chemical changes during a two-stage digestion technique for biogas production from combinations of...

Resources, Conservation and Recycling, 3 (1990) 217-230 Elsevier Science Publishers B.V./Pergamon Press p l c - - Printed in The Netherlands

217

Chemical changes during a two-stage digestion technique for biogas production from

combinations of organic wastes

M.M. E1-Shinnawi l*, M. E1,Houssieni 2, S.A. Aboel-Naga 1, and Sohier Fahmy 2 ~ Department of Soil Science, Faculty of Agriculture, Menufiya University, Shibin El K6m (EgYPO

2Soils and Water Research Institute, Giza (EgYpO

(Received October l, 1988; accepted in revised form November 1, 1989 )

ABSTRACT

El-Shinnawi, M.M., EI-Houssini, M., AboeI-Naga, S.A. and Fahmy, S., 1990. Chemical changes during a two-stage digestion technique for biogas production from combinations of organic wastes. Resour. Conserv. Recycl., 3:217-230.

Constant amounts (200 g) of air-dried rice straw (RS), maize stalks (MS), and cotton stalks (CS), each mixed with 0, 100, 200 and 400 g fresh (moist) cow dung (CD), were first predigested under microaerophilic conditions at various intervals, i.e. 1, 2, 3 and 4 weeks (stage 1, acidogenesis). After each predigestion interval, elutriates of the materials were introduced into laboratory biogas fermenters and incubated for a week (stage 2, methanogenesis). Concentrations of some constituents were determined after each predigestion interval and at the end of the fermentation period. The rates of biogas production were also monitored.

In the first microbial and biochemical phase, the rice straw mixtures produced the greatest amounts of volatile (organic) solids, followed by the maize stalks and cotton stalks mixtures. Acetic, propionic, and butyric acids were the major detectable fatty acids formed, during this phase, from all feedstocks. Total volatile fatty acid contents produced from the waste combinations showed the order RS > CS > MS. Acetic acid, the prime fatty acid produced, was highest with cotton stalks, less with maize stalks, and least with rice straw mixtures. NH~ -N release showed a similar order to that of the acetic among the combinations examined. In the second phase, the greatest evolution of biogas was obtained from the feed materials predigested for one week. The efficiency of substrates in generating biogas was highest for the maize stalk combinations, in relation to both the volatile solids consumed ( 1300-1445 l/kg), and the fatty acids formed ( 1311-13451/kg). Increasing the amounts of cow dung in the feed raised the concentrations in all cases.

INTRODUCTION

Biogas generation from crop residues, one of the major biomass sources, faces some problems concerning the digester costs and the reliability of fermentation. For instance, crop residues have low density and are therefore

*To whom correspondence should be addressed.

Elsevier Science Publishers B.V./Pergamon Press plc

218 M.M. EL-SHINNAWI ET AL.

bulky and need digesters of large volume. Biodegradability of cellulosic materials under anaerobic conditions is low, which means long retention times and thus a slow rate of biogas production. Other technical and financial problems may also be encountered. To overcome such obstacles, attention was drawn to splitting the two stages of digestion into a microaerophilic, acidogenic phase and a strict anaerobic, methanogenic phase. Elutriates were removed after the optimum period of predigestion (the first stage) and trans- ferred into small-sized biogas fermenters (the second stage).

The magnitude of this problem in Egypt is shown by the annual figures for biological wastes [ 1 ]: 13.6 × 106 tons of crop residues (containing 57.2 × 1012 kcal ), 4.0 × 106 tons of cattle dung (containing 16.8 × 1012 kcal ) and 3.2 × 106 tons of garbage "landfill" (containing 12.9 × 1012 kcal ).

