Surface Flow Systems Work Like Natural Wetlands
This is the second of three articles on constructed wetlands. To read the first one click here. They are reprinted with some modification from Pipeline, a quarterly publication of the National Small Flows Clearinghouse at West Virginia University, Morgantown, WV.
In the United States and around the world, there are two main types of constructed wetlands used for wastewater treatment: surface flow wetlands (also called free-water-surface wetlands) and subsurface flow wetlands.
Of the two constructed wetland designs, surface flow systems most resemble natural wetlands both in the way they look and the way they provide treatment. Both designs can be used to treat wastewater from individual and community sources, but surface flow wetlands are usually more economical for treating large volumes of wastewater.
Wetlands are areas on land where the ground maintains saturated conditions for much of the year. As the name suggests, surface flow wetlands stay saturated enough to maintain a shallow level of water and wastewater (four to 18 in. deep) above the soil exposed to the atmosphere.
Wetland plants also are present in surface flow systems, and natural forces, such as wind, sun, rain, and temperature, affect the plants, the water, and the treatment processes in these systems, just as they do in lagoons and natural wetlands.
Although the natural forces and treatment processes at work in wetland systems are always somewhat unpredictable, treatment is controlled to a great extent through some carefully planned design features.
For example, the size and configuration of surface flow systems are carefully based on such information as the performance of existing systems, estimates of the volume and strength of the wastewater to be treated daily, and/or estimates of how long the wastewater needs to remain in the wetland to receive treatment (the hydraulic residence time). System designers also consider climatic factors, such as average temperatures, evapo-transpiration rates, and precipitation amounts, to predict and maintain the level of water in the system.
However, because there are so many variables to consider, some which are less predictable than others, and because no design approach has been established as being the single most effective, engineers usually take a combination of factors into account when designing wetland systems. In addition, they usually size surface flow systems generously as a precaution to ensure that treatment standards are always met, even in the face of such unforeseeable events as unusually prolonged periods of rain or extreme cold.
System designers often design a single system to include multiple wetland basins called cells. Two or more cells providing the same level of treatment may be operated side by side or alternately (parallel operation) to allow for periodic maintenance of each cell, or multiple cells may be operated in series to provide an improving level of treatment in each consecutive cell. Some communities even use hybrid systems that include both surface flow and subsurface flow wetland cells.
Ideally, wetland systems should be sited as closely as possible to, and down slope from, the septic tank, aerobic unit, lagoon, or other facility that will be providing primary wastewater treatment. This way, the wastewater can travel to the wetland by gravity. If there isn't enough suitable land available nearby, it may be necessary to pump the wastewater to the wetland, which can add to the overall costs for construction, operation, and maintenance.
Most surface flow wetland cells are self-contained rectangular-shaped basins surrounded by banks on all sides. The inlet and outlet are located on opposite sides, as shown in the diagram.
Exceptions include certain systems in arid climates designed for no discharge, which do not have an outlet, and systems used for advanced treatment that open directly to natural wetland areas.
The bottoms of surface flow wetland cells should be somewhat free of bumps and ridges and have a slight down grade (approximately 0 to 0.5%) to assist the flow of wastewater through the cell by gravity. Unless nature has already provided a site with these features, the cell usually must be excavated either with a backhoe or by hand.
The cell bottom also needs to be self-contained to prevent wastewater from seeping into the groundwater below and the surrounding environment. So, for sites with soils that are not naturally dense or watertight, it often is necessary to line the bottom of the cell with clay, bentonite, or a synthetic liner, and then add soil or other material on top of the lining to form a substrate that will support the growth of wetland plants.
Surface flow systems can treat wastewater much more efficiently than natural wetlands. The reason for this is that the rate and pattern in which wastewater flows through the system is controlled by design.
In natural wetlands, wastewater tends to flow through relatively narrow, well-established channels and may never even come in contact with a large portion of the wetland area. To prevent this short-circuiting of the wastewater flow, and to make the best use of every inch of cell space, the wastewater in constructed wetlands should be evenly distributed across the width of each cell. Wastewater enters surface flow cells by means of perforated distribution pipes, gated pipes, or a series of weirs at the inlet. At the outlet, most systems have control valves and other devices to help operators to adjust the water level.
Even the rectangular shape of the cells and the amount and placement of wetland plants is designed to optimize wastewater flow and treatment. The ratio of the cell's length to its width (the aspect ratio) usually ranges from 2:1 to 4: 1, but may be higher depending on the site and other factors.
