Engineering Parameters in the Design of Evapotranspiration Beds
There is a variety of standard technologies available to treat and dispose domestic wastewater. These include connections to a local publicly-owned treatment works (POTW), private treatment plants, and septic tank and leach field systems. Evapotranspiration (ET) systems can provide an additional wastewater management alternative for appropriate locations. The use of an ET system should be considered at locations where alternative treatment or disposal methods are either technically or economically inappropriate (Maurer 1976). The use of ET beds has increased as residential housing developments have expanded into areas not suited for standard treatment methods (Ingham 1980).
An ET system consists of a septic tank for solids removal followed by one or more ET beds to evapotranspirate remaining liquids. The physical components of an ET system include septic tank, distribution piping, bed liner, fill material, monitoring wells, overflow protection and surface cover. ET beds are lined to prevent infiltration of wastewater and potential subsequent contamination of local groundwater.
Both fill material and surface cover can affect the efficiency of an ET bed. The fill material should be sand and gravel selected and placed to facilitate distribution of wastewater within the bed and the wicking of the liquid towards the surface. Surface vegetation should have good water uptake and thrive in the local climate. Surface soil should be a stop soil and support the selected vegetation. These components should be designed in accordance with applicable regulations and good engineering practices.
The remaining design parameter, not described above, is the ET bed area and volume. A wide variety of guidelines have been developed for the sizing, design, and use of an ET system (Beck 1979, Bennett 1978, Bernhart 1985, Ingham 1980, Salvato 1983, Ward 1977). State and local regulatory agencies may also have guidelines for ET system use in their area. These guidelines are typically biased to minimize the likelihood of an ET system overflowing due to insufficient surface area for evapotranspiration or volume for storage. Careful engineering design can extend the applicability of ET systems beyond those guidelines.
A site-specific water balance can support the design of an ET system tailored for a specific location and usage (Pence 1985, Salvato 1983). The water balance can be described as follows.
(precipitation inflow) - (wastewater loading) - (evapotranspiration) = (accumulation)
Precipitation inflow, wastewater loading and evapotranspiration are site specific; the development of these parameters is discussed below. Accumulation indicates the storage volume required during time periods when ET is less than the total inflow. The water balance can be performed for an entire year, on a monthly basis, or a more frequent basis (Brandes 1980). Water balance calculations gain precision as the input variables are better defined and as the time basis is shortened.
The components of the water balance (precipitation inflow, wastewater loading and ET rate) can be developed to reflect site-specific conditions if data are available. However, it is often difficult to obtain this data due to either the lack of hard information or constraints on the required research. In this case, a conservative design basis is appropriate. When resources are available to develop site-specific data, a more detailed evaluation and site-specific design may be possible. The following engineering parameters should be evaluated during design.
Drainage: The area surrounding the ET beds should be graded to prevent precipitation run-on. Small swales or culverts are often appropriate drainage modifications.
The surface of the beds should be mounted to enhance precipitation run-off. Surface grading is typically several percent. Surface run-off of precipitation depends on several factors: surface soil type and compaction, vegetation cover, surface slope, rate of precipitation, and soil saturation. Run-off can be estimated using correlations from the Soil Conservation Service or the Rational Method, or measurements of a test bed. A conservative ET system design assumes no run-off. However, run-off correlations typically indicate that there will be some fractional runoff from a sloped surface. For example, with a surface slope of 4 percent, runoff estimates range from several percent up to 25 percent, depending on land conditions and the precipitation event.
When the ground is frozen, there is essentially a 100 percent run-off. However, it has been found that ET beds tend to remain unfrozen when ambient temperature is below 32 deg F due to subsurface microbial activity and the heat content of the wastewater inflow (Bernhart 1985).
Precipitation: Precipitation is primarily measured as rain and snow, with snow being measured as both depth of snow and water content of snow. Precipitation is measured by weather stations operated by various governmental agencies. The amount of precipitation can vary within a small area due to local variations in elevation, terrain and water bodies. Therefore, precipitation data should be obtained from the nearest weather station that is representative of the site's climactic conditions. If on-site data is collected, a sufficiently long history needs to be analyzed to provide a statistically valid record.
