The cost-effectiveness of the combined electrocoagulation (EC) and electroflotation (EF) process was investigated on a pilot scale in this project. Four parts of research were involved: the effects of the substrate geometry and operating conditions on the stability of the EF anode, Ti/IrO₂-Sb₂O₅-SnO₂; the development of online water quality monitoring system; the modeling and optimization of the combined process. _{2 2 5 2} ^-1...[ Read more ]

The cost-effectiveness of the combined electrocoagulation (EC) and electroflotation (EF) process was investigated on a pilot scale in this project. Four parts of research were involved: the effects of the substrate geometry and operating conditions on the stability of the EF anode, Ti/IrO₂-Sb₂O₅-SnO₂; the development of online water quality monitoring system; the modeling and optimization of the combined process.

Experimental results showed that anodes with large mean curvature have high electrochemical stability. Although increasing temperature results in a decrease in the stability of Ti/IrO₂-Sb₂O₅-SnO₂, the anode with a mean curvature of 200 m^-1 still shows acceptable service life even at 70 ℃. This tolerance of high temperature was attributed to the thermal expansion difference between the substrate and the coating layer, the redox window for Ir(V)/Ir(IV) conversion, and the redox reversibility of Sb and Sn species in the coating layer.

A water quality monitoring system incorporating a UV/Vis spectrometer and a turbidimeter was employed to monitor the Chemical Oxygen Demand (COD), Total Suspended Solids (TSS) and Oil & Grease (O&G) concentrations of the effluents from the Chinese restaurant on campus and the EC-EF pilot plant. Boosting-Iterative Predictor Weighting-Partial Least Squares (Boosting-IPW-PLS) was developed to construct the COD, TSS and O&G prediction models. The monitoring system was tested satisfactorily in field with high correlation coefficients and small RMSEP values, as 0.945, 0.965, 0.945 and 141, 30.2, 34 mg/L for COD, TSS and O&G measurements, respectively.

The effluent COD, TSS and O&G models of the EC-EF pilot plant obtained through response surface methodology were in the quadratic polynomial forms with 25, 30 and 27 statistically significant terms included, respectively. ANOVA of the experimental data shows the effluent COD, TSS and O&G models were significant with large F-values. Moreover, the standard deviations of the prediction error of effluent COD, TSS and O&G in the confirmation experiments were 73, 23.8 and 17 mg/L, respectively, indicating that the response surface models obtained were appropriate for the operating condition optimization.

Two optimization strategies were proposed and tested on the combined EC-EF process from the aspects of quality control and operating cost control. Experimental results show that the quality-based optimization strategy is effective for stage operation but the treatment performance is not guaranteed in the transient periods. The cost-based optimization strategy was found to be capable of giving high cost saving with environmental compliance in theory and experiments. Up to HKD 1.1/m³ water or 44% of the original trade effluent surcharge (TES) could be saved through the proposed cost-based optimization strategy, showing the cost-effectiveness of the EC-EF technique.