemented slurries, respectively. As can be noted from the tables, the steady-state methane production rates increase only slightly with retention time. This may be due to the fact that, as HRT increases, the effect of decreasing substrate concentration on reaction rate is counterbalanced by the increase in microbial mass concentration. However, when the steady-state methane production rates for the digestion of basic, alkali treated and nitrogen supplemented slurries for a specific HRT are compared with one another, significant differences can be observed. It can be noted that the steady-state methane production rate depends on the type of slurry digested and increases in the following or- der; basic, nitrogen supplemented and alkali treated slurry. The reason why the production rate is lowest for the basic slurry, is that it involves no pretreatment. Therefore, the results in- dicate only the potential of methane production from untreated wheat straw slurry.

Higher methane production rates obtained from the nitrogen supplemented slurry are primarily due to higher biogas produc- tion rate as the methane percentages from both basic and nitrogen supplemented slurries are not significantly different (Tables 4 and 6). This is an expected trend since nitrogen sup- plement enhances the microbial growth and hence also the total gas production.

Methane production rates from the alkali treated slurry are the highest since both the gas production rates and methane percen- tages are higher than those from basic and nitrogen sup- plemented slurries. These results demonstrate the positive effect of hydrolysis on anaerobic biodegradability of wheat straw. This is attributed to the fact that alkali treatment decreases the association of lignin with carbohydrates and hence increases the substrate accessibility and anaerobic biodegradability.

Chem. Eng. Technol. 12 (1989) 96- 102 99

O--Q H R T = 8 DAYS bd H R T = 10 DAYS - HRT = I5 DAYS

I 15 10 15 20 25 30 35 4 0 45 Time (day)

Fig. 2. Variation of methane production rate with time for the digester fed with basic slurry.

Time (day1

- H R T = 8 DAYS - H R T = 10 DAYS - HRT = 15 DAYS - 2.4 x-, 0 -0

al

al 0,

U

L

t; 2.0 .-

m E

E

.o 1.2

e

& 1.6 u

c c 0

'p

0.8 0) E 0

f s" 0.4

0 1 4 8 12 16 20 24 28 32 36 4 0 44

-.- - H R T = 8 DAYS - HRT = 15 DAYS - HRT = 10 DAYS - 2.4

x-, 0 -0

al

al 0,

U

L

t; 2.0 .-

m E

E

.o 1.2

e

& 1.6 u

c c 0

'p

0.8 0) E 0

f s" 0.4

0 1 4 8 12 16 20 24 28 32 36 4 0 44

Time (day1

Fig. 3. Variation of methane production rate with time for the digester fed with alkali treated slurry.

). - HRT = 8 DAYS I- - HRT = 10 DAYS c - HRT = 15 DAYS

0 'p

al g 1.0 0, 'p .- m E , 0.8

E n - c

0.6 0 3 'p e

0.4 al E 0 f E" 0.2

1 4 8 12 16 20 24 28 32 36 40 44 48 Time (day)

Fig. 4. Variation of methane production rate with time for the digester fed with nitrogen supplemented slurry.

100 Chem. Eng. Technol. 12 (1989) 96- 102

Table 4. Steady state effluent characteristics and gas production in the increases the microbial growth but not the substrate ac. digester fed with basic slurry (5% wheat straw-water mixture). cessibility.

Parameter Hydraulic retention time

15 days 10 days 8 days 3.2 Comparison of Properties of Feed and Efluent Slurries

Total solids [kgim’l Volatile solids [kglm’l COD [kglm’l Kjeldahl-N [kdm’l Alkalinity w m 3 1 PH Carbon Ikg/m31 Methane I%1 Gas production

m’im’ digester day m’/kg VS fed m3/kg VS used

48.0 21 .o 20.0 0.434

1.78 7.3

18.25 31.2

0.492 0.250 0.492

49. I 21.3 26.0 0.469 I .70 7.3

19.10 33.7

0.420 0.120 0.297

48.7 21.8 29.8 0.252 1.72 7.3

19.85 25.0

0.450 0.102 0.266

Table 5. Steady state effluent characteristics and gas production in the digester fed with alkali treated slurry

Parameter Hydraulic retention time

15 days 10 days 8 days

Total solids Volatile solids COD Kjeldahl-N Alkalinity

Carbon Methane

PH

[kgim’] 33.0 35.4 [kg/m3] 17.0 17.4 [kg/m3] 7.0 6.0 [kg/m31 1.330 0.434 w m 3 1 1.90 I .82

7.0 7.0 [kg/m’] 1 1 . 1 13. I [%I 45 62

37.2 18.0 8.0 0.196 1.83 7.0

14.2 60

Gas production m3/m3 digester day 1.670 1.100 0.910 m’ikg VS fed 0.696 0.31 I 0.206 m3/kg VS used 0.943 0.614 0.420

Table 6. Steady state effluent characteristics and gas production in the digester fed with nitrogen supplemented slurry.

