L-5228 8-99

On-site wastewater treatment systems

Loam soil

Wick Crushed stone

Two-compartment Evapotranspiration septic tank bed

Figure 1: An evapotranspiration bed system. Evapotranspiration bed Bruce Lesikar Extension Agricultural Engineering Specialist The Texas A&M University System

n evapotranspiration (ET) bed treats wastewater by using evapo- There are two types of ET beds: transpiration — the loss of water from the soil by evapo- lined and unlined. In lined systems, the ET bed is lined with a natural clay, ration and by transpiration from plants growing there. A synthetic or concrete liner. A liner is ET beds are used where the soil cannot treat wastewater before it required if the surrounding soil is very percolates to groundwater, such as in rocky soils, or where the soil pre- permeable, such as in sandy gravel or karst limestone. vents wastewater from percolating from the application field, such as in heavy clay soils. Unlined systems can be used in highly impermeable soils such as ET systems are designed accord- can tell you what the rates are in your heavy clays. In unlined systems, ing to local evapotranspiration and area. ET bed systems can be smaller wastewater is disposed of by a rainfall rates, which vary across in drier regions of the state compared combination of evaporation, transpira- Texas. The local authorized agent, to the same size household in wetter tion and absorption, which is often generally the local health department locations. These systems do not work called an evapotranspiration/absorp- or regional office of the Texas Natural in very wet areas where more rain tion (ETA) system. Resource Conservation Commission, falls than is evaporated or transpired. Treatment the rock. A loam soil is added to fill the bed to within two inches of the In ET bed systems, solid materi- top. Selecting the proper soil is als are removed from the wastewater extremely important in building an ET by a septic tank. Then the wastewater system. (State regulations classify the is distributed throughout the ET bed soil as a class II, loamy soil.) The soil system. There, final treatment and draws the water toward the surface disposal occur when the water faster than coarse sand. evaporates and plants use nutrients in the effluent and release moisture Wicks incorporated into the rock through transpiration. media draw water continuously from the rocks into the soil and toward the As the water evaporates, salts, surface area, where it evaporates or is minerals and solids from the effluent taken up by plants. A wick is a accumulate in the bed. During very column of soil that extends through wet periods when evapotranspiration the rock media to the bottom of the is low, ET beds store water until drier bed. The total wick area should be 10 periods when it evaporates and to 15 percent of the bed surface and transpires. should be uniformly spaced through- out the bed. Design After the loamy soil is in place, An ET bed contains storage the final two inches are filled with Grass cover trenches, loam backfill around the sandy loam and mounded in the trenches and sandy loam soil over the center with a slope of 2 to 4 percent is important top of the loam backfill for grass toward the outside of the bed. The last growth. Generally, the required bed step is to plant vegetation specially for transpiration surface area is divided between two selected to transpire the most water, beds, which allows for switching such as bermudagrass or St. August- of wastewater between the beds to avoid overload- ine grass. Placing grass sod over the ing. bed may be the best approach to A liner and sand cushion are establishing grass there. Using seed placed in the ground, and the storage may let the mounded soil wash away system is set on the bed bottom. during heavy rainfall before the grass Generally, the storage system consists is established. Larger plants with of a bed of rocks or gravel of a shallow root systems, such as ever- green bushes, may also be used to uniform size ranging from 3/4-inch to 2 inches in diameter, filling the bed to help take up water. a depth of 12 inches or less, depend- If you use grasses with dormant ing on the bed’s overall depth. periods, be sure to provide adequate Distribution pipes are placed no more vegetation on the beds during these than 4 feet apart and no more than 2 periods. A common solution is feet from the bed walls. The top of the overseeding with winter grasses to distribution pipe must be flush with provide year-round transpiration. the top of the rock media. How to keep it working Other types of media such as tire chips, or storage systems such as A valve connecting the two beds leaching chambers, may be used for allows you to alternate the wastewater the storage trenches. inflow between each bed. When one bed becomes saturated, turn the valve A water-permeable soil barrier (a to send effluent into the other under- geotextile filter fabric) is placed over loaded bed. An inspection port added 2 to 3 inch perforated monitoring pipe

Surface to drain Sandy loam-slope

Geotextile Loam soil Loam soil fabric

18" min 3 to 10 inch 36" max distribution Sand pipe cushion 12" Crushed stone

Alternate PVC liner crushed stone bed 4 foot 2 foot space maximum spacing

Figure 2: Evapotranspiration beds should be built as shallow as possible, from 18 inches to a maximum of 36 inches deep. to each bed will help you determine system. Maintain the sloped Divert each bed’s water levels during use. cover on the system to help rain Covering the port prevents insects, run off the bed. rainfall runoff small animals and unauthorized ✓ Check the vegetation growing on people from getting to the bottom of around the system the system as the system matures. the bed. You may need to use salt-tolerant Here’s how to maintain your ET grasses, such as bermuda-grass, bed properly: because salt accumulates in the system. Water leaves salts in the ✓ Mow the grass cover regularly. soil when it evaporates. Harvest- Grass cover is important for ing the salt-tolerant grasses may transpiration of wastewater. reduce the salts in the system if Overseed with a cool-season the plants can accumulate the salt grass to provide transpiration in in their leaves. The potential for winter. If you do not maintain the high salt concentrations depends grass cover, the system will on how much salt is in the water probably fail. supply. ✓ Divert rainfall runoff around the ✓ Develop good water conservation system. The system is designed to habits at home. Excessive water handle normal rainfall entering use overloads the system and from the top of the system, but causes failure. excessive rainfall will overload it. Rainfall runoff from buildings To keep the beds aerobic and and paved areas can add too prevent clogging, build them as much water to the ET bed. This shallow as possible, from 18 inches water must be diverted around the to a maximum of 36 inches deep. The On-Site 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: draoB noitavresnoC retaW dna lioS etatS saxeT etatS lioS dna retaW noitavresnoC draoB APESU )h(913 margorP Texas On-Site Wastewater Treatment Research Council Texas Agricultural Extension Service Texas Natural Resource Conservation Commission Texas Agricultural Experiment Station stcejorP noitartsnomeD ytilauQ retaW ADSU retaW ytilauQ noitartsnomeD stcejorP saxeT etiS-nO retawetsaW noitaicossA Consortium of Institutes for Decentralized Wastewater Treatment USDA Natural Resources Conservation Service