Simultaneous methanogenesis and phototrophic bacterial growth in relatively dry sewage sludge under light

Anaerobically digested sewage sludge with a variety of moisture content, namely 81%, 86%, 90% and 98%, were anaerobically cultured at 35 degrees 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.