POWER’s first Water Award goes to a plant that developed an innovative solution to a common problem: the economic and environmentally responsible disposal of flue gas desulfurization wastewater.
U.S. coal power plants are finding that they need to comply with an increasing number of stringent environmental regulations, and while nobody in any industry looks forward to additional regulatory burdens, new rules can prompt new thinking about familiar processes that result in unexpected benefits. Westar Energy’s approach to handling wastewater from its upgraded flue gas desulfurization (FGD) system is a perfect example.
A Kansas Coal King
Westar Energy (Westar) is an investor-owned utility serving nearly 700,000 customers in east and east-central Kansas and is the largest energy provider in the state. It owns or purchases power from coal, natural gas, nuclear, landfill gas, and wind generation facilities. Westar’s Jeffrey Energy Center (JEC) is one of the fleet’s four coal-fired plants; the others are LaCyne, Lawrence, and Tucumseh.
JEC, located near St. Mary’s, in northeast Kansas, is owned by Westar (92%) and Great Plains Inc. (8%). Unit 1 began operation in 1978, Unit 2 in 1980, and Unit 3 in 1983. The plant burns low-sulfur coal and was recognized by the Powder River Basin Coal Users’ Group in 2003 as that group’s Plant of the Year.
Big Ideas from a Big Plant
At 1,857 MW, JEC is the largest power plant in Kansas (Figure 1). With great size comes great scrutiny, and JEC, like many coal plants, has had to upgrade its environmental systems over the years to comply with federal and state regulations. JEC is also an example of how, over time, new energy-related issues move to the top of the list of society’s concerns.
In a profile of the new constructed wetlands system, Westar notes that at the dedication of Unit 1, then-Vice President Walter Mondale pointed to the JEC as representing the energy wave of the future. At the time, shifting from oil-fired generation—when many plants relied on imported oil—to domestic coal was seen as, and was, a major improvement.
Fast forward to our current energy and environmental situation: As Westar notes, coal “is now often begrudgingly accepted as a necessary part of our energy portfolio. While the political landscape still includes a few who celebrate the use of coal, more are critical, and the majority avoid a stance if given the opportunity. In reality, if America is going to retain affordable, reliable electricity, coal is a necessary element. It’s our job to balance the cost, the environmental impact and the operational effectiveness. Often that balance requires innovation and partnership. The JEC wetlands is a story of both.”
Westar says that nearly 25% of the original cost of JEC was spent on air quality control measures, including burning low-sulfur coal exclusively. Among the most recent environmental upgrades was rebuilding and upgrading the FGD systems on all three units.
The original scrubbers were designed to remove 60% of sulfur dioxide (SO2) emissions. The new limestone slurry scrubbers were designed to remove 95% of SO2. Work on the project (by URS, with Burns & McDonnell acting as owner’s engineer/construction management) began in the third quarter of 2007 and was completed in the second quarter of 2009. All three upgraded scrubbers are in service and are meeting or exceeding emission rate expectations. Westar says the new system is delivering a 97% reduction in SO2 emissions. (Co-benefit mercury emissions were reduced by at least 25%, and particulate matter was reduced by at least 20%.)
Installation of the new scrubbing system triggered the state water antidegradation standard. This requirement drove the need for evaluation and installation of new water control technologies for the FGD wastewater. Prior to installation of the new system, Westar had been dewatering slurry, landfilling the gypsum, and discharging water to Lost Creek after clarification and treatment for mercury removal.
The FGD system discharge required evaluation and treatment for constituents that include sulfate, selenium, mercury, and arsenic. Westar partnered with the Kansas Department of Health and Environment (KDHE) to establish an agreement that temporarily allowed the scrubber wastewater to discharge to Lost Creek while Westar investigated potential methods for treatment.
As Westar engineers looked for a cost-effective way to handle the wastewater, they settled on an approach that marries biology and chemistry: a constructed wetland treatment system (CWTS).
Finding the Best Fit
Before Westar’s environmental and engineering staff decided upon the CWTS, with the help of Burns & McDonnell, they researched a number of innovative ways to effectively treat the discharge:
- Underground deep well injection.
- Process through falling film evaporators and crystallizer.
- Process through reverse osmosis and crystallizer.
- Process with falling film evaporators, using the brine to condition fly ash for disposal in an on-site landfill.
- Treatment with sulfate precipitation and a CWTS, with water effluent sent back to the plant for reuse.
