Biomass and Bioemrgy Vol. 5, No. 5, pp. 369-377, 1993 0961-9534/93 $6.00+0.00 Printed in Great Britain. All rights reserved C 1993 Pergamon Press Ltd SOLID PHASE FERMENTATION OF UNTREATED LEAF BIOMASS TO BIOGAS H.N. CHANAKYA,* SUSHAMABORGAONKAR,G.MEENA~~~ K.S.JAGADISH ASTRA (Centre for the Application of Science and Technology to Rural Areas), Indian Institute of Science, Bangalore 560 012, India zyxwvutsrqponmlkjihgfedcbaZYXW (Received 6 January 1993; revised received 6 Augusr 1993; accepted 24 August 1993) Abstract-Solid phase fermentation is a simple alternative to overcome problems in conventional slurry-based biogas digesters such as feedstock addition, floating scum, feed stratification and spent material discharge. The present study demonstrates the potential for a semi-continuous mode of operation. Laboratory scale solid phase digesters were operated with intact, untreated leaf biomass feedstock in a weekly-fed, batch mode without spent material removal for a period of 300 days. High biogas production rates were achieved by a twice daily sprinkling of the biomass bed with recycled liquid phase containing biodegradative bacteria. Results indicate that acidogenesis and methanogenesis occur at different layers of the decomposing bed (top and bottom, respectively). This simplifies feeding so that untreated leaf biomass may simply be added on top of the existing biomass bed in the digester without the need for feeding through a conventional liquid trap. Over 92% of the gas production was from the solid phase (biomass bed). Dry and fresh feedstocks gave gas yields of 295 and 343 L biogas kg-’ total solids. respectively, at levels up to OS-O.6 L L-i digester day-‘. Suppression of gas production in the biomass bed occurred when the volatile fatty acid (VFA) levels of the digester liquid exceeded 6 g . L-‘. Therefore during start up care has to be taken to ensure the development of the required population of anaerobic microorganisms in order to prevent the build up of VFA levels in the sprinkled liquid phase. Keywords-Solid phase, rural biogas digesters, volatile fatty acids, leaf biomass feedstocks. List of abbreviafions-VFA-volatile fatty acids (g acetic equivalent), TS-Total solids; SGY-specific gas yields (L gas kg-’ TS), SRT-solids retention time (days), GPR-gas production rate (L gas L-’ digester. day). 1. INTRODUCTION The use of cow dung for biogas production is very well established.’ However, cow dung is a limited resource even in countries like India’ and to produce larger quantities of biogas, it is inevitable that other resources such as leaf biomas?’ have to be utilized. This paper explores a new approach to the production of biogas using untreated whole leaf biomass as feedstock since powdering leaf biomass is uneconomic for rural applications. It has been reported’ that adequate leaf biomass can be made available for biogas production, after meeting the fodder requirement of the livestock population. Preliminary experiments carried out at ASTRA made it clear that conventional biogas plants cannot use leaf biomass feedstocks. The solid biomass and water in conventional biogas plants remain distinct and separate out. *Author to whom correspondence should be addressed. Operations such as feeding of the biomass and the removal of spent feedstock pose insurmountable problems in such plants. A radically different approach has been used here, based on earlier attempts’*” to carry out solid phase digestion of biomass. In these earlier experiments, acidogenesis and methanogenesis were attempted in different reactors. The acid phase reactor contained a decomposing biomass bed with periodic sprinkling of leached liquid and the leachate from the reactor was passed through an upflow methanogenic reactor. The first experiment utilized a batch mode operation while in the second experiment the biomass feedstock was fed periodically, without the removal of spent material. These experiments demonstrated the feasibility of solid state conversion of biomass to biogas. However, the acidogenic and methanogenic phases could not be separated in a stable manner suitable for rural application. However, the biomass bed exhibited a strong tendency to combine acidoge- nesis and methanogenesis within the same reactor. It was considered that a solid phase 369