By Gordon Balch
Historically, conventional wastewater treatment technologies were designed to remove solids, kill pathogens, and rely on microorganisms to biologically oxidize simple organic and inorganic compounds.
The complexity of wastewaters is increasing as more chemicals are being used in domestic products and industry. Treatment challenges can arise from the presence of nonconventional contaminants that are often difficult to remove, and with high strength wastewaters (e.g., ethylene glycol) that may cause operational upset to conventional treatment processes.
Many of the nonconventional contaminants now found in wastewaters originate from a variety of domestic products (e.g., personal care products, pharmaceuticals, household cleaners), chemical wastes from manufacturing processes (organic solvents) or commercial services (e.g., airplane deicing fluids). A number of these compounds are challenging to treat biologically because of their cytotoxic properties or their unreactive chemical structures.
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There is a growing awareness that new treatment processes are needed, particularly those involving technologies that can treat compounds on-site at the source of generation, rather than sending them to centralized municipal treatment facilities. There, at best, the treatment may be ineffective, and, at worst, it may cause operational problems for the biological processes of the wastewater facility.
Electrochemical advanced oxidative processes (EAOPs) are one class of alternative wastewater treatment technologies showing great promise. This class of treatment processes functions by sending a strong electrical current between two electrodes submerged in the wastewater.
This is done to evoke an oxidation reaction that is based on the use of electricity, rather than biological organisms. The electro-oxidation process stimulates the loss of electrons (oxidation) in some compounds and the gain of electrons (reduction) in others, which can lead to breaking molecular bonds and the degradation of the unwanted compound.
Typically, the oxidation of compounds takes place on the surface of the anode (negative electrode). In some cases, this process may also generate highly reactive oxygen species, such as the hydroxyl radical that causes the indirect oxidation of compounds within the wastewater. The magnitude of oxidative reactions taking place by either direct oxidation on the surface of the anode, or via indirect processes involving the hydroxyl radical, is dependent on both the composition of the anode and the chemicals being treated.
There are a variety of characteristics inherent to electro-oxidation processes that make them particularly attractive when dealing with difficult compounds. In a recent review article, Garcia-Segura et al. (2018) said advantages include:
- Operation is conducted under ambient temperature and pressure, negating the need for special temperatures or pressures;
- The compact nature of the electro-chemical cell means that the physical footprint is often smaller than many other technologies;
- No auxiliary chemicals are needed, which eliminates transportation and storage costs;
- No secondary waste streams that require additional treatment are produced;
- The technology can be easily combined with conventional treatment systems;
- Operation can be fully automatized;
- Capital and operational costs are cost competitive with other technologies.
Ethylene glycol removal by electro-oxidation technology
Ethylene glycol is one example of an industrial compound often generated in large volumes. The annual worldwide production of ethylene glycol in 2004 was estimated to be greater than 18 kilotons (Hosseinpour et al. 2016). It is used in many applications, including antifreeze, airplane deicing fluids and solvents.
High strength ethylene glycol wastewaters are one class of compounds that are difficult to treat using conventional wastewater treatment methods. The high biochemical oxygen demand of ethylene glycol means that most conventional plants do not have the capacity to treat it aerobically. As such, anaerobic degradation is often required, which can take weeks to accomplish.
Xogen Technologies Inc. has developed an electro-oxidation cell designed for the treatment of challenging wastewater constituents. A benchtop version of the electro-oxidation cell has been investigated at Fleming College’s Centre for Advancement of Water and Wastewater Technologies (CAWT) in Lindsay, Ontario, to better define the range of wastewater constituents that are amenable to this technology, as well as to identify operational parameters for increased efficiencies. In one CAWT investigation, a commercial airline’s toilet (lavatory) wastewater, stabilized with ethylene glycol, was tested, using the electro-oxidation cell.
The initial concentration of ethylene glycol in the toilet water was greater than 26,000 mg/L. Within a period of only two hours, the concentration of ethylene glycol was reduced by 28%. Although this work is only preliminary, it does indicate that electro-oxidation demonstrates good potential as a rapid alternative to the treatment of wastewaters with a high concentration of ethylene glycol.
More work is underway to maximize the system’s treatment performance, including determining optimum retention times within the cell and determining the ratio between the surface area of the electrodes and the volume of wastewater treated. It is believed that the information gained from these investigations will lead to greater treatment efficiencies and prove crucial in the manufacturing and operation of larger commercial units.
Electro-oxidation technology has shown great potential for use as an on-site treatment technology for challenging wastewaters. Its small footprint and ease of installation are also advantageous. Preliminary investigations suggest that the electro-oxidation cell is an advanced treatment technology suitable for a wide range of challenging chemical and biological contaminants. Furthermore, the technology does not produce many of the greenhouse gases commonly associated with conventional systems, making it better for the environment.
Gordon Balch, PhD, is a research scientist/professor with the Centre for Advancement of Water and Wastewater Technologies at Fleming College, Frost Campus. This article appears in ES&E Magazine’s June 2019 issue.