Disinfection of Wastewater Effluent: A Review of Current Techniques
This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.
Report no. UWRAA 101
Disinfection is the process of reduction of pathogens of concern to humans, animals, or plants to acceptable levels of risks of transmission of disease. This report critically examines the processes available for disinfection of pathogen-containing wastewater and comparing their effectiveness, practicality, reliability, effects on the environment and human health and the costs involved. It also details the circumstances under which disinfection should be practised. The disinfection methods generally considered for use in Australia consist of chemical methods (chlorine, chlorine dioxide and ozone), physical methods (UV irradiation and membrane microfiltration), and biological methods(ponds).
When determining disinfection needs for wastewater treatment plant effluent, it is the pathogens present in the effluent that need to be inactivated. Care should be taken in using bacterial indicators such as E. coli or thermotolerant coliforms to design and control disinfection processes as these indicator organisms are known to be more susceptible to some disinfection processes than are pathogens. The effectiveness of disinfection methods should not relate to removal of indicator organisms alone. Research has shown that there are large variations in pathogen removal through processes in wastewater treatment plants. Literature values should not be used to assess pathogen removal. Site specific data should always be obtained.
To optimise disinfection, it is advisable to reduce suspended solids levels to the most practicably achievable levels for the system, to reduce organic compounds entering the wastewater treatment plant as these are difficult to remove in secondary treatment plants without advanced treatment and to reduce inorganic compounds that interfere with the disinfection method.
In relation to specific disinfection methods a number of conclusions have been reached. Briefly the effectiveness of chlorine for inactivation of viruses, helminths and protozoa is lower than for bacteria and depends to a large extent on having the appropriate conditions, viz. optimum pH, adequate chlorine contact time and low levels of ammonia and suspended solids. Ultraviolet irradiation which is gaining popularity in Australia is effective for disinfection of bacteria and viruses, but has yet to be fully assessed for inactivation of protozoa and helminths.
Also problems exist when treating effluent with suspended solids greater than 20mg/L. Reliability of UV equipment was raised as a major concern by authorities considering UV irradiation as a disinfection method. Although very effective, chlorine dioxide has only been sparingly used in Australia because of its complexity which requires constant supervision and high operating costs. The use of ozone for disinfection of wastewater has not been practised in Australia to date. Membrane microfiltration is gaining popularity in Australia although it is relatively complex, requiring a high degree of maintenance and system control to provide continuous disinfection. Ponds have been traditionally used in Australia and remain the disinfection method of choice by some authorities, however they have not been purposely designed for disinfection.