Critical factors affecting biological phosphorus removal in dairy wastewater treatment plants

Operational data from nine dairy wastewater plants (DWTPs) in Wisconsin were collected and analyzed to determine reasons for poor enhanced biological phosphorus removal (EBPR) efficiency. Several factors affecting EBPR performances in dairy were identified. Since many dairies operate five days a week, DWTPs suffer low F/M ratios during off-days, leading to sludge bulking and unstable EBPR efficiency. The most pronounced factor affecting EBPR performance was uneven organic loading caused by lack of an equalization tank or too a small volume to alleviate the fluctuation in flow and organic loading. The other factor was imbalance of nutrients, especially nitrogen. The other factors include sudden change of pH in a matter of hours and higher temperature (> 30°C) in the summer. The high temperature was thought to shift microbial population and thus lead to the loss of EBPR capability. Unexpected discharge of cleaning solution was another problem causing poor EBPR and COD removal efficiencies. Laboratory-scale tests confirmed the effects of the COD/P ratio (organic loading) and pH on EBPR efficiency. Microscopic examination showed the presence of tetrad-arranged coccoid cells, called G-bacteria in five out of nine DWTPs. Rhodocyclus-related PAOs were also detected from Fluorescent in situ hybridization (FISH) analysis.