In addition, system designers must estimate the amount of head loss wetland vegetation is likely to cause in flow rates. The placement of the plants can be planned and arranged as well. For example, some surface flow cells are designed to have areas of open water as well as areas of dense vegetation to allow wind and sunlight to reach parts of the cell to influence flow and treatment.
Most systems are designed for the wastewater to flow once through the system. However, systems can be designed to treat the waste stream more than once.
As soon as wastewater enters a surface flow wetland cell, natural processes immediately begin to break down and remove the waste materials in the water.
For example, before the wastewater has a chance to travel very far in the wetlands, much of the small suspended waste material is physically strained out by submerged plants, plant stems, and plant litter in the wetlands The roots, stems, leaves, and litter of wetland plants also provide a multitude of small surfaces where wastes can become trapped and waste-consuming bacteria can attach themselves.
As is true in most natural environments, wetland systems are teeming with life. Bacteria are among the most plentiful life forms in wetland systems and are believed to be responsible for providing most of the wastewater treatment that occurs.
Aerobic bacteria thrive in wetlands wherever oxygen is present--usually in the water, especially near the surface. Wind, rain, wastewater, and anything else that agitates the water surface can add oxygen to the system.
Conversely, anaerobic bacteria thrive where there is little or no oxygen. In surface flow cells, oxygen is scarce in the lower substrate and soil. In systems that maintain deep water levels, there may be an anaerobic zone near the cell bottom.
When the different types of bacteria in wetlands consume waste particles in the water, they convert them into other forms, such as methane, carbon dioxide, and new cellular material. Some of these substances, in turn, are used as food by plants and bacteria in the wetland.
Worms, protozoa, insects, and other organisms live in wetlands as well, and many of these organisms also contribute to treatment or to maintaining conditions in the wetland conducive to treatment.
For any of the natural processes in wetlands to be successful, the wastewater must remain in the system long enough for treatment to occur and for viruses in wastewater to die-off naturally.
Typically, the hydraulic residence time for wastewater in surface flow systems is five to ten days or more. The exact time needed is estimated based on wastewater strength, the level of treatment desired, climatic factors, and how efficiently biological treatment processes are expected to work. System size is based on this information.
One of the most important factors affecting treatment is temperature. Biological treatment processes tend to speed up in warm weather and slow down in cold weather. In cold climates, systems must be large enough to accommodate the longer hydraulic residence times needed for treatment.
While most experts agree that plants are important to treatment in wetlands, they do not completely understand exactly to what extent. But plants help treatment processes in several ways. Even in winter, when wetland plants appear dead, they often are dormant but still contribute to treatment.
Some of the important contributions of wetland plants that were already mentioned include their role in filtering wastes, regulating flow, and providing surface area for bacteria and treatment. In addition, floating plants, such as water lilies, and emergent plants, such as cattails, shade the water surface and control algae growth.
Plants also contribute to treatment by taking up nutrients, metals, and other substances and retaining them. However, many of these substances can accumulate again in the wetland when plants die and can be released from the system if the wetland is flooded by excessive storm runoff or an increase in hydraulic load. Plant harvesting usually is not a practical solution in surface flow systems.
In some cases, two or more growing seasons may be required before plants are established enough in the system to realize their full treatment potential.
When properly designed and operated, surface flow wetland systems effectively reduce biochemical oxygen demand (BOD), suspended solids, nitrogen, metals, trace organics, and pathogens in the wastewater to levels that meet environmental standards. Phosphorus removal usually is minimal, however, and there always is a small amount of residual organic matter in the effluent from dead plant materials. Depending on the level of treatment and local requirements, effluent from surface flow wetlands may be disinfected or discharged directly into the environment.
The performance of wetland systems is more variable than that of many other systems and is affected by changing temperatures and other factors. However, when adequately designed, performance should always remain within regulatory requirements. Performance may be lower in cold weather and during the initial startup of the system, before wetland plants have become adequately established.
Surface flow wetlands have few operation and maintenance requirements, but maintenance must be performed properly to ensure system performance. Operation may entail alternating cells or adjusting water levels. Some systems may have banks and berms that need to be maintained, and inlet and outlet structures that should be cleaned periodically. Mosquitoes and burrowing animals present problems in some systems. Different control methods are available, including natural solutions, such as trapping and relocating animals, introducing fish that eat insect larvae, and building bat houses.
Editor's Note: The third and final article in this series will appear in the near future. It will discuss how subsurface wetlands can be applied to treat smaller flows.