Comparison on treatment strategy for chemical cleaning wastewater: Pollutants removal, process design and techno-economic analysis

Chemical cleaning wastewater (CCW) usually consists of pickling wastewater (PW) and alkaline cleaning wastewater (ACW), and the strategy of separate treatment or combined treatment affects pollutant removal efficiency and cost. In this study, separate and combined treatment of real PW and ACW generated from an on-site cleaning campaign were investigated. A neutralization - fluoride removal - coagulation - oxidation process was constructed for PW and mixed wastewater (MW) treatment, and operational conditions of each process were optimized. The optimal mixing ratio of PW and ACW in the primary neutralization tank was 3:7, which obtained a near neutral pH, efficient chromaticity and turbidity removal and good settling performance. The neutralized MW and PW were both adjusted pH to 9.5 to precipitate metal ions as hydroxides. After fluoride precipitated as CaF₂, the fluoride removal rates of MW and PW were both 99.9%, respectively, and polyaluminum chloride was dosed to improve the settleability of CaF₂. Then sodium hypochlorite oxidization was employed to remove NH₃-N and soluble COD. Techno-economic analysis based on pilot-scale tests showed that separate treatment of PW and ACW obtained better effluent quality than combined treatment. The total cost of combined treatment (37.44 $/m³) was greatly higher than that of separate treatment of PW and ACW (18.20 $/m³). This study proposed a cost-effective strategy for CCW treatment, and suggested that neutralization with acidic or alkaline wastewater should be systematically considered for technical and economic feasibility.