The amount of rain that percolates into an ET bed is estimated from the rainfall event and the run-off estimate, as discussed above. The amount of snow that percolates into the bed depends on weather as well as the run-off estimate. Percolation inflow into an ET bed is often most critical in winter and spring months, when ET rates are at their lowest.
Snow percolation is a function of the water content of the snow (inches water per inch of snow), sublimation of snow prior to melting, and when the snow melts. Each of these components can be estimated.
The water content of snow is variable between location, time of year, and specific snow fall event. Weather station data may include rain fall, snow fall, and water content, all measured in inches. During months with only snow fall, the water content per inch of snow can readily be calculated. During months when both rain and snow fall occur, the water-content contribution from snow must be estimated.
Sublimation of snow depends primarily on characteristics of the snow, local relative humidity, wind velocity and air temperature (Croft 1944, Kittredge 1953). The rate of sublimation is typically equivalent to several hundredth inches of water per day. Sublimation occurs prior to snow melt and decreases the water content available to percolate into an ET bed. The rate of snow melt is primarily a function of air temperature. Air temperature can be measured as "degree-days." One degree-day is a 1 deg F increase for one day of the daily temperature above a reference temperature. The daily temperature may be either the daily mean or maximum; the reference temperature is often freezing (32 deg F).
A correlation between degree-days and snow melt is then used to determine when and how much of the accumulated snow melts (Garstka 1958). Delayed snow melt is important for two reasons. One reason is that a delay provides time for sublimation to occur. The other reason is that the bulk of delayed snow melt typically occurs during spring months. The available storage volume in ET beds and the ET rate will vary between winter and summer months. Engineering design calculations need to account for these changing parameters.
Wastewater Inflow Rate: The inflow rate of domestic wastewater is a site-specific parameter. The flow rate can either be estimated based on standard water usage rates (Bennett 1978) or measured with a flow meter. For residences that have non-discharging water uses, such as irrigation or hot tubs, water used at the residence may not equal the wastewater flow. Design guidelines typically incorporate an average wastewater inflow rate for ET bed design. Daily, weekly, or monthly wastewater flow rates can be used for a more site-specific design, particularly at residences where water usage is not consistent. A safety factor to account for peak flows or future increased site usage may also be incorporated into the design flow rate.
Evapotranspiration Rate: The ET rate is the rate that liquids evapotranspirate from an ET bed. Over the course of a year, ET must exceed the inflow of precipitation and wastewater for the ET system to operate without overflowing. The bed design can incorporate storage for limited periods of time when this is not the case.
The ET rate is a function of many variables, including air and ET bed temperatures, wind exposure, relative humidity, vegetation type and activity (Penman), fill material of the ET bed, height of liquid in the ET bed, and snow debris cover over the ET bed. The ET rate can be estimated from correlations that incorporate one or more of these variables (Hasfurther 1978, Penman 1956). Commonly used parameters to estimate ET rates are pan and lake evaporation. Alternatively, empirical ET rates can be determined from test beds by monitoring total inflow and water levels within the test bed.
State or local agencies may provide ET rates for use in the design of bed storage volume and surface area. These rates are typically the yearly average rate and tend to underestimate the ET rate; this provides a safety factor in the design of an ET system. In instances where local guidelines provide ET rates too low to support the use of an ET system, carefully researched ET rates may be high enough to allow the design and use of an ET system.
Historically, ET systems have been used year-round in warm-weather and dry areas or during the summer months in colder and wetter areas. Expansion of ET systems to year-round use in cold-weather or wet areas may require an increase in ET bed capacity to provide storage during periods when the ET rate is less than the precipitation and wastewater inflow. ET rate estimates range from less than 0.005 gallons per day per square foot (gpd/sf) to 0.5 gpd/sf (Beck 1979), depending on location and time of year. Estimates of cold-weather ET rates range from 0.03 gpm/sf in Colorado and northern Nevada (Bennett and Linstedt 1978, Bernhart 1973) to 0.095 gpd/sf in Toronto (Bernhart 1973).
ET systems are a potential alternative for domestic sewage disposal at locations where the annual ET rate exceeds precipitation and wastewater inflows. A site-specific water balance can be used to determine the surface area and storage volume of an ET system if sufficient design and data are available. Careful evaluation of the engineering parameters may extend the applicability of ET systems beyond the conservative guidelines provide by state and local regulatory agencies.
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2. Bennett, Edwin and K. D. Linstