Regulatory uncertainty, changing resources, and an industrywide drive to cut costs and boost flexibility and efficiency are among a growing list of challenges that are prompting new approaches to treat power plant wastewater.
The past decade has been brutal for coal and nuclear generators. Charged with compliance of a slew of environmental rules promulgated by the Bush and Obama administrations, the economics of the sectors were dented further by falling prices of natural gas, weak power prices, and more recently, an upsurge of renewables. To adapt to the changing market, a number of plant owners have moved to adjust their supply portfolios by shutting down older, inefficient units, and outfitting newer units with more-efficient technologies, modifying them with better cycling capabilities, or converting them to natural gas.
In this environment, finding opportunities to trim costs has become especially important, but doing so requires a rethink of where to sink investments. And planning must take into account a growing number of moving parts, such as when proposed environmental regulations will come into force and what they will require, as well as which technologies and options are available, or appear promising, and whether they are affordable. That’s especially true when considering how to treat and dispose of power plant liquid waste, which encapsulates a broad range of streams that are dependent on a plant’s location, water quality, fuel type, type of combustion technology, mode of operation, cooling technology, and treatment options.
The characteristics of power plant wastewater generated depend on how the water has been used and how surface water is collected and drained. But they also depend on how prone discharged water is to contamination, for example, from demineralizers; lubricating and auxiliary fuel oils; trace contaminants in fuel; and chemicals used to manage water quality, such as chlorine and biocides.
Then, it is no less prudent to understand water conservation goals, and take into account available measures to prevent, minimize, and control wastewater effluents. Generally, higher-volume wastewater streams in electric power generating plants include cooling tower blowdown; ash handling wastewater; wet flue gas desulfurization (FGD) system discharges; and material storage runoff. Low-volume wastewater includes air heater and precipitator wash water, boiler blowdown, boiler chemical cleaning waste, flood and yard drains and sumps, backflush from reverse osmosis, and ion exchange boiler water purification units.
Shifting Rules and Priorities
How power plants have opted to treat wastewater has also generally hinged on government guidance and compliance with discharge rules, which vary by region. In the U.S., those guidelines have grown stricter—but are currently subject to political uncertainty. The U.S. Environmental Protection Agency (EPA) in September 2015 finalized revisions to the 1974-promulgated Steam Electric Power Generating Effluent Guidelines (ELG), which had last been amended in 1982, setting the first federal limits on the levels of toxic metals—including arsenic, lead, mercury, selenium, chromium, and cadmium—in wastewater that can be discharged from power plants, as well as new requirements for wastewater from FGD, fly and bottom ash, flue gas mercury control, and coal and petroleum coke gasification. The agency reasoned, citing recommendations from a detailed study of power plant wastewater conducted by the Bush administration, that the “new technologies for generating electric power and the widespread implementation of air pollution controls over the last 30 years have altered existing wastewater streams or created new wastewater streams at many power plants, particularly coal-fired plants.”
However, the rule remains mired in federal court, and the Trump administration has said it would review and “potentially revise” the 2015 rule’s more stringent “best available technology” limitations and pretreatment standards for existing sources for bottom ash transport and FGD wastewater. An EPA spokesperson in April told POWER that the agency was “working expeditiously” on the review and potential revision, noting that a final rule proposed in September 2017 extended compliance dates for those aspects of the 2015 final rule from November 2018 to November 2020.
At the same time, the EPA is also reconsidering the “Hazardous and Solid Waste Management System; Disposal of Coal Combustion Residuals”—commonly known as the Coal Combustion Residual (CCR) rule. Promulgated in April 2015, that rule regulates the disposal of CCR as a non-hazardous solid waste under Subtitle D of the Resource Conservation and Recovery Act, establishing minimum federal criteria for determining which CCR landfills and surface impoundments qualify as “sanitary landfills” and may continue to operate, and which landfills and surface impoundments are “open dumps” and must close. The Trump administration moved to reconsider the rule last September, owing to concerns about aspects of the rule voiced by the Utility Solid Waste Activities Group. This March, the EPA proposed more than a dozen significant changes to the Obama administration’s final 2015 rule, essentially allowing states to operate CCR permit programs in lieu of federal regulations. A second proposed rule with more changes is expected later this year. Litigation of the rule also continues in federal court.
As engineering, environmental, and construction firm HDR Inc. noted, the CCR rule focuses on engineering design of CCR impoundments and landfills, and the solid waste stored in them. “There’s a natural intersection” between that rule and the ELG rule—which is a technology-based rule to govern leachate and water discharges from CCR surface impoundments and landfills—that “should be understood before developing a compliance strategy,” the company advises. Meanwhile, along with the CCR and ELG rules, new rules governing air regulations are setting timeframes that are “driving changes to the water quality and wastewater quality, and they’re putting a completely new framework around the limitations and specific requirements on handling products,” Electric Power Research Institute (EPRI) Senior Technical Leader Jeffery Preece told POWER in April.
