The Effect of pH on Enterococci Removal in Water Lettuce, Duckweed and Algal Ponds

A batch scale experiment was conducted to determine the effect of pH on enterococci removal rate. The batch experiment was conducted using pH 4, 5, 7, 9, and 11 treatments under light and dark conditions. Enterococci concentrations and DO were measured every day and every other day respectively for nine days. A bench scale continuous system was also constructed to determine the environmental conditions and its effects on enterococci removal. The system comprised of water lettuce, duckweed and algal ponds operating in parallel with each system having a series of four ponds and a total retention period of 28days after two days of anaerobic pre-treatment. After two months of operation, temperature, pH, DO, TDS, and enterococci populations were monitored. Enterococci die-off rates at pH 4, 5, 7, 9 and 11 in the light/dark were (expressed as kd -1 ); 2.1/2.1, 2.1/1.5, 2.1/1.5, 2.1/1.4 and 1.1/1.0, respectively. DO levels in the batch incubations were low, between 0.17 mg/L at pH 4 (light) to 0.56mg/L at pH 7 (light). Low pH of 4.4 was obtained in the water lettuce pond system. Neutral conditions were observed in duckweed pond system. pH values > 9 were observed in the algal pond system. Enterococci decreased from 7.8x10 6 to < 500/100mL in all pond systems after 28 days of treatment with no significant differences between treatments.