Synergetic effect of gamma irradiation and moisture content on decontamination of sewage sludge

Samples of concentrated municipal sewage sludge, stored for 2, 4 and 6 months, with moisture contents of 2%, 20%, 40%, 60%, and 80% were exposed to doses of 0, 1, 2, 3, 4 and 5 kilogray (kGy) of gamma irradiation. Immediately after irradiation, total microbial count and bacterial pathogens in sewage sludge were determined. The results indicated that in all tested sewage sludge samples, bacterial pathogens including Enterobacter sp., Klebsiella sp., Salmonella sp., and Escherichia coli were initially detected. All doses of gamma irradiation reduced the total counts of microorganisms. D10 of total count decreased with increase in the moisture content of the sewage sludge. The lowest lethal dose for tested bacterial pathogens was 5 kGy in air dried sewage sludge. In addition for wet sewage sludge having more than 40% moisture, the lethal dose was 1 kGy, for samples taken at different storage periods 2, 4 and 6 months, and therefore the cost per unit could be decreased to half when wet sewage sludge (about 50% moisture) was used.