Updated Review of Sewage Disinfection Techniques Final Report
Report No DWI0400

Jul 1992

EXECUTIVE SUMMARY

Consultants in Environmental Sciences Ltd. (CES) was commissioned by the Department of the Environment to review the current applicability of four processes used for disinfection of sewage:

• microfiltration;
• ultraviolet radiation (UV);
• ozonation; and
• lime treatment

These are employed to varying extents in the UK water industry for the disinfection of combined and separate sewage. The objectives of the study were to investigate the available data on the processes in respect of efficacy of microorganism inactivation, environmental effects and capital and operating costs.

In terms of micro-organism removal microfiltration can be very effective, achieving good reductions even in effluents of low quality. However, the reliable application of this technology at full scale to low quality effluents is yet to be fully demonstrated, although considerable development work is currently being undertaken. UV also appears to be effective in reducing numbers of indicator organisms, pathogenic bacteria and viruses. Its application is limited to effluents of good quality, ie essentially secondary effluents. However, it should also be noted that, following UV disinfection, reactivation of bacteria may occur in some cases.

Only limited useful data are available to assess the application of ozone despite its widespread use in the USA, where it is normally applied to effluents treated to a greater degree than secondary standard. Ozone appears to give a moderate degree of inactivation of pathogenic bacteria and viruses in secondary effluents.

Clariflow acts as a primary treatment process as well as achieving a high degree of disinfection at high pH. The process seems to be effective in removing pathogenic bacteria, spores and cysts from raw sewage. Data for viral inactivation are scarce, with different reports stating reasonable or poor removal.

With respect to the practical aspects of disinfection systems, microfiltration and ozonation are perhaps the most complex systems of those examined in this study. In this respect large scale microfiltration experience is limited to one plant in Australia, and the process is essentially still under development. The other systems are more established in terms of practicability, with UV being by far the simplest system to install and operate.

The available information on environmental effects of the processes is somewhat limited. Microfiltration and UV are essentially physical treatment processes and are considered unlikely to produce substances of significant detriment to the environment. However, UV has been shown to cause the formation of by-products and mutagenic compounds under certain conditions. Ozone can form small quantities of toxic by-products and may leave a residual of a toxic nature; both may be expected to have some impact on the aquatic environment. The Clariflow process produces an effluent of a high pH value, and large quantities of sludge (approximately 33% more than normal primary treatment, depending on water hardness). All processes use electrical power to various degrees.

Current total system costs (p m3) for disinfection plants serving S,000 and 50,000 population to achieve a target of 4 log reduction are given below. These include the amortised capital costs and operating costs for 365 days per year of operation.

The above costs indicate that UV is economically very attractive for the disinfection of secondary effluents. Although microfiltration appears to be the most expensive option, the manufacturers expect costs to reduce with further development. Also it must be recognised that microfiltration yields several other benefits in terms of general effluent quality, the value of which has not been included in the analyses. Furthermore, the Clariflow costs are for the disinfection of primary treated effluents, and the costs quoted are the marginal additional costs of Clariflow relative to those of primary treatment. Again Clariflow has potential benefits other than disinfection.

The report concludes with a review of novel treatment processes and further research needs.

Copies of the Report are available from FWR, price £35.00 less 20% to FWR Members