Advancing Characterization of Municipal Wastewater Treatment Implications to Aquatic Phosphorus and Eutrophication in Receiving Streams

Abstract

Municipal wastewater treatment remains a critical intervention for reducing phosphorus inputs to receiving waters and mitigating nutrient loadings to prevent eutrophication. However, treatment performance is still commonly evaluated using total phosphorus, which provides limited insight into the environmental relevance of discharged phosphorus because it does not distinguish between dissolved, particulate, reactive, and potentially bioavailable forms. This limitation is particularly important for advanced treatment systems that reduce total phosphorus but may also alter the balance and relative distribution of specific phosphorus fractions discharged to receiving waters. Under these conditions, concentration-based assessment alone may obscure whether residual phosphorus is likely to remain inert or become available after discharge. The goal of this research was to advance the characterization and interpretation of phosphorus in municipal wastewater effluents by evaluating how treatment affects phosphorus form, release behaviour, and contributions to receiving waters. Full-scale wastewater effluents were characterized across secondary and tertiary treatment configurations using solids collection, phosphorus fractionation, and controlled desorption experiments. These measurements were used to assess the composition and bioavailability of effluent particulate phosphorus, quantify changes associated with tertiary cloth filtration, evaluate phosphorus release behaviour from effluent solids in relation to established sediment-based concepts, and apply a multi-form phosphorus evaluation approach with hydrologic regime analysis to evaluate conditions under which WWTP discharges may materially influence bioavailable phosphorus concentrations post-discharge. Collectively, the findings demonstrated that wastewater phosphorus relevance depends not only on how total phosphorus is removed, but on how remaining phosphorus is partitioned and transformed after discharge. Effluent particulate phosphorus was dominated by reactive inorganic forms, indicating that residual solids can represent a bioavailable phosphorus pool rather than an inert particulate fraction. Tertiary treatment reduced total and particulate phosphorus and lowered the release potential of effluent solids, but did not eliminate dissolved phosphorus inputs or the capacity of remaining solids to act as phosphorus sources. Desorption behaviour was highly influenced by temperature, while pH effects were most evident outside circumneutral conditions. When these experimentally derived behaviours were evaluated under receiving water conditions, the contribution of bioavailable phosphorus was controlled strongly by dilution, background phosphorus concentration, and effluent flow. This thesis demonstrates that total phosphorus removal alone is insufficient to characterize eutrophication-relevant wastewater performance. A more complete assessment requires explicit consideration of phosphorus speciation, particulate phosphorus bioavailability and post-discharge release, and receiving water context. By linking treatment performance to phosphorus form and flow patterns, this work provides a stronger basis for interpreting wastewater phosphorus controls in relation to downstream bioavailable phosphorus exposure, without assuming that concentration changes alone directly predict ecological response.