Making Long-Term Decisions
Yet another key factor forcing power plant owners to rejigger their water management strategy concerns water availability. The World Research Institute this April noted that 47% of the world’s thermal power plant capacity—mostly coal, natural gas, and nuclear—are located in highly water-stressed areas, which are vulnerable to power disruptions from drought and increased competition among water users. “Even some new facilities that go into existing areas may not have the same access requirements or access rights to water, and so they’re looking at alternative water sources,” Preece noted. “That becomes a challenge not only for operations but in the treatment and handling of any waste byproduct residues,” he said.
Given the regulatory uncertainty and a drive to engage in environmental stewardship, industry is generally planning for “what’s coming down the road” over the next five to 20 years, Preece said. “There’s no longer a silo of managing the different regulations. It really has to come together holistically,” he said. EPRI is working to help industry identify pathways that enable technology development and applications that are more reliable and cost-effective to optimize operations across air to water to water solids, he noted, but industry is also engaging with government and other industries to manage opportunities better across multiple facets. The collaboration is perhaps achievable now—compared to 20 years ago—owing largely to innovations in big data management, which are allowing integration of different components, models, and perspectives, including from individuals, companies, and researchers, he said.
The Vendors’ Perspective
The changing wastewater treatment landscape is offering new opportunities for wastewater treatment vendors, too, though tight spending at power plants, especially in the U.S., is limiting technology development and offerings. Marek Herrmann-Nowosielski, senior vice president of Product Development at Heartland Water Technology—a firm that develops and markets proprietary wastewater treatment technologies for several applications, including oil and gas and power generation—who has focused much time in the Chinese market, noted key differences between that country’s approach to technology adoption compared to the U.S. “The U.S. market is a brownfield market, where new plants that are being built are combined cycle, solar, and wind.” China, where new coal plants are being built extensively, is “very hungry for technology and willing to invest and build new plants that demonstrate new technology as well,” he said. “As a result of that, they’re armed with some better solutions because they are more open to more solutions.” China has progressed development of biological selenium treatment as well as reverse osmosis, he noted.
According to Michael Pudvay, business development manager for global wastewater technology firm Veolia Water Technologies, the shift in generation technologies, paired with lax demand and regulatory uncertainty in the U.S., is also affecting business development of power plants. “Then there’s an economic driver where a lot of these coal plants are just not economical to run anymore,” he said. “The market is in flux because of all these things.” That uncertainty, Pudvay added, is affecting the wastewater treatment business. “If you look at forecasts, you expected more CCR/ELG–compliant projects, and I think right now a lot of those are somewhat on hold.” Veolia, which offers wastewater treatment for combined cycle projects, is presently finding project opportunities, but he noted that “forecasting out in the future, it may not be as many projects.” Future generation in the U.S., will likely comprise wind and solar, which have minimal water demands, and peaker plants, with no cooling towers, he noted. “So, you don’t see water demand being as huge as it used to be. Even some big combined cycle plants are going to air cooling, so they don’t have cooling towers.”
Tightening environmental rules are meanwhile transforming customer needs, Brett Housley, regional sales manager for the power market at WesTech Engineering, told POWER. “Our customers are very aware of these limits, and it means they are requesting higher guarantees from us for a clean water result. They are very serious about their responsibility to keep waters as free of pollution as possible,” he noted. Customers are looking to “purchase an entire system not just the components,” he said. “They want vendors to play a more significant role and take greater responsibility in the complete package process.” WesTech has responded by “being more innovative,” presenting a full turnkey system and providing guaranteed performance. Another trend he noted is an uptick in demand for mobile treatment solutions for temporary projects. “We have installed some in the southeastern U.S.,” where companies are motivated to meet demands for cleanup from coal ash ponds, even at retired coal plants, he said.
Innovations in Technology
Finally, new approaches to wastewater treatment involve taking into account vast technology leaps that have been achieved over the past decade. As it promulgated its 2015 ELG rule, the EPA noted that more than 80% of coal units built over the last 20 years moved to avoid generating ash transport wastewater discharging with dry bottom ash handling systems or closed-loop systems, and many plant owners were increasingly using more advanced technologies to treat wastewater, such as chemical precipitation and biological treatment as an alternative to surface impoundments, which essentially allow particles in wastewater to settle out over time.
Surface impoundments—also known as an “ash ponds” or “settling ponds”—don’t remove pollutants dissolved in water, such as selenium, boron, and magnesium, which are likely to be present under acidic conditions. Often, they are also ineffective at removing particulates, because the composition of fly ash and scrubber wastewater can impede the settling process, the EPA said. Impoundments are also subject to seasonal turnover—causing resuspension of solids that have settled into lower layers—and it documented cases of drinking water contamination caused by impounded wastewaters leaking into groundwater.
