The effect of pH on enterococci removal in Pistia-, duckweed- and algae-based stabilization ponds for domestic wastewater treatment

A batch scale experiment was conducted to determine the effect of pH on enterococci die-off. A continuous flow system was also established to determine the environmental conditions and their effects on enterococci removal. The batch experiment was conducted for pHs: 4, 5, 7, 9, and 11 under light and dark conditions. Enterococci and DO were measured every day and every other day respectively for nine days. Pathogen removal rates at pH 4, 5, 7, 9 and 11 in the light/dark were (expressed as d^-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 were low, between 0.17 mg/L at pH 4 (light) to 0.56 mg/L at pH 7 (light). The continuous flow system consisted of Pistia (water lettuce), duckweed and algal treatments in series of four ponds with a total retention period of 28 days after two days of anaerobic pre-treatment. After two months of operation, temperature, pH, DO, TDS, and enterococci populations were monitored. A low pH of 4.4 was obtained in the Pistia ponds. Neutral conditions were observed in the duckweed system, while pH values >9 were observed in the algal system. Enterococci decreased from 7.8×10⁶ to <500/100 ml in all treatment systems after 28 days of treatment with no significant differences between treatments.