A considerable amount of research and testing is underway to identify viable thermal evaporative solutions for treating challenging FGD wastewaters as part of compliance with the EPA’s new ELGs
By Todd Whiting
The U.S. Environmental Protection Agency (EPA) issued the final effluent limitation guidelines (ELGs) rule in September of 2015 for the steam electric power sector, establishing technology-based regulations that target reductions in toxic metals and other harmful pollutants from power plant wastewater discharges. The ELGs are based on technology improvements in the steam electric power industry over the last three decades and establish new requirements for wastewater streams from flue gas desulfurization (FGD), fly ash, bottom ash, flue gas mercury control (FGMD), and gasification of fuels.
The new rule sets stringent effluent limits on arsenic, mercury, selenium, and nitrogen for FGD wastewater and zero discharge of pollutants from ash transport water and FGMD wastewater. Additionally, rigorous limits are placed on arsenic, mercury, selenium and total dissolved solids in coal gasification wastewater based on evaporation technology, and strict controls are established on any new coal or petroleum coke plants that may be built in the future.
Although EPA’s rule does not require power plants to use specific technologies to comply, the new ELGs were developed based on the performance of what EPA determined to be the best available technology (BAT) for treating each specific process wastewater stream. For achieving compliance, EPA estimates that roughly 12 percent of steam electric power plants will have to make new investments including design modifications, process changes, facility expansions, or installing new treatment systems.
Challenges with FGD Wastewater
Quite possibly, the most challenging aspects to the new ELGs is addressing requirements related to treating FGD wastewaters. These difficult-to-manage discharge streams can be very different from site to site, with contaminant compositions influenced by a range of factors including coal type, makeup water quality, as well as the specific technology used in the plant for removing sulfur dioxide from exhaust flue gases.
According to Jonathan Shimko, director of power & water solutions at Tetra Tech, FGD wastewater is one of the biggest concerns for most people in the power industry due to the inconsistency of these waste streams. “The composition of FGD wastewater is highly dependent on the fuel source used in the plant,” Shimko said. “For this reason, FGD wastewater can exhibit significant regional variability-a station in the West might face a completely different set of challenges than a station in the East due to differences in the coal type. As one example, eastern bituminous coals tend to have higher chloride and sulfur content, generating an FGD product that can be more difficult to handle than subbituminous or lignite coals found in the central and Gulf regions.”
In addition to those constituents, coals can also contain varying ratios of other components such as calcium, magnesium, or dissolved silica, which will ultimately affect the FGD wastewater.
Since treating FGD wastewater is case-by-case, station-by-station, and fuel source-by-fuel source, a universal approach that could work for any situation is not readily available. “As such, addressing FGD wastewater requires a specific evaluation of the water quality, the quantity of water, and the systems that are in place,” Shimko said.
Shimko advises that as a first step, power plants should develop a thorough water balance at their sites, followed by an understanding of the plant’s water needs, including the way that water can be managed within the confines of the rule.”
FGD Treatment Strategies
For existing coal-fired plants larger than 50 MW, the ELGs set limitations on FGD wastewater for arsenic, mercury, selenium, and nitrate-nitrite as N and specify a BAT combination of chemical precipitation treatment followed by anoxic/anaerobic biological treatment. Power plants can also address FGD wastewater by implementing certain process changes rather than installing new wastewater treatment systems. According to the EPA, some power companies have expressed interest in approaches that would use FGD wastewater from a wet scrubber as makeup for the sorbent injected into a dry scrubber, and as a result, completely eliminate FGD wastewater discharge.
As another alternative, power plants have the option to participate in EPA’s voluntary incentives program for treating FGD wastewater, which provides deferred compliance to December 31, 2023 for plants that agree to meet stringent ZLD limitations based on the performance of thermal evaporation technologies. While the extension allows plants more time to gain a better understanding of their FGD waste streams and develop the most effective approach, larger plants anticipating future regulations may opt to pursue thermal evaporation as an insurance policy, as thermal evaporation would most likely cover any future regulations or contaminants that the EPA may target-such as boron.
