Abstract
Sub-critical water hydrolysis produced organic acids at 5-10 min from excess sludge. To make clear that sub-critical water hydrolysis is superior as a pretreatment, fermentation experiments were conducted in an anaerobic sewage sludge digester for three different samples: sub-critical water hydrolyzed excess sludge, model substrate in which the concentrations of the main components were close to those in sub-critical water hydrolyzed excess sludge, and acetic acid aqueous solution. In sub-critical water hydrolyzed excess sludge experiment, methane was produced drastically rapidly compared to the non-treated excess sludge. In synthetic sample experiment, about 40% of methane was produced in 5 days. Only the organic acid (mainly acetic acid) was completely consumed in 1 day. However, alanine and glycine were consumed about 60% in 5 days, and also cysteine and pyroglutamic acids were consumed to 40% in 5 days. Amino acids were not suitable for methanogen to produce biogas. When 0.01 M acetic acid was feeded into the digester, nearly 100% of acetic acid was converted to methane and carbon dioxide in 1 day. Also, approximately linear rate of methane formation (8.5 µmoles CH4 hour-1) was observed among 0.5, 1 and 3 hours of incubation. In this experiment the conversion efficiency of acetic acid to methane was nearly100%; 4.0, 8.5 and 32 moles of acetic acid were added to the culture, and 4.0, 8.5 and 32 µmoles CH4 were present in headspace after incubation. The overall results suggested that high speed and high efficient methane fermentation process was obtained by using sub-critical water hydrolysis reaction as a pretreatment.
