Water Velocity Distribution and Its Impact on the Performance of an Electrocoagulation Reactor for Drinking Water Treatment
Authors: Nouri-Khorasani Amin , Mcbeath Sean T , Mohseni Madjid , Wilkinson David P. 
Published: February 2020

Iron electrocoagulation (EC) is a promising alternative to more expensive chemical coagulation for remote communities that frequently face boil water advisories[1]. In EC, the iron anode dissolves in water producing iron hydroxide and other iron-based species that facilitate the removal of natural organic matter (NOM), as per the reactions below:

Anode:
Fe(s)→Fez+(aq)+ze-

Cathode:
2H2O(l)→H2(g)+2OH-(aq)

Solution:
Fez+(aq)+zOH-(aq)+xNOM(aq)→Fe(OH)z(NOM)x(s)

The current density distribution in an EC cell is heterogeneous due to the local variation of [Fez+] and its effect on the local anodic equilibrium potential, according to the Nernst equation. Therefore, the reactor hydrodynamics affect current density distribution and the resulting performance of the EC cell.

This research focused on the theoretical and experimental analyses of the velocity distribution within a pilot scale EC reactor. The steady-state water velocity distribution was simulated using COMSOL Multiphysics. The flow turbulence was simulated using the k-ε model. The reactor walls and the open interface with air were modeled as no-slip and slip boundaries, respectively. Water entry and outlet were modeled as constant flow and constant gauge pressure boundary conditions, respectively. Model validation was performed using partial electrode assembly design, similar to the work of Stumper et al [2]. Segments of the EC reactor were masked by an adhesive, insulating Kapton sheet. Masking of the electrode limited the reaction to the uncovered portions, while not interfering with the hydrodynamics of the reactor.[3]

The modeling results show the water velocity distribution in the cell, which leads to removal of [Fez+] from the electrode/electrolyte interface and shifts the electrochemical equilibrium potential. This shift leads to a variation in the distribution of iron dissolution in the reactor. The results also indicate that the flow homogeneity increases with an increase in the inter-electrode distance, as well as with a decrease in water flow rate. Partial electrode assembly experiments verified the simulation results for a range of inter-electrode distances and water flow rates. Figure 1 shows a typical water velocity distribution, current mapping in the reactor, and comparability of the simulation and experimental results.

The validated model shows the heterogeneity in flow distribution and can predict the useful lifetime of an iron electrode before its substitution or break off due to accelerated corrosion in specific areas. Such predictions enable optimum capacity, design, and operation for the reactor. The results of this project can lead to improved electrocoagulation of water for NOM removal, which can benefit communities that rely on surface water sources.

References:

S. Vasudevan and M. A. Oturan, Environ. Chem. Lett. 12, 97 (2014).

J. Stumper, S. A. Campbell, D. P. Wilkinson, M. C. Johnson, and M. Davis, Electrochim. Acta 43, 3773 (1998).

S. T. Mcbeath, Pilot-Scale Iron Electrocoagulation for Natural Organic Matter Removal, University of British Columbia, 2017.

Figure 1