Gamma Radiation Technology for Hygienization of Municipal Dry Sewage Sludge

Abstract

Sewage is the wastewater discharged from domestic premises consisting mainly of human waste. Sewage typically contains more than 99.9% water and about 0.05% solid. The solid part results in the formation of sludge. Largely, sludge is disposed in an unorganized manner resulting in environmental pollution and spread of diseases. The sludge produced carries a heavy microbiological load including pathogens and therefore its disposal has been a challenge to the urban development authorities. Indian cities and towns together are generating an estimated sewage load of 38,254 million liters per day (MLD), out of which 11,787 MLD is treated at sewage treatment plants (STP) with a capacity gap of 26,737 MLD (Ministry of Urban Development 2013). Considering 0.05% solid content, the total potential of sludge generation from the sewage is 19,127 tons per day. High energy ionizing radiation technology has a great potential for hygienizing municipal sewage sludge and makes it safer for use or disposal. In this study, dry sludge hygienization process was studied and compared with wet sludge hygienization. Dry sludge in plastic bags was hygienized in a terminal process using standard fully automatic irradiation plant without manual intervention. Dry sludge containing 75–80% solid irradiated at an average dose of 8 kGy, showed the absence of indicator organism E. coli/total coliforms even after 10 months of study period. Heavy metal concentrations in the domestic sewage sludge were observed to be much below the United States Environmental Protection Agency (US EPA) and Ministry of Urban Development, New Delhi (MOU) norms (Ministry of Urban Development 2013; United States Environmental Protection Agency). Inoculation of the hygienized sludge with Rhizobium, Azotobacter, and Phosphate solubilizing bacteria showed 100–1000 times higher growth in comparison to growth in unhygienized sludge. A comparison between dry and wet sludge (4–6% solid) irradiation shows that it is more practical to hygienize dry sludge which is economical, reliable, and scalable to treat 100–500 tons of city dry sludge. Use of radiation technology for sludge hygienization can significantly contribute to “Clean India Mission”. The technology is available with the department.