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POWER’s 2020 Water Award was awarded to Saltworks Technologies for successfully piloting a new application of a 50-year-old technology in a coal-fired power plant. The company's solution is expected to reduce the cost of treating flue gas desulfurization (FGD) wastewater by as much as 50% by targeting chlorides and circulating water.

Since its introduction in the 1960s, electrodialysis (ED) technology has been widely used in industrial fields, and now it has become the second most widely used membrane desalination technology. The traditional technique involves applying a direct current electric field to make positive ions pass through the cation exchange membrane in one direction, while negative ions pass through the anion exchange membrane in the opposite direction. Recently, it benefited from an innovation that helped keep the membrane clean by allowing the polarity of electrodes and hydraulic channels to "reverse".

✔ Research collaboration has led to the development and testing of meaningful flue gas desulfurization (FGD) wastewater treatment solutions.

✔ The dechlorination solution can realize the recycling of wastewater and avoid the installation of complicated and more expensive evaporation facilities.

✔ The technology improves existing, proven industrial processes and is customized for power plant applications.

But in 2010, Saltworks Technologies Inc., a Canadian water treatment solution provider, set out to further improve reverse electrodialysis (EDR) technology. As it told POWER in May, although the initial development work produced a new type of ion-exchange polymer membrane with ductility and high conductivity, because it hopes to install it in its next-generation EDR plant, it further The membrane was modified to make it selective for monovalent ions. Saltworks calls the resulting technology monovalent electrodialysis reversal (mEDR, Figure 1). “Under an electric field, these membranes selectively allow monovalent anions (such as chloride ions) to pass through the membrane and concentrate in the brine wastewater stream, while blocking multivalent ions, such as sulfate,” explained Malcolm Man, executive vice president of business development.

1. Saltworks Technologies' unit price electrodialysis reversal (mEDR) stack and skid are modular in design, easy to expand, project integration and maintenance. The mEDR equipment is fully automated and has a self-cleaning function to maintain optimal performance. The company's highly elastic and malleable monovalent ion exchange membranes are critical to this process. Courtesy: Saltworks Technologies Inc.

Although Saltworks has focused its new technology application on mining wastewater for the first time, its efforts are in line with the research of the Electric Power Research Institute (EPRI), a non-profit organization that brings together scientists and engineers from academia and industry To help solve related challenges. To electricity. Since 1996, EPRI has been exploring various flue gas desulfurization (FGD) wastewater issues and is studying the clarification of factors that affect the cost of FGD wastewater physical and chemical treatment. Given that the Obama administration has just issued final emission limit guidelines and standards for steam power point source categories, the study is timely, and technology-based guidelines limit the total dissolved solids (TDS) in FGD wastewater.

"Mining water and FGD wastewater have similar chemical properties, and both are highly scaled in gypsum," Saltworks said. This is why when EPRI introduced the FGD application to Saltworks, the company began to adjust the process and started small-scale testing with EPRI's insight and support.

But instead of focusing on the entire range of TDS components, “the team focused on the bad actors that limit the FGD cycle and cause wastewater discharge in the first place: chloride,” Mann said.

The FGD system of coal-fired power plants is used to reduce sulfur dioxide air emissions, which will produce wastewater because chlorides accumulate in the circulating water of the sulfur scrubber. Most FGD units include some wastewater treatment, combining polymer and coagulation through medium pH precipitation, followed by a filter press to remove heavy metals and fluoride. This process is called "triple box". However, it still releases TDS-rich treated water, including chloride, selenium, and other ingredients. (For an in-depth understanding of the existing FGD wastewater treatment system, please refer to "Reducing Costs and Waste in FGD Wastewater Treatment" in the April 2017 issue of POWER.)

