Bioresource Technology 40 (1992) 179-181

Short Communication

Effects of Adsorbents on Anaerobic Digestion of Water Hyacinth- Cattle Dung

Abstract

In an effort to improve the anaerobic digestion of water hyacinth-cattle dung the effects of various adsorbents have been studied. A trend of enhanced gas production with high methane content and lower effluent BOD and COD, was found with increasing doses of the adsorbents gelatin, polyvinyl alcohol, powdered activated charcoal, pectin, kaolin, silica gel, aluminium powder, bentonite and talc powder.

Key words: Water hyacinth, cattle dung, adsor- bents, anaerobic digestion, biogas.

INTRODUCTION

In India, many water bodies have been damaged due to excessive growth of aquatic weeds, particu- larly the water hyacinth (Echhornia crassipes), to the extent that they are of no further use. Today, its utilization is considered an important part of weed management. Among possible uses, anaero- bic digestion of water hyacinth (WH) resulting in the production of biogas, a valuable source of energy, has attracted much attention in the recent past (Obeid, 1975; Gupta & Lamba, 1976; Wolverton & McDonald, 1976; Saraswat & Khanna, 1986). Furthermore in recent years WH has come to be used as a replacement for cattle dung (CD), mainly due to the limited availability of the latter (Obeid, 1975; Wolverton & McDonald, 1976; Saraswat & Khanna, 1986). In practice, however, a problem arises due to the nature of the organic content of WH and digester performance has not been consistently good. Our earlier studies showed that W H : C D used in the ratio of 7 : 3 on a dry-weight basis gives a high pro- duction of biogas with enriched methane content (Madamwar et aL, 1990). However, there is still a

growing interest in maximizing extraction of methane for energy recovery from WH so that farmers will be more likely to adopt this process. Therefore, further process improvements are needed.

It has been reported that addition of powdered activated carbon results in an increase in total gas production with high methane content (McCon- ville & Maier, 1978; Madamwar & Mithal, 1985, 1986). Based on a review of the literature, it is evident that activated carbon is responsible for improved digestion. The surface of the activated carbon provides adsorption sites where substrate can accumulate, thereby providing high localized substrate concentration. These areas of adsorp- tion provide a more favourable growth environ- ment for the bacterial-substrate system (McConville & Maier, 1978). No detailed study, however, seems to have been done so far on the effect of activated charcoal and other adsorbents on anaerobic digestion of WH. Therefore it was felt necessary to study this aspect with the ultimate aim of improving the gas production with increased methane content in biogas fermenta- tion. Experiments carried out with this object in view are reported in this paper.

METHODS

Resources All chemicals used were of analytical grade. Cattle dung and water hyacinth were obtained locally.

Anaerobic digestion Several bench-scale anaerobic digesters were used. Each vessel consisted of a 10-1itre glass reaction bottle, having a working volume of 6 litres and containing 7% total solids (TS) (mixture of WH-CD, 7:3, w/w). All the digesters were maintained at 37 _+ I°C in a thermostat. Gas was collected and measured by displacement of acidified, saturated salt solution, making due cor- rection for atmospheric pressure and tempera- ture. The digesters were fed on a semicontinuous basis (once per day) with a mixture of WH (dried at 60°C and powdered to 50 mesh) and CD in the

179 Bioresource Technology 0960-8524/92/S05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain

180 V. Patel, A. Patel, D. Madamwar

ratio of 7:3 (w/w) containing 7% TS (w/v) with a retention time of 8 days (loading rate of 8.75 g TS per litre of digester per day) which was found to be most suitable in a previous study (Madamwar et aL, 1990). Prior to feeding an equal quantity of sludge was withdrawn from the bottom of the digester. Adsorbents were incorporated with the feed sludge.

A fresh digester was always started by prepar- ing a mixture of powdered WH and CD in the ratio of 7:3 (w/w) to give a final TS concentration of 7% (w/v) and using a 10% inoculum from a running biogas digester of the same type. Steady- state was denoted by a fairly constant gas produc- tion. Experiments were carried out in triplicate for each adsorbent and for each concentration and average data are presented.

Analysis Gas composition was analysed by CIC gas-liquid chromatography with a stainless steel chromosorb 2 column and thermal conductivity detector (Varel et aL, 1980).

Feed and effluent slurry were routinely analysed for pH, volatile acids, BOD, COD, TS and volatile solids as per standard procedures (APHA, 1976).

