Integrated risk assessment of industrial wastewater in a Bangladeshi export processing zone

Industrial wastewater from export processing zones may comply with regulatory standards while concealing ecological risks not captured by routine monitoring. This study aimed to (1) evaluate effluent quality and regulatory compliance in the Chattogram Export Processing Zone (CEPZ), Bangladesh; (2) develop an integrated risk assessment framework integrating the Effluent Quality Index (EQI), ecotoxicity, metal speciation, and geographic information system (GIS) analysis; and (3) identify high-risk discharge corridors. Treated effluents (n = 6) and drinking water were analyzed for physicochemical parameters, Pb and Cr, acute ecotoxicity using Daphnia magna (EC₅₀), metal speciation (Visual MINTEQ), and spatial distribution against ECR 2023 and WHO guidelines. Although pH and metal concentrations complied with regulatory limits, 5-day biochemical oxygen demand (95-180 mg L^-1) exceeded the ECR limit (30 mg L^-1) by up to 600%, while chemical oxygen demand (240-326 mg L^-1) exceeded the 200 mg L^-1 limit by 63%. The washing-finishing sector exhibited the highest pollution burden (EQI = 3.10) and toxicity (EC₅₀ = 18.5%). Metal speciation showed that 45-88% of regulated heavy metals occurred as bioavailable organic complexes. EQI correlated with EC₅₀ (R² = 0.84, p < 0.01); GIS identified a 2-km high-risk corridor, potentially exposing 50,000-75,000 residents.