Electrocoagulation for cost-effective wastewater treatment

Electrocoagulation (EC) is a wastewater treatment technology capable of removing suspended solids, dissolved organic matter and nutrients, faecal indicator bacteria as well as heavy metals, oils and other organic contaminants. EC has been most widely used for the treatment of industrial wastewater, including textile, oil, paper, and dye wastewaters.

EC generates coagulants in situ by electrolytic oxidation of metal anodes. Iron or aluminium plates are commonly used for the anodes, releasing iron (Fe2+) and aluminium (Al3+) ions into the wastewater which hydrolyse to polymeric hydroxides. Polymeric hydroxides are excellent coagulants for the removal of various wastewater pollutants. Coagulation involves charge neutralization of negatively charged contaminants followed by the formation of flocs that either settle or float. Therefore, a subsequent solids removal stage (e.g., clarifier or Dissolved Air Flotation) is required.

EC efficiency can be improved by optimizing operational parameters including: electrode spacing, electrode orientation, periodic electrode polarity reversal, current density (A/m2) and contact time. Particularly, the removal efficiency of TSS and particulate BOD (including algae which are negatively charged) by EC is mainly dependent on the amount of Fe2+ or Al3+ ions generated from the anode. Therefore, greater removal can normally be achieved at higher current density. Phosphate ions (PO43-) are neutralized by the polymeric metal hydroxides which also directly bind to suspended P contaminants. These then aggregate and settle with the flocculated solids. All nitrogen compounds can be removed to some degree by EC. For example, organic nitrogen is removed with the flocs of TSS. EC can promote inactivation of microorganisms including faecal coliforms and viruses by rupturing their membranes and then coagulating them into settleable flocs. Furthermore, EC can remove heavy metals as metal hydroxides and other organic compounds including pesticides and halogenated hydrocarbons.

Many laboratory-scale EC trials have been conducted to determine optimum design and operation parameters for efficient wastewater treatment. However, currently there is no information on full-scale application of EC technology available in peer-reviewed scientific literature. This study tested a laboratory-scale EC unit for the treatment of wastewater pond effluent. Pond water samples (~20 L) were collected from an oxidation pond on three occasions and each sample was used on the same day for the laboratory experiments. The effect of different EC currents (between 0.4A and 3A) on the water quality of the wastewater pond effluent was investigated in terms of the removal of organic matter (TSS and BOD5), nutrients (nitrogen and phosphorus), and faecal coliforms. Physico-chemical parameters including temperature, pH, dissolved oxygen (D.O.), conductivity, turbidity and %UV transmittance (UVT) were also measured before and after the EC treatment.

This study showed that the laboratory-scale EC unit typically achieved >90% removal of TSS, BOD5 and TP, >95% removal of DRP, 50-80% removal of TKN, and 2-3 log removal of E. coli at a EC current of 0.8-1.6A. Full-scale EC unit power consumption would be ~0.4 kWh/m3 wastewater which would cost NZ$0.12/m3 wastewater (based on the current average power cost of NZ$0.30/kWh). This research indicates that EC is an efficient and potentially cost-effective option for treating wastewater pond effluent since the EC can provide a combined removal of organic matter, phosphorus and disinfection (replacing chemical flocculation/coagulation and UV treatment) and produce a readily dewaterable sludge.