But while vapor-compression evaporation technologies may offer a way to mitigate potential risks from future and more stringent federal or state regulations, adopting this strategy requires power plants to contend with a whole new set of cost, equipment, and operational challenges. For this reason, a great deal of industry effort is focused on the development of solutions that address the unique technical considerations of thermal treatment.
“Compared to chemical-biological treatment for surface discharge, thermal systems require a much higher capital investment,” Shimko said. “But, power plants also need to consider the long-term operation and maintenance (O&M) costs, as thermal is an energy-intensive process. The other big risks are related to scaling and corrosion from potentially high sulfate and chloride concentrations in the FGD wastewater. For this reason, the materials of construction are very important.”
Corné Pretorius, an associate with Golder Associates, echoes these same concerns. “It takes a tremendous amount of energy to evaporate water, especially when it’s a concentrated salt solution,” he said. “On the chemistry side, the concern is that industrial solutions tend to produce precipitates when they are highly concentrated. So, from a thermal evaporation perspective, scaling and corrosion risks need to be managed very closely to avoid equipment complications.”
Brine concentration technologies
One approach that is increasingly being explored as a strategy for mitigating the high operating costs associated with thermal systems is the use of advanced membrane filtration processes that can further reduce residual waste products prior to evaporation. “Advanced membrane technologies are now becoming commercially available that enable plants to pre-concentrate brine material,” Pretorius said. “By minimizing waste volumes, the requirement for thermal water reduction is lower, resulting in a shorter evaporation stage.”
On the back-end of the treatment train, a significant amount of industry research is also focused on the development of solutions for responsibly managing concentrated residual wastes leftover from thermal evaporative processes.
Pretorius, whose work focuses on brine encapsulation and waste solidification, said brine concentrates can be blended with fly ash and other ingredients to form materials that harden much like concrete, reducing potential environmental risks associated with contaminant leaching and enabling for more environmentally-responsible landfill disposal. “The whole idea is to immobilize contaminants in a high strength material that demonstrates structural integrity and very low permeability,” he said.
Another option with managing brine is deep-well injection. “The availability of a deep well depends on the geology of the formation, and in some cases, brine pre-treatment may be required prior to disposal to facilitate continued use of the formation,” Pretorius said.
Yet another approach similar in scope to vapor-compression evaporation is crystallization, a high-energy process which optimizes the formation of crystals. “Instead of producing a combined mixed-salt precipitate through brine evaporation, crystallization processes result in a purer sodium sulfate, calcium sulfate, limestone, or calcium carbonate that potentially can be reused in industry,” Pretorius said.
But the challenge that comes with pursuing this option is the additional equipment, the expensive materials of construction, and the added complexity of managing a process for optimal crystallization of salts.
“That added complexity works to offset the benefit that would otherwise be realized by selling those resources back on the market, as they are not high value materials,” Pretorius said. “But still, crystallization can make sense in some situations-particularly when a local market is available, as transporting low-value commodities over long distances can quickly erode any financial gains.”
EPRI research evaluates potential technologies
As a resource to the coal-fired industry, the Electric Power Research Institute (EPRI) is conducting extensive research and testing of potential and commercially-available treatment technologies that could be implemented in plants for meeting performance standards as set by the new ELGs. EPRI’s programmatic research includes laboratory bench investigations, proof-of-concept pilot tests, and demonstration projects at or near commercial-scale focusing on a range of treatment issues including performance, reliability, cost-effectiveness, and thermal efficiency. In particular, efforts are heavily focused on evaluating promising technologies-including thermal evaporation processes-that could be utilized for treating FGD wastewater.
Pilot demonstrations are being carried out at partnering power plants, which include the Springerville Generating Station in Springerville, Arizona; the Water Research Center (WRC) at Georgia Power’s Plant Bowen; and Hoosier Energy’s Merom Generating Station located in Sullivan, Indiana.
