Subsurface Wetlands Treat Smaller Flows
Subsurface flow systems are the most common type of constructed wetlands used to treat household wastewater onsite. They are particularly well suited for on-lot treatment because they require less land area than surface flow wetlands and they usually can be designed to attractively blend in with backyard landscaping. Wastewater is treated below ground in these systems, so they also are less likely to release odors or attract mosquitoes, pests, or pets and children looking for a place to play.
Subsurface flow wetlands are known by a variety of names (rock plant filters, rock-reed filters, and vegetated submerged beds, for example), and they also are used by many businesses, residential developments, and small communities. Although they have certain advantages over surface flow systems, subsurface flow wetlands tend to be most cost-effective for treating small- to medium-size wastewater flows.
Like surface flow systems, subsurface flow systems consist of one or more rectangular treatment basins or trenches, called cells, which may be operated in parallel or in series. It is common for subsurface systems to be designed with multiple cells operated in parallel to allow the cells to be alternated and rested during maintenance. The bottoms of subsurface flow cells may be slightly sloped (up to 0.5%) to assist the flow of wastewater through the system, and a natural clay or synthetic lining may be necessary for certain sites with high groundwater or permeable soils.
In addition, like other wetlands, subsurface flow wetlands are natural systems that don't require energy to perform treatment. When possible, systems are located near to and down slope from the septic tank or other primary treatment unit to avoid the need to pump the wastewater to the system. They also should be sited away from downspouts and natural drainage areas to avoid excess water from entering the system.
However, subsurface flow systems also differ from other wetlands in many ways because they are designed to provide wastewater treatment entirely below ground.
Each subsurface flow wetland cell is filled with a treatment media, such as rock or gravel, which is placed on top of the soil or lining on the cell bottom. The depth of the media layer is usually about one to two feet. In properly functioning systems, the wastewater flows just below the media surface and remains unexposed to the atmosphere while it saturates the layers below. The saturated media and soil, together with the wetland plants roots, create conditions below the surface of the system that are conducive to treatment.
Treatment in subsurface flow systems is more efficient than in other wetland systems because the media provides a greater number of small surfaces, pores, and crevices where treatment can occur. Waste-consuming bacteria attach themselves to the various surfaces, and waste materials in the water become trapped in the pores and crevices on the media and in the spaces between media. Chemical treatment also takes place as certain waste particles contact and react with the media.
Biological treatment in subsurface flow wetlands is mostly anaerobic because the layers of media and soil remain saturated and unexposed to the atmosphere. Cattails, bulrushes, and reeds—the plant species commonly used in constructed wetland systems—are able to grow extensive roots even in these anaerobic conditions. The area where the roots grow is called the root zone, and usually includes the upper six to 12 in. of the media. But in cells that are alternated or allowed to rest periodically, or in which the water level is regularly lowered, the roots sometimes reach throughout the media layer.
Wetland plant roots contribute oxygen to the cells, which allows some aerobic treatment to take place in the root zone. The plants further contribute to wastewater treatment by providing additional surfaces where bacteria can reside and where waste materials can become trapped. Plants also take up and store some of the metals and other pollutants in the wastewater.
Choosing the treatment media is one of the most important design considerations for subsurface flow wetlands. It is usually best to use materials that are available locally to reduce construction costs. Buying, inspecting, transporting, and placing the media in the system can add significantly to the cost of subsurface systems and are the reason they tend to be less economical than surface flow wetlands for treating large wastewater flows.
In addition, engineers and system designers must carefully consider the type, size, uniformity, porosity, and hydraulic conductivity of the media material. These characteristics affect the flow of wastewater in the system and system performance.
In the U.S., gravel and rock are the most common media used in subsurface flow wetlands. Whatever type or size media material is chosen, the most important concern is that it be as uniform in size as possible. When different size media are placed together in systems, the result is fewer spaces between the media, and finer materials settle into the remaining spaces, leaving little room in the system for the wastewater to flow.
Therefore, to prevent system clogging, the treatment media should be sorted and measured to ensure its uniformity. It also should be washed to remove any dirt, leaves, or fine materials, and inspected by the engineer or consultant for cleanliness before being applied to the treatment bed.
A method used to analyze the size and uniformity of treatment media includes sorting them through a series of mechanical sieves of diminishing size. These characteristics are expressed as the media's "effective size" and "uniformity coefficient." The effective size of medium-size gravel is about 32 millimeters, whereas the effective size of coarse gravel is about 128 millimeters in diameter.
Certain characteristics of the media determine, in large part, the rate and pattern of flow in subsurface flow systems and the efficiency of treatment. Gravel in the small- to medium-size range tends to work better than coarse gravel because it offers a greater number of surfaces where biological treatment can take place. Medium-size gravel also is not as likely to become clogged by the accumulation of any solids in the wastewater as is fine gravel or sand. In addition, the smaller spaces between medium-size gravel tends to provide better support for plant growth than the large spaces between coarse gravel. Also, medium-size gravel is more likely than coarse media to promote the slow, even, non-turbulent flow of wastewater through the system that is desirable for treatment.
Once the type and size of the media is chosen, its porosity and hydraulic conductivity should be verified in a field or laboratory test. These characteristics are used together with information about the wastewater and the site to design the system.
Most subsurface flow wetlands are designed so that wastewater travels through the length of the cell one time to receive treatment. Typical wastewater retention times range from two to six days.
The inlet and outlet areas of subsurface flow systems are sloped more dramatically than the rest of the treatment bed and are filled with coarse gravel or rock to prevent clogging in these areas (see diagram). Wastewater enters the system either above the gravel at the inlet or below it through perforated pipe or weirs. It is important that the wastewater is distributed as evenly as possible as it flows into the cell to prevent short-circuiting.
Subsurface flow cells are usually designed with aspect ratios (length to width) of three to one or less. Wider cells tend to be more cost-effective because long narrow cells must be deeper and require more treatment media, which add cost. Also, the wastewater is less likely to back up in wider cells if too much water enters the system or if the rate of flow changes.
System designers usually use formulas, such as Darcy's law or Ergun's equation, to estimate the rate of flow through subsurface flow cells. However, it is impractical for consultants to rely on one set of tools alone when designing subsurface flow systems. Instead, they employ a combination of approaches and built-in safety factors to ensure system performance.
For example, the bottoms of cells are sloped slightly to permit drainage, and the level of wastewater in the cells often can be controlled at the outlet by means of an adjustable pipe, which can be swiveled up or down to allow the cell to drain to the desired level. Engineers also tend to "over design" these systems to offset any unforeseen or unpredictable factors that can affect treatment.
Subsurface flow systems that are properly designed, operated and maintained can effectively reduce the biochemical demand (BOD), suspended solids, nitrogen, metals, and other polluting materials in wastewater to levels that meet environmental standards for discharge or disposal. However, like surface flow designs, subsurface flow phosphorus removal is minimal, which may be a concern in some areas.
Also similar to surface flow systems, subsurface flow wetlands are simple to operate and maintain. Their performance may be lower before wetland plants have become adequately established and during prolonged periods of cold weather.
Depending on the level of treatment and local requirements, effluent from subsurface flow wetlands may be disinfected, discharged directly into the environment, or directed to a soil absorption field for further treatment.
Editor's Note: To order any of the following products, call the National Small Flows Clearinghouse (NSFC) at (800) 624-8301 or (304) 293-4191, fax (304) 293-3161, e-mail nsfc_orders@estd.wvu.edu, or write NSFC, West Virginia University, P.O. Box 6064, Morgantown, WV 26506-6064. Be sure to request each item by number and title. A shipping and handling charge will apply.
Constructed Wetlands General
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