This is a joint publication of Purdue University and the Indiana State Department of Health as part of the on-site wastewater disposal project. Extension paper coming soon.
Designing a Constructed Wetland, Building a Constructed Wetland, Maintenance and Monitoring, More Information, Images
Constructed wetlands can be a good alternative to conventional on-site wastewater disposal systems, which usually consist of a septic tank and a soil absorption field. They may be appropriate for environmentally sensitive areas as well as areas where soils are not suitable for conventional trench systems or space limitations exist.
Constructed wetlands are classified as a pretreatment technology, meaning that they treat the septic tank effluent prior to discharge to an absorption field where final polishing occurs. Pretreatment eases the burden on the soil absorption area by lowering the chemical and biological strength of sewage. It serves as a safety net, filtering out most intestinal pathogens. Additionally, it may buffer the soil during short periods of saturation, extend the life of the system and allow repair or renovation of a failing system when there is no other alternative.
This publication describes general procedures for developing a constructed wetland for a single-family residence (figures one and two). This guide is designed to be used with the diagrams.
Designing a Constructed Wetland
Locate and flag utilities. Provide temporary fences or barriers around absorption field site to prohibit traffic and avoid compaction. Remove trees, shrubs, grass and other vegetation where necessary. If it is necessary to remove soil, reserve for later use. Fill soil should be free of rocks, plants, debris, and frozen clods over two inches.
Grade and level area to be used for constructed wetland cell. Compact fill to the required elevations, filling in any disrupted or low-lying areas. Mechanical compaction is necessary to ensure proper base preparation. The soil may need to be dried or moistened as maximum compaction occurs when the soil is moist.
Constructed Wetland Cell
Size the wetland cell at one gallon per square foot or a five-day detention time. This sizing refers to the area between the inlet and outlet manifold pipes. If multiple systems are to be built, conduct a void ratio test on the gravel to determine the correct detention time. Lengths to width ratios are 2:1 (or less). For example, a two bedroom home would require 300 square feet of wetland area with dimensions of 25 by 12 feet. A constructed wetland cell for a three bedroom home may have dimensions of 30 by 15 feet. (Note: This sizing has been generated using assumptions valid only for small (< 750 gal/day) residential systems. Sizing is temperature dependent, and equations have been solved for Indiana winter conditions. Sizing generally follows plug flow equations. Details on sizing a range of systems can be found in Reed et al., 1995.)
The requirements for the absorption field will be determined by a soil examination. Consult the Indiana State Department of Health (ISDH) to determine specific sizing regulations for both the wetland and the absorption field. The soil absorption field may be downsized to 1/3-1/2 of the size required without pretreatment. However, the possibility of downsizing is dependent upon the hydraulic conductivity of the soil, depth to groundwater, and other factors. Absorption field downsizing is not possible at all sites.
Building a Constructed Wetland
Tanks must either have at least two compartments or contain an effluent filter to prevent transport of solids or scum to the wetland system. The tank should be polyethylene, reinforced concrete or fiberglass capable of withstanding lateral pressures for intermittent periods between pump-out and liquid filling. The septic tank should contain an internal baffle and connecting tees accessible by surface access risers (figure three). If using an existing septic tank, visually inspect the existing tank. It should be a minimum of 1,000 gallons with the inlet and outlet piping in good repair, and no signs of leaking.
After a new tank is set in the excavation, you may wish to install 1-2 inches of polystyrene insulation (blue board) so that it covers the sides of the tank. Cut the insulation so it may be removed to permit access to the tank. Tank insulation is more important for Northern Indiana.
Prior to completion of backfilling, and after installation of piping, fill the tank with water and check for leaks. The tank must hold water level for 24 hours. All connections into and out of the tank and the access riser must be sealed to maintain water tightness.
Septic tank effluent may either flow to the wetland cell by gravity or it may be pumped. Utilize gravity flow whenever possible. If a pump is required, select a pump for the distribution piping and elevation involved. Pumps should be equipped with automatic restart. Protect the pumps against thermal overload. It is best to add septic tank effluent to the wetland in small doses. Large doses may induce clogging of the gravel.
Constructed Wetland Cell
The bottom of the wetland should be level when excavation is complete.
Rip-rap, 3-6 inches in size can be placed around the septic tank riser and any cleanouts and monitoring wells to protect them from accidental damage from vehicular traffic. Make sure that surface water is diverted around and away from the wetland cell (figures seven and eight). One to two inches of rigid insulation on the sides of the wetland cell is recommended for all of Indiana. The insulation will help stabilize the wetland during freezing and thawing cycles.