Westar wanted to treat the discharge with the most environmentally friendly and least costly solution. Analysis showed the lowest-cost alternative was a constructed wetland paired with sulfate precipitation pretreatment (Table 1).
|Table 1. Treatment options. Evaluation of the estimated 15-year net present value costs of technically viable alternatives showed a constructed wetland treatment system (CWTS) was the best option. Source: Westar Energy|
In the chosen process, the FGD wastewater would first be treated in a traditional wastewater treatment plant to remove sulfate. Then it would be introduced into the wetland process that was engineered and targeted for removal of metals. The wetland discharge, having been effectively treated for constituents of concern, could then be returned to the plant for reuse.
Although this was the least expensive and most environmentally friendly solution, a constructed wetland system had never been utilized in this application and was untested.
Westar began by developing a pilot system to mimic biological processes occurring in natural wetlands, with the ultimate goal of deploying a full-scale system.
Getting Environmental Groups on Board
The JEC site has long been a special one for both wildlife and people. Approximately 7,700 acres of JEC’s total 10,500 acres are leased for grain or hay production, and nearly all of that acreage is open for public fishing, hunting, and hiking. Water pumped from the Kansas River fills two lakes comprising more than 600 acres for plant make-up water. These provide some of the state’s best fishing during warm months, and the larger lake offers waterfowl hunting in winter. Handicapped-accessible docks at both lakes and an accessible waterfowl blind make the area attractive for handicapped duck and goose hunters.
Additionally, The Oregon Trail Nature Park was constructed for public use on the plant site near the Oregon Trail to showcase the natural ecosystems of Kansas. It includes two ponds, three nature trails, a shelter house, and picnic areas.
Before the utility got serious about the constructed wetlands project, it contacted the local Sierra Club chapter and members of Friends of the Kaw, Kansas Riverkeeper. The intent was to engage and educate them before requesting permission from the KDHE to modify its discharges temporarily for the experimental pilot phase. Westar notes that these local chapters of nationally active organizations have significant interest in Kansas River water quality, and their support was essential to the project’s approval.
Westar hosted educational tours for the groups, which resulted in their support for this treatment approach as the most environmentally sound. The utility also updated the groups on trial results throughout the experiment. When Westar approached KDHE, having already obtained support from these environmental groups helped demonstrate that the chosen approach was appropriate.
The two-acre pilot wetland system, designed to treat roughly 10% of the FGD wastewater, was installed in December 2010. It included three cell types: free water surface cells, vegetated submerged bed cells, and vertical flow bed cells (Figure 2). As Westar describes the differences:
|2. Pilot phase. The pilot wetland system was installed in December 2010 and included three cell types (left to right): free water surface cells, vegetated submerged bed cells, and vertical flow bed cells. Courtesy: Westar Energy|
- Free water surface cells function in a manner similar to a permanently flooded marsh, with a shallow water depth and a combination of aquatic species such as cattail, bulrush, water lily, and arrowhead.
- Vegetated submerged bed cells function similarly to a fully saturated marsh with high ground water levels and plant species such as switch grass, inland salt grass, and sedges.
- Vertical flow bed cells are similar to vegetated submerged bed cells except that incoming water is applied evenly over the surface of the cell, allowing vertical infiltration instead of horizontal flow.
Water was transferred through equal-size cells in series through two parallel trains, allowing each cell to be monitored for individual effectiveness. With a wide range of target constituents, Westar needed to understand each cell’s strengths and weaknesses to design a full-scale system.
The pilot cells were constructed in summer 2010, wetland vegetation plugs were transplanted in November, and the utility expected to start seeing treatment benefits in spring 2011. However, monitoring showed removal of metals right away, most notably in those cells designed to move flows vertically through the plant root zones and soil layers (the vertical flow bed cells).
Constituent removal continued in spring, and the plants “exploded to fill the wetland surface area by summer 2011,” according to a Westar report. Removal rates for the various cell designs were compared with those for other treatments. Kansas State University faculty and students did both field and lab work to help the utility understand the capture mechanisms and magnitudes.
Overall, 19 water quality constituents were shown to have been effectively treated through the pilot wetland project, including selenium, mercury, fluoride, nitrate, and nitrite, which are constituents of concern. The only constituents of concern not treated effectively were chloride and sulfate. Chloride levels were historically low enough that treatment is not necessarily required, while pairing the wetlands with a targeted sulfate precipitation process would overcome the lack of sulfate removal by the wetlands.
Westar and Burns & McDonnell evaluated final treatment options and concluded that full-scale wetlands combined with some supplemental chemical removal of salts presented the most economical and environmentally benign approach. The KDHE approved this plan in summer 2012.
In mid-2012, Westar decided to proceed with the full-scale wetlands. Through 2013 the pilot project was the testing ground that led to the design and construction of the full-scale, 24-acre constructed wetlands. That full-scale project, completed July 2014, now treats 100% of the site’s scrubber wastewater discharge (Figures 3 and 4).
|3. Under construction. This shot shows one of the cells in the full-scale wetland under construction. Cells are lined with a composite liner consisting of clay and HDPE flexible membrane liner. Courtesy: Westar Energy|
|4. Thriving wetland. Water piped from the flue gas desulfurization system at the plant enters the wetland cells from below the surface. Courtesy: Westar Energy|
The extensive pilot research led to an optimized full-scale design consisting of two parallel vertical flow cells (19.2 acres combined) followed in series by two parallel vegetative submerged cells (4.5 acres combined). These cell types proved the most effective at broad-based removal of target constituents.
Testing on the pilot system demonstrated that the constituents are trapped within the soil of the vertical flow cells, and sizing of the wetland is based on the quantity of removed constituents that can be stored per mass of soil. Once a cell is no longer able to remove and contain the constituents, the soil can be removed and landfilled. As each cell is synthetically lined, closure in place is also an option.
The full-scale wetland was designed to mimic nature with one significant exception.
While the vertical flow cells proved exceptional at capturing constituents of concern, their concentration at the surface raised the question of exposure for the myriad species of wildlife that would be attracted to the wetland. Westar was concerned that the captured constituents might be moved away from the wetlands via wildlife movements.
High concentrations at the surface would also necessitate more frequent removal and replacement of the plants and upper soil layer. Since the water was to be pumped between cells, Burns & McDonnell consultants suggested filling from the bottom of the cells and collecting the treated water at the surface for a “bottom up” approach. This solved both concerns by concentrating constituents several feet below the surface, protecting wildlife, and greatly reducing maintenance.
Avoiding negative effects on wildlife was a concern because of high concentrations of common as well as some uncommon species in the area. The endangered Least Tern has a large nesting colony near the wetlands, and JEC contains more than 7,000 acres open to the public for fishing and hunting and managed cooperatively with the Kansas Department of Wildlife, Parks and Tourism.
In a video about the system produced by the utility, Andy Evans, Westar manager of plant support and engineering, calls the CWTS “engineering with nature.” Brad Loveless, executive director of environmental services, explains that the wetland “relies on natural features that have been going on for thousands of years.”
Westar has found that the CWTS offers multiple benefits when compared with alternatives.
A Natural “Green” Solution. The full-scale wetland uses available, naturally occurring soil, plant materials, and soil microbes. The wetland will capture energy from the sun and remove carbon dioxide from the atmosphere. Under controlled soil moisture conditions, each wetland cell can be managed to enhance the interrelationship of soil microbes with plant roots, which increases and maximizes the inherent ability of the system to chemically process, take up, and sequester inorganics and metals found in scrubber wastewater effluent.
Energy Savings. The full-scale wetland provides significant savings in energy usage when compared with other options such as zero liquid discharge or deep well injection. Less energy spent on equipment and processes means more energy is available for customers. The wetland, for example, represents less than 5% of the annual energy costs required by falling film evaporators.
A Sustainable Solution. Constructed wetland systems normally require more land area than mechanical treatment systems (which means they will not be appropriate for all sites) and a grow-in period for the development of plants, roots, and soil microbes. Once they become fully functional, however, constructed wetlands are designed to be “passive” and long-lasting sustainable treatment systems. They require very little energy and maintenance compared with mechanical treatment and are more economical to operate and maintain.
A Social Solution. Many critical stakeholders were found to prefer wetlands and the unique functions and values that they provide to society. For example, wetlands perform as biological filters that help keep streams, rivers, ponds, and lakes clean. They also provide valuable habitat for diverse wildlife.
Water Conservation. The wetland allows the treated water to be reclaimed for reuse at the plant. (For the growing number of power plants around the world faced with water availability constraints, this may be a solution worth considering.)
Favorable Economics. The wetland treatment system is expected to result in net present value benefits of $40 million over 15 years in comparison with alternative zero liquid discharge treatments. This savings includes both capital and operating savings that Westar says will benefit customers through lower rates.
A Model for Future Systems
Westar has already shared its development challenges and successes through numerous industry technical presentations and site tours for interested utilities. It does so to provide a model of an alternative water treatment process for the rest of the industry that offers both improved performance and significantly lower costs than other alternatives. Both of those metrics are increasingly important to fossil-fired generating units.
Although site-specific factors always play a role in the viability of alternative treatment approaches, for Westar, the bottom line is clear: Total loaded costs of the full-scale wetland system and sulfate removal were $36.2 million.
The Edison Electric Institute recognized the importance of the JEC CWTS in June by giving its annual Edison Award to the project. POWER is proud to join the chorus of congratulations for Westar and its staff with our first Water Award! Kudos for innovative thinking, a design that makes no sacrifices, effective stakeholder communication, and timely implementation. ■
— Gail Reitenbach, PhD is POWER’s editor (@GailReit, @POWERmagazine).