High rate algal ponds (HRAPs) are an adaptation of waste stabilisation ponds, in which increased treatment efficiency and reduced pond area are achieved by optimising conditions for algal photosynthetic oxygen production. If the treated effluent is subsequently to be reused for irrigation of crops and recreational areas, from a public health viewpoint it is important to reduce the risk of contamination by pathogenic organisms. The microbiological quality of effluents is traditionally assessed by enumerating the coliform group of organisms. Die-off rate constants (Kb) of 0.35 − 2.34 d−1 were measured for E.coli in outdoor ponds operating on synthetic sewage mix with acetate as the carbon source (260 kg COD ha−1 d−1) at a range of depths from 12 to 34 cm. There were significant positive effects of pH, irradiance and pond depth on Kb. Light penetration throughout the pond depth was calculated and shown to be a useful parameter for the prediction of Kb. Multiple regression equations describing the influence of these environmental parameters on die-off rate constant were derived. Dry matter concentration negatively affected light penetration but had a positive influence upon pH. Pond operating conditions to maximise E.coli die-off are discussed.

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