Sulphur and phosphorous transformations were studied in two pilot-scale wastewater storage and treatment reservoirs on the basis of four experiments carried out at EXTRABES in northeast Brazil. The reservoirs were fed with the effluent from an anaerobic pond and after filling they were allowed to rest until faecal coliform numbers dropped below 100 cfu/100 ml (10 times below the WHO guideline for unrestricted irrigation). During the filling periods sulphate was reduced to sulphide which reached high values by the beginning of the rest period, decreasing thereafter due to its oxidation, mainly to sulphate. Based on these results a rest period of at least two months is required for sulphide to fall to safe levels from the point of view of biological toxicity. Total phosphorus removal in the reservoirs was low, 9–33%; soluble orthophosphate concentrations remained virtually unchanged, especially when the initial organic loading was high.

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