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Wastewater treatment plant (WWTP) discharges are a primary anthropogenic source of particulate and dissolved phosphorus that can contribute to nutrient spiralling and eutrophication, threatening downstream aquatic ecosystem health and drinking water treatability. Because fine solids are the primary vector for phosphorus transport in aquatic systems, this study departs from typical analyses of total phosphorus in WWTP discharges by providing new insights regarding the relative bioavailability of particulate phosphorus (PP) forms within them. A continuous-flow centrifuge (CFC)-based method was adapted for suspended solids collection from secondary (2°) and tertiary (3°) effluents at two WWTPs. These solids were then analyzed using chemical sequential extraction to quantify key PP forms to inform bioavailability. Non-apatite inorganic phosphorus (NAIP)-the most readily bioavailable phosphorus form-was the predominant PP fraction (typically >90 %) composing the wastewater effluent solids. It was predominantly bound to metal oxides. Benchmarking indicated that wastewater effluent solids can contain 10 to 100 times more NAIP than other commonly reported sources of PP, such as aquatic sediments from landscapes with varying levels of disturbance. Their impacts on receiving water quality are case-specific, depending on the relative magnitude and timing of NAIP loading from these and other sources. Although the relative composition of PP did not substantially differ between effluents from 2° and 3° treatment, NAIP concentrations in effluents from 3° treatment by cloth filtration were approximately 50 % lower than those in effluents from 2° treatment. These first-of-their-kind (1) application of a CFC to collect WW solids and (2) characterization PP forms and their bioavailability in WWTP effluents provide essential insight for understanding and modeling effluent-streamwater P dynamics and support targeted watershed management during sensitive environmental conditions (e.g., low flows) when receiving waters are most vulnerable.
Vianini et al. (Fri,) studied this question.