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 evide nce that sunlight is the single most important factor in WSP disinfection. Much of the uncertainty in the literature regarding pond disinfection may r eflect the interaction of sunlight with other factors, including dissolved oxygen (DO) and pH, which fluctuate diurnally within WSPs owing to algal me tabolism. Our experiments with WSP effluent (conducted in small, stirred re actors with well-controlled physico-chemical conditions) showed that differ ent faecal indicators are inactivated by different components of the solar spectrum, and the rates of sunlight inactivation have differing dependencie s on physico-chemical conditions. For example, F-specific DNA phage was ina ctivated only by solar UV-B (300-320 nm) at a rate unaffected by other fact ors, whereas enterococci and F-specific RNA phage were inactivated by a wid e range of wavelengths (300-550 nm) by (DO-dependent) photo-oxidation. Sunl ight inactivation of faecal coliforms was particularly complicated: at pHs < 8.5 only solar UV-B (300-320 nm) caused (slow) inactivation, but at highe r pHs, the inactivation rate increased and a wider range of wavelengths (30 0-550 nm) contributed - suggesting photo-oxidative damage to membranes whic h sensitises faecal coliforms to high external pH. Our findings on the diff erent influences of physico-chemical conditions for different indicators su ggest difficulties in interpreting microbiological quality of WSP effluent in terms of a single indicator micro-organism. However, clearly disinfectio n in WSPs may be enhanced by increasing sunlight exposure, for example with shallower ponds or increased residence times.