Electrocoagulation in the treatment of industrial waters and wastewaters

Chemical coagulation is commonly used in raw water and wastewater treatment plants for the destabilization of pollutants so that they can be removed in the subsequent separation processes. The most commonly used coagulation chemicals are aluminum and iron metal salts. Electrocoagulation (EC) technology has also been proposed for the treatment of raw waters and wastewaters. With this technology, metal cations are produced on the electrodes via electrolysis, and these cations form various hydroxides in the water depending on the water pH. In addition to this main reaction, several side reactions, such as hydrogen bubble formation and the reduction of metals on cathodes, also take place in the cell. In this research, the applications of EC were investigated in raw water treatment and wastewater applications.The surface water used in this research contained high concentrations of natural organic matter (NOM). The effect of the main parameters-current density, initial pH, electric charge per volume, temperature and electrolysis cell construction-on NOM removal was investigated. In the wastewater treatment studies, the removal of malodorous sulfides and toxic compounds from the wastewaters and debarking effluents were studied. In addition, the main parameters of the treatment, such as initial pH and current density, were investigated. Aluminum electrodes were selected for the raw water treatment, whereas wastewaters and debarking effluent were treated with iron electrodes.According to results of this study, aluminum is more suitable electrode material for EC applications because it produces Al(III) species. Metal ions and hydroxides produced by iron electrodes are less effective in the destabilization of pollutants because iron electrodes produce more soluble and less charged Fe(II) species. However, Fe(II) can be effective in some special applications, such as sulfide removal. The resulting metal concentration is the main parameter affecting destabilization of pollutants. Current density, treatment time, temperature, and electrolysis cell construction affect the dissolution of electrodes and hence also the removal of pollutants. However, it seems that these parameters have minimal significance in the destabilization of the pollutants besides this effect (in the studied range of parameters). Initial pH and final pH have an effect on the dissolution of electrodes, but they also define what aluminum or iron species are formed in the solution and have an effect on the ζ-potential of all charged species in the solution. According to the results of this study, destabilization mechanisms of pollutants by EC and chemical coagulation are similar.Optimum dissolved organic carbon removal and low residual aluminum can be obtained simultaneously with EC, which may be a significant benefit of EC in surface water treatment compared with chemical coagulation. Surface water treatment with EC can produce high quality water, which could be used as potable water or freshwater for industrial applications. In wastewater treatment applications, EC can be used to precipitate malodorous sulfides to prevent their release into air. Technology seems to be able to remove some toxic pollutants from wastewater and could be used as pretreatment before treatment at a biological wastewater treatment plant. However, a thorough economic and ecological comparison of chemical coagulation and EC is recommended, because these methods seem to be similar in pollutant destabilization mechanisms, metal consumption, and removal efficiency in most applications.