Chemical changes during a two-stage digestion technique for biogas production from combinations of organic wastes

Abstract

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 4 +-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 (1300–14451/kg), and the fatty acids formed (1311–13451/kg). Increasing the amounts of cow dung in the feed raised the concentrations in all cases.