By Scott Wallace
Industrial site managers are facing a wide variety of challenges these days, ranging from wastewater treatment to stormwater management, how to redevelop brownfields and manage life cycle costs. With the global economy slowing down, everyone is taking a hard look at their bottom line and realizing that in many cases, we cannot carry on with “business as usual.”
Oddly enough, these economic challenges are resulting in a renewed interest in wetland treatment systems. The economics of wetlands can be summed up in the phrase “plants and bacteria work for free, and people and machines don’t.” Where site managers have the option of trading land for mechanical complexity, wetlands can often be harnessed to do the same job as more conventional (and higher cost) treatment systems. Savvy managers are realizing that there are a variety of “wetland” technologies out there, and different types of wetlands can be combined to offer a customized treatment solution.
Free Water Surface Wetlands
When most people think of building a wetland, the vision that first comes into their mind is an open-water marsh, maybe with some cattails growing in it and a few ducks swimming around. These systems, called Free Water Surface (FWS) wetlands are essentially man-made equivalents of natural marshes. This concept grew out of the North American experience of the 1970s, when people starting paying closer attention to the natural environment and realized the benefits that wetlands were providing. Free Water Surface wetlands are a good option for projects that are combining multiple uses such as water management, green space, recreational use, and wildlife habitat. Free Water Surface wetlands can handle higher sediment loads than other types of wetlands, and this makes them a popular option for stormwater management.
The Free Water Surface wetland in Faribault, Minn., is used by a power plant for stormwater harvesting and cooling water management. The wetland is part of a popular community park featuring trails, wildlife viewing areas, and other recreational features.
Subsurface Flow Wetlands
Another, very different concept of constructing wetlands was developed in Germany during the 1960s and 1970s. These systems, termed Subsurface Flow (SSF) wetlands use beds of gravel or peat. The trademark feature of these wetlands is that water is not exposed during the treatment process. This makes them very suitable for applications where odors may be an issue, or where mosquitoes may be a concern. Both horizontal flow and vertical flow applications have been developed. These Subsurface Flow wetlands remain popular in Europe.
Designing wetlands that do not have any exposed water has lead to some interesting design twists and challenges. In cold climates, engineers have realized that adding a mulch layer on top of the wetland bed insulates the system, allowing for year-round operation in cold regions like Canada. Subsurface Flow wetlands can also be designed with reactive materials in the bed. A common example is the use of limestone in the wetland bed, which can stabilize and buffer the pH of water being treated.
One major challenge facing airport managers is the contaminated runoff generated by deicing airplanes. New regulations are being put forth by the U.S. Environmental Protection Agency that requires collection and treatment of this runoff. The cold temperatures and variable nature of deicing operations makes treatment in conventional systems difficult. An innovative solution being implemented by the Niagara Frontier Transportation Authority is the use of Subsurface Flow constructed wetlands for treatment of deicing runoff. Because the water is not exposed during the treatment process, the system is not attractive for birds. This opens up opportunities to build treatment units within the airport property.
Mixing and Matching Wetland Types
Different types of treatment wetlands can also be combined into an overall treatment process. This “mix and match” approach allows the advantages of different wetlands to be combined to meet complex, multi-stage treatment goals. Certain types of treatment challenges, such as contaminated groundwater or landfill leachates, often involve a combination of different pollutants; for instance metals combined with petroleum hydrocarbons.
Due to the variable nature of landfill leachate for example, a treatment system must be able to receive and properly treat a wide range of parameters over a varying range of concentrations. For this reason, a complete leachate system must include a number of unit processes that target removal of a certain group of parameters. Wetland treatment systems are used in concert with other processes to completely treat a full range of leachate parameters. In particular, wetlands are used for bacterial-mediated degradation of some of the more difficult to degrade organics. Aerobic and anaerobic zones can be engineered in the wetland to expedite the degradation of xenobiotic compounds. Subsurface wetlands with properly sized gravel media provide a stable surface for attached growth bacteria, which allows the bacteria to be resident in the wetland and acclimate to the variable load.
Petroleum Hydrocarbon Remediation
A good example of this approach is a former refinery site in Casper, Wyo., where three different treatment stages were combined into the overall treatment process.
The treatment challenge faced at the Casper site was a combination of high iron concentrations and residual petroleum hydrocarbons left over from refinery operations. The three-stage process used included a cascade aerator (to oxidize groundwater), Free Water Surface wetlands for iron precipitation and removal, and a series of Subsurface Flow wetlands for removal of petroleum hydrocarbons such as benzene, toluene, ethylbenzene, and xylene. The entire refinery site was converted to an 18-hole golf course, and the treatment wetlands were integrated into the golf course design.
The former refinery site in Casper, Wyo., was converted to an 18-hole golf course during brownfield redevelopment. Treatment wetland units were integrated into the golf course for the treatment and management of legacy contaminants.
Getting the Size Right
Constructed wetlands are still an evolving science. For instance, about 90 percent of the papers written on the performance of wetland treatment systems were published in the last 15 years. Consequently, many engineers and scientists are not familiar with the technology or how it operates. Fortunately, there have been a several textbooks published on the engineering design of treatment wetlands in recent years, making it much easier to access information that was formerly scattered across many scientific journals.
Pilot Testing Comes into Play
However, for some applications, just knowing what has been written before is not enough. This happens in instances where the contaminant is outside of the scientific literature, or in instances where there is a complex mixture of contaminants present. This often occurs with landfill leachates, where the liquid present in the landfill (leachate) is often a mixture of organic compounds, ammonia and metals, depending on the nature of the solid waste placed in the landfill. A multi-stage treatment approach is often required, and pilot testing can be used to explore design concepts, develop site-specific treatment parameters, and optimize the overall treatment process. The pilot-scale wetland system is being used for process optimization. This system is treating leachate from a landfill near Chicago, Ill.
Wetlands of the Future
Constructed wetlands are one of the “green” technologies that are beginning to transform our economy. Because plants and bacteria work for free, the economics of wetland treatment is inherently different than technologies that rely on energy and chemical inputs. In fact, many companies are realizing that the energy savings of wetlands is a path forward to reducing carbon dioxide emissions, and in some cases, claiming carbon credits.
The greening of brownfield sites is an important step in reclaiming the post-industrial landscape. With their ability to manage water, remove contaminants, create green space and wildlife habitat, wetlands will continue to play an expanding role in industrial site management.
Environmental managers and engineers are looking for treatment solutions that work for the long haul. Designing systems to handle future flows with lower life cycle costs is a goal in most cases. Engineered wetlands deliver on this goal and integrate the treatment necessity into the facility often enhancing the site for plants, animals, and people.