Kinetics of anaerobic digestion of labaneh whey in a batch reactor

Abstract

In this work, anaerobic digestion of labanah whey was carried out in a 100 L batch reactor (RE-BIOMAS) at temperature of 30-40°C and pH 6 - 7. During the experiments, the biogas production and chemical oxygen demand (COD) concentration were recorded with time. During fermentation of labaneh whey, the pH drops dramatically due to the accumulation of volatile fatty acids that inhibits the activity of methanogens, resulting in a low gas yield and low methane content of the biogas. In a 28 days batch experiment at 36°C and pH 6.5, COD removal efficiency of 84% was achieved at initial COD of 18,000 mg/l. The cumulative biogas production was 20 L. Experimental data were fitted to the four kinetic models: Monod, Logistic, Contois and Tessier models. Comparison was made between model predictions and experiments for COD concentration. Tessier model gave marginally better fit than other models tested. Kinetic and stoichiometric coefficients were determined for the four kinetic models using Matlab nonlinear optimization function. Diluted labaneh whey (about 8000 mg/l COD) was treated to almost complete COD removal in a 10 days retention time and producing about 20 L of biogas. The COD removal was described well by first order kinetics with respect to substrate concentration. The modified Gompertz equation was used to describe the cumulative production of biogas with time. The equation kinetic constants were determined for labaneh whey and for diluted whey.   Key words: Labaneh whey, biogas, anaerobic digestion, methane production, dairy wastewater, modified Gompertz equation.