Comply with strict environmental water discharge limits with electrochemical water treatment

Combining electricity and water might not sound like the most intelligent thing to do, but when electricity is used correctly it can be a helpful tool in wastewater treatment. Electricity has been used for water treatment for the past few decades, but limitations in technology and high operating costs compared to other treatment processes have prevented its wide-scale use as a major wastewater treatment method. However, the tightening of environmental restrictions for wastewater discharge has brought a new drive to develop electrochemical water treatment (EWT) processes.

The basic principle behind EWT processes is very simple: electrodes are used to apply an electrical current to water to get the required reaction. Despite its simplicity, the result can be quite hard to predict as the process is a mixture of water and electrochemistry.

Different types of EWT

EWT processes can be roughly divided into three different types: electro-coagulation (EC), electro-flotation (EF), and electro-oxidation (EO). However, there is no clear differentiation between the types and some authors use the terms interchangeably, meaning extra attention must be paid when reading relevant literature.

Electro-coagulation, where process electricity is used to dissolve metal from a sacrificial electrode (anode) into the treatment water, is probably the most well known of these methods. The dissolved metal then reacts with both the impurities in the water and the water itself, which causes coagulation. The treatment with EWT does not add salts to the water as conventional precipitation processes do. The most common electrode materials for EC are iron and aluminum because of their proven efficiency, affordable price, and wide availability.

In electro-oxidation, an inert electrode coated, for example, in titanium or boron-doped diamond is used as the anode. In EO the dissolution reaction is replaced by oxygen formation from the electrical dissociation of water. In some cases, chlorine might be formed, but it readily reacts to hypochlorite that acts as a strong oxidant making the process more efficient.

The gases generated by the dissociation of water on the electrodes during EWT can be used to float either the formed particles in the case of EC or other impurities in the water. This is called electro-flotation. Usually EF is seen as part of EC or EO, but it can also be implemented as its own process.

Target applications for EWT

Recently EWT processes have been studied in various industries and with various different pollutants. The most common applications studied are the removal of metal ions/oxyhydroxides, oil removal, and the removal of organic matter. In general, when compared to traditional precipitation processes, EWT technology gets more competitive as concentrations get lower.

The biggest potential of EWT technology is in the wastewater polishing process for substances that are hard to remove using traditional precipitation processes, such as oxyanions like arsenic, antimony, and selenium, or metal residuals like cadmium, nickel, and copper.

Factors affecting EWT

The main factors affecting EWT are current density or applied current, and residence time in cell or flowrate. Current density [A/m²] is the amount of current applied to a cross-sectional area of the electrode and is the main factor that defines which electrochemical reactions take place on the electrode surface. It also determines the rate of electrode dissolution, bubble generation, and potential in the cell, so it greatly affects the economics of the treatment.

Charge loading [C/m³, As/m³] defines the amount of electricity applied to an electrolyte by volume. It is directly proportional to the amount the electrode has dissolved, i.e. it states the dosage of the metal. Charge loading consists of the applied current and flowrate, thus combining both the main parameters of the process. It can also be said that charge loading is the treatment level of the process.

EWT technology has been proven to work in a wide temperature and pH range. Furthermore, the EWT process naturally drives the pH towards a slightly alkaline (pH 8-9) level, neutralizing the solution and usually allowing a direct discharge. One good thing to remember when designing an EWT system is that aluminum electrodes can be passivated more easily at certain pH levels, whereas iron electrodes are more flexible.

The composition of the water under treatment and its purification needs determine the final level of applied current and flowrate. It is therefore important to have a definite target in mind so the EWT process can be designed and optimized. In addition, ionic concentrations (i.e. impurities) affect the conductivity of the water, determining the system voltage and the treatment cost.

EWT results and costs

In all of the following examples, treatment level means charge loading. As previously mentioned, the EWT process works most efficiently as a polishing process. If the initial concentration of impurities is less than 10 mg/l we can expect treated water to reach residual levels of 20 μg/l. Figure 2 gives an example of water that is well suited for EWT technology. Initially, there weren’t many impurities in the water, but the discharge limit was as low as 20 μg/l for nickel.

Of course, most water has many more dissolved compounds in it. Figure 3 shows how different compounds have a different reaction order. For example, arsenic and nickel react before antimony. In general, EWT is not a selective method and the treatment result depends on total dissolved solid that react in the EWT process.

If the impurity levels are high, it is recommended to carry out a pre-treatment before EWT, for example neutralization or a ferric arsenate process. Figure 4 shows how a simple pH modification affects the purification results. By selecting a suitable pre-treatment we can ensure that EWT works efficiently, gaining savings in operational expenditure.

The operational expenditure of EWT is made up of electrical consumption and electrode consumption. Electrode consumption is directly proportional to the current applied according to Faraday’s law. Electrical consumption also depends on the applied current, as well as on the conductivity of the wastewater and the cell design itself. This makes approximating operating expenditure difficult as it varies so much case-by-case. Power consumption is typically around 0.7–1.2 kWh/m³, whereas the consumption range of iron electrodes is usually between 0.5 and 0.2 kg/m³. Figures 5 and 6 show an example of the relation between treatment level and operational expenditure. Generally, ever-lower residual concentrations can be reached but the OPEX of treatment will rise at the same time. Usually, an optimal point of operation can be found which balances treatment results with cost.

Outotec EWT-40

The Outotec EWT-40 electrochemical treatment unit is the latest evidence of Outotec’s intensive research and development into water treatment applications. Outotec has combined its unique understanding of water treatment, process design, electrolysis, and hydrometallurgy into a cost-effective modular product. The electrochemical water treatment solution is a highly automated process, which minimizes the need for personnel while ensuring high water treatment performance. Outotec EWT units can be combined to different solid or liquid separation units for fine particle removal to meet the required limits.

Outotec EWT solutions can handle everything from the removal of arsenic, selenium, and antimony, to trace metals and organic matter. Our EWT solutions are available as a process island with full maintenance, spare parts, and operational support services. EWT is also another example of our containerized cPlant concept. The cPlant concept is based on pre-fabricated and functionally tested modules inside container-sized steel frames that can be easily transported and installed, and quickly connected to the process. Outotec can also offer complementary services and complete solutions from laboratory-scale test work to on-site piloting, conceptual and feasibility studies, basic and detailed engineering, as well as developing a bespoke solution covering the entire process.

Conclusion

Electrochemical water treatment technology offers a solution for dilute effluents or those that have already gone through some treatment but still do not reach strict environmental discharge limits. EWT is most effective against compounds that are hard to remove using traditional water treatment processes, namely antimony, arsenic, cadmium, and selenium.

There are three ways electrochemical water treatment helps our customers:

• Meet environmental permits
• Decrease operational costs
• Simplify operation principle

The technology can also be implemented as an end-of-pipe solution for existing plants that are looking for a way to secure their discharge levels.

The article written by Niko Isomäki, Product Manager, Industrial Water Treatment