Pilot-Scale Electrocoagulation

Project Overview

Canadian remote communities are most often those who are affected by poor water quality and boil water advisories. A major issue is the applicability of traditional water treatment technologies to unconventional applications (small-scale and inaccessible communities). Their inaccessibility presents difficulties for supplying needed chemicals involved in traditional treatment processes such as coagulations and flocculation. One potential technology that has the means of addressing the aforementioned challenges facing small water systems is electrocoagulation (EC). EC is an alternative technology to traditional chemical coagulation, and may be suitable to replace it in certain circumstances, such as for small-scale operations. Its appeal as a potential small system process is due to the elimination of the chemical supply chain required for traditional chemical coagulation, its seemingly easy operation and maintenance, as well as purported low operating costs. EC is an electrochemical process producing coagulant chemicals on-site and on-demand, may be an alternative technology to traditional coagulation suitable for small and remote communities.

The basis of EC involves the in-situ production of coagulant species, in the form of metal hydroxides, through the electrochemical process of slowly dissolving a metal electrode into water [see figure below].

Electrocoagulation process: (a) half-cell reactions, (b) coagulant and floc formation

One or more electrochemical cells are comprised of sacrificial anodes (typically aluminum and/or iron) and inert cathodes (stainless steel) placed in solution – the water to be treated. By use of an external DC power supply, current is passed through the electrochemical cell initiating the slow dissolution of metal cations (Mez+) into the solution at the anode-solution interface. On the cathode side of cell, the electrolysis process reduces water molecules to hydroxide anions (OH–) and hydrogen gas (H2). These two electrochemical half-cell reactions occur simultaneously. As the process continues, metal cations and hydroxide anions interact in solution and combine to synthesize various metal oxides and metal hydroxides, collectively referred to as metal hydr(oxides). It is these metal hydr(oxides) that that function as the coagulants in an EC process.

Summary of Results

The efficacy of iron EC for the removal of HA was first investigated at bench- scale low and medium flow rates of 1.35 and 5 LPM. Four variables were monitored: flocculation time, metal loading, current density and inter-electrode gap. The following outline the key research outcomes from the in-lab, bench-scale EC experiments:

  • At both flow rates, flocculation time was found to have no effect on DOC or UV- abs-254 reductions.
  • Clear trends in both DOC and UV-abs-254 were unable to be determined with respect to inter-electrode gap and current density. A number of unmonitored factors could be responsible for the results yielded, such as effect of DO, pH and iron speciation effects.
  • Metal loading was found to have the greatest impact on HA removal, with respect to all other variables tested. A clear trend of increased reductions in both DOC and UV-abs-254 were yielded with increased ML. Increases in UV-abs-254 were observed initially at lower MLs, which was confirmed to be due to the presence of high dissolved residual iron concentrations after filtration using AA- spectroscopy.
  • High chloride and humic concentrations were found to function as ligands increasing the solubility of iron, thereby providing the mechanism contributing to the high concentrations of dissolved iron present after EC at lower MLs.
  • The EC process has a low energy footprint, whereby the conditions providing the great reductions in DOC and UV-abs-254 at the highest ML also tended to be the least energy intensive (low current density and inter-electrode gap).

After the bench-scale investigation, EC was assessed at a pilot-scale with experiments being undertaken at 10 LPM on raw surface water in the community of VAID. Although results did not reach the maximum DOC and UV-abs-254 reductions achieved at lab-scale, consistent reductions in both parameters were achieved for all conditions tested. The following outline the key research outcomes achieved during pilot-scale experiments:

  • Reduction differences at the greatest two MLs tested were negligible, with an average DOC and UV-abs-254 reduction of 37.2±4.2% and 54.7±0.9%, respectively.
  • SUVA reductions as low as 1.71 L/m!mg were achieved from an initial SUVA of 2.20 L/m!mg, reaching previously published minimum values suggested as being feasible using EC
  • The maximum reductions in DOC and UV-abs-254 were achieved at the highest ML, utilizing a 4-cell configuration and 1 mm inter-electrode gap (DOC/DOC0=42.9±6.6% and UV-abs-254/UV-abs-2540=55.8±0.8%).
  • Through HPSEC analysis, it was found that EC has preferential removal of larger MW fractions of NOM, a phenomenon that has been documented for both CC and EC in prior research.
  • The conditions which achieved the greatest reductions in DOC and UV-abs-254 (4-cell, 1 mm gap) were also the least energy intensive. At these conditions, operations require 0.621 kWh of energy per cubic meter of water treated. Statistically similar results can be achieved at a lower ML, for a significantly lower energy consumption rate of 0.480 kWh/m3.

Current density distribution over the electrode surfaces was determined for 1, 2 and 10 mm inter-electrode gap conditions using the partial electrode method. This in- situ technique of current density determination was utilized in order to investigate the reactor and configurations utilized during bench- and pilot-scale experiments. CFD models were generated to evaluate the water velocity patterns for the various inter- electrode gaps and flow rates. The following outline the key research outcomes achieved from utilizing the partial electrode method for current distribution determination:

  • 35 and 5 LPM flow rate experiments were in good agreement for both horizontal and vertical segments.
  • At both flow rates, increased current uniformity was observed with an increase of the inter-electrode gap, at the expense of reduced overall current densities.
  • CFD models showed fluid flow uniformity also increased with an increasing inter- electrode gap.
  • In general, regions of the electrode that were occupied by higher velocity or more uniform flow yielded lower current densities. In contrast, regions of the electrode that were occupied by low flow velocity or non-uniformity, which tended to yield higher current densities.
  • Although current distribution was maximized at an inter-electrode gap of 10 mm, the reduction in current density greatly increased the energy requirements of the process.
  • Future design iterations for the EC reactor should more carefully consider the fluid dynamics of the system in order to maximize both current densities and current distribution. In practice, areas of highly undistributed flow and consequently highly variable current during continuous EC operations could repeatedly exhaust certain areas of an electrode. This would create a greater operating cost, due to more frequent electrode replacement.

Publications

  • McBeath, S.T., Hajimalayeri, A., Jasim, S., Mohseni, M. (2021) Coupled electrocoagulation and oxidative media filtration for the removal of manganese and arsenic from a raw ground water supply. Journal of Water Process Engineering 40, 101983.
  • McBeath, S.T., Nouri-Khorasani, A., Mohseni, M., Wilkinson D.P. (2020) In-situ determination of current density distribution and fluid modeling of an electrocoagulation process and its effects on NOM removal for drinking water treatment. Water Research 171, 115404.
  • McBeath, S.T., Wilkinson D.P., Graham, N.J.D. (2019) Application of boron doped diamond for the anodic oxidation of pesticide micropollutants in a water treatment process: A critical review. Environmental Science: Water Research & Technology 5, 2090-2107.
  • McBeath, S.T., Mohseni, M., Wilkinson D.P. (2018) Pilot-scale iron electrocoagulation treatment for natural organic matter removal. Environmental Technology.
  • McBeath, S.T., Dubrawski, K., Mohseni, M., Wilkinson, D.P., (2015). Pilot-Scale Electrocoagulation for Natural Organic Matter Removal. Vatten – Journal of Water Management and Research 71, 231-238.