Report No DWI0486

Review of Operational & Experimental Techniques for the Removal of Bacteria, Viruses & Pathogens from Sewage Effluents


Sept 1988


Pressure for compliance with the EEC Bathing Water Quality Directive has prompted investigation into methods of disinfection for marine discharge of wastewater. Disinfection may be an alternative to discharge through long sea outfalls or may provide a temporary solution while outfalls are constructed. Chlorination is the established disinfectant for both water (eg. UK) and wastewater (eg. US), but increasing concern over its environmental impact has led to development of alternatives including UV; ozone; lime; peracetic acid; chlorine dioxide; bromine chloride and gamma irradiation.

The Department of the Environment commissioned CES to undertake a comparative evaluation of disinfection techniques. The objectives of the study were to review both current operational techniques and experimental techniques for the inactivation of bacteria, viruses and pathogens in sewage effluents, with particular reference to marine discharges. The inactivation efficiency of the various techniques was to be compared and the actual or potential capital and operating costs evaluated. In addition, promising techniques for future application and aspects of disinfection requiring further research were to be identified.

The report covers firstly the properties of individual operational or experimental disinfectants, reviewing their production and application; chemistry; mechanism of disinfection; inactivation efficiencies; factors affecting efficiency; and environmental impact. Operational and cost data from pilot and full scale trials, both reported in the literature and provided by UK Water Authorities are presented. Subsequent sections of the report comprise a cross comparison of disinfectant alternatives in terms of their inactivation efficiency, environmental impact and cost; and a brief summary of Water Authority attitudes to disinfection. Factors affecting the choice of disinfectant have been discussed and areas for future research identified.

  1. Operational disinfection systems
  2. Experimental disinfection systems
  3. Possible disinfection techniques

    Embryonic techniques considered included bromine; heat; sensitized photo-oxidation; quaternary ammonium compounds; activated carbon adsorption; protein precipitants; filtration; and ultrasound. Although several have been evaluated at pilot scale for the disinfection of wastewater, technical limitations (filtration, ultrafiltration) or cost considerations (activated carbon, heat) mitigate against their use at full scale. None were considered to provide a viable technique which could be developed for application within the next 5 years, although ultrasonication could be a useful pretreatment method for ozonation and UV irradiation.
  4. Comparisons of alternative disinfectants

    Comparative summaries of both operational and experimental disinfection systems are given in Tables 1 and 2.
  5. Water Authority attitudes to disinfection

    A summary of disinfection trials carried out by Water Authorities is given in Table 4. The general concensus among Water Authorities would appear to be to opt for long sea outfalls wherever possible. Even with adequate screening and disinfection, the public acceptability of swimming in sewage polluted waters is questionable. Although there is a great deal of interest in the potential for disinfection, there is a general reluctance for its adoption on a long-term basis because of the lack of proven efficacy and the high operating costs. The awareness of environmental impact observed by water authorities, both in-house and increasingly amongst consumers, also tends to mitigate against disinfection. It is however being seriously considered, as for example by South West Water, as a short term measure while outfalls are constructed.
  6. Conclusions

    There are several processes available which can disinfect sewage to achieve coliform levels permitting discharge through comparatively short outfalls. Estimates of UK costs show prices ranging from approximately 1.6 p/m3 to 19 p/m3 for year-round disinfection or 2.0 p/m3 to 27 p/m3 for six monthly operation. These costs would reduce slightly for very large outfall schemes, and in some cases such as Clariflow, are associated with additional benefits such as increased BOD, 55 and heavy metal removal. In this respect (although not included in the present remit), it would be of interest to compare the costs of disinfection options with those for a long sea outfall. The figures presented should also be viewed in the context of the costs levied by Water Authorities for full biological treatment for subsequent discharge to an inland watercourse, typically approaching 25 p/m3. In relation to this, those processes at the lower end of the cost range indicated may be attractive if they can be shown to be environmentally acceptable.

    A major drawback with disinfectants is the formation of hazardous by-products. It is difficult to interpret existing information in the context of the marine environment rather than in relation to human consumption, since much of the toxicological data for chlorinated by-products relates to exposure via drinking water. The processes with the least by-products are either very costly or not presently effective for raw sewage. The only option without associated by-products is a long sea outfall. Until further evidence proves otherwise, it is considered that disinfection by chemical means should only be used on a short term basis, while-an outfall is being constructed. It is interesting to note the apparent recovery of marine organisms from the effects of chlorinated discharge in the Welsh Water study; this suggests that seasonal disinfection may be less environmentally detrimental than year-round treatment. Nevertheless, the possible establishment of reservoirs of pathogens in sediments during the winter months would have to be reviewed at individual locations.

    Where standards are imposed rigorously and beaches have to remain open for five years or so until an outfall scheme (or land-based treatment) comes on-stream, a choice of disinfectant may well have to be made. If economic considerations prevail, chlorine compounds (specifically hypochlorite) are attractive. Seasonal application and dechlorination to preclude residual toxicity could be used to reduce environmental impact. However, experience at Weston-Super-Mare, where dosages at the upper end of the chlorine range in Table 3 are applied and compliance is still not always achieved, has to be borne in mind. Although about three times more expensive than gaseous chlorine, the lower degree of by-product formation associated with chlorine dioxide suggests that it merits further investigation as a wastewater disinfectant.

    If economic aspects are not overriding and minimal environmmental impact is considered important, the Clariflow process could have application. However, construction time, land acquisition, sludge disposal and an overall cost approaching that of land-based treatment would have to be considered. Ozone is a potentially attractive option in economic comparison to Clariflow, but its efficacy on crude sewage is not well documented. The intermediate option in terms of both cost and likely impact is PAA, however it is not possible to assess the latter factor adequately on the basis of current data.
    An examination of embryonic technologies for sewage disinfection identified no systems which showed sufficient development potential for use within the next five years. Aspects identified for further research include the following:

Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.