| The algae blooms in natural water bodies is a worldwide issue, which poses a severe challenge to the safety of drinking water supply. Traditionally, pre-treatments by bluestone, ozone, chlorine dioxide or chlorine are usually employed to enhance algae removal in the chemical coagulation unit in conventional water treatment processes. However, some inherent limitations have been revealed for these pre-treatment methods. Electro-coagulation-flotation(ECF) process has been attracting more and more attentions in water and wastewater treatment. When compared with traditional chemical coagulation, the ECF process exhibits several advantages: the coagulant generated in-situ in the ECF is more active, and no additional anions would be introduced in the finished water; there is no need for transportation, storage, and mixing of the coagulant as that in traditional coagulation; in ECF process, the unit process of flocculation, flotation, oxidation and reduction could be accomplished in one single reactor, leading to the simpler set-up, higher integrity, and reduced footprint.In this research, the removal effectiveness of ECF process for algae, natural organic matter, nitrogen and phosphorus was investigated, due to the fact that the concentrations of these pollutants in eutrophic water usually exceed the drink water standard. The influence of process conditions and water quality parameters, such as current, electrolysis time, the electrode gap, initial p H, temperature, the concentrations of pollutants and ions on the pollutants removal efficiencies were firstly examined by orthogonal experiments. Then, the main process conditions/water quality parameters were systematically analyzed in terms of the pollutants removal efficiencies, the energy consumption and the variety of solution p H during ECF process to achieve high pollutants removal and low energy consumption simultaneously. Among which, the current and electrolysis time are most important factors. When the algae density was 1.0~1.5×109 cells/L, the optimum current was determined as 60 m A, and the electrolysis time required for complete algae removal was 45 min, with the energy consumption of 0.3k Wh/m3. When 15mg/L humic substances were treated, optimum current and electrolysis time were found to be 60 m A and 30 min, respectively. The DOC and UV254 in the treated water were lowered to 0.4mg/L and 0.061, with the energy consumption as low as 0.19 k Wh/m3. As for 10mg/L of phosphorus, 82.2% of phosphorus removal was achieved under the current of 60 m A and electrolysis time of 60 min, with the energy consumption of 0.18 k Wh/m3. For the removal of nitrate, the current of 90 m A was shown to be proper for reducing the generation of nitrite.Furthermore, the fundamental mechanisms of ECF for the removal of various pollutants in eutrophic water were studied and discussed from several aspects. Firstly, the release of aluminum ions, aluminum species and the zeta potential of aluminum flocs in the ECF process were discussed to illuminate the flocculation removal mechanism. Secondly, the electro-oxidation involed in the ECF and its effect on pollutants were discussed. Besides, the electric field action and electro-reduction effect may also contribute to the removal of pollutants in the eutrophic water.During the ECF process, it was demonstrated that different mechanisms were involved in the removal of different types of pollutants. The algae cells were effectively removed in the ECF process, mainly through flocculation and flotation. Besides, synergetic oxidation effect of the electrochemically generated active chlorine(about 0.3mg/L) and the electric field could inactivate the algae cells efficiently, which also enhanced the algae removal. However, the main mechanisms for natural organic matter and phosphorus removal were shown to be flocculation. On the other hand, the removal of nitrate and nitrite in the ECF was mainly through the electro-reduction taking place at the cathode, which converting the nitrogen species to N2 eventually.The effectiveness of ECF and conventional coagulation-sedimentation for the treatment of eutrophic water was systematically compared through adjusting the Al dosages and solution p H. Results showed that under the same Al dosages, much better algae removal was achieved by the ECF process. When the initial p H is ≤ 8, better removal of natural organic matter was also achieved by the ECF process as compared with the conventional coagulation-sedimentation. Furthermore, as compared with the significant p H decrease during conventional coagulation-sedimentation treatment, only a little p H variation(from 7.4 to 8.4) was observed for the ECF; and there is no need to adjust the p H in the treated water.According to the research results, it could be concluded that the ECF process is effective for the removal of algae cells, natural organic matter, nitrogen and phosphorus in the micropolluted water containing algae. When considering the advantages of ECF process over traditional chemical coagulation, such as excellent water treatment efficiency, simple set-up configuration, and high mobility, it is promising for this novel process to be employed in medium/small scale decentralized water treatment facilities, or in the emergency treatment facilities for the eutrophic water, to improve the safety and reliability of water supply. |
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