Although the effluent quality met the regulation standards, the discharge pipeline of the semiconductor wastewater treatment plants in this study was partially blocked. In order to manage the pipeline blockage, the relationship between the blockage and the coagulation-flocculation process was investigated. Additionally, the automatic control of the coagulant dosing was proposed. The results of SEM/EDX analysis, jar tests, and intensive sampling indicated that the blockage was primarily formed by organically bound-Al as a result of overdosing polyaluminum chloride (PACl) and anionic polymer. In this case, the possible deposition mechanism was mainly post-aggregation of residual Alb-OM. Adjusting PACl, pH, and polymer at the optimum dosage would produce the least deposits. PACl was more effective at removing turbidity than ferric chloride, but it had a similar performance in removing NPDOC. Adding anionic polymer as a flocculant enhanced turbidity removal but did not affect Cu and Ni removal. Moreover, it also improved the sludge compactness. In contrast, changes in pH (8 to 9.5) did not significantly affect the removal efficiency. The Cu and Ni were greatly removed by adsorption on silica particles and the formation of metal hydroxides. At low initial turbidity, high soluble Cu allowed better turbidity removal but resulted in greater sludge production. Accordingly, the optimum polymer dosage and pH were determined to be 1 mg/L and 8.5, respectively. In addition, jar test and intensive sampling result suggested that PACl dosage depended on initial turbidity (at high initial turbidity) and initial soluble Cu (at low initial turbidity). Based on these results, the automatic control scenario for optimal PACl dosing was established. The models showed a satisfactory agreement with the experimental data with adjusted R2 > 0.75.

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