The concentration of the volatile fatty acids formed during the acidogenic phase has been found to be of importance for successful methanogenesis. McCarty and Mclnerney [2 ] stated that the level of volatile fatty acids should remain below 2000 mg acetate/l for efficient fermentation. However, Hob- son et al. [3 ] noted that a well balanced digester working on farm wastes could cope with inputs of up to 600 mg acetate/l. Kroeker et al. [ 14 ] reported that acute methanogenic toxicity occurred at un-ionized volatile acids concentration of between 30 and 60 meq/l, which corresponded to 1800-3600 mg/1 as acetic. On the other hand, Van Velsen et al. [ 5 ] observed no adverse effect of the volatile acids concentration as high as 5000 mg acetic acid/1.

The present work was carried out to study the effects of splitting the digestion process of some crop residues mixed with varying amounts of cow dung into separate phases-namely, microaerophilic acidogenesis and strictly anaerobic methanogenesis-on the constituents of the digesting materials and on biogas production.

MATERIALS A N D M E T H O D S

Rice straw (RS), maize stalks (MS), cotton stalks (CS), and cow dung (CD) were used as substrates in this study. The initial analysis of such materials was made following the methods recommended by Horwitz [ 6 ]. Data are presented in Table 1. The crop residues were air-dried and pulverized. Samples of 200 g of each of the plant materials were supplemented with 0, 100, 200, and 400 g of fresh (moist) cow dung, mixed thoroughly and then placed in 41 glass bottles. After addition of 21 of water and mixing, the bottles were closed with loose covers to maintain microaerophilic conditions and to allow the escape of CO2 formed. Treatments were performed in duplicate and incubated at room temperature (25-30°C). At intervals of 1, 2, 3, and 4 weeks, during the primary acidogenic phase, one litre of each elutriate was withdrawn from the predigestion bottles. It was introduced into laboratory biogas fermenters of 4 1 each (Fig. 1 ) [7], having a red-clay-fixed bed to

BIOGAS PRODUCTION FROM ORGANIC WASTES

TABLE 1

Properties of crop residues and cow dung used

219

Raw material TS = (%)b VS = (%) OC" (%) TN" (%) C/N ratio

Rice straw 85.57 80.41 46.64 0.50 93.84 Maize stalks 84.40 93.59 54.28 0.74 73.85 Cotton stalks 85.50 94.72 54.94 1.00 54.99 Cow dung 20.78 68.38 39.66 1.48 26.82

Raw Water- Crude Fat Hemi- Cellulose material sol. subs. protein (%) cellulose (%)

(%) (%) (%)

Lignin (%)

Lignin/ cellulose ratio

Rice straw 14.74 3.11 1.88 25.42 31.95 18.09 1.77 Maize stalks 17.03 4.59 1.61 19.65 35.79 20.31 1.76 Cotton stalks 10.75 6.24 1.72 27.87 31.29 24.87 1.26 Cow dung 13.06 9.24 6.99 32.35 21.54 17.74 1,21

"TS = total solids; VS = volatile solids; OC = organic carbon; TN = total nitrogen. bOn basis of 70°C dried matter.

~ For gas sampling~l-

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Fig. 1. Schematic diagram of the laboratory biogas digestion unit.

activate the process, and then kept in a walk-in incubator at 35°C for one week. The first litre of elutriate removed was replaced with a litre of water. The succeeding volumes removed were replaced with the previously removed volumes that had already undergone methanogenesis, in a recycling manner.

Concentrations of the individual fatty acids were measured weekly, for four


weeks of predigestion, using gas-liquid chromatography [ 8 ]. At each interval, as well as at the end of the methanogenic phase, the following estimations were carded out on the elutriates:

( 1 ) Total and volatile solids (TS and VS) were measured according to the recommendations of the American Public Health Association [9].

(2) Total volatile fatty acids (VFAs), were determined by steam distillation of the liquor, which was acidified with H2SO4, and then back titrated with NaOH [ 10 ].

(3) Ammoniacal nitrogen was measured by steam distillation of the ef- fluent, made alkaline with NaOH [ 11 ].

