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Evapotranspiration (ET) is a method of onsite wastewater treatment and disposal that offers an alternative to conventional soil absorption systems for sites where protection of the surface water and groundwater is essential. An ET system is unique in its ability to dispose of wastewater into the atmosphere through evaporation from the soil surface and/or transpiration by plants, without necessarily discharging it to the surface water or groundwater reservoir. However, in certain cases, the ET concept also offers flexibility by combining seepage with evaporation as and alternative option.
Why Use an Evapotranspiration System
An ET system is a feasible option in semi-arid climates and locations where the annual evaporation rate exceeds the annual rate of precipitation and wastewater applied. The ET System can eliminate all or most of the effluent from being discharged into an environment with limitations on nitrogen discharges or impervious soils.
Evapotranspiration is the net water loss caused by the evaporation of moisture from the soil surface and transpiration by vegetation. For continuous evaporation, three conditions must be met. (1) There is a latent heat requirement of approximately 590 cal/g of water evaporated at 15 C, (2) A vapor pressure gradient is needed between the evaporative surface and the atmosphere to remove vapor by diffusion, convection, or a combination of the two, (3) There must be a continuous supply of water to the evaporative surface.
Evapotranspiration is also influenced by vegetation of the disposal field. Theoretically, evapotranspiration can remove high volumes of effluent in the late spring, summer and early fall, especially if large silhouette and good transpiring bushes are used.
There are three main types of evapotranspiration systems; (1) evapotranspiration (ET), (2) evapotranspiration/absorption (ETA) and mechanical.
Evapotranspiration Bed
The ET system is the most commonly used evapotranspiration system used. The main components are (1) a treatment unit, usually a septic tank and (2) an ET bed with wastewater distribution piping, a bed liner (unless the soils are determined to be impermeable), fill material, monitoring wells, overflow protection and a surface cover. Vegetation has to be planted on the surface of the bed to enhance the transpiration process.
The clarified effluent from the septic tank flows into the lower portion of a sealed ET bed that has a continuous impermeable liner (or dense impermeable sub-soils) and carefully selected sands. Capillary action in the fine sand causes the wastewater to rise to the surface and escape through evaporation as water vapor. In addition, vegetation transports the wastewater from the root zone to the leaves, where it is transpired as a relatively clean condensate. This design allows for complete wastewater evaporation and transpiration with no discharge to nearby soil.
The second type of evapotranspiration system is known as an evapotranspiration/absorption (ETA), which is an unsealed bed where evaporation and transpiration are the primary means of disposal, but percolation is also used. This design provides discharge to both the atmosphere and subsurface.
The third type of evapotranspiration system is the use of mechanical devises. There are two types of mechanical evaporation systems. The first is a rotating disk mechanical evaporation unit. The disks rotate slowly so that the moisture on their wetted surfaces can evaporate into the air moving over the unit.
The second type is a concentric cylinder unit where forced air enters at the center of the cylinder, and moves outward through wetted cloth wraps and is discharged as vapor.
Mechanical systems use a very small amount of electricity and require a minimal amount of maintenance.
Accurate estimates (daily, weekly and monthly) of flow rates must be calculated as part of the design process to prevent overloading problems associated with under-sizing or over-sizing a system. A water budget should be completed which shows that the ET bed will not saturate to grade nor have a saturated level higher than the capillary rise of the soil. The design flow rate should include a factor of safety to account for peak flows or future increased site use.
The use of the ET system can be constrained by limited land availability and site topography. For year-round, single-family homes, ET systems generally require about 4,000 to 6,000 square feet of available land.
Slopes greater than 15% can be used if terracing, serial distribution, and other necessary design features are incorporated.
The most important performance consideration of any ET system is the rate of evaporation. This is largely affected by climatic conditions such as precipitation, wind speed, humidity, solar radiation and temperature. Since these factors continually change from time to time, evaporation rates will also vary significantly, which must be considered in the design.
Although most of the precipitation will be absorbed into the ET bed, hydraulic overloading could occur if more water enters the system than is evaporated. Provisions for long-term storage of excess water can be expensive. Therefore the evaporation rate must exceed the precipitation rate, which makes and ET system suitable for areas of relatively low rainfall. For ETA systems, the climate requirements are not as well defined, although the soils must be able to accept all of the influent wastewater if net evaporation is zero for any long period of time.
The sand in the ET bed must be fine enough to draw up water from the saturated zone to the surface by capillary action. The potential for capillary rising must be slightly more than the depth of the bed. However, the sand should not be too fine or the bed can be clogged by solids from the wastewater. The capillary rise of the chosen material must be known to justify the design. Typical acceptable materials have a capillary rise between 18 and 36 inches.
The vegetation that is used in an ET system must be able to handle the varying depths of free water surface’s in the bed. Grasses, alfalfa, broad-leaf trees, and evergreens are some of the vegetation used in ET beds that have been known to increase the average annual evaporation rate from the ET bed to a rate higher than that for bare soil. Grasses and alfalfa also result in nearly identical or reduced evaporation rates as compared to bare soil in the winter and the spring when evaporation rates are normally at a minimum. Similarly, topsoil has been shown to reduce evaporation rates. Some evergreen shrubs have resulted in slightly higher evaporation rates than bare soil throughout the year. Water seekers with hair roots, such as berries, are not recommended because the distribution pipes could become clogged. Dark gravel or rock (typically volcanic cinder) has been used instead of vegetation in some applications. The dark colored rock absorbs heat and accelerates evaporation.
Soil permeability affects the performance of ETA beds that involve seepage into the soil in addition to evaporation. A portion of pretreated wastewater is absorbed and treated by the soil. Generally, the wastewater must travel through 2 to 4 feet of unsaturated soil for adequate treatment before reaching the groundwater. Since the effluent in an ETA bed is considered equivalent to the discharge form the pretreatment unit the minimum vertical separation required is dependent upon the degree of pretreatment included in the complete system.
Regular operation and maintenance for ET and ETA systems is usually minimal, mostly involving typical yard up-keep such as trimming the vegetation. The septic tank is used for pretreatment it should be maintained as indicated in previous sections. The following is a list of recommended operational practices:
· Ensure all storm-water is diverted around the system.
· Use only high transpiration plants that are suitable for wetness at ground level.
If an ET or ETA system is properly installed on a suitable site, scheduled maintenance should rarely be needed except in cases of poor operating practices such as irregular septic tank pumping.
Some Advantages and Disadvantages are listed below.
Advantages:
· ET systems may overcome site, soil, and geological limitations or physical constraints of land that prevent the use of subsurface wastewater disposal methods.
· The risk of groundwater contamination is reduced with ET systems that have impermeable liners.
· ET systems can be used to supplement soil absorption for sites with slowly permeable, shallow soils with high water tables.
· ET systems could be used for seasonal application, especially for summer homes or recreational parks in areas with high evaporation and transpiration rates.
· Reductions in horizontal and vertical setbacks are permitted.
· There is no need for a reserve area.
Disadvantages
· ET systems are governed by climatic conditions such as precipitation, wind speed, humidity, solar radiation, and temperature.
· ET systems are not suitable in areas where the land is limited or where the topography is irregular.
· They have a limited storage capacity, and are thereby unable to store much winter wastewater for evaporation in the summer.
· There is a potential for overloading from infiltration and precipitation.
· Care must be taken to assure the liner is not torn during construction.
· ET systems are generally limited to sites where evaporation exceeds annual rainfall by at least 24 inches.
· Transpiration and evaporation can be reduced during the winter when the vegetation is dormant.