POWER MAGAZINE: Water and Wastewater Treatment Technology Update
Water is the lifeblood of a thermal power plant. As such, obtaining clean and pure makeup water and dealing with wastewater has been a requirement since the first steam generating unit went into operation. As rules and regulations change, new technology is often necessary to meet more restrictive guidelines. The desire for energy savings, more reliable treatment methods, and solutions to water availability challenges can also lead to innovations.
Reverse osmosis (RO) is a widely used technology in the power industry. Developed in the 1950s, the first commercial RO plant began operating in 1965. The process uses a semipermeable membrane to purify water by applying pressure to overcome osmotic pressure, forcing water from a region of high-solute concentration through the membrane to a region of low-solute concentration. A newer membrane technology that may not be as familiar to readers is forward osmosis (FO).
Fast Forward to Forward Osmosis
The first FO water treatment plant was built in 1998 for use on landfill leachate; today, research and development continues to refine the process. While not as common as RO, FO systems are proving to offer a new solution for some challenging situations. Boston-based Oasys Water recently installed a system to treat a Chinese coal-fired power plant’s flue gas desulfurization (FGD) wastewater (Figure 1).
Lisa Marchewka, vice president of strategy and marketing for Oasys, explains, “We use membrane technology, but instead of using hydraulic pressure to force water through a membrane, we instead use a high-molarity ‘draw’ solution that pulls freshwater across the membrane rather than pushing it on the surface.”
The key ingredient in the system is the draw solution. Oasys uses ammonium bicarbonate, which is an off-the-shelf product available in bulk. Although ammonium bicarbonate is not completely harmless, it is a relatively safe product that was once used in homes before modern day baking powder became available. In fact, Oasys obtains its product from the well-known baking soda company Arm & Hammer.
Feedwater enters the FO system at one end of the membrane module (Figure 2). The draw solution flows on the opposite side of the membrane, counter to the direction of feedwater flow, and pulls water molecules through the membrane. The draw solution becomes more and more diluted until it exits the module and is directed to the thermal process.
In the thermal recovery device, the diluted draw solution is heated to evaporate only the draw solutes, leaving behind the clean, purified water. Because evaporation of the water is not required in the thermal column (Figure 3), less energy is consumed than would otherwise be necessary. Another advantage of this arrangement is that no impurities enter the thermal process, therefore scaling and foaming are not a problem.
By design, the closed loop system should not require additional ammonium bicarbonate to be added. The plant has typical mechanical components though, such as tanks, valves, pumps, and piping, so there is always the potential for leaks or a component failing. For that reason, Oasys suggests that additional draw solution be kept on hand.
Benefits of FO
Oasys says its FO system offers some advantages over other more common water treatment options. According to Marchewka, in RO systems used for seawater desalination, the typical water recovery rate is only about 50%. In other words, for every two gallons of seawater taken into a system, one gallon of purified water is produced and one gallon of reject water is discharged back to the source.
The FO process can be used to take the reject from a seawater desalination RO system and concentrate that to achieve an additional 80% recovery. Therefore, combining the two systems can result in an overall recovery of 90%.
RO systems also are limited in the salinity that they can handle. Once the system reaches its maximum hydraulic pressure, water can no longer be pushed through the membrane to achieve recovery. In contrast, FO technologies can treat water up to 150,000 ppm of total dissolved solids—four times the maximum for conventional RO systems—and concentrate it to over 280,000 ppm. So not only can much higher recovery be achieved using FO—because it is not limited by an osmotic gradient—but it also operates at a lower pressure, which offers an energy savings.
Thermal systems, such as multiple effect distillation, multi-stage flash, or mechanical vapor recompression, offer another option for desalination of seawater and brine concentrating. Although thermal systems can be designed to work well in many situations, they have limitations of their own.
For one thing, thermal systems are capital intensive to install. The materials used have to be capable of handling the corrosive effects of seawater, so they are frequently constructed of more expensive alloys. The energy consumed by a thermal system is also much higher than in FO systems.
In thermal systems, the feedwater must be heated to its vaporization temperature, which requires significant energy. The vapor is then condensed to produce the distillate. In that process, impurities in the water can cause scaling or foaming, resulting in a very maintenance-intensive operation. As noted previously, only the draw solution and clean water enter the thermal recovery column of the FO system, which eliminates this problem.
Innovative FO Uses
Although FO and RO may sound like rival systems designed using similar technology—the membrane portion of an FO system does look nearly identical to that of an RO system, at least on the outside—Oasys views its FO system as more of a complement to RO systems rather than a replacement for them. It suggests FO systems are better able to compete directly with thermal evaporation systems.
“The focus of the company, right now, is more on industrial high-salinity recovery projects, specifically in zero-liquid discharge, or near zero-liquid discharge systems,” said Marchewka.
In addition to the FGD wastewater treatment system Oasys installed at the Changxing Power Plant, it has another FO system already operating in China. That system has the flexibility to be used for seawater desalination or for treating cooling tower blowdown, depending on the plant’s needs.
Through a partnership with National Oilwell Varco (NOV), Oasys’ technology is being deployed in the oil and gas industry too (see this issue’s cover photo). NOV says the system is suitable for onshore unconventional shale plays, and it markets the solution as a means of treating exploration and production wastewaters. It touts that these streams can be converted to freshwater quality, fully treated for reuse in new drilling and completion fluids or for surface discharge in remote areas where disposal options have traditionally been limited and expensive.
Oasys says it is the first company to deploy an FO-based brine concentrator. The company can also imagine using the technology for things like brackish desalination and other municipal applications.
One final advantage that really benefits operators is the FO system’s ability to handle variation. Marchewka noted that the company has learned from its experience in China that the water chemistry from the FGD process is quite variable—seasons, load, and various other operating parameters all factor in. Although changes can be problematic for many systems, because the FO system operates at lower pressure and pulls the water across the membrane with the draw solution, it is much less prone to fouling and scaling, and it can handle the challenge.
“It actually gives operators a nice benefit when dealing with fluctuations and changes in water quality and water chemistry,” says Marchewka.
In late 2014, Oasys Water sold the world’s first zero liquid discharge (ZLD) forward osmosis (FO) based brine concentration system to treat flue gas desulfurization wastewater. This story, and the innovative approach, has caught the eye of those in the industry, and thus Oasys was approached by Power Magazine to discuss how the MBC system can benefit operators over traditional evaporation. The above article is an excerpt from the featured cover article, which can be found online.