Faecal bacterial dynamics during flood events were studied in the Topehaehae Stream near Morrinsville, New Zealand, in a catchment used for grazing dairy and beef cattle. During the rising limb of a natural flood event, E. coli bacterial concentration rose by more than 2 orders of magnitude and peaked at 41,000 cfu/100 mL. E. coli correlated closely with turbidity over the flood event, and both variables peaked close to the time of maximum flow acceleration rather than peak flow. An artificial flood on the same stream, created by releasing water from a supply reservoir during fine weather with no wash-in from the catchment, produced a broadly similar pattern of faecal contamination (peak E. coli = 12,500 cfu/100 mL). This and other evidence suggests that direct deposition of faecal matter by cattle in the stream channel may be of similar or greater importance than wash-in from land. The flood experiments have been useful for constructing a model of faecal bacterial yields, and they imply that exclusion of livestock from stream channels may appreciably improve water quality.
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.
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