Effect of different neutralizing treatments on the efficiency of an anaerobic digester fed with deproteinated cheese whey

Abstract

SUMMARY Deproteinated whey, prepared from sweet whey powder, was digested anaerobically in a downflow fixed-bed reactor at 35 °C. Different pH control treatments were applied over a period of 210 d. Initially NaOH was added at a concentration of 68 mequiv. l -1 substrate. This was successfully replaced by 80 mequiv. Na 2 CO 3 l -1 which resulted in a 15·5% increase in biogas production and a 6·7% increase in CH 4 production. A decrease in the quantity of Na 2 C0 3 l -1 added led to signs of instability and a drop in CH 4 production. The stepwise addition of urea restored the stability, and a further increase of urea, to a level of about 20 mequiv. l -1 , made it possible to replace Na 2 C0 3 completely. When all pH control measures were omitted, complete failure resulted within 10 d. Methane yields, under stable conditions, varied between 0·321 and 0·272 m 3 kg -1 chemical oxygen demand (COD) removed which corresponds to 92 and 78% of the theoretical yields. Carbon balances indicated that between 60·8 and 68·5% of the carbon fed was converted to biogas. No toxicity was observed at a carbon: nitrogen (C/N) ratio of 7·7. This was in contrast to previous results where toxicity had occurred at the same C/N ratio but at a higher concentration of Na.