Biogas generation of the fermenting elutriates was monitored by means of the displacement technique using water acidified with H2SO4 (2%) to pre- vent the solubilization of CO2 [7 ]. Evolution of biogas started on the first day of the second stage, and ceased entirely after 5 to 6 days.

RESULTS A N D DISCUSSION

Changes in concentrations of constituents

Total and volatile solids (TS and VS) Changes in total and volatile solids during the two-stage digestion of the

crop residues mixed with varying amounts of cow dung are illustrated in Fig. 2. The results indicate that prolongation of the acidogenic phase reduced the contents of both TS and VS. Application of increasing ratios of CD to biomass enhanced the level of organic solids in the mixtures at each predigestion interval. Combinations of CS produced the lowest concentrations of solids at the biomethanation stage after each predigestion interval of the acid formation stages. The treatments of MS attained the highest figures for TS, but intermediate ones for VS (Fig. 3, Table 2 ). Values of both TS and VS observed at the termination of the methanogenic phases showed decreases throughout.

The loss rates of VS, of the different combinations tested, showed no defi- nite trends by advancing the predigestion interval (acidogenic stage), except the severe reduction in the last week (Fig. 3). The greatest loss rates were shown by CS treatments, followed by RS and MS, respectively. The overall loss rates of VS ranged between 18 and 25% (Table 2). Generally, negative correlations appeared between the total amounts of VS introduced-throughout the methanogenic phase-and their loss rates (Table 2 ).

The changes observed depended on the type of plant residue and the level of co-substrate (animal waste). The initial C/N ratio of the materials being digested was inversely proportional to the rate of VS disappearance, i.e., the smaller the ratio, the higher the loss of VS (Table 2, Fig. 3 ). Sax et al. [ 12 ] noted that the loss rate of VS depends on the system of application, type of

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Fig. 4. Changes in the content of the individual fatty acids in the elutriates of crop residues (200 g) mixed with varying amounts of cow dung during different intervals of the acidogenic phase (up to 4 weeks). (I) Rice straw; (II) maize stalks; (III) cotton stalks. A: 0 g CD; B: 100 g CD; C: 200 g CD; D: 400 g CD. ( • ) Acetic acid; ( • ) propionic acid; ( X ) butyric acid.

tion duration and type of feed stock. CS and MS were higher than RS in acetic concentration, whereas propionic and butyric acids showed indefinite trends among the crop residues. Increasing the amount of CD increased the content of each individual fatty acid in the mixtures.

Figure 5 shows the contents of the total VFAs for each substrate just before and after the stage of biogas generation. VFAs contents decreased by extend- ing the acid formation stage of all organic combinations exposed to microaerophilic digestion prior to the strict anaerobic fermentation for methanogesis. CD amendments enhanced VFA production in the mixtures. Combinations of RS produced the highest concentration of VFA, whereas those of CS and MS were respectively lower. Maximum VFA levels, i.e., 56 meq/1 were in- duced by the mixtures of RS plus 400 CD for one week acidogenesis. Such values lay within the safe and preferred range [ 3,4 ]. At cessation of the methanogenic phase, a sharp drop occurred in VFA concentrations, reaching zero in many cases. The rate of disappearance was parallel to that of initial accumulation.

It is evident from the results that the intensity of acid formation increases by increasing the quantity of CD in the waste mixtures. Since CD is a rich

BIOGAS PRODUCTION FROM ORGANIC WASTES 225

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Fig. 5. Changes in the total volatile fatty acids during methanogenic stage of the elutriates of crop residues (200 g) mixed with varying amounts of cow dung and predigested for different periods (up to 4 weeks). For legend see Fig. 2.

226 M.M. EL-SHINNAWl ET AL.

source of active bacterial agents with their energy and nutritional require- ments, augmenting its content increases the rate of acid release from both sources, i.e., from itself and from the combined crop residue. In this connec- tion, Hobson and Shaw [15] and Chengdu [ 16 ] stated that the ruminant wastes contained, as a rule, sufficient anaerobic acid-producing and methane- forming bacteria.

