Abstract
The literature indicates a considerable degree of ambiguity and contradiction in the efficiency of various wastewater disinfectants. This largely reflects on the absence of standardization in these studies and the use of only a single disinfctant and virus rather than multiple studies. It thus seemed appropriate to carry out a comparative study on several commonly used wastewater disinfectants against representative enteric viruses. The chosen disinfectants were chlorine, chlorine dioxide, ozone and peracetic acid because they represent those most likely to be useful in wastewater disinfection. The selected viruses were bacteriophage f[2] and poliovirus 1 which have been commonly used, also echovirus 1 and Coxsackievirus B5 to extend the range of enteroviruses and finally simian rotavirus (SA11) and human rotovirus, the latter, being one of the most important enteric viral pathogens present in wastewater. The effect of various parameters which might influence the efficiency of the disinfectants such as pH, temperature, suspended solids and peptone concentration as well as the effects of combining disinfectants were investigated. The results obtined from this study indicated that lower levels of disinfectants were required than most of those levels already reported in the literature which was probably due to the quality of effluent used and emphasized the need to standardize this before disinfection studies. Chlorine dioxide, although more expensive than chlorine, may be a useful alternative to chlorine because its virucidal efficacy was only slightly affected by changes in pH, temperature and presence of organic matter. Furthermore it does not react with organic matter to form some classes of chlorinated organic compounds considered hazardous to public health and it was easy to obtain reliable assay of its residual concentration. Ozone may also be a promising alternative to chlorine because its efficacy is not greatly affected over the pH range 6-10 which is normally that encountered in effluent and temperature (2-30°C). Furthermore, it does not leave a toxic residual and it adds dissolved oxygen to water. However, its disadvantages are the high capital cost for the establishment of the plant, difficulties in its reliable assay in effluent, it requires good quality effluent and it does not maintain a stable residual. Peracetic acid is another possible alternative disinfectant. It does not form toxic by-products, effluent does not exert a demand for it which leaves it available for disinfection. However, the high concentrations required reduced the pH of the effluent and increased the BOD5 of the treated product. It was found that the combination of disinfectants did not produce a synergistic effect but only a complementary, additive or combined effect. It was interesting to note that the different viruses reacted differently to each disinfectant which explains the argument against the use of model or indicator virus studies. In this regard, it was particularly worth noting that the human rotavirus isolate was the most resistant virus to disinfectants although surprisingly simian rotavirus was the least. Therefore, for the proper evaluation of a wastewater disinfectant it would be advisable to test it against as many representative enteric viruses as practicable.