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 water resource recovery facility (WRRF) 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. This 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 facilities 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 operational 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 tertiary treatment, and membrane separation processes.
The results of this study suggest that the effluent total reactive phosphorus (TRP) concentration has, of the operational characterizations that the team assessed, the strongest statistical association with the total effluent BAP concentration (r2 = 0.81) with an average total BAP to TRP ratio of 0.61 ± 0.24. The results of this work should encourage water quality modelers and total maximum daily load (TMDL) permit writers to consider the importance of BAP when assessing the likely ecological impacts of effluent discharges from municipal nutrient removal facilities. These results also 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%). The research team also characterized the bioavailability of a variety of well-defined P containing compounds. These results clearly showed the operationally defined P classification scheme from classic chemical methods is problematic. Algal phosphorus uptake experiments also suggest that P species with high bioavailability, including some organic P species, are unlikely to persist in natural surface waters because their uptake kinetics are very rapid. Finally, these results further suggest recalcitrant P compounds, such as humic-metal-P complexes, phytic acid, and/or apatite may be the dominant components of the recalcitrant dissolved P pool in effluents identified in this and other studies.
This title belongs to WERF Research Report Series
ISBN: 9781780405551 (eBook)
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