Simultaneous production of biogas and volatile fatty acids through anaerobic digestion using cereal straw as substrate

Abstract

Cereal straw is one of the most abundant wastes worldwide, with 30.000 million tons produced per year. Bioconversion of this residual material into carboxylates by anaerobic digestion could potentially replace conventional production based on fossil feedstocks (oil). In this work, fundamental issues of this bioconversion have been explored, including: different kinds of straw (wheat, barley and rye), biomass size reduction, mass balances and modelling of the different steps of the digestion. Under optimum conditions, 44% of the raw material was effectively converted into VFAs (mainly acetate) when barley was used as substrate. Wheat and rye straw presented lower conversion rates due to the higher lignin content compared to barley straw. According to the modelling proposed, methanogenesis and hydrolysis presented very similar reaction rates, which resulted in a simultaneous production of VFAs and biogas. In view of these results, a process integration is proposed where biogas covers the thermal needs of the biotransformation of barley biomass into VFAs.