Sulphur and phosphorus transformations in wastewater storage and treatmentreservoirs in northeast Brazil

Sulphur and phosphorous transformations were studied in two pilot-scale was tewater storage and treatment reservoirs on the basis of four experiments c arried out at EXTRABES in northeast Brazil. The reservoirs were fed with th e effluent from an anaerobic pond and after filling they were allowed to re st until faecal coliform numbers dropped below 100 cfu/100 mi (10 times bel ow the WHO guideline for unrestricted irrigation). During the filling perio ds sulphate was reduced to sulphide which reached high values by the beginn ing 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 bio logical toxicity. Total phosphorus removal in the reservoirs was low, 9-33% ; soluble orthophosphate concentrations remained virtually unchanged, espec ially when the initial organic loading was high.