BIOGAS GENERATION FROM FOOD-PROCESSING WASTES

Biogas generation and changes in chemical oxygen demand (COD), total and volatile solids (TS, VS), volatile fatty acids (VFAs) and ammonia (NH4+) were monitored during a 40-day fermentation period in 60-1 anaerobic contact-type reactors, of carrot, orange, phaseolus, pea and tomato wastes. The reactors were first loaded with a spent slurry of previously digested cattle dung and then a daily feed with 2.35 l of the waste slurry was incorporated for two cycles at a 20-day hydraulic retention time (HRT). Biogas generation exhibited greater rates in the first cycle with a higher percentage of CO2 component. Average rates of volumetric biogas evolution were 0.998, 0.991, 0.885, 0.814 and 0.499 m3 m-3 day-1 for orange, phaseolus, tomato, pea and carrot wastes respectively. The daily mean rates of biogas output, collectively for all materials examined, were found to be 0.840 m3 m-3 of fermenting volume and to be 325, 362, and 281 m3 t-1 for TS, VS, and COD respectively. The bioconversion rates of VS ranged between 20.2% for carrot and 43.8% for phaseolus wastes. An exponential increase of total VFAs was recorded especially during the second cycle of fermentation, reflecting an instability in the fermentation process under these conditions of operation. Liberation of NH4+-N occurred, recorded at highest concentration (815 mg l-1) with legume wastes, whilst tomato, carrot, and orange wastes followed in descending order.