Removal of pathogenic organisms from the effluent of an upflow anaerobic digester using waste stabilization ponds

A series of four pilot-scale, shallow waste stabilization ponds (WSPs), comprising one facultative followed by three maturation ponds with a total design retention time of 20 days, was monitored to observe its ability to remove pathogenic organisms from the effluent of an upflow anaerobic sludge blanket (UASB) digester. The UASB reactor received strong domestic wastewater from the shanty district of a city in north-east Brazil. The raw wastewater had a very high concentration of intestinal nematode eggs of which, on average, 89.6 percent were removed in the UASB reactor. No intestinal nematode eggs were recovered in the effluent of the first maturation pond, making it suitable for restricted irrigation. The removal of eggs in the first pond exceeded predictions made using a recently published model. Faecal coliforms (FC) were reduced by 4.7 log units on average in the pond series -- the final effluent being suitable for unrestricted irrigation. pHs exceeding 10 were attained in the final maturation pond at the sunniest time of day. There was a significant correlation between levels of pH and FC in the ponds, the latter being ≤ 1000 per 100 ml when the former was ≥ 9.1. The removal of FC in the ponds was linear over the range of pH encountered. The findings are consistent with recent work by others suggesting that FC removal in ponds is multi-factorial. The UASB reactor, with a retention time of 7 h, is an efficient primary treatment alternative to an anaerobic pond in a WSP series receiving an extremely strong domestic wastewater. There are potential advantages of using the former in preference to the latter in a series of ponds.