“Much of what we are doing at these sites is focused on vetting different thermal volume reduction technologies that could be readily adopted,” said Kirk Ellison, technical leader with EPRI’s Water Management Technology program. “We are looking at novel technologies beyond traditional brine concentrators and evaporators to investigate their potential effectiveness and whether they offer better performance in terms of lower energy consumption, decreased operational and maintenance requirements, and less scaling potential. The energy that is required for concentrating brine wastes can be extremely high, so finding innovative technologies that can reduce energy consumption is very important.”
Ellison emphasized that a significant driver for EPRI’s work is dedicated to the management of by-products that remain following thermal processes. “Volume reduction technologies are a critical piece, but they do not represent the total solution,” Ellison said. “There is still a solid salt waste stream that needs to be disposed of in an environmentally-responsible manner. Without that end-game in mind, it’s hard to properly choose the right technology upstream of that.”
With the timing for ELG compliance fast approaching, Ellison advises that utilities focus on collecting quality and comprehensive data at their plants, which can go a long way to making more informed decisions.
“Many utilities face a tremendous amount of work prior to implementing new treatment technologies and must first evaluate their material balances, their water balances, as well as the wastewater chemistry at their site including how it varies over time,” Ellison said. “Much of this is a data issue, and the more that utilities can gather, the better off they will be in selecting the best approach and the most effective technology.”
EPRI demonstrations evaluate Purestream’s AVARA system
In 2016, three pilot demonstrations were conducted at each of EPRI’s demonstration sites investigating the commercial viability of Purestream’s Advanced Vapor Recompression (AVARA) system for achieving compliance with the EPA’s ELGs, and specifically in relation to addressing FGD wastewater requirements.
A patented innovation of traditional mechanical vapor recompression science, the AVARA system is a modular, fully-automated, thermal evaporation system that easily integrates into existing treatment trains and is designed to concentrate brine and remove chlorides and heavy metals from industrial wastewaters such as FGD wastewater. The system achieves significant volume reduction and distilled water recovery, enabling for discharge or reuse.
The first demonstration was conducted at the Springerville Generating Station evaluating the effectiveness of the AVARA system for reducing volume of cycled-up cooling tower water (similar in characteristics to FGD wastewater) from one of the plant’s evaporation ponds. The second AVARA pilot was performed at the WRC, where FGD wastewater was treated and concentrated brine generated from the process was encapsulated with fly ash and tested for dry disposal options. The third AVARA pilot, conducted at Hoosier Energy’s Merom Generating Station, included a 60-day evaluation of the performance of a 35-gpm AVARA system for treating FGD wastewater.
Results of the pilot tests demonstrated the capacity for the AVARA system to consistently treat and concentrate up FGD water for meeting ELG standards. Specifically, data gathered from the Merom Plant demonstration revealed that the AVARA system achieved 91.4 percent distillate recovery in processing 2,754,172 gallons of FGD wastewater. Waste brine in the system was concentrated up to between 180,000 and 200,000 ppm with 93% uptime maintained over the 60-day pilot. The AVARA system also demonstrated the ability to handle variable feed water and fluctuating flow rates, and energy use data suggested that the AVARA system can reduce equipment and operations costs by 40 percent compared to traditional thermal evaporation processes.
One of the main challenges in treating FGD wastewater at the Merom site included scaling from high levels of calcium sulfate in the feed water. During the pilot test, the high concentrations of calcium sulfate in combination with the thermal process created tenacious scale on the surfaces of the AVARA core heat exchangers. However, the AVARA system demonstrated the ability to still run and deliver consistent, successful results even as scaling occurred.
The ability of the AVARA unit to maintain performance under scaling can be attributed to the system’s novel design-during treatment, the cores remain fully immersed in process wastewaters, which serves to sustain heat transfer effectiveness and optimize thermal efficiency. The modular configuration of the AVARA cores also proved advantageous in terms of minimizing downtime as the system can be shutdown, drained, existing cores removed, new cores installed, and tanks refilled in 24 hours, at which operations can continue.
The AVARA’s anticipated cartridge core life will vary, but based on planned mechanical, chemical, and operational modifications, 180 days between core exchanges is expected. Design changes and modifications to the AVARA system are currently underway to increase the process efficiency of exchanging cartridge cores.
Todd Whiting is vice president of operations for Purestream Services.