Ammoniacal nitrogen Changes in NH~--N within the various intervals of the first stage, i.e., aci-

dogenesis, as well as at the end of the second stage (methanogenesis) are shown in Fig. 6. The greatest NH~- -N amounts were detected after the second week of the first stage; otherwise, no marked differences were observed among the other predigestion intervals. Cow dung supplements elevated the NH~--N contents of the mixtures. Generally, CS combinations revealed the highest rate of NH~- - N formation, MS was second and both were higher than RS in most cases. Contents of NH~--N were diminished at the end of the methanogenic phase. The rate of decrease ranged between 30 and 55%.

The initial C /N ratio of crop residues (Table 1 ), as well as the amount of

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Fig. 7. Volume of biogas generated within methanogenic stage of the elutriates of crop residues (200 g) mixed with varying amounts of cow dung(CD) and predigested for different periods (up to 4 weeks). For legend see Fig. 2.

CD added, affected the ammonification rate of the mixtures. Decreases in NH~- - N by the digestion process are attributed to volatilization of NH3 and/ or assimilation by the fermentative bacteria to which ammonium serves as the main nitrogen source [ 14,17 ].

Biogas generation

The rates of biogas evolution from the elutriates of fermenting feedstocks for different incubation periods are shown in Fig. 7. The volume of biogas

228 M . M . E L - S H I N N A W l E T AL.

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Fig. 8. Cumulative biogas production during methanogenic stage of the elutriates of crop residues (200 g) mixed with varying amounts of cow dung and predigested for different periods (up to 4 weeks). (I) Rice straw; (II) maize stalks; (III) cotton stalks. ( • ) 0 g CD; ( • ) 100 g CD; ( × ) 200 g CD; ( • ) 400 g CD.

decreased as the predigestion interval of the acidogenic phase advanced. Highest values were obtained in MS mixtures, followed by CS and RS, respectively. The extent of diminution was maximum from the first to the second predigestion week then from the third to the fourth, which ranged between 35 and 75%, whilst from the second to the third week it ranged between 10 and 20% only. Addition of CD stimulated biogas production with all treatments. Cumulative biogas production from the examined feedstock elutriates, plotted in Fig. 8, denotes the superiority of MS over CS and RS.

The efficiency of biogas production as a function of either VS introduced, VS consumed, or VFAs formed is shown in Table 2. The data reveal that CS treatments were superior in responding to VS fed (248-302 1/kg) especially at the highest CD amounts, while those of MS were intermediate. In respect to VS consumed, MS was first (1300-1455 1/kg) and CS came second. This pattern was also obtained for the rate of biogas evolved for VFAs fed, when MS combinations gave the greatest amounts, i.e., 1311-1345 1/kg. RS treatments occupied the last position in all cases.

The chemical composition of plant residues and amount of animal waste


influenced the patterns of results obtained. For instance, the C / N ratio played a major role in the breakdown rate of the substrates examined [ 18-21 ]. Ap- plication of cow dung, having a small C / N ratio, encouraged the digestion process, and thus improved the production of biogas as it is a rich source of the facultative and strictly anaerobic bacteria responsible for acidogenesis and subsequent methanogenesis [ 15 ]. Maramba [7 ] reported that cellulose, starch, sucrose, acetic acid, and pig manure can give theoretically 926, 926, 877, 833, and 789 1 biogas/kg volatile solids Consumed according to their carbon contents.

From our results, it can be seen that one-week predigestion of feedstock materials under microaerophilic conditions proved to be the optimum period of acidogenic phase and subsequently strict anaerobic methanogenesis. The ratio 1 : 1 air-dried crop residue to fresh (moist) cow dung seemed to be eco- nomically acceptable. The type of plant residues governed the biochemical processes mainly through the C / N ratio the original constituents.

