Towards a mechanistic understanding of pond disinfection

A broad overview of mechanisms of disinfection of waste stabilization ponds (WSPs) is based on a review of the literature on indicator micro-organisms in ponds, including our own recent experiments. There is appreciable evidence that sunlight is the single most important factor in WSP disinfection. Much of the uncertainty in the literature regarding pond disinfection may reflect the interaction of sunlight with other factors, including dissolved oxygen (DO) and pH, which fluctuate diurnally within WSPs owing to algal metabolism. Our experiments with WSP effluent (conducted in small, stirred reactors with well-controlled physico-chemical conditions) showed that different faecal indicators are inactivated by different components of the solar spectrum, and the rates of sunlight inactivation have differing dependencies on physico-chemical conditions. For example, F-specific DNA phage was inactivated only by solar UV-B (300-320 nm) at a rate unaffected by other factors, whereas enterococci and F-specific RNA phage were inactivated by a wide range of wavelengths (300-550 nm) by (DO-dependent) photo-oxidation. Sunlight inactivation of faecal coliforms was particularly complicated: at pHs < 8.5 only solar UV-B (300-320 nm) caused (slow) inactivation, but at higher pHs, the inactivation rate increased and a wider range of wavelengths (300-550 nm) contributed - suggesting photo-oxidative damage to membranes which sensitises faecal coliforms to high external pH. Our findings on the different influences of physico-chemical conditions for different indicators suggest difficulties in interpreting microbiological quality of WSP effluent in terms of a single indicator micro-organism. However, clearly disinfection in WSPs may be enhanced by increasing sunlight exposure, for example with shallower ponds or increased residence times.