Abstract
1. This thesis is concerned with raising the safety of sewage treatment plant effluents, and particularly those from waste stabilisation pond systems in developing countries where there is a growing necessity to use wastewater in agriculture and aquaculture. 2. Waste stabilisation ponds systems have grown considerably in popularity in the last 25 years, both in developed and developing countries. Their popularity owes much to the fact that they are seen as an appropriate technology which is straightforward to construct, requiring little skill to operate and maintain, and potentially offering efficient treatment, including high pathogen removal efficiencies. 3. Although sold as a simple technology, the mechanisms which control the efficiency of waste stabilisation pond (WSP) treatment are exceedingly complex and rather little is known about them. As for the pathogens, a review of their removal and die-off revealed that, although work on the mechanisms indicates what the basic factors are, their relative importance remains to be determined and, with the exception of residence time, they cannot be controlled in the pond environment. All that can be safely said at present is that their efficient removal and die-off is primarily dependent on sufficient time and temperature, and die-off is enhanced by a high pH. 4. Unfortunately it seems that, in promoting this technology, some of the fundamental rules which have been applied in other areas of hydraulic engineering have been ignored or, at least overlooked, in the case of WSPs. The most widely used WSP design equations for pathogen removal were derived from first order bacterial die-off in a completely mixed reactor. The assumption that completely mixed reactor faecal coliform bacteria die-off kinetics may be used to reliably predict pathogen removal and therefore design WSP systems is shown to be wrong on several counts in this thesis. 5. A review of excreted human viruses is presented which focuses on those which occur in wastewater and what knowledge exists about their non-host survival. In spite of the fact that quantitative techniques for the recovery of animal viruses from aquatic environments are still in their infancy, and therefore subject to substantial errors, enough progress has been made to state without any doubt that they cannot be reliably indexed by thermotolerant (faecal) coliform bacteria. They generally survive better than faecal coliforms, move more efficiently through the WSP systems with only modest reduction in numbers, and therefore design equations based on faecal coliform die-off kinetics are inadequate for viruses as a group. 6. Some of the fundamental differences between a range of common entero-pathogenic, enteric bacteria and viruses were investigated under controlled laboratory conditions. The influence of elevated temperatures and a range of pH, was used to determine inactivation times and death rates. Additional investigations were undertaken to define the hydrophobicity and hydrophilicity of the bacteria and viruses. It was demonstrated that all the viruses were much more hydrophilic and that this, together with better survival, may largely explain why viruses pass more efficiently through sewage treatment plants. 7. The technical difficulty and cost of quantitative recovery of viruses from environmental samples means that, as with bacterial pathogens, it would be highly desirable to have a reliable indicator of virus pollution and treatment efficiency. Although the viruses used in the laboratory studies were bacterial viruses (bacteriophages) it had been demonstrated in other studies that the survival characteristics of animal pathogenic viruses and bacteriophages in water and wastewater are generally much more similar to each other than either group is to the bacteria. It was argued that the bacteriophages might therefore be a good model for evaluating treatment process efficiency for virus removal. It was further argued that if they also survive well in sewage then they could be used as hydraulic tracers to investigate the hydraulic residence time at each stage of treatment. 8. It was logical to establish a collection of phages likely to be rare in the wastewater environment and evaluate their ability to survive in the different types of sewage which they would encounter in waste stabilisation ponds. This validation study confirmed the fact that they could survive much longer than the faecal indicators, and for sufficient time to make their use as process efficiency indicators and tracers feasible. 9. The bacteriophages chosen had the additional advantages that, a) they could be grown to extremely high titres (1013/ml) making them far more sensitive than all other tracers used in hydraulic studies, and b) they could be recovered and cultured using the most basic laboratory facilities. They could even be grown without a laboratory incubator in tropical climates, which was important (given the basic facilities available) in some of the developing country study sites. Five distinct phages were eventually used in tracer studies overseas, of which only one had ever been used before. 10. The overseas developing country work was based on the hypothesis that many WSP systems produce a poor quality effluent and do not meet the WHO faecal coliform guidelines for reuse because they are inadequately designed and/or constructed. To test this hypothesis it was necessary to develop a diagnostic evaluation methodology. This involved establishing criteria for the selection of WSP systems for study, carrying out a pre-diagnostic to obtain all available information concerning the selected systems, and the application of intensive field evaluation methods in which performance deficiencies and their causes were identified. 11. Central to the hypothesis was the belief that under-performance, if identified, would be caused principally by hydraulic short-circuiting associated with inadequate design criteria and/or adverse meteorological conditions. Consequently the selection of WSPs included a variety of designs of varying complexity in a range of climatic conditions. As the project progressed it became clear that wind conditions were very important and therefore sites in Colombia were chosen where wind was typically slight. Work continuing beyond this thesis include sites in Mexico where wind is more intense and variable.