Parameter Hydraulic retention time

15 days 10 days 8 days

Total solids Volatile solids COD Kjeldahl-N Alkalinity PH Carbon Methane

[kgim’] 45.8 [kg/m’] 23.7 [kg/m’] 12.0 [kg/m’] 0.616 Ikgim’l 1.84

7.4 [kgim’] 11.8 [%1 33

48.5 23.6 10.0 0.834 I .80 7.4

14.0 31

47.0 24.0 12.0 0.588 I .82 7.4

15.0 28

Gas production m3/m3 digester day 1.600 1.240 1 .Ooo m’/kg VS fed 0.670 0.351 0.266 m3/kg VS used 1.950 1.061 0.707

Methane production rates obtained from alkali treated slurries are higher than those from nitrogen supplemented slurries. This shows that provision of the nitrogen requirement of digestion

Since the total solids concentration in the digester had to be kept at 5 % during the experiments, it was essential to withdraw the effluent with the same concentration as that of the slurry loaded into the digester. However, the total solids concentrations in the steady-state effluent slurries were found to be lower than those in the fed slurries. The losses in the total and volatile solids were found to be due to gasification of organic material during diges- tion. The comparison of values in Table 3 with those in Tables 4 to 6 shows that the total and volatile solids reductions decrease in the following order; alkali treated, nitrogen supplemented and basic slurries. This sequence is consistent with that follow- ed by total gas yields and hence with gasification.

Comparison of the COD value in Table 3 with those in Tables 4 to 6 shows that the gas production increases with COD reduc- tion. This can be explained as follows: substrate stabilization, which is important in the digestion of wastes, is directly related to COD reduction. The only way, in which COD reduction can occur in an anaerobic digestion process, is through the removal of organic material from waste, such as by evolution of methane and carbon dioxide. Hence, the total gas production is directly proportional to the COD reduction. The comparison of COD values for different retention times in each experimental set shows that, as expected, the COD reductions increase with hydraulic retention time.

The nutrient concentrations are of interest when the effluent slurry is to be used as a fertilizer. Comparison between the nitrogen concentrations in the feed and in the effluent slurries of the three sets (Tables 3 to 6) shows that the concentration of nitrogen, as a fraction of the dry mass, is higher in the effluent slurry than in the undigested feed. This results simply from the conservation of nitrogen as a nutrient during digestion and gasification of some organic material.

With regard to the pH adjustment during digestion experiments, it has been found that the basic and nitrogen supplemented slur- ries require more frequent adjustment than the alkali treated slurry. This is caused by the group of bacteria, which affect primary breakdown of organic matter and produce volatile fatty acids faster than it is possible for the methanogenic bacteria to remove them [ I I ] . The decrease in pH inhibits digestion because methanogenic bacteria are killed at low pH and the digestion eventually ceases. However, alkali treatment satisfies the requirement of alkalinity and thus removes the problem of continuous pH adjustment.

3.3 Kinetic Data for Digestion of Wheat Straw Slurries

Kinetic data for the semi-continuous digestion of wheat straw are not available as yet. Since the digestion of wheat straw is difficult, due to the latter’s high cellulose content, fermentation stress occurs when the straw content of straw-manure mixtures

Chem. Eng. Technol. 12 (1989) 96- 102 101

increases [8]. Hence, the kinetic data of the wheat straw may form a basis for the design of a digester for processing a mixture of substrates containing wheat straw. Therefore, the experimen- tal measurements were applied to a substrate utilization kinetic model in order to obtain the kinetic data for the digestion of wheat straw. Due to low methane yields, obtained in this study, it was not possible to measure the bacterial cell mass concentra- tion. Therefore, substrate concentrations were measured in terms of COD or VS. The effluent substrate concentration depends on the influent substrate concentration if either COD or VS is used as an indicator of substrate concentration [12]. Therefore, the steady-state measurements, obtained from the digestion experiments, were applied to the kinetic model developed by Chen and Hashimoto [13] which permits the determination of ultimately attainable methane production, minimum retention time, kinetic constant and volumetric methane production rate.

For the evaluation of ultimately attainable methane production, the following equation based on Contois kinetics [ 141 has been used'' [13]:

010, - " 1 1 + K

where B denotes the volume of methane produced per g COD added to the digester, B, is the volume of methane produced at infinite time per g COD, i.e. the ultimately attainable methane production, K is the kinetic constant while I9 and 0, are the retention time and minimum retention time, respectively.

The measured data contained in Table 7 have been plotted in ac- cordance with Eq. ( I ) . Since the variation of B vs 1/19 is found to be linear, linear regression analyses were applied for the determination of B, for the three sets of experiments. These values of ultimately attainable methane production decrease in the following order: alkali treated, nitrogen supplemented and basic slurry. These results indicate a beneficial effect of pretreatments on the ultimately attainable methane production.

Table 7. Variation with time of measured COD, VS concentration. total gas production, calculated methane production rate and methane production per g COD.

