Disinfection of sewage by ultraviolet irradiation

Disinfection of sewage from an activated sludge process, and of sewage from an activated sludge process followed by chemical precipitation, has given very high killing rates for the indicator organisms examined. For coliforms, Escherichia coli and faecal streptococci, a killing rate of more than 999.99% was usually obtained, and the residual number of microorganisms in the irradiated sewage was very low. The killing rate increased with the irradiation dosage, depending on the intensity of the irradiation, ie the water quality, and on the irradiation period, ie the water flow. The study demonstrates that ultraviolet irradiation may be an alternative method to chlorination of sewage.