RESULTS AND DISCUSSION

Steady-state performance efficiencies are shown in Fig. 1, gas production tended to a maximum with increasing concentration of additives, while COD removal peaked and residual BOD, COD and acids were minimized. An example is gelatin,

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Fig. 1. Enhancement of anaerobic digestion of WH-CD in presence of (ai gelatin, (b) pectin, (c) talc powder, (d) polyvinyl alcohol, (e) powdered activated charcoal, (f) aluminium powder, (g) kaolin, (h) silica gel, (i) bentonite, o, gas production; o, methane content; T, COD removal; zx, BOD; ~7, COD; t~, volatile acids. Operational conditions: temperature 37 ± I°C; RT, 8 days; loading rate, 8"75 g TS (WH:CD, 7:3, w/w) per litre of digester per day.

Anaerobic digestion of water hyacinth- cattle dung 181

with the optimum being at 7 g/litre. Gelatin also produced a higher methane content in the digester gas. As much as 67% methane was present in the gas in a gelatin-dosed digester (Fig. 1). Process stability, as evidenced by lower volatile acids, con- sistently increased with increased levels of gelatin. Average acid concentration ranged from 1.35 g/ litre in the digester with no gelatin to 1"13 g/litre in a gelatin-dosed digester. In our studies pro- pionate was found to accumulate in large quan- tities in the digester without gelatin, however, addition of gelatin reduced the amount of pro- pionate (data not given).

Process performance can also be judged by effluent BOD and COD values (Fraser, 1977; Varel et al., 1980; Madamwar & Mithal, 1986). As shown in Fig. 1, addition of 7 g/litre of gelatin gave 14.0 g/litre BOD and 24.6 g/litre COD, much lower values than in the control without gelatin, indicating greater biodegradation. Figure 1 shows that addition of gelatin increased the efficiency of COD removal.

The other adsorbents gave similar results, in- dicating better process performance and better process stability in the presence of adsorbents. However, addition of vanadium pentoxide was found to be detrimental to the digester (data not given).

Thus, in general, addition of adsorbents can improve digestion performance, though a clear mechanism of their action cannot be given. Whether these quantities could be economically used in practice is to be determined.

ACKNOWLEDGEMENT

Thanks are due to the DNES, New Delhi, India for financial support.

REFERENCES

Fraser, M. D. (1977). The economics of SNG production by anaerobic digester of specially grown plant matter. In Proc. Symp. on Fuels from Biogas and Wastes. Institute of Gas Technology, Chicago, IL.

Gupta, O. P. & Lamba, P. S. (1976). Some aspects of utiliza- tion of aquatic weeds. In Aquatic Weeds in S.E. Asia, ed. W. Junk. The Hague, The Netherlands, pp. 361-7.

Madamwar, D. B. & Mithal, B. M. (1985). Adsorbents in anaerobic digestion of cattle dung. Indian J. Microbiol., 25, 57-8.

Madamwar, D. B. & Mithal, B. M. (1986). Effect of pectin on anaerobic digestion of cattle dung. Biotechnol. Bioengng, 38,624-6.

Madamwar, D. B., Patel, A. & Patel, V. (1990). Effect of tem- perature and retention time on methane recovery from water hyacinth cattle dung. J. Ferment. Bioeng., 70, 340-2.

McConville, T. & Maier, W. J. (1978). Use of powdered activated carbon to enhance methane production in sludge digestion. Biotechnol. Bioeng. Symp., No. 8, pp. 345-59.

Obeid, M. (1975). Utilization of water hyacinth -- introduc- tion. In Aquatic Weeds in Sudan, ed. M. Obeid. National Council for Research, Sudan and US National Academy of Science, Washington, DC, pp. 101-3.

Saraswat, N. & Khanna, P. (1986). Methane recovery from water hyacinth through anaerobic activated sludge process. Biotechnol. Bioengng, 828, 240-6.

Varel, V. H., Hashimoio, A. G. & Chen, Y. R. (1980). Effect of temperature and retention time on methane production from beef cattle waste. Appl. Environ. Microbiol., 40, 217-22.

Wolverton, B. C. & McDonald, R. C. (1976). Don't waste water weeds. New Scientist (USA), 71 (1913) 318-29.

Vikram Patel, Anami Patel & Datta Madamwar* Department of Biosciences, Sardar Patel University, Vallabh Vidyanagar - 388120, Gujarat, India

(Received 12 March 1991; revised version received 15 April 1991; accepted 23 April 1991 )

APHA (1976). Standard Methods for the Examination of Waterand Waste Water, 14th edn. American Public Health Association, Washington, DC. *To whom correspondence should be addressed.