Let’s clean up toxic water by electricity

SciComm. Group Blog
Jul 25, 2020
3 min read

Author: Sonia R. (IG: @chemiphillic_randhawa)

SCG ID: 2507202016

Wastewater is a significant issue for our environment, as well as for many industries. Electrochemical oxidation processes are revolutionary in the field of wastewater treatment as they can clean up complex wastewater containing toxic chemical pollutants. This process results in 99% removal of chemical oxygen demand and 96% discolouration of the wastewater. It involves the treatment of wastewater with electricity using specialised electrodes. It uses two electrodes to break down non-biodegradable pollutants that remain after biological treatment. Electric current is passed through the electrodes, oxidants are produced during the treatment in situ either directly at the surface of the electrode or indirectly from chemical compounds in the treated water, generating hydroxide radicals (*OH), which attack the refractory molecules. The primary advantage of these methods is that it does not require any chemicals to be added to the water so, it could also be called “green technology” methods.

Unit energy consumption is ranged from 0.4 to 1.2 kWh per cubic meter of wastewater treated, and from 3.7 to 7.2 kWh/kg BOD removed. In small treatment plants, there is little ability to adjust energy use in response to varying influent flow rate. Hence, unit energy consumption is inversely related to plant inflow.

Mechanism

Electro-oxidation may occur either by direct oxidation by hydroxyl radicals produced on anode´s surface or by an indirect process where oxidants like chlorine, hypochlorous acid and hypochlorite or hydrogen peroxide/ozone are formed at electrodes by following reactions:

2Cl- → Cl2 + 2e-

Cl2 + H2O → HOCl + H+ + Cl-

HOCl → H+ + OCl-

H2O → *OH + H+ + e-

2*OH → H2O2

H2O2 → O2 + 2H+ + 2e-

O2 + *O→ O3

Indirect oxidation occurs when active species are generated anodically to destroy pollutants. In this, metal ions are oxidized on anode from a stable state to a reactive high valence state which in turn attack pollutants directly and may also produce hydroxyl free radicals to promote degradation. In direct electro-oxidation (anodic oxidation) oxidation of pollutants in the electrolytic cell occurs at the electrode surface or by direct electron transfer to the anode. Also, powerful oxidants called reactive oxygen species (ROS) can be formed from water discharge at the anode. Anodic oxidation has some benefits over indirect oxidation as there is no need to add chemicals to the treated solution or producing less secondary pollution.

The advanced electro-oxidation processes are good for breaking down drug wastes left behind, after biological treatment. Researchers tested the technology on three types of water coming from different treatment processes: conventional, membrane bioreactor, and a treatment process that separates wastewater, including faeces, at the source. They focused on a particular molecule, Paracetamol also known as acetaminophen (one of the world's most widely used drugs).

However, these techniques are not capable of removing all harmful compounds from wastewater. Therefore, novel methods should be developed for using together with these techniques to improve the purification results.

References:

University of Sydney. (2020, July 7). Engineers use electricity to clean up toxic water: Powerful electrochemical process destroys water contaminants, such as pesticides. ScienceDaily. Retrieved July 16, 2020 from www.sciencedaily.com/releases/2020/07/200707113206.htm
Young, D. F., & Koopman, B. (1991). Electricity use in small wastewater treatment plants. Journal of environmental engineering, 117(3), 300-307.
Institut national de la recherche scientifique - INRS. (2020, July 9). Using electricity to break down pollutants left over after wastewater treatment: Advanced electro-oxidation process. ScienceDaily. Retrieved July 16, 2020 from www.sciencedaily.com/releases/2020/07/200709085301.htm
Särkkä, H., Bhatnagar, A., & Sillanpää, M. (2015). Recent developments of electro-oxidation in water treatment—a review. Journal of Electroanalytical Chemistry, 754, 46-56.