Methane Production From Alkaline Food Waste.

Abstract

Anaerobic digestion of an alkaline food waste for methane production was investigated. The waste generated by potato processing contained significant quantities of carbohydrate degradation products and sodium carbonate. Anaerobic digestion was determined to be a reliable process for waste treatment and methane production. The high sodium levels did not inhibit anaerobic digestion at a substrate concentration of 1.0 wt % alkaline peel solids. Single and two-stage (separate acid, methane stages) anaerobic digestion systems were studied. In two-stage bench-scale studies at 37(DEGREES)C, methane production averaged 0.28 m('3)/kg COD feed or 0.65 m('3)/(m('3)(.)d) in a settled bacterial sludge methane fermenter with a 4-day retention time. The effect of temperature on methane production was found to follow an Arrhenius relationship, with an activation energy of 92.1 kJ/mol. A pilot-scale anaerobic digestion system with a 15.4-m('3) fermenter demonstrated the feasibility of a proposed anaerobic pond system for a food processing plant. With a 1.0 wt % alkaline solids feed and a 9-day retention time, methane production averaged 0.16 m('3)/kg solids or 0.18 m('3)/(m('3)(.)d). Various modes of operation, including single and two-stage fermentation, and continuous and semi-continuous feed, resulted in similar performance. Reaction rates were found to follow zero-order kinetics, and averaged 0.6 kg COD/(m('3)(.)d). The methane fermenter was very sensitive to temperature fluctuations, but insensitive to feed variations. A sensitivity model of the fermentation illustrated these results. A novel process for single cell protein production via the acid stage of anaerobic digestion was explored. Bacterial cell and protein yields of 0.18 and 0.09 kg/kg glucose consumed were obtained at a retention time of four hours. Organic acids produced included a high proportion of butyric acid, along with acetic, propionic and iso-butyric acid. Increases in glucose concentration to 5.0 wt % in the feed resulted in lower yields, and indicated product inhibition. A simple salts media was shown to be sufficient for single cell protein production.