By Brian M. Davis, Ph.D., P.E.
Wastewater treatment can present challenges to the engineer for numerous reasons: the chemistry may be complex and variable, and flows can change with the addition or subtraction of production facilities. In addition, high capital cost outlays for mechanical wastewater treatment system additions and significant long-term operations and maintenance costs can result in lifecycle cost analysis numbers that are sobering to plant managers.
Engineered wetland wastewater treatment systems present an opportunity for plant managers and engineers to meet these challenges head on. These systems are capable of handling complex waste streams with significantly smaller operations and maintenance demands when compared with mechanical treatment systems. Recent advancements in engineered wetland technologies have produced a powerful, green tool for treating industrial and domestic wastewater—one that trades land for the complexity and cost of a mechanical system.
An engineered wetland incorporates a saturated, subsurface flow media-filled bed reactor, lined with an impermeable liner and often equipped with a Forced Bed Aeration™ system to enhance oxygen delivery. The bed media can be designed to adsorb wastewater constituents such as metals, or can be oxygenated to microbially oxidize organic contaminants. Hydraulic design techniques from the chemical engineering discipline are used to size the engineered wetland for optimal biodegradation and/or sequestration of organic and inorganic wastewater constituents. Pilot-testing can provide reaction rate parameters, which are used for sizing of the full-scale engineered wetland treatment system.
Engineered Wetland Technologies
The most basic operating premise of an engineered wetland can be summarized by the phrase, “Plants and bacteria work for free, but people and machines do not.” Although an engineered wetland is planted with selected wetland plants, the bacterial population in the gravel bed–which performs at least 90 percent of the treatment–is truly a “build it and they will come” proposition. Bacteria thrive in the aerated gravel bed, which is hydraulically isolated (using an impermeable liner) from the surrounding terrain, thereby creating an ideal, niche environment for aerobic bacterial respiration.
Aerobic respiration requires sufficient dissolved oxygen in the gravel bed. A Forced Bed Aeration™ system provides the oxygen through a series of aeration tubes that are emplaced at the bottom of the gravel bed. A blower, sized according to the area and depth of the engineered wetland and the organic loading to the gravel bed, provides air at a specified flowrate and pressure to the aeration tubing.
The engineered wetland is sized based on the wastewater flowrate and concentrations of chemical parameters for treatment. A mass loading to the engineered wetland is calculated for parameters such as biological oxygen demand (BOD) and ammonia. The residence time and aeration requirements for treatment of such parameters to effluent guidelines are calculated, and the engineered wetland is sized accordingly. A similar process is followed for parameters such as benzene, toluene, ethylbenzene, and xylenes (BTEX), and other organic compounds. The gravel bed is covered with an insulating mulch layer, enabling continuous operation through the year.
Designed for both warm and cold-weather operation, an engineered wetland can provide significant wastewater treatment lifecycle cost savings; employing a powerful, green technology to do the same work as a steel and concrete mechanical wastewater plant. The picture below shows a comparison of the energy requirements to treat a gallon of domestic wastewater for a number of treatment technologies, ranging from the most passive to the most energy intensive. Engineered wetlands also have much lower energy requirements than mechanical treatment systems, including activated sludge systems (Kadlec and Wallace, 2008). The trade-off needed to achieve this reduction in energy requirements is land: the more passive the system, the greater area required.
Unlike the land devoted to a mechanical treatment system, an engineered wetland can be easily blended into the natural environment, providing an aesthetic amenity to the community. Wetland plants native to region are planted in the engineered wetland during the construction phase. They are maintained through a regular operations and maintenance program that also incorporates blower maintenance and monitoring for compliance reporting. Each engineered wetland, while designed for a specified wastewater flowrate and mass loading, incorporates all of these design elements into an environmentally friendly package.
For Rural Communities
The City of Greenville, in northwest Iowa, is a community of 43 homes and two businesses. The city was being served by individual septic systems, the majority of which did not meet code requirements. The poor condition of the septic tank systems resulted in wastewater entering a local receiving stream. The design solution consisted of new septic tanks connected to a small diameter gravity collection system, along with a 9,900 gallon per day horizontal subsurface flow engineered wetland with Forced Bed Aeration™ for treatment prior to discharge of treated effluent to the receiving stream. A construction photo of the engineered wetland, as shown below, demonstrates emplacement of the impermeable liner.
The picture below shows the engineered wetland after planting. The engineered wetland provides an aesthetically pleasing, low operations and maintenance treatment system for this rural community while enhancing the local environment.
The management of aircraft deicing fluids (ADFs) and associated stormwater and snowmelt is a challenge for airports across much of the U.S. The discharge of ADFs without discharge controls can result in environmental degradation of receiving water bodies. The development and implementation of discharge controls is a topic of increasing concern to airport managers and regulators as the U.S. Environmental Protection Association is currently in the process of developing effluent guidelines for airport discharges of ADFs in stormwater. A proposed rule is scheduled for release by U.S. EPA in December 2009.
Although ADF-contaminated stormwater can be treated in a municipal wastewater treatment system, the cost per pound of BOD treated can be expensive. Engineered wetlands are a viable option for the on-site treatment of ADFs. A 4.6 acre, vertical subsurface flow engineered wetland is currently nearing construction completion at the Buffalo Niagara International Airport in New York State. This system, with a maximum design flow of 1.2 million gallons per day and with Forced Bed Aeration™, is designed to treat ADFs and stormwater and snowmelt at the airport prior to discharge to a receiving water body.
Due to the high BOD concentration in the ADFs, a vertical subsurface flow design was chosen to optimize distribution of the organic load to the media. The engineered wetlands are designed to blend seamlessly into the area adjacent to the runways. The subsurface flow design lends itself to airport applications, as there is no water at the surface, thereby minimizing the risk of attracting birds and other wildlife. Construction photos of the Buffalo Niagara International Airport engineered wetlands are shown below.
Engineered wetlands are an increasingly popular wastewater treatment technology. Recent advancements in engineered wetland technologies have produced a powerful, green tool for treating industrial and domestic wastewater—one which trades land for the complexity and cost of a mechanical system. As energy efficiency increases in importance, the lower operations and maintenance costs of engineered wetlands make this solution more attractive on a lifecycle cost basis. The aesthetically pleasing appearance of engineered wetlands, combined with lower energy requirements, will result in greater emphasis on this technology in the future.