Wastewater treatment and influent characteristics shape phosphorus fractions and metal composition in sewage sludge fertilizers: a full-scale multi-facility monitoring study

Recovering phosphorus (P) from sewage sludge is increasingly important, but treatment configuration and influent characteristics shape the quality of the recovered product, including its P solubility and metal loading. We examined how treatment processes and sewer type govern the composition and temporal variability of sludge-derived fertilizers at three full-scale facilities differing in biological treatment, anaerobic digestion, and coagulant dosing. Fertilizers were sampled approximately weekly for seven months and analyzed for total, inorganic, citrate-soluble, and water-soluble P and 21 metals. Redundancy analysis related elemental composition to organic matter content, precipitation, and seasonality, while regression analysis related P solubility to metal-to-P ratios and metal-balance indices. Total P differed among facilities but varied little within facilities. By contrast, metals varied substantially, particularly at the combined-sewer facility, possibly reflecting stormwater-driven urban runoff. Anaerobic digestion was associated with metal enrichment. Single metal-to-P ratios mainly reflected inter-facility differences, whereas the combined use of log[(Fe + Al)/(Ca + Mg)] and log(Ca/Mg) retained explanatory power for citrate- and water-soluble P after inter-facility differences were removed, showing that relative metal balance better captures P solubility than single-metal abundance. Together with repeated monitoring, these dual metal-balance indices offer a practical framework for assessing sludge-derived fertilizer quality.