Disinfection of sewage sludge cake by gamma-irradiation

Abstract

Disinfection of municipal sewage sludge cake by gamma-irradiation was reviewed. Total bacterial count in the sludge cake did not vary markedly throughout all four seasons in Japan, and it was in the range of 1.6 × 10 8/g to 4.1 × 10 9/g. Coliform count in aerobically activated sludge was form 1.8 × 10 7/g to 4.8 × 10 8/g, while in anaerobically digested sludge it was less than 8.3 × 10 7/g. The dose to reduce the coliforms to undetectable levels ranged from 0.3 to 0.5 Mrad, depending on the season. In addition, it was observed that no coliforms reappeared in 0.5 Mrad irradiated sludge cake during storage at room temperature (6 – 16°C) and at 30°C. The adequate disinfection dose is therefore considered to be 0.5 Mrad. Pseudomonas cepacia was a predominant bacterium in non-irradiated sludge cake. In a range of 0.5 to 0.7 Mrad, the residual flora consisted of Bacillus species, and radioresistant Deinococcus proteolyticus, Deinococcus radiodurans and Pseudomonas radiora were isolated from sludge cake irradiated at dose levels of more than 1 Mrad. Bacterial regrowth and the growth of Escherichia coli K-12 seeded in irradiated sludge cake are discussed.