<p>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 &gt; 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 (<i>p</i> &lt; 0.05). No significant temporal trends were observed for nickel (Ni), cadmium (Cd), zinc (Zn), copper (Cu), and lead (Pb), with <i>p</i> values exceeding 0.05 and low explanatory power (<i>R</i>² &lt; 0.16). All monitored&#xa0;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.</p> Graphical Abstract <p></p>