L-5230 9-08

Onsite wastewater treatment systems

Constructed wetland Soil absorption Two-compartment field septic tank

Figure 1: A constructed wetland system. Constructed wetlands

Bruce Lesikar Professor and Extension Agricultural Engineer The Texas A&M System

constructed wetland system for domestic wastewater treatment Constructed wetland systems is designed to mimic the natural wetland treatment processes by should be designed and built to blend A Mother Nature. This system uses plants and microbes to improve the into the home’s landscaping. The best wastewater quality. way to achieve this goal is to deter- mine where the wastewater treat- Natural wetlands generally have removing the settling solids and ment system will be located before visible water in the system. However, floating solids (oils and greases) the house is built. Effective planning for those at homes, the water flows from the wastewater; before building the house simplifies beneath the media surface, which ✓ A constructed wetland, which is the system and helps you enjoy it. limits contact between residents and a bed of graded stone, with water For example, a key factor in de- wastewater. beneath the surface, where aquatic termining whether or not your system The constructed wetland waste- plants are grown. It removes nu- uses gravity or a pump to move the water treatment system has three trients, organic matter, suspended water between the system components main components that work together solids, and pathogens; and is the location and elevation of the to purify wastewater: ✓ A final treatment and disper- plumbing exit from the house. If the ✓ A septic tank, which is an en- sal system, which disperses the plumbing exit is too low, you will need closed watertight container that wastewater into the soil for final to install a pump to lift the wastewater provides primary treatment by treatment and dispersal/reuse. back to near the ground surface to plants provide oxygen to the bed and have a gravity flow system. remove some of the nutrients. Other Also, the pipes transporting the processes such as filtering, microbial wastewater must be installed with a decomposition, and attachment to slight slope to help wastewater flow particle surfaces also help remove through the system. If the yard slopes pollutants. The longer the water is de- in the same direction as the waste- tained in the wetland, the more pol- water flow, the wetland will be near lutants are removed and the better the the soil surface, thus increasing its quality of water leaving the wetland. aesthetic value. After flowing through the media, The wetland consists of a bed of wastewater exits the wetland through graded stone where aquatic plants are a water-level control sump. The sump grown. The wetland cell is generally allows the water level to be adjusted, an earthen basin lined with com- because the wetland water level must pacted native clay, bentonite clay, be lower than the media surface to concrete, PVC, hypalon, or PondGard prevent odors. Ethylene Propylene Diene Terpolymer The water then flows into the soil (EPDM) Rubber. The media bed itself absorption field, the last treatment is usually washed rock but can be any stage. If it cannot enter the field by porous material that resists being cor- gravity, it enters a pump tank and roded or dissolved by wastewater. is pumped to the absorption field, The media bed has devices to where the soil absorbs it. Microbes distribute wastewater entering the and plants growing in the soil use the system and to collect wastewater remaining nutrients in the water. Constructed leaving it. The water to be treated Design wetland systems flows horizontally through the bed, remaining below the media surface. All components of the wetland should be Plants in the wetland must be system must be designed properly. able to survive in a saturated medium. This fact sheet focuses on the wet- designed and built to Although soft- and hard-tissue plants land. For information on septic blend into the home’s can be used, some experts believe that tank sizing and final treatment and hard-tissue plants are better because dispersal systems, see the Extension landscaping they may provide a pathway for oxy- publication L-5227, Septic Tank/Soil gen to enter the wetland during winter Absorption Field. and reduce the need for plant main- Wetland systems remove bio- tenance. However, residents prefer logical materials, suspended solids, soft-tissue plants because they gener- nutrients and pathogens from the ally have flowers and are colorful. wastewater. To determine a wetland’s Remember that if you use soft-tissue size, consider: plants, your system may need more ✓ Temperature of the system, maintenance. which affects how fast it removes nutrients; and Treatment ✓ Amount of waste. A wastewater Wastewater is treated by the sep- treatment system must be de- tic tank first. The tank, which should signed to treat the most wastewa- have two compartments, should be ter that a residence generates. sized appropriately to allow enough This information is used in an time for the settling solids to separate equation to determine the hydraulic from the wastewater. detention time, which is the amount Upon leaving the septic tank, of time the wastewater needs to stay in wastewater enters the wetland. It is the wetland. It is generally expressed believed that pollutants entering the in days. The longer the wastewater wetland are removed from the water stays in the wetland, the more time by microbes living on the surfaces that microbes and plants have to treat of the media and plant roots. The the water. Generally, the water needs Cleanout Adjustable standpipe