Exp. I i O COD Total CH, 7" B set [day- ' ] [g per gas prod. [% ] [ I CH,/I [ I CH, per

day] liday per day] g COD]

0.067 3.93 0.492 31.2 0.154 0.039 I 0.100 5.90 0.420 33.7 0.142 . 0.024

0.125 7.38 0.450 25.0 0.113 0.015

0.067 3.93 1.670 45.0 0.751 0.191 I1 0.100 5.90 1.100 62.0 0.682 0.1 16

0.125 7.38 0.910 60.0 0.540 0.073

0.067 3.93 1.600 33.0 0.528 0.134 111 0.100 5.90 1.240 31.0 0.384 0.065

0.125 7.38 1.000 28.0 0.280 0.038

Once the values of B, are determined, the minimum retention times and kinetic constants are obtained from an alternative form of Eq. ( I ) :

Using the measured data, Eq. (2) has been plotted as 0 vs BI(B, - B ) for each set. Values of 0, and K were subsequently found from the results of linear regression analyses.

Once 8, values are calculated, the maximum specific growth rates, hm = I/I9,, can be evaluated. Table 8 presents the calculated parameters B,, %,, ,urn and K . This table shows that, at 55 "C, the minimum retention time is about six days. It can be seen that 8, values are not significantly affected by pretreatment.

As can be seen from Table 8, the K values are almost indepen- dent of the type of digested slurry. This is due to the fact that VS concentrations in the slurries fed to the digesters are the same and that the K value is only a function of influent VS con- centration [ 131. The slight differences in K values may be at- tributed to experimental errors. Methane production rates were then predicted by substituting the calculated kinetic parameters into the following equation 1131:

where yv denotes the volumetric methane production rate and STo is the total influent substrate concentration. Measured and predicted volumetric methane production rates are shown in Table 9. As can be seen from the table, the two sets of values are in a reasonably good agreement. This shows that the kinetic

Table 8. Kinetic constants for reactors fed with basic. alkali treated and nitrogen supplemented slurries.

Exp. B, om +In K set [ I CH,/g COD added] [day] [day '1

I 0.0666 I1 0.3260 111 0.2440

6.25 0. I60 0.965 6.41 0.156 0.953 6.94 0.144 0.959

Table 9. Comparison of measured and predicted methane production rates.

Exp. 0 predicted 7. measured Error set [day] [ I CH,II day] [ I CH,/I day] [ % I

15 0.154 0. I54 0 I 10 0.149 0. I42 5

8 0. I10 0.113 - 3

15 0.749 0.75 I -0.3 I1 10 0.712 0.682 4

8 0.497 0.540 - 8

15 0.526 0.528 - 0.4 111 10 0.453 0.384 18

8 0.247 0.280 - 12 1) List of symbols at the end of the paper

102 Chem. Eng. Technol. 12 (1989) 96- 102

model is suitable for the determination of ultimately attainable methane production, minimum retention time and kinetic con- stant in the digestion of wheat straw slurries.

Details of application can be found elsewhere (151.

4 Conclusions

This study reports on the effect of hydraulic retention time, alkali treatment of wheat straw and nitrogen supply on the fermentation of 5% wheat straw-water mixtures in semi- continuous mode at 55 "C. The following conclusions were drawn on the basis of this study:

Methane production rate increases with retention time for each slurry. Methane yield depends on the type of slurry digested and in- creases in the following order: basic, nitrogen supplemented and alkali treated slurry. The results of the digestion of alkali treated slurry show that alkali treatment decreases the association of lignin with carbohydrate portion and, there- fore, increases the anaerobic biodegradability. Nitrogen supplement, on the other hand, enhances microbial growth and thus also the gas production rate but not the substrate accessibility. The total gas production rate and hence the COD reduction rate increase with retention time. Pretreatments (alkali treatment or nitrogen supplement), performed on the wheat straw, result in significant increases of the ultimately attainable methane production. The measured methane production rates are in a reasonably good agreement with predictions of the kinetic model of Chen and Hashimoto [12].

Acknowledgement

Received: February I , 1988 [CET 1221

Symbols used

I CH, (at STP) producedlg COD I CH, (at STP) produced/g COD as 0 + m chemical oxygen demand kinetic constant influent total COD concentration volatile solids concentration hydraulic or average solids retention time minimum hydraulic or average solids retention time maximum specific growth rate of micro-organisms volumetric methane production rate

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[2] Smith, R.T., Hein, M.E., Greiner, T.H.,J. Anim. Sci. 48(1979)No.

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[4] Jones, D.D., Jones, B.A., Day, D.L., Truns. ASAE (1968) pp.

[5] Sievers, P.M., Brune, D.E., Truns. ASAE (1979) pp. 537-541. [6] Pavlostathis, S.C., Gossett, J.M., AIChEJ. 27(1985) pp. 334-344. 171 Klass, D.L., Ghosh, S., 177th National Meeting. American Chemicul

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[ I l l Trevelyan, W.E., Trop. Sci. 17(1975) No. I , pp. 193-209. 1121 Chen, Y.R., Hashimoto, A.G., Biotechnol. Bioeng 22 (1980) pp.

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ASAE 20 (1977) NO. 2, pp. 336 - 340.

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The investigation reported in this paper was sponsored by the 269 - 282. Middle East Technical University Research Foundation under [14] Contois, D.E., J . Gen. Microbiol. 21 (1959) p. 40. contract AFP No. 86-03-04-02. [I51 Tosun, A,, Thesis. Middle East Technical University, Ankara 1987.