Line the wetland bed with an impermeable liner made of polyvinylchloride (PVC), high-density polyethylene (HDPE), polypropolene (PPE) or other approved materials. Liner is usually 20-30 mil depending upon regulations. A 45-mil butyl rubber liner that has been treated for UV resistance may also be acceptable. It is important that the liner is sunlight and weather resistant or covered to protect from UV degradation. It should be free of pinholes and defects. Check the liner for holes before placement and leaks after placement by ponding water over the liner to a depth of at least 12 inches above the top of the inlet and outlet pipes. Leave this water in the cell to protect the liner from damage during placement of the gravel. Before placing the liner, it is a good idea to line the wetland excavation with geotextile fabric (four ounces per square foot) or 2-3 inches of sand to protect the liner from punctures. It is important that the liner does not leak where the pipe enters and exits.
Piping is usually PVC. Clean all joints and openings prior to cementing. Test all the joints in water under operating pressure before burial. Do not glue the vertical riser on the outlet end of the wetland (figure 4) within the water level control sump. This should be removable so that the wetland can be completely drained if desired.
Gravel must be of a minimum Mohs hardness of six. There are three different gravel sizes in the wetland.
Materials delivered to the site should be at least 94 percent free of material smaller than #10 sieve. Gravel should be screened and washed. Gravel with excessive fines is likely to cause plugging and subsequent failure of the wetlands. Be careful to keep soil out of the gravel.
The inlet distribution and outlet collection pipes are placed at the bottom of the wetland gravel. The 1 ½ -3 inch rock should be placed first over the collection and distribution piping to a depth of 24 inches and extend out at least 6 inches from the pipes. The lined basin should be filled with water first, so that the impact of falling rock on the liner will be minimized, and more clearly indicate finish grades.
The ½-l inch gravel should be placed next between the banks of previously placed 1 ½-3 inch rock to a depth of 18 inches.
Place six inches of pea gravel on top of the ½-1 inch gravel. Finish grade must be level. Again, filling the basin with water to a depth of 24 inches will aid in determining finish grades. Fill in low places (where water is visible) and grade the high spots such that the finish grade is within specified tolerances. Gravel depth should be no greater than 24 inches (figures five and six).
It has been reported that if the pea gravel is put over the 1 1/2 - 3 inch gravel, the pea gravel infiltrates the 1 1/2-3 inch gravel and contributes to clogging.
Level Adjusting Basin or Sump
The sump is located at the outlet end of the wetland and provides a mechanism to adjust the water level to 2 inches below the surface of the gravel. One way to adjust the water level in a constructed wetland cell is by manually raising or lowering PVC pipe in the slip-joint (socket) elbow located in the bottom of the level-adjusting basin (figure four).
Cattails (Typha), bulrush (Scirpus), rushes (Juncus), and sedges (Carex) are usually the preferred species for the wetland treatment area. (Note: Reeds (Phragmites) are considered an undesirable invasive species in Indiana.) Look for wetland perennials with deep, dense fibrous root systems and winter tolerance. Locate the wetland where it receives full sunlight. Consult your extension botanist, horticulturist, or local wetland plant nursery for recommendations. Select two to five different species. Plant same species together. Use locally grown and adapted plants whenever possible. Other wastewater treatment technologies may be more appropriate for heavily shaded sites that cannot be modified.
Do not select soft tissue ornamentals, such as lilies, for the treatment area. They require harvesting and may increase BOD5 (five-day biological oxygen demand) and nitrogen concentrations. If these plants are desired, place a small decorative planting shelf immediately adjacent to the treatment area (figure two).
Make sure that the plants are free of disease and mold. Keep roots moist at all times. Plants should be inserted into the pea gravel bed to a depth of 2 to 4 inches with the shoots slightly exposed. Roots must be placed in water.
Planting should be done six weeks prior to building occupancy. Rows may be 18 inches apart and staggered 9 inches. Rows should be perpendicular to the direction of the flow.
Planting should occur at air temperatures above 40oF. Planting should be completed before August 24 (Northern Indiana) or September 1 (Southern Indiana) for best performance.
If after six weeks the plants do no seem to be taking hold and growing, replant in-between the original plants in a similar pattern.
