As the industry turns to greener processes, the use of electrochemical technologies such as electrocoagulation is gaining more traction for the removal of contaminants from industrial wastewater. Electrocoagulation can be effective in some applications, but the technology also has its constraints. Here, we unveil some of the myths & truths about electrocoagulation.
Chemical Coagulation and Electrocoagulation
Coagulation is the process of destabilizing suspensions by neutralizing the surface charges of suspended solids and colloids to enhance solid-liquid separation. Suspensions are difficult to treat because these particles do not settle or separate easily; they are forced into suspension by the repulsive electrostatic forces created between them due to their similar surface charges. Coagulants neutralize the surface charges to enable their separation from liquid. The process is called chemical coagulation if salts of coagulants are used and electrocoagulation if coagulants are generated electrolytically.
In electrocoagulation, electricity is applied to consumable electrodes that are in contact with the wastewater. The electrodes dissolve to generate aqueous complexes of coagulants. Two of the most common and efficient coagulates used in electrocoagulation are hydroxylated iron and aluminum.
Electrocoagulation Advantages vs Chemical Coagulation
Chemical and electrocoagulation are not fundamentally different processes as they both rely on the coagulating properties of the chemicals used in the process. Any contaminant that can be removed by electrocoagulation can also be removed by chemical coagulation although the removal may not be to the same extent or with the same efficiency.
Electrocoagulation is generally more efficient than chemical coagulation. The in-situ generation of coagulants as well as the presence of charged electrodes play an important role in the higher efficiency of electrocoagulation. This results in the generation of a smaller volume of solid by-product with a low water content.
Water treated using electrocoagulation will also have a lower total dissolved solids (TDS) concentration than water treated with chemical coagulation. Unlike chemical coagulation, salt reagents are not used in electrocoagulation where the reagents are generated in-situ. As a result, counter ions present in salt reagents are not introduced into the electrocoagulation treated water.
Electrocoagulation Cannot Remove Everything from Mine Water
The mining industry has long used coagulation to remove suspended solids. Coagulation is typically applied prior to filtration or sedimentation in the overall wastewater treatment process. The technology is also effective for the treatment of oily water to remove turbidity, colour and silica.
During electrocoagulation or chemical coagulation, some dissolved metals and metalloids species will co-precipitate or adsorb onto the surface of the hydroxylated coagulants. This affinity enables the use of coagulation in some instances to reduce the concentration of heavy metals and metalloids in mine water. Oxyanions of pentavalent arsenic, antimony, vanadium, molybdenum and tetravalent selenium (selenite) are examples of dissolved species that can be partially removed using chemical coagulation or electrocoagulation.
Additionally, during electrocoagulation a portion of the coagulating species can exist in a more reduced form such as ferrous iron as opposed to ferric iron. As a result, electrocoagulation will allow for partial removal of more oxidized contaminant species such as chromium (VI).
Electrocoagulation Cannot Remove Hexavalent Selenium (Selenate) from Mine Water
Not all contaminants can be removed with coagulation. One of these contaminants is hexavalent selenium (selenate), which is highly topical for the mining industry as new and impending regulations are being enacted to regulate it. Due to the oxidizing condition of most mining processes, selenium presence in mine waters is primarily in the form of selenate, an oxyanion with a chemical structure that is less reactive and responsive to treatment in comparison to selenite. Removing selenate from mine waters requires it to be first reduced to selenite before it can be removed with chemical coagulation or electrocoagulation.
Coagulation Operation Conditions Is Not Universal
During coagulation and specifically with electrocoagulation, precise tuning of the operating conditions is required to achieve consistent removal of contaminants. But not all contaminants that can be removed with electrocoagulation can be removed at the same time and with the same operating conditions. To meet removal objectives, a multi-stage electrocoagulation set-up – each with their own specific operating conditions – may be required.
Electrocoagulation Considerations beyond Cell Shape and Configuration
Electrocoagulation cells with different designs and sizes are commercially available. Monopolar and bipolar configurations have been used in the past. Regardless of the shape, size and configuration of the electrocoagulation cells, the main electrocoagulation consumables are power and the electrodes. Depending on the wastewater chemistry, coagulant dose and treatment plant location, electricity costs can become prohibitive. And because the process relies on adsorption and co-precipitation, the volume and stability of the solid waste by-product generated during treatment can also be limiting due to disposal and management requirements.
While chemical and electrocoagulation processes are suitable and perform reliably for suspended solids removal, their application to remove dissolved species including heavy metals is limited and must be investigated on a case-by-case basis.
Farzad Mohamm, MSc, PhD