REFERENCES

1 Alaa El-Din, M.N., EI-Shimi, S.A., Mahmoud, M.H. and Abdel-Aziz, I.M., 1983. Biogas for the Egyptian country. Rep. submitted to the Egyptian Ministry of Agriculture and FAO of the United Nations. Giza, Egypt, pp. 7-12 (in Arabic).

2 McCarty, P.L. and Mclnerney, R.E., 1961. Salt toxicity in anaerobic digestion. J. Water Pollut. Control Fed., 33:399-415.

3 Hobson, P.N., Robertson, A.M. and Mills, P.J., 1975. Anaerobic digestion of agricultural wastes. A.R.C. Res. Rev., 1: 82-96.

4 Kroeker, E.J., Schulate, D.D., Spading, A.B. and Lapp, H.M., 1979. Anaerobic treatment process stability. J Water Pollut. Control Fed., 51: 518-529.

5 Van Velsen, A.F.M., Lettinga, D. and Ottelander, D.D., 1979. Anaerobic digestion of pig- gery waste. Neth. J. Agric. Sci., 27: 255-269.

6 Horwitz, W., 1970. Official Methods of the Association of Agricultural Chemists. AOAC, Washington, DC, 1 lth ed.

7 Maramba Jr., F.D., 1978. Biogas and Waste Recycling, The Philippine Experiences. Maya Farms Division, Liberty Flour Mills, Inc. Metro Manila, The Philippines.

8 Wujick, W.J. and Jewell, W.J., 1980. Dry anaerobic fermentation Proc. 10th. Biochemistry and Bioengineering Syrup. Gatlinburg, TN, USA. Wiley, New York, pp. 43-65.

9 American Public Health Association, 1976. Standard Methods for the Examination of Water and Waste Water. Washington, DC, 14th ed.

10 Neish, A.C., 1952. Analytical methods for bacterial fermentation. Rep. No. 46, National Research Council of Canada.

11 Chapman, H.D. and Pratt, P.F., 1961. Methods of Analysis for Soils, Plants and Water. Div. of Agricultural Sciences, University of California, Riverside, CA.

12 Sax, R.I., Mourice, H. and Pette, K.C., 1980. Production ofbiogas from waste water of food processing industries. Conf. Industrial Energy Conservation Technology and Exhibi- tion. Houston, TX.

13 Ghosh, T.S. and Klass, D.L., 1978. Two-phase anaerobic digestion. Process. Biochem., 13: 15-24.

230 M.M. EL-SH1NNAWl ET AL.

14 Mclnerney, M.J. and Bryant, M.P., 1981. Review of methane fermentation fundamentals. In: D.L. Wise (Editor), Fuel Gas Production from Biomass. CRC Press, Florida.

15 Hobson, P.N. and Shaw, B.C., 1973. The anaerobic digestion of waste from an intensive pig unit. Water Res., 7: 437-449.

16 Chengdu, S., 1979. Biogas Technology and Utilization. Sichuan Provincial Office of Biogas Development, China.

17 Bryant, M.P., 1974. Nutritional features and ecology of predominant anaerobic bacteria of the intestinal tract. Am. J. Clinical Nutrit., 27:1313-1322.

18 Acharya, C.N., 1958. Preparation of fuel gas and manure by anaerobic fermentation of organic materials. Bull. No. 15, Indian Council of Agricultural Series, New Delhi, India, pp. 5-8.

19 Singh, R.B., 1971. Biogas Plant. Generating Methane from Organic Wastes. Ajitmal, Eta- wah (U.P.), India. Gobar Gas Research Station, New Delhi, India, pp. 16-17.

20 Hill, D.J., 1979. Effect of carbon:nitrogen ratio on anaerobic digestion of dairy manure. Agric. Wastes, 1: 267-279.

21 Liu, Ke-xin and Chen Guang-gian, 1981. Studies on the biogas fermentation of Chinese rural areas. In: S.O. Emejuawe et al. (Editors), Proc. 6th Int. Conf. on Impact of Applied Microbiology (GIAM-VI).

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