Thermal Modification of Water Treatment Residuals for Enhanced Phosphorus Removal: A Systematic Review and Meta-Analysis

Phosphorus pollution from municipal and agricultural sources drives freshwater eutrophication globally, yet conventional removal technologies permanently immobilize phosphorus in metal laden sludge, precluding nutrient recovery. Water treatment residuals (WTRs) aluminum and iron rich byproducts of coagulation flocculation processes represent abundant, low-cost precursors for phosphorus adsorbent development. This systematic review and meta-analysis quantitatively evaluated the effectiveness of WTR modification techniques, with particular focus on thermal modification, for enhanced phosphorus removal from wastewater. Following the PRISMA 2020 guidelines, systematic searches of Scopus, Science Direct, Engineering Village, and ProQuest (2000-2024) identified 32 studies meeting predefined eligibility criteria, of which 10 reporting sufficient statistical parameters were included in quantitative meta-analysis. Standardized mean differences (SMD) with 95% confidence intervals were calculated using random effects REML models, with subgroup analyses stratified by modification method, treatment temperature, material composition, and physical form. Thermal modification exceeding 300°C emerged as the most effective enhancement strategy, yielding a statistically significant subgroup effect size of SMD = 1.24 [95% CI: 0.62, 1.87] (p<0.001; I² = 1.95%). The overall pooled effect was non-significant (SMD = 0.32, p = 0.64; I² = 83.40%), attributable to methodologically heterogeneous pooling including air drying (SMD = −2.25, p<0.001) rather than absence of genuine thermal enhancement. Powdered WTRs outperformed granular forms (SMD = 0.97, p = 0.01). Maximum adsorption capacity reached 37.8 mg P/g, following pseudo second order kinetics (R² = 0.9923) and Langmuir isotherm behavior (R² = 0.9889). Operational cost was confirmed at 0.74 USD/kg P removed, which was substantially lower than conventional adsorbents (1.60-20.26 USD/kg P) with environmental reductions in carbon footprint (87%), energy consumption (81%), and water usage (82%). Thermal modification above 300°C establishes thermally activated WTRs as technically viable, cost effective, and environmentally sustainable materials for wastewater phosphorus management, with future research priorities including field scale validation, regeneration optimization, and circular nutrient recovery.