According to EPRI, whose Water Management Technology program is invested in identifying, evaluating, and demonstrating technologies to support cost-effective and reliable treatment of water discharged from power plants, factors that affect the eventual technology or treatment choices made by plant owners are deeply dependent on their distinct suitability to a plant as a solution and must consider the chemistry, conditions, and water availability limitations at individual plants. EPRI’s Preece also noted that treatment commonly involves several sequential steps, which may include several physical, chemical, physiochemical, and biological processes.
Over the past decade, however, EPRI and other entities have identified key technologies that could reshape power plant wastewater discharge and treatment. Many have been tested at the Water Research Center (WRC) at Georgia Power’s Plant Bowen, a research facility established in 2012 by EPRI, Southern Co., and 14 other utilities (Figure 1).
Zero-Liquid Discharge (ZLD). A water treatment process in which all wastewater is purified and recycled—and leaves zero discharge at the end of a treatment cycle—ZLD involves an advanced method that utilizes ultrafiltration, reverse osmosis, evaporation/crystallization, and fractional electrodeionization. According to the International Energy Agency’s Clean Coal Centre, some countries—like China—require new coal plants to be ZLD, which will boost the growth of ZLD technologies. “However, ZLD can result in complex systems that are difficult to operate. Some water treatment technologies for meeting ZLD, such as brine concentrators, can have a relatively high energy consumption, and high capital and operating costs,” it said in February. “The use of membrane-based systems that reduce the amount of wastewater treated by brine concentrators and other thermal evaporation techniques can help lower energy consumption and reduce operating costs.” The center also notes that more generators are considering selling salt solids that result from the process to decrease operating costs.
Thermal Evaporation. Traditional evaporation solutions for treating wastewater to reduce liquid volumes (often as part of a ZLD treatment train) have proven to be cost-prohibitive, operationally challenging, and/or resource intensive. However, research is showing promise. One EPRI study at the WRC to evaluate the efficacy of an adiabatic evaporator (used commercially at numerous sites, including a 1,500-MW coal plant in the Midwest) for FGD wastewater treatment/concentration using flue gas from the 952-MW power station as a source of thermal energy for evaporation revealed that the option may suit sites implementing a ZLD treatment train (Figure 2).
Biological Processes. Biological processes, which use bacteria to degrade dissolved organic substances, have been in use for more than a decade, and the EPA describes them as the “best available technology” for selenium and nitrogen. However, their uptake has been limited to fewer than six facilities in the U.S. A major transition to these technologies will require detailed study to understand “how they apply, how we understand the operations and the different types of technologies, the biochemical fundamentals of how and why these specific microbes denitrify and reduce selenium, what are the conditions that cause upset, and how do we prevent and mitigate that,” Preece said. EPRI’s focus for now is on how to improve the “fundamentals,” he added, including analytical methods and ways to monitor and control these technologies in real-time, and gauge fluctuations and variability from different fuel sources.
Physical/Chemical Technologies for Metals and Solids Removal. EPRI has evaluated a number of different technologies with different chemicals that target specific removal of metals and published guidelines for physical/chemical treatment meant to inform operators and engineers about fundamentals of the technology. Preece said that document, a combination of about 10 years of research, finds that opportunities exist for treatment with these technologies ahead of biological, membrane, and thermal system treatment.
Advanced Membrane Technologies. As they become more commercially available, these technologies may potentially enable plants to pre-concentrate brine material and minimize waste volumes. At the same time, they require less thermal water reduction, resulting in a shorter evaporation stage. For Preece, the benefits of the technology are especially important to cost-efficient water recovery. “Membrane technology offers an advantage in that the energy requirement is relatively low compared to a thermal system, but they’re limited much more on the chemistry,” he said. More research is underway to identify reliability and functionality of new membrane materials and new components that may enable a higher clean water recovery without requiring extensive—and costly—cleaning or replacement, he said. “What we can do to push the boundary of membrane technology will be important—and not just for our industry,” he said, noting development was also happening in agriculture and industrial applications.
Encapsulation. This process involves mixing brines with fly ash along with binders such as lime to produce a material—ranging from a low-moisture soil-like consistency to a high-moisture flowable paste—that can be transported to a landfill. As the product solidifies in the landfill, its constituents stabilize chemically and physically. While the technology has been around for more than 30 years, EPRI researchers are investigating the chemistry and mineralogy of the initial ingredients, the reactions, and the final products, along with the entire chain of activities, which involves mixing materials, transporting, and landfilling the product, and monitoring its long-term stability. Preece said encapsulation shows promise as a reliable waste management technology, but field tests over the next two years will determine whether it is technically feasible and cost-effective. ■
—Sonal Patel is a POWER associate editor.