Solid phase fermentation of untreated leaf biomass to biogas

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 −1 total solids, respectively, at levels up to 0.5–0.6 L · L −1 digester day −1. Suppression of gas production in the biomass bed occurred when the volatile fatty acid (VFA) levels of the digester liquid exceeded 6 g · L −1. 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.