Since repeated use of scrubber water in multiple cycles can lead to chloride accumulation, operators will release or "drain" the FGD wastewater and usually add cleaner dilution water to keep the chlorides below the set level. Saltworks stated: "Chloride emission levels range from 10,000 milligrams per liter (mg/L) to as high as 30,000 milligrams per liter," and pointed out that the increase in chloride concentration will bring some potential problems, such as problems related to corrosion. And hinder the absorption of sulfur dioxide.

However, if the chloride can be removed by some kind of "chloride kidney", this will enable water reuse, less makeup water and up to 90% of the FGD wastewater discharge, the company said. The minimum discharge volume may also become easier to manage. "EPRI has been committed to exciting developments that can directly solidify wastewater without treatment because the amount of wastewater is very small," Man points out. More importantly, “this method also avoids the complicated treatment systems required to treat FGD wastewater because there is no need to meet strict discharge regulations. On the contrary, it meets the simpler'low chloride' regulations,” he said.

The company stated that under the mEDR process, “because sulfate is removed from the brine instead of being concentrated with calcium, it avoids expensive soda ash softening.” Due to the elasticity of organic and suspended solids (less than 20 microns), mEDR also requires less pretreatment, while concentrated brine (in a single membrane step) is as high as 180,000 mg/L.

Technically, the chloride can be removed to less than 200 mg/L. However, reducing the chloride to 1,200 mg/L to 1,500 mg/L can prove more economical, mainly because at chloride concentrations below 1,200 mg/L, the membrane flux decreases and the resistance increases, increasing the membrane area ( Larger factories) and the company stated that power requirements.

Saltworks stated that in order to evaluate the economics of the process, it reviewed and tested "a variety of options to inform the appropriate development path." If it is combined with traditional reverse osmosis and evaporator processes (including chemical softening, seawater reverse osmosis and evaporation) Compared with Ultra High Pressure Reverse Osmosis (UHPRO) (a newer process involving chemical softening and 80 bar seawater reverse osmosis), compared with 120 bar UHPRO-mEDR, "the most promising stage is to reduce the cost of FGD wastewater treatment. Change in nature.” Although the evaporator can produce higher concentrated brine waste at 180,000 mg/L TDS than UHPRO (130,000 mg/L), the evaporator process cost is higher than UHPRO. However, according to Saltworks' testing of FGD water, the chemical softening used in these two processes accounts for nearly 50% of the total cost of ownership (including capital plus operating costs) of the treatment.

Saltworks said that so far, it has conducted two important tests, and both show good potential. First, it tested the mEDR process on off-site micro-pilots in 2017 to prove its feasibility and provide initial performance data. In early 2019, with the support of the U.S. Department of Energy, it conducted a larger-scale field test of 3 cubic meters per day at the Southern Company's coal-fired power plant in Georgia, using a full-scale chimney system.

The pilot was completed in June 2019, running 24/7 for 60 days with 100% uptime. The larger pilot has achieved the goal of "removing up to 85% of chlorides so that the treated FGD wastewater can be recycled back to the scrubber" through the internal power plant recycling process. It is worth noting that the pilot also showed that the process can "produce a non-scaling concentrated brine that can be directly solidified with fly ash and other additives, or can enter the evaporator crystallizer without pretreatment to achieve zero liquid Emissions," Mann said. And, as expected, it successfully treated highly fouled FGD wastewater-without chemical softening-and eliminated the costs associated with chemical and sludge treatment and disposal.

The company told POWER that now that the pilot has been confirmed, the next step is to conduct a larger-scale and longer-term demonstration. "We need to ensure that the chloride-treated water can be recycled for a long time, characterize new and lower in situ emissions, and determine the management of these lower-volume wastes," it said.

If future tests are successful, the company envisions mEDR can be adopted by coal-fired power plants around the world. At the same time, it can also be extended to the recovery of desalinated wastewater in the oil and gas industry; in the mining industry, it can remove chlorides in mine wastewater and process streams to prevent equipment corrosion; in industry, it can be used for corrosion protection.​​​​ ■

— Sonal Patel is the senior associate editor of POWER.

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