3/8 inch pea gravel

1% bed slope 4 inch inlet 4 inch outlet pipe pipe

Figure 2: A constructed wetland is a bed of graded stone where water flows beneath the surface and plants grow on top. to stay in the wetland system for 2 to width. That calculation gives you the 3 days. wetland length-to-width ratio, which The relationship between how should range from 2 to 3. much water is stored and how long it An example: Say you need a wet- stays in the wetland determines how land to be 360 square feet. You could much surface area the system needs. build it 3 feet wide by 120 feet long, In general, a constructed wetland in but that would give a length-to-width Texas needs about 1 square foot of ratio (120 divided by 3) of 40 — way Plants in the surface area for every gallon of waste- too high. If you built it 12 by 30 feet water it receives a day. A wetland (divide 30 by 12), the ratio would be wetland must be for an average three-bedroom house 2.5 — which falls into the accepted able to survive in producing 300 gallons of wastewater a range of between 2 and 3. day would need a surface area of 300 When enlarging the wetland, a saturated medium square feet. keep the length-to-width ratio rela- Physical limitations, such as tively constant by extending both wetland depth, bottom slope, and the width and the length. If it stays length-to-width ratio, also can affect relatively constant, the flow per unit the wetland’s performance and sizing. of cross-sectional area increases less The depth of a wetland can vary, but than if the wetland size were in- it generally ranges from 1 to 2 feet. A creased by adding length only. 1-foot-deep wetland has double the Gravity pulls the water through surface area for the same storage vol- the wetland. The slope of the wetland ume as a 2-foot-deep wetland. bottom also helps the water move The wetland should have enough through the wetland. In residential cross-sectional area for water to flow systems, the slope could be used to into it. Water flows through the wet- help flush solids from the pore spaces land by passing from one pore space in the media. Common slopes in resi- to the next while remaining below dential systems can range from 0 to 1 the surface of the media. If the total percent. Bottom slopes for municipal cross-sectional area of pore spaces is systems need to be designed based on too small, wastewater cannot enter the hydraulic loading rates. media, and the water level rises to the surface of the wetland, causing odors. How to keep it working One way to determine if you Each of the constructed wetland have enough cross-sectional area to system’s three main components — accept the flow of wastewater is to the septic tank, constructed wetland, divide the length of the wetland by its and final treatment and dispersal system — has its own maintenance the pores needed for the water to requirements. See the Extension flow through the wetland bed. To publication, L-5227, Septic Tank/Soil do this, remove plants from the Absorption Field, for tips on how to wetland and replant. Manage the maintain the septic tank and final water level to help break down The longer treatment and dispersal system. the fixed film on the media. Also, In general, maintain the septic prevent soil from entering the the wastewater stays tank periodically to make sure solids wetland, because soil can fill the in the wetland, are removed efficiently. Pump the pores. If the media is filled with tank every 2 to 3 years, depending on soil or solids, you will need to the more time that the size of the home and the number remove the media and replace microbes and plants of people living there. If an effluent with clean media. filter is located at the exit of the septic ✓ Control the water level. You may have to treat tank, clean the filter periodically. need to adjust the water level in the water Here’s how to take proper care of the wetland so that it is below the constructed wetland: the media surface. When it is too ✓ Manage the wetland as a rock close to the gravel surface, the garden. To keep it looking good, wetland smells bad. Lowering remove dry material as plants the outlet pipe in the water level freeze in the winter. control sump will lower the water ✓ Keep the pore spaces in the me- level in the wetland and prevent dia open. Removing plant roots odors. from the wetland helps maintain

The Onsite Wastewater Treatment Systems series of publications is a result of collaborative efforts of various agencies, organizations and funding sources. We would like to acknowledge the following collaborators: Texas State Soil and Water Conservation Board USEPA 319(h) Program Texas On-Site Wastewater Treatment Research Council Texas AgriLife Extension Service Texas Commission on Environmental Quality Texas AgriLife Research Consortium of Institutes for Decentralized Wastewater Treatment USDA Natural Resources Conservation Service