The Bioavailable Phosphorus (BAP) Fraction in Effluent from Advanced Secondary and Tertiary Treatment

Due to the widespread severity of eutrophication in surface waters, there is a strong impetus to require ultra-low effluent phosphorus (P) concentrations (i.e., <100 µg L-1) in many municipal wastewater treatment plant discharges. Chemical addition, with filtration or membrane separation, is commonly used to meet these low targets and therefore most of the effluent P from tertiary P removal facility is in the soluble phase. Our study examined the bioavailability of phosphorus (BAP) in the effluents of advanced phosphorus removal treatment systems using algal bioassay experiments. Effluent BAP was determined for 17 full-scale wastewater treatment plants representing a wide range of phosphorus removal technologies, including enhanced biological phosphorus removal and chemical coagulant addition in secondary and tertiary treatment processes. The phosphorus in the effluent samples was operationally characterized as particulate or dissolved, and reactive or nonreactive P using filtration and chemical characterization. A standard bioassay was used to determine the BAP of both total and soluble fractions. The P fractions were then statistically compared to the BAP concentrations. The nutrient removal technologies tested included alum and ferric based chemical P removal, enhanced biological P removal (EBPR), single and two-stage filtration, and membrane separation processes.

The results indicate that the bioavailability and P species composition varies with the nutrient removal process and that in most cases a large portion (>50%) of the effluent P was recalcitrant to algal growth. Comparisons between different technologies indicate higher chemical doses, which also achieved lower effluent P concentrations, decreased the fraction of the phosphorus that was bioavailable (BAP%). Our results also suggest that the effluent total reactive phosphorus (TRP) concentration has, of the operational characterization we assessed, the strongest statistical associated (r2 = 0.81) with the total effluent BAP concentration with the average BAP to TRP ratio of 0.61 ± 0.24. The results of this work should encourage water quality modelers and TMDL permit writers to consider the importance of BAP when assessing the likely ecological impacts of municipal nutrient removal facility effluent discharges.