Razi University

   Abstract Automotive industry wastewater is considered a complex and challenging environmental concern due to the simultaneous presence of organic pollutants (COD), nitrogenous compounds (nitrate and nitrite), phosphate, and total suspended solids (TSS). In this study, the performance of a continuous, integrated electrocoagulation (EC) system for the simultaneous removal of these pollutants from both real and synthetic automotive wastewater was evaluated. The primary objective was to design and optimize a continuous electrocoagulation reactor with various electrode configurations and to determine the optimal operational conditions to achieve maximum removal efficiencies alongside minimal sludge generation.For this purpose, raw wastewater was sampled and characterized from four process streams: hot/cold rinse, phosphating, degreasing 1, and degreasing 2. Due to accounting for an 80% volume fraction and facing accessibility constraints, the hot/cold rinse stream was prepared synthetically. A continuous plexiglass reactor with an effective working volume of 5880 mL was designed, equipped with 12 cylindrical rod electrodes of iron and aluminum with an inter-electrode spacing of 1 cm. Three electrode configurations—Al–Al, Fe–Fe, and hybrid Fe–Al (6+6 combination)—were assessed under a continuous-flow regime. The experiments were designed using Response Surface Methodology (RSM) coupled with a Central Composite Design (CCD), comprising 20 experimental runs for each configuration. The independent variables included initial pH (5.5–8.5), current intensity (1.0–2.5 A), and hydraulic retention time (10–25 min), while the evaluated response variables were the removal percentages of COD, phosphate, nitrate, nitrite, turbidity, and settled sludge volume. Statistical analysis was performed using Analysis of Variance (ANOVA) and quadratic regression model fitting.The results demonstrated that initial pH was the predominant and governing parameter controlling the process, exhibiting a significant, non-linear impact on pollutant removal efficiencies and sludge generation. In the Fe–Fe configuration, favorable COD and turbidity removals were obtained; however, sludge volume increased drastically under alkaline conditions. The Al–Al configuration showed superior performance in removing turbidity and phosphate, though its COD removal efficiency remained limited. The hybrid Fe–Al configuration, leveraging the synergistic interaction between iron and aluminum ions, established a well-balanced trade-off between simultaneous multi-pollutant removal and sludge volume mitigation.The optimum operating window for the Fe–Al configuration was achieved at an initial pH of 7.0–7.9, a current intensity of 1.5–2.19 A, and a hydraulic retention time of 19.3 min. Under these optimal conditions, removal efficiencies reached 87.5% for turbidity, 40.56% for COD, 49.12% for nitrate, and 39.25% for nitrite, with a settled sludge volume of approximately 580 mL. Additionally, phosphate removal reached up to ~80% across select optimal regions. For comparison, the optimal operating conditions were identified at pH = 6.7–7.5, current = 1.0–2.1 A, and HRT = 13.6 min for the Fe–Fe system, and pH = 5.8–7.8, current = 1.5–2.0 A, and HRT = 17.5 min for the Al–Al system.Overall, the continuous EC system utilizing the hybrid Fe–Al configuration is proposed as an efficient, robust, and scalable alternative for automotive wastewater treatment owing to its effective simultaneous removal of CNP and TSS, reduced sludge production with enhanced dewaterability, minimized chemical consumption for pH adjustment, and operational stability under continuous-flow regimes. These findings provide a solid engineering foundation for the industrial scale-up of continuous electrocoagulation reactors treating complex automotive effluents.