J. Gen. Appl. Microbiol., 51, 27–33 (2005)

The recycling of organic wastes is required from the viewpoint of environmental issues. Sewage sludge makes up about 30% of the organic wastes in Japan, and at present, the ratio is increasing (Study group on bio-waste recycling, 1999); therefore, it is important to treat and utilize the sewage sludge. Conventional treatment methods such as land filling and ocean dumping have been regulated to protect environmental water. Anaerobic digestion produces energy in the form of biogas and fertilizers; therefore, it is one of the most effective treatment systems for wet organic wastes. There are lots of reports concerning the anaerobic digestion of organic wastes with relatively high moisture contents (Salminen and Rintala, 2002). Organic wastes from municipal waste have been evaluated using anaerobic digestion by a number of researchers (Held et al., 2002; Mata-Alvarez et al., 2000). In general, anaerobic digestion at a relatively higher organic concentration has an advantage compared with that of a lower concentration. Dry anaerobic digestion has been found to produce a larger volume of methane than wet digestion without wastewater treatment (Lissens et al., 2001). Dewatered sewage sludge contains a high concentration of nitrogen, and ammonium was released during anaerobic digestion. A high concentration of ammonium in degrading dewatered sewage sludge is thought to inhibit the methanogenesis (Fujishima et al., 2000). Lay et al. reported that methanogenic activity dropped from 100% to 53% Simultaneous methanogenesis and phototrophic bacterial growth in relatively dry sewage sludge under light