If planting cannot take place earlier than six weeks before the expected date of the first hard frost, then postpone planting until spring. If the wetland will receive wastewater prior to planting, spread mulch two inches deep over the gravel bed. Select mulch that will not contaminate the wetland with undesirable weed species. The mulch can be covered with a woven biodegradable netting or jute to hold it in place. Prior to mulching, place a durable material over the gravel to prevent the mulch from clogging the wetland. The mulch will help insulate the bed to prevent freezing during the winter.
Plants typically take two to three years to fully mature.
Watering and Fertilizing
If effluent is not immediately available after planting, flood the wetland so that the water is within 2 inches of the top of the gravel. Apply an all-purpose water-soluble plant food at the manufacturers lowest recommended rate for lawns every three weeks until effluent is available. Do not let the water reach a level above the top of gravel, as plant survival will be less than 50 percent under these conditions.
The minimum period for watering and fertilizing the plants should be six weeks. Watering may be accomplished by leaving the building water supply on and running faucets continuously or by inserting a hose into the front end of the wetland. The water level must be maintained at 1-2 inches below the surface of the gravel for at least one full growing season at all times. Failure to do so will result in the death of the plants and the requirement to replant.
When constructing the absorption field, include 3-4 inch diameter monitoring wells in the trenches. These wells consist of PVC pipe with holes or slots located on the bottom 3-6 inches to allow water movement into the pipe. Place the pipe in the trench or bed before the addition of gravel. The bottom of the monitoring wells are not capped.
Maintenance and Monitoring
Constructed wetlands require minimal maintenance. However, they should be inspected every six months. The most common maintenance activities are pulling out undesirable plant species such as willow tree saplings, removing dead vegetation (not dormant vegetation), and cleaning pipes. Other maintenance activities include replanting, fertilizing, cleaning/brushing screens and pipes, and installation of barriers to exclude deer. The need to control turtles and burrowing animals has also been reported.
Septic tanks should be inspected and pumped every three to five years depending on the tank size and number of people in the home. If applicable, level the distribution box and check any pumps associated with the system. Effluent filters must be periodically cleaned and replaced. Do not mow grass on the absorption field shorter than three inches.
If residents are planning to be away from the home for more than six to seven days, arrange to have water added to the wetland cell with a hose every two to three days.
Cap all exposed pipes and maintain the water level 2 inches below the wetland's gravel surface to control/eliminate odors. The gravel at the inlet end of some systems has clogged, requiring the first 1-2 feet of gravel to be replaced. Dosing with small doses may prevent this. Another management technique used to prevent clogging is draining the wetland two to three times a year during the growing season. Simply unplug the wetland, allow it to completely drain, and immediately re-plug. This also encourages root penetration to the bottom of the wetland, particularly in the first few years of operation.
If Monitoring is Required
Constructed wetlands are still considered experimental in Indiana. The Indiana State Department of Health may require monitoring. It would be useful to note that a considerable amount of water will be lost through evaporation and transpiration (evapotranspiration) in the summer. Therefore, judging wetland performance based solely on concentrations coming out of the wetland may be misleading, as the wetland will both treat and concentrate the wastewater. If monitoring is required, measure the flow going into and out of the wetland. From these measurements mass balance can be calculated. When this information is combined with the concentrations from the inlet and outlet of the wetland, performance can be more clearly evaluated.
Natural Systems for Waste Management and Treatment, Second Edition, S.C. Reed, R.W. Crites, and E.J. Middlebrooks. 1995, McGraw Hill.
Constructed Wetlands Wastewater Treatment Systems for Small Users including Individual Residences. Second Edition. G.R. Steiner, J.T. Watson. 1993. Tennessee Valley Authority, Water Management Resources Group. (Available from National Small Flows Clearinghouse, Design Manual Number 65: General Design, Construction, and Operation Guidelines, Publication WWBLDM65. (800) 624-8301.)
Residential Sewage Disposal, Indiana State Department of Health, 2 North Meridian Street, Indianapolis, IN 46204, (317) 233-7177.
National Small Flows Clearinghouse, (800) 624-8301. West Virginia University, P.O. Box 6064, Morgantown, West Virginia 26506-6064. http://www.estd.wvu.edu/nsfc/nsfc_homepage.html
**Catherine Taylor and Joe Yahner, Agronomy, Purdue University, Don Jones, Agricultural and Biological Engineering, Purdue University; Michael Ogden, Southwest Wetlands Group, 901 W. San Mateo, Suite M, Santa Fe, NM 87505; and Alan Dunn, Indiana State Department of Health
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