Areas without municipal sewers normally rely on large areas of porous soils to absorb household wastewater. As residential densities increase and resultant wastewaters overload groundwater sources and aquifers, cleaner, more space-saving methods will be sought.
Constructed wetlands simulate natural wastewater treatment systems, using flow beds to support water-loving plants. The roots of these plants help provide an aerobic environment to aggressively break down contaminants. Constructed wetlands can offer an affordable solution to wastewater for sites with some of the following characteristics: warm climate, failed conventional absorption field, narrow or oddly-shaped lot, high water table, low soil percolation, high organic matter/suspended solids in wastewater and enough unshaded area. Because the systems are custom designed, they are applicable for all projects ranging from a one-bedroom house to a whole town.
There are different types of constructed wetlands. Subsurface flow is the most common for residential, as it keeps sewage effluent underground. Surface flow is sometimes more economical. Discharge systems allow some water to flow out of the wetland, whereas non-discharge can absorb all effluent. Different layouts include single-cell, dual-cell in series, or multiple-cell (parallel or in series). Subsurface dual-cell discharge is common for small residential applications.
Wetlands can be custom designed and built, or purchased as a system. Some system components can retrofit existing septic systems. The components of a complete system include: a filtered, two-cell septic tank (or two plain tanks, or a stabilization pond); a bermed or retained cell(s) that contains an impermeable liner, a gravel substrate, mulch and water-loving plants; a distribution system including header pipe, distribution pipe within the cell, collection pipe, water level control structure, various cleanouts and possibly pumps; and a drainage field if required by regulatory agencies. Treated water is high quality and could, in the right conditions, be directly released to a river or aquifer. Low-flow plumbing fixtures can act as a "pretreatment" method to minimize required cell area.
Constructed wetlands are site-specific; expert design and additional calculations to determine the economics are advised. Because year-round flow is necessary to sustain the plants, constructed wetlands are not appropriate for seasonal residences. In colder climates larger cells are needed for freeze-prevention design, and efficiency will be compromised. On steep slopes, cut-and-fill may be necessary to keep the effluent flow slow enough for proper absorption.
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Under the right site conditions, constructed wetlands can be an affordable alternative to conventional wastewater treatment systems.
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Constructed wetlands can be an effective natural method for wastewater treatment.
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There have been good examples of installations across the country, and the agencies listed tend to be helpful and well-advised. Easily-obtainable construction materials are used.
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If the local health department does not recognize the technology or will not allow adequate reduction or elimination of the drainage field, costs will increase. Local policy may require enough drainfielding so as to make the total treatment system larger than if a conventional system was used.
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Wetlands can be custom designed and built, or purchased as a system. Some system components can retrofit existing septic systems. The components of a complete system include: a filtered, two-cell septic tank (or two plain tanks, or a stabilization pond); a bermed or retained cell(s) that contains an impermeable liner, a gravel substrate, mulch and water-loving plants; a distribution system including header pipe, distribution pipe within the cell, collection pipe, water level control structure, various cleanouts and possibly pumps; and a drainage field if required by regulatory agencies. Treated water is high quality and could, in the right conditions, be directly released to a river or aquifer. Low-flow plumbing fixtures can act as a "pretreatment" method to minimize required cell area.
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Costs vary enormously depending on the chemical qualities of the wastewater and the site conditions. A complete system for a house (not including design) can range from $2,000 to$10,000. Downsizing the leach field can offset other costs depending on codes and local regulations. A properly constructed and maintained wetland can last much longer than conventional septic systems.
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