Alternative disinfectants for wastewater effluents: viable or prohibitively expensive?

The permissible residual chlorine limit for discharge of treated wastewater effluents may be reduced substantially in the foreseeable future, although the coliform standard of zero for wastewater discharges will not be changed. In addition, many organisations are contemplating moving away from chlorine gas disinfection for reasons such as safety and formation of undesirable disinfection by-products (DBPs). Umgeni Water together with the Water Research Commission conducted a project to evaluate various alternative disinfectants in comparison to chlorine for wastewater disinfection. The disinfectants investigated included ozone, ultra-violet (UV) irradiation (low and medium pressure), a mixed-oxidant generator, as well as peracetic acid, chlorine and bromine used alone and in combination with UV irradiation or ozone. The various disinfectants were tested on secondary clarified wastewater effluent at both laboratory and pilot scale, the effluent before and after disinfection being subjected to a range of tests to assess the impact of the treatment on various microbiological indicator organisms, including parasitic cysts and oocysts, as well as on typical water quality parameters and the organic matter in the effluent. Based on the results of these tests ozone, medium pressure UV, peracetic acid and bromine were identified as promising alternatives to chlorine. A cost assessment was conducted in which bromine and peracetic acid were shown to be prohibitively expensive, but UV and to a lesser extent ozone, remain promising alternatives to chlorine.