Continuous Multi-cell Electrochemical Reactor For Pollutants Degradation

Industry wastewater contains various kinds of toxic and refractory organic pollutants. With the rapid social development, the quantity of the discharged industry wastewater increases year by year, while the discharge standards become more and more strict. The traditional methods for wastewater treatment can hardly meet the requirement. Therefore, techniques that can efficiently degrade toxic and refractory pollutants are imperative.In recent years, electrochemical oxidation has attracted great attention due to its high treatment efficiency, fast reaction rate, and ease of operation. Large amount of researches have shown that this technique has good application prospects. Up to now, most works have focused on electrode property improving and pollutant degradation efficacy examining, but very few efforts have been paid to the electrochemical reactor investigation. In this thesis, a novel continuous multi-cell electrochemical reactor was proposed, and its application in wastewater treatment was systematically studied. The reactor consists of ten cells. Each cell could be easily detached from the reactor, and thus the number of cells could be determined depend on the situation. Each cell contains two PbO2/Ti mesh anodes and three stainless steel cathodes. The inter electrode gap was as short as 2 mm to reduce the cell voltage and improve the mass transfer. When the reactor is in operation, each cell could be assumed to be a continuous stirred tank reactor (CSTR) due to the agitation of bubbles. Ten CSTRs in series could achieve equivalent function of a plug flow reactor. Major research works include four parts:hydraulic characterization of the reactor, performance of the reactor in treating simulated phenol wastewater, performance of the reactor in treating actual textile wastewater, and mathematical models for pollutants degradation.Experimental results showed that the multi-cell reactor is quite close to a plug flow reactor, and the effect of current density (j) and hydraulic retention time (HRT) on the flow pattern was not significant. At j=100 A/m2 and HRT=30 min, the dispersion number was 0.09, the Morrill index was 2.55 and the percentage of dead volume was about 5.2%.The multi-cell reactor performed well in treating simulated phenol wastewater, and performed much better than the conventional single-cell reactor. The effluent COD of the multi-cell reactor was between a quarter and half of that of the single-cell reactor, while the current efficiency was 6-9 times higher than that of the single-cell reactor. Compared with the conventional batch reactors using PbO2/Ti electrode, the reaction time could be saved by about 40%-80% by using the multi-cell reactor.The multi-cell reactor has good performance in treating the biologically treated textile effluent. At the best operational condition of j=120 A/m2 and HRT=30 min, the COD of the wastewater could be reduced from 238-249 mg/L to 43-54 mg/L; effluent pH was in the range of 6.37-6.48, and effluent Pb content was below 0.1 mg/L. The current efficiency and energy consumption were 42.5-43.6% and 16.2-16.6 kWh/m3, respectively.Plug flow model with axial dispersion and tanks-in-series model were built, and phenol, acetic acid, maleic acid, and methyl orange were used to test the accuracy of the models. The plug flow model with axial dispersion could well fit the COD variation along cells. The deviation of the calculations from the experimental data increased slightly with the cell number, the current density and the pollutant concentration; among four kinds of pollutants, the deviation of calculations for acetic acid were about 6-20 mg/L; the deviation of calculations for methyl orange were about 13-38 mg/L; the deviation of calculations for the other two kinds of pollutants were in between this range. The tanks-in-series model could well fit the effluent COD of the reactor, and the calculations of this model were nearly the same as that of the plug flow model with axial dispersion.