On the whole, wetlands used for wastewater treatment do not have a sparkling reputation in the United States. The coupling of non-technical enthusiasm with unrealistic operational expectations has resulted in numerous systems that do not reliably achieve effluent targets. However, recent development and use of Forced Bed Aeration (FBA) in aerated wetlands has provided proven, consistent, and controllable results for new and retrofit applications.
The downside with passive wetlands is that oxygen transfer is limited by surface area exposure; a typical transfer rate of 2 to 4 grams of oxygen per square meter per day (m2/d) is prudent for design. For this reason, the sizing of a wetland is proportionate to the influent load and the engineering approach is not unlike that for sizing a facultative lagoon.
In the last decade, engineers have been experimenting with aeration systems to improve wetland performance. FBA has been used to great success in post-lagoon nitrification, airport de-icing, and groundwater remediation. In these areas, the incorporation of aeration to traditionally un-aerated wetlands has resulted in systems that are reliable and achieve a high degree of performance.
The technical challenge is unique; there are few similar scenarios in which an engineer is trying to distribute a large volume of air, at low pressure, over a large area. It is like trying to build a constructed wetland over a one-acre air hockey table.
So, what is the best approach to uniformly aerate a large, shallow bed? Aeration of lagoons and treatment plants is well understood. Blowers and diffusers are designed to supply oxygen effectively to deep water. But shallow water needs a different approach because the footprint makes delivering oxygen throughout the area difficult. The trick is selecting the right tubing and airflow rate to distribute small quantities of air in a very uniform pattern.
The result is a recent emergence of hybrid “natural treatment” systems that occupy the middle ground between entirely passive technologies, like constructed wetlands, and those that are completely mechanical, like an activated sludge plant. This compromise of technological extremes has resulted in the successful construction and operation of several innovative projects. A common thread in each project is the trade between land and mechanical complexity — noting that some land was swapped for a blower. But overall, the systems require relatively low operations and maintenance attention and reliably achieve effluent targets.
FBA commonly is used in subsurface flow wetlands, which are simply beds of aggregate or crushed rock or stone through which water is directed. The appeal of this type of wetland is that it provides an environment for bacteria to remain resident in a bed while water passes through. Similar, more mechanical systems have been in circulation for the last 100 years and include an array of technologies that stem from single-pass sand filters to biological aerated filters. Despite the name on the box, the bacteria are responsible for the treatment, and creating the correct environment is crucial to maintaining treatment. Aerated wetlands simply take a squat, boxy technology and stretch it out.
The stretching out is important to note, however, because one of the more desirable features of FBA systems is that they do not require sludge management. The explanation for this is the low “areal” loading rates that promote minimal biofilm growth and in situ aerobic digestion of biosolids. Loadings are kept below 200 grams biochemical oxygen demand per square meter per day (BOD5/m2/d). Below this level, the aggregate void spaces remain unclogged. Above this level, the beds generate excessive “biomat” that hydraulically limits throughput.
Another aspect of FBA systems is that they are designed in such a way that the water is not exposed during treatment, making them ideal for applications in which the water must be shielded from climatic influences. This is critical for the design of performance-based systems in which reliability is paramount, particularly for facilities that are unable to discharge to sewage plants. Management of produced water from oil wells, treatment of spent deicing fluids at airports, groundwater remediation, and tailings water from gold mines are all examples of where FBA systems have been employed for onsite treatment. By using understood hydraulic and thermodynamic principles grounded in chemical engineering, designers are creating wetland “reactors” that are stable and provide predictable, high levels of performance.
Langenreichenbach (LRB), Germany Research Facility — Academics are currently investigating the use of aeration in wetlands systems. Research conducted in 2011 at Helmholtz Center for Environmental Research in Langenrechenbach, Germany, demonstrated the side-by-side performance of different wetland configurations — including wetlands with FBA. The results illustrated the significant improvements associated with adding aeration to the beds.
With more than one year of performance data collected to date, the LRB research team is redefining the relative benefits of wetland aeration for nitrogen removal. Research results to date indicate that aerated wetlands can have a dramatically smaller footprint area and still achieve superior treatment performance (Figure 1). The LRB work clearly demonstrates the relative benefits of aerating passive treatment systems.
Buffalo Niagara International Airport (BNIA), Buffalo, N.Y. — With more than 2.5 meters of snowfall per year, BNIA ranks as one of the “big sprayers” in terms of annual use of airplane deicing fluid. Located 30 minutes from Niagara Falls and one and a half hours from Toronto, Canada, the busy airport receives contaminated stormwater and snowmelt for six months of the year. To provide treatment for the land-limited airport, an underground treatment system utilizing FBA was installed just outside of the “Object Free Area” next to a runway.
The treatment system has been in operation for two years and represents one of the major applications of aerobic treatment systems for glycol-based deicing fluids. The full-scale system is designed to remove 4,500 kg/d of BOD5. Unique to the system is its ability to handle a wide fluctuation of influent concentrations while still maintaining a high level of treatment (greater than 90 percent). Performance results from the 2010-2011 deicing season are provided in Figures 2 and 3.
Pump and treat remediation site, Casper, Wyo. — British Petroleum (BP) in Casper, Wyo., constructed a petroleum hydrocarbon remediation system. The site includes an office park, river front trails, and a whitewater kayak course. The Casper system provides treatment of as much as 11,400 m3/d of gasoline-contaminated groundwater. It blends into the middle of a premier golf course and is anticipated to operate effectively for longer than 100 years. This project design includes aerated subsurface flow wetland cells for BTEX removal. Results from 2004 to 2006 illustrate the ability of the aerated system to consistently achieve excellent benzene removal (Figure 4).
Aerated wetland systems are bridging the gap between traditional mechanical treatment systems and passive wetlands. The degree of operator control that FBA provides is allowing for new applications of treatment wetlands. At the current stage of development and with the documentation of successful performance of these systems, it is apparent that the range of industrial applications will continue to expand. Changing regulatory climates and available datasets proving the efficacy of treatment wetland systems will support their use. As the range of industrial applications increases, so will the requirement to improve existing methods and to create new solutions for the increasingly complex mixtures of pollutants under constrained process and site availability conditions.
Scott Wallace P.E., is president of Naturally Wallace Consulting. He is a globally recognized leader in the design of ecological systems including constructed wetlands, decentralized wastewater systems, stream bank stabilization, and control of nonpoint source pollution. The author of papers in numerous technical and environmental publications, Wallace is the co-author of “Treatment Wetlands Second Edition.” Wallace and his team were awarded the 2009 Diamond Award for Engineering Excellence in Water Resources from the American Council of Engineering Companies (ACEC) for design of a natural system to treat spent deicing fluid at Buffalo International Airport. They also received the 2005 Grand Award for Engineering Excellence from ACEC for design of a hydrocarbon remediation wetland for BP in Casper, Wyo. He can be contacted at email@example.com.
Mark O. Liner, P.E., senior engineer for Naturally Wallace Consulting, specializes in onsite wastewater treatment for industrial facilities with an emphasis on airport deicing, mines, landfill leachate, and groundwater remediation. He can be contacted at firstname.lastname@example.org