Burns & McDonnell's seasoned scientists and engineers set out to discover whether microbial action in earthen materials and indigenous plants could naturally treat a wastewater stream produced by environmental control equipment at Westar Energy's Jeffrey Energy Center in St. Marys, Kan.
What they learned through a successful pilot program could shape how coal-fired plant operators across the country consider alternatives to managing increased and more stringent environmental regulations at a much lower cost.
"Everyone is excited about the potential here," says Jared Morrison, manager, water and waste programs, Westar Energy.
Addressing a Need
Coal remains one of the most abundant energy sources in the world, but it requires advanced emission control technologies to be cleanly used for generating electricity.
Built in 1978, the Jeffrey Energy Center is the largest coal-fired generating plant in Kansas, with three 800 MW units on-site. Each operates with a wet flue gas desulfurization (FGD) scrubber, a system that removes sulfur oxides from the exhaust flue gas to meet federal emission regulations. The FGD system produces a thick mixture of gypsum byproduct that contains high levels of suspended solids, dissolved solids, nutrients and metals that can be toxic to the environment.
In accordance with federal and state regulations, Westar built a wastewater treatment facility to treat the mixture, but the company needed to consider options to further reduce levels of selenium and mercury.
Westar, which serves more than 690,000 Kansas customers, turned to Burns & McDonnell for advice that led to an award-winning project setting the bar for energy companies to advance innovative treatment programs and preserve environmental quality.
Burns & McDonnell has worked with Westar since 2009 after being hired to study wastewater treatment upgrades in the three Jeffrey Energy Center units. That relationship led to discussions on how Westar could treat the remaining discharge using the most environmentally friendly and least costly methods. Westar had considered multiple options. Dennis Haag, a Burns & McDonnell scientist, collaborated closely with Brad Loveless, executive director of environmental services at Westar and his team, on an idea to use nature to do the work. The team came up with a concept for a constructed wetlands treatment system that began as a pilot program in 2010 and is now a fully built, functioning system.
"The system needs time to demonstrate how well it performs and all that it can do," says Chris Snider, a Burns & McDonnell project manager. "We are hopeful it will advance the coal industry's understanding of how engineered natural-based systems can be a cost-effective alternative for handling coal plant waste streams."
Pilot Program Success
For centuries, constructed wetlands have been used as a sink for waste. They act as natural, biological filters and are a viable option for helping to solve a wide range of environmental and water quality issues. But tailoring constructed wetlands to handle certain types of waste stream from coal-fired plants is uncharted territory, and its success hinges on a handful of factors.
"We felt like it was possible and we believed in it," Snider says. "We were thrilled to have a client willing to think outside the box, explore the possibilities and make a significant investment in a pilot study."
The pilot project required a $3 million investment so the team could evaluate and compare constructed wetlands against other treatment alternatives. In this case, Burns & McDonnell sought to process and blend the water from the Kansas River with the pretreated FGD wastewater in order to remove various elements, including mercury and selenium, the primary targeted elements selected for treatment. The pretreated wastewater needed to travel from the site through the constructed wetlands where it could get clean enough to be discharged back to the river or reused in the plant.
In this kind of system, the different species of plants work along with soils and microbial activity to reduce the concentrations of metals in the wastewater. The metals are attenuated in the soils and vegetation.
How the System Works
Haag's design began with 11 species of plants from cattails to water lilies and bulrush concentrated in a 2-acre area on the Jeffrey Energy Center site, centered on more than 10,000 acres full of rolling hills dotted with sunflowers and ponds.
The pilot system evolved into eight wetlands cells designed with two parallel — side by side — treatment lines blending 18,000 gallons per day of Kansas River water with the same amount of pre-treated FGD wastewater that was processed over a seven-day cycle. The design allowed the system to operate during maintenance and in the event of a line failure.
A team of engineers, chemists and agronomists from Kansas State University evaluated the system throughout the pilot. Researchers were impressed with the high levels of heavy metals removed by the constructed wetland cells.
"It's complex and fascinating redox chemistry," says Ganga Hettiarachchi, an associate professor of soil and environmental chemistry in the Department of Agronomy at K-State. "We were quite impressed with the selenium retention capabilities in the treatment soils."
The K-State team developed laboratory scale column studies that demonstrated the treatment performance for the constituents of concern.
"One of the coolest things about this project is how we built upon our pilot experience and combined that with the K-State column study outcomes, including the concept of upward vertical flow in the treatment soil," Snider says.
Findings Lead to Full-Scale Program
The Jeffrey Energy Center pilot project proved a constructed wetlands treatment system is a viable technology for polishing relatively low chloride FGD wastewater. Not only did the plants in all the pilot wetland cells survive, they actually thrived in extreme conditions, and within very high levels of salinity, boron and fluoride. The pilot system itself also operated with very few disruptions, minimal maintenance, and operated in the winter without significant issues associated with the water freezing.
The technical success of the wetland treatment option overall, compared with other alternatives, relied on a detailed plant/water balance Burns & McDonnell prepared with Westar's help. "Westar wanted the flexibility to either reuse the wetland effluent or discharge it," says Katie Bland, senior environmental engineer with Burns & McDonnell. "We were able to show both Westar and Kansas Department of Health and Environment that this technology would be able to achieve those goals."
Based on the pilot's success, Westar opted to move forward with the design and construction of the 28-acre constructed wetlands treatment system that was approved by KDHE.
Using key takeaways from the pilot model, the design team developed an equalization pond to enhance the consistency of water quality treated and allow for storage during maintenance or an emergency.
The team also created a new innovative wetland design that pairs vegetated submerged bed (VSB) cells stacked on top of vertical flowbed (VFB) cells to create a tiered filtering system. The new innovative wetland cell design allows either vertical upflow or downflow through the treatment soil. Hydraulic conveyance through the treatment soil was a key design issue, and the University of Arkansas assisted in testing treatment soil permeability during full-scale construction.
Similar to the pilot, the team designed the full-scale system with parallel treatment lines that include one of the two VFB and VSB cells. The two VFB cells are further split into four treatment cells each, helping to subdivide the system for maintenance purposes. The design enables wastewater to be recycled and reused at the plant and be released into the subsurface, or middle of the wetlands.
"Instead of the birds and other wildlife being exposed to the pretreated wastewater, it travels through the first VFB cell for initial treatment before there is any exposure on the wetland surface," Snider says. "This innovative design helps to reduce ecological risk."
The full-scale project was completed in July 2014 and continues to be monitored for efficiency by Westar. Well supported by stakeholders, regulators and environmental groups, the project is gaining national attention, including the 2014 Edison Electric Institute Award, which honors distinguished leadership, innovation and a project's contribution to the advancement of the electric industry.
"There are a lot of unknowns at power plants; things change a lot and we're going to be learning how this system handles those changes over time," says Paul Von Hertsenberg, a plant support engineer at Westar Energy. "We expect positive results, and we're excited to have it up and running."
For more information, contact Chris Snider, 816-822-3534.