Performance of a facultative waste stabilization pond treating dairy shed wastewater

A facultative waste stabilization pond treating anaerobically pre-treated dairy shed (milking parlor) wastewater was studied in order to investigate the oxygen demand transformation ability of the pond, the oxygen demand distribution between total and soluble components and to assess nutrient removal performance. Total BOD5 levels entering the pond averaged 159 g/m(3) and effluent soluble BOD5 levels averaged 18 g/m(3), representing an 89% removal of the total incoming BOD5. On a soluble BOD5 only basis the removal through the pond averaged 81%. These data represent a reasonably good level of oxygen demand transformation performance for a facultative pond system. The proportions of soluble BOD5 to total BOD5 averaged 0.61 for influent to the pond and 0.25 for effluent from the pond. Total COD concentrations entering and leaving the pond were relatively high, averaging 884 g/m(3) and 618 g/m(3) respectively, yielding COD:BOD5 ratios of 5.6:1 and 8.5:1. Suspended solids were associated with 41% and 75% of the total effluent COD and BOD5, respectively. Ratios of insoluble BOD5:SS averaged 0.20 and 0.27 for influent and effluent, respectively, whilst biodegradable fractions were estimated at 0.21 g-BSS/g-SS and 0.28 g-BSS/g-SS. Nitrogen removals were relatively low, at 25% for total kjeldhal nitrogen, giving an average effluent concentration of 129 g/m(3), while for ammonia the figures were 30% and 100 g/m(3), respectively. Nitrogenous oxygen demand was not typically exerted in the pond. A low degree of total and dissolved reactive phosphorus removal occurred, with effluent concentrations averaging 23.9 g/m(3) and 12.7 g/m(3), respectively. At an estimated applied total organic loading rate of 30 kg-BODu/ha-d the pond was well underloaded in relation to both conventional design practice and maximum theoretical algal oxygen production. Tertiary treatments able to remove suspended solids offer the potential for substantial reduction in effluent BOD concentrations. In order to meet proposed water quality criteria in New Zealand, effluent dilutions of up to 1850:1 were indicated from this study, with dissolved inorganic nitrogen (DIN) and dissolved reactive phosphorus (DRP) identified as the key parameters. Future research into the reduction of dissolved inorganic nitrogen and dissolved reactive phosphorus in dairy shed wastewaters is recommended.