Abstract
1. This thesis describes the establishment of a comprehensive monitoring programme instituted and operated by the author on the sewage treatment system on Grand Cayman, Cayman Islands on behalf of the Water Authority of the Cayman Islands. 2. The monitoring programme was established in 1988 and has run for more than seven years. It includes the essential chemical, physical and biological parameters required to monitor wastewater lagoon performance. 3. The programme was established with the practical aim of ensuring that the final effluent met microbiological and chemical guidelines which would permit the reuse of effluent for irrigation. 4. The study revealed that high saline groundwater intrusion into the sewers is such that the effluent cannot be reused at present. Repairs to the sewerage system reduced the salinity and flow in the final two years of the study but the salinity is still above the level which would permit healthy plant growth. Throughout much of the life of the system the groundwater intrusion also caused the hydraulic loading on the treatment plant to be at or above the maximum design flow. 5. The sewage treatment system is dominated by an unbalanced sulphur cycle resulting in high levels of hydrogen sulphide being produced largely from seawater-derived sulphate. 6. Since saltwater and salinization are increasingly important considerations in water and wastewater treatment, and have been shown to profoundly influence aerobic pond performance, more detailed insight into the mechanisms was required. These mechanisms are reviewed and investigated. 7. The diversity of phytoplankton and zooplankton normally associated with aerobic maturation ponds was greatly reduced by hydrogen sulphide generated in the facultative ponds. This aggravated the hydraulic overload and resulted in poor performance with respect to faecal coliform removal in the maturation ponds. 8. For more than 4 years the final effluent quality was not adequate for reuse because the anticipated faecal coliform removal of 99.99% was not achieved, and thus was inadequate for reuse (>1000 faecal coliforms cfu/100 ml). Consequently, a fundamental study of lagoon hydraulic retention time was undertaken to validate and assess whether the design specifications and operating conditions are appropriate for the faecal coliform reductions required. 9. Serratia marcescens bacteriophage survival in vitro in raw sewage and effluent from the facultative and maturation pond was studied to investigate and compare survival characteristics with faecal coliform bacteria and with viruses and also to assess the prospects for using phage as a tracer of dispersion and retention time. 10. Phage death was shown to be affected primarily by high pH with rapid death above pH 9 associated with algal photosynthesis. Survival times in other conditions were adequate to permit Serratia bacteriophage to be used as a full-scale, hydraulic tracer. 11. Primary facultative pond 1.1 was dosed, in vivo, with a Serratia marcescens bacteriophage suspension. In-pond analysis of dispersion was conducted in addition to measurements of the phage in the effluents of each pond in the treatment system. The results were used to determine the dispersion characteristics of flow in the ponds and pond outlets samples were used to calculate the mean retention time. 12. Short-circuiting was identified experimentally and a numerical model using physical data of the facultative pond and climatic data, was subsequently applied. The retention time resulting from the application of this model and that shown from the tracer study were more closely correlated than either of them with the design retention time. The factors of wind direction and windspeed were identified as having a potentially major, negative influence on the hydraulic regime of the ponds when the wind varied from the east to north. 13. A series of engineering design change recommendations is presented which are intended to improve pond performance by improving the hydraulics of the system. 14. A comparison of the design performance in terms of pathogen indicator removals and biochemical reduction was carried out using data obtained from the routine monitoring programme and specialised intensive studies. There was good agreement between the level of short-circuiting defined experimentally and the actual removal of faecal indicators in the facultative ponds. This confirmed that retention time is the most important determinand controlling indicator reduction.