Total removal of coliforms and E. coli from domestic sewage by high-rate pond mass culture of Scenedesmus obliquus

Abstract

The single celled alga Scenedesmus obliquus was cultured in biologically treated domestic sewage in small experimental ponds (4·5 m 2) with a depth of 25 cm. Attention was focused on the removal of total coliforms and E. coli—indicators of water pollution by human faecal contamination—from sewage effluent using a mass culture of microalgae ( Scenedesmus obliquus). Batch, continuous and intermittent flow systems were studied. Results of batch culture experiments showed total removal of E. coli within 4 days. In the control ponds, in which Scenedesmus obliquus was not inoculated, and natural growth allowed to take place, E. coli was detected even on the tenth day. The results obtained confirmed reports in the literature which concluded that high pH values (above 9·4) were responsible for E. coli removal; pH values were always higher than 9·4 on the third day, reaching a maximum of 11·1 around the fifth day in batch culture systems. Semi-continuous flow culture systems were operated under three dilution rates: 0·5 day −1, 0·33 day −1 and 0·25 day −1. Results indicated that total removal of E. coli was not achieved even at a dilution rate of 0·25 day −1, even though pH values above 10·0 were observed. Under intermittently fed systems, two dilution rates, 0·5 day −1 and 0·25 day −1, with respective feeding intervals of 1 and 2 days, were studied. Total removal of E. coli was observed at a dilution rate of 0·25 day −1. These results have proved that a contact time of 2 days at pH 11·0 is adequate for total removal of E. coli from high-rate algal ponds.