The enhancing effect of aerobic thermophilic (AT) bacteria on the production of biogas from anaerobically digested sewage sludge (methanogenic sludge) was investigated. Sewage sludge (5%, w/w) was incubated at 65°C with shaking for a few months to prepare the AT seed sludge. AT sludge was prepared by incubation of the AT seed sludge (5%, v/v) and sewage sludge (5%, w/w) at 65°C with shaking. The addition of this AT sludge (1.2% Ϯ 0.5% of total volatile solids) to methanogenic sludge enhanced the production of biogas. The optimum volume of the addition and the pretreatment temperature of the AT sludge for optimum biogas production were 5% (v/v) and 65°C. Batchfed anaerobic digestion was covered with the addition of various AT sludges. The AT sludge prepared with the AT seed sludge improved the biogas production by 2.2 times relative to that from the sewage sludge addition. The addition of sludge without AT seed sludge weakly enhanced biogas production. An aerobic thermophilic bacterium (strain AT1) was isolated from the AT seed sludge. Strain AT1 grew well in a synthetic medium. The production of biogas from the anaerobic digestion of sewage sludge was improved by the addition of 5% (v/v) AT1 bacterial culture compared with that from the sewage sludge addition. The addition of AT1 culture reduced the volatile solids by 21%, which was higher than the 12.6% achieved with the sewage sludge addition. The AT1 bacterial culture enhanced the biogas production more than the AT seed sludge. The phylogenetic analysis of the 16 S rRNA gene revealed that strain AT1 is closely related to Geobacillus thermodenitrificans (100% sequence similarity). The improvement in the production of biogas with the AT sludge could be caused by thermophilic bacterial activity in the AT sludge.

Anaerobically digested sewage sludge with a variety of moisture content, namely 81%, 86%, 90% and 98%, were anaerobically cultured at 35°C under light. Phototrophic bacteria grew in the 86% moisture sludge (bacteriochlorophyll a, 0.46 g/L), 90% sludge (bacteriochlorophyll a, 0.36 g/L) and 98% sludge (bacteriochlorophyll a, 0.04 g/L) with methane production. Phototrophic bacteria could not grow in the 81% moisture sludge (bacteriochlorophyll a 0.004 g/L). Phototrophic bacteria could assimilate about 46% of the extracellular ammonium in the 90% moisture sludge. Phototrophic bacteria utilized organic compounds competing with methanogens; therefore, methane yield from the 90% moisture sludge under the light conditions was lower than that under the dark conditions. Phototrophic bacteria could grow in anaerobically digested sludge with relatively low moisture content, and assimilated extracellular ammonium in the sludge. The quality of digested sludge with phototrophic bacterial biomass for fertilizer could be improved compared with that without phototrophic bacterial biomass.