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

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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. Na2CO3l-1 which resulted in a 15·5% increase in biogas production and a 6·7% increase in CH4 production. A decrease in the quantity of Na2C03l-1 added led to signs of instability and a drop in CH4 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 Na2C03 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 m3 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.