In situ studies on waste stabilization ponds (in Portugal) showed that faecal coliform numbers were lowest at positions in the water column where pH, temperature, dissolved oxygen and algae were high. Numbers were not always lowest at the surface where light intensities were highest or in ponds where light penetration had increased through algal grazing by Daphnia. Laboratory studies showed that pH values approaching 9.0 or above increased faecal coliform die-off particularly under nutrient-poor conditions. Elevated temperatures enhanced the pH effect but the level of dissolved oxygen made little difference. Streptococcus, Salmonella and Campylobacter isolates all behaved similarly to the faecal coliforms. Daphnia grazing of the algal population in maturation ponds may reduce the microbial quality of the final effluent. These findings are discussed in relation to pond design.

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