Two-compartmentseptic tank

Loam soil

Crushedstone

Evapotranspirationbed

Wick

On-site wastewater treatment systems

Evapotranspiration bedBruce Lesikar

Extension Agricultural Engineering SpecialistThe Texas A&M University System

ET systems are designed accord-ing to local evapotranspiration andrainfall rates, which vary acrossTexas. The local authorized agent,generally the local health departmentor regional office of the Texas NaturalResource Conservation Commission,

L-52288-99

A n evapotranspiration (ET) bed treats wastewater by using evapo-transpiration — the loss of water from the soil by evapo-ration and by transpiration from plants growing there.

ET beds are used where the soil cannot treat wastewater before itpercolates to groundwater, such as in rocky soils, or where the soil pre-vents wastewater from percolating from the application field, such as inheavy clay soils.

can tell you what the rates are in yourarea. ET bed systems can be smallerin drier regions of the state comparedto the same size household in wetterlocations. These systems do not workin very wet areas where more rainfalls than is evaporated or transpired.

There are two types of ET beds:lined and unlined. In lined systems,the ET bed is lined with a natural clay,synthetic or concrete liner. A liner isrequired if the surrounding soil is verypermeable, such as in sandy gravel orkarst limestone.

Unlined systems can be used inhighly impermeable soils such asheavy clays. In unlined systems,wastewater is disposed of by acombination of evaporation, transpira-tion and absorption, which is oftencalled an evapotranspiration/absorp-tion (ETA) system.

Figure 1: An evapotranspiration bed system.

TreatmentIn ET bed systems, solid materi-

als are removed from the wastewaterby a septic tank. Then the wastewateris distributed throughout the ET bedsystem. There, final treatment anddisposal occur when the waterevaporates and plants use nutrients inthe effluent and release moisturethrough transpiration.

As the water evaporates, salts,minerals and solids from the effluentaccumulate in the bed. During verywet periods when evapotranspirationis low, ET beds store water until drierperiods when it evaporates andtranspires.

DesignAn ET bed contains storage

trenches, loam backfill around thetrenches and sandy loam soil over thetop of the loam backfill for grassgrowth. Generally, the required bedsurface area is divided between twobeds, which allows for switchingbetween the beds to avoid overload-ing.

A liner and sand cushion areplaced in the ground, and the storagesystem is set on the bed bottom.Generally, the storage system consistsof a bed of rocks or gravel of auniform size ranging from 3/4-inch to2 inches in diameter, filling the bed toa depth of 12 inches or less, depend-ing on the bed’s overall depth.Distribution pipes are placed no morethan 4 feet apart and no more than 2feet from the bed walls. The top of thedistribution pipe must be flush withthe top of the rock media.

Other types of media such as tirechips, or storage systems such asleaching chambers, may be used forthe storage trenches.

A water-permeable soil barrier (ageotextile filter fabric) is placed over

the rock. A loam soil is added to fillthe bed to within two inches of thetop. Selecting the proper soil isextremely important in building an ETsystem. (State regulations classify thesoil as a class II, loamy soil.) The soildraws the water toward the surfacefaster than coarse sand.

Wicks incorporated into the rockmedia draw water continuously fromthe rocks into the soil and toward thesurface area, where it evaporates or istaken up by plants. A wick is acolumn of soil that extends throughthe rock media to the bottom of thebed. The total wick area should be 10to 15 percent of the bed surface andshould be uniformly spaced through-out the bed.

After the loamy soil is in place,the final two inches are filled withsandy loam and mounded in thecenter with a slope of 2 to 4 percenttoward the outside of the bed. The laststep is to plant vegetation speciallyselected to transpire the most water,such as bermudagrass or St. August-ine grass. Placing grass sod over thebed may be the best approach toestablishing grass there. Using seedmay let the mounded soil wash awayduring heavy rainfall before the grassis established. Larger plants withshallow root systems, such as ever-green bushes, may also be used tohelp take up water.

If you use grasses with dormantperiods, be sure to provide adequatevegetation on the beds during theseperiods. A common solution isoverseeding with winter grasses toprovide year-round transpiration.

How to keep it workingA valve connecting the two beds

allows you to alternate the wastewaterinflow between each bed. When onebed becomes saturated, turn the valveto send effluent into the other under-loaded bed. An inspection port added

Grass coveris important

for transpirationof wastewater

to each bed will help you determineeach bed’s water levels during use.Covering the port prevents insects,small animals and unauthorizedpeople from getting to the bottom ofthe bed.

Here’s how to maintain your ETbed properly:

✓ Mow the grass cover regularly.Grass cover is important fortranspiration of wastewater.Overseed with a cool-seasongrass to provide transpiration inwinter. If you do not maintain thegrass cover, the system willprobably fail.

✓ Divert rainfall runoff around thesystem. The system is designed tohandle normal rainfall enteringfrom the top of the system, butexcessive rainfall will overload it.Rainfall runoff from buildingsand paved areas can add toomuch water to the ET bed. Thiswater must be diverted around the

system. Maintain the slopedcover on the system to help rainrun off the bed.

✓ Check the vegetation growing onthe system as the system matures.You may need to use salt-tolerantgrasses, such as bermuda-grass,because salt accumulates in thesystem. Water leaves salts in thesoil when it evaporates. Harvest-ing the salt-tolerant grasses mayreduce the salts in the system ifthe plants can accumulate the saltin their leaves. The potential forhigh salt concentrations dependson how much salt is in the watersupply.

✓ Develop good water conservationhabits at home. Excessive wateruse overloads the system andcauses failure.

To keep the beds aerobic andprevent clogging, build them asshallow as possible, from 18 inchesto a maximum of 36 inches deep.

Figure 2: Evapotranspiration beds should be built as shallow as possible, from18 inches to a maximum of 36 inches deep.

Divertrainfall runoff

around the system

2 to 3 inch perforatedmonitoring pipe

GeotextilefabricLoam soil

Sandcushion

PVC liner

2 footmaximumspacing

4 footspace

Alternatecrushed stone

bed

12"

18" min36" max

Loam soil

3 to 10 inchdistribution

pipe

Surface to drainSandy loam-slope

Crushed stone

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:

margorP )h(913 APESUdraoB noitavresnoC retaW dna lioS etatS saxeTTexas On-Site Wastewater Treatment Research Council Texas Agricultural Extension ServiceTexas Natural Resource Conservation Commission Texas Agricultural Experiment Station

noitaicossA retawetsaW etiS-nO saxeTstcejorP noitartsnomeD ytilauQ retaW ADSUConsortium of Institutes for Decentralized Wastewater Treatment USDA Natural Resources Conservation Service