Tanneries relocation effect on heavy metals content in sewage sludge in Egypt: a focus on chromium contamination.

Heavy metal contamination in sewage sludge (SS) threatens safe agricultural reuse. This study assesses the impact of relocating tanneries from Magra El-Oyoun to Robbiki Leather City (RLC) on heavy metals especially chromium (Cr) in sludge from Cairo's El-Gabal El-Asfar Wastewater Treatment Plant (GAWWTP). Before July 2020, Cr-rich tannery effluent caused elevated sludge Cr (median: 800 mg kg⁻1; max: 1750 mg kg⁻1), exceeding safe limits and posing environmental/agricultural risks. Post-relocation, Cr plummeted (median: 77 mg kg⁻1; max: 140 mg kg⁻1)-a > 90% reduction-bringing concentrations within safe limits. Other metals showed variable trends: Hg increased (median: 0.05 to 1.10 mg kg⁻1) while Arsenic (As) exhibited statistically significant temporal trends (p < 0.05). No significant temporal trends were observed for nickel (Ni), cadmium (Cd), zinc (Zn), copper (Cu), and lead (Pb), with p values exceeding 0.05 and low explanatory power (R2 < 0.16). All monitored metals (Hg, Ni, Cd, Zn, Cu, Pb, As) remained below permissible limits. While Cr posed historical risks that have now been resolved, the other metals-despite minor fluctuations-present no current threat. This confirms the success of industrial intervention and underscores the need for ongoing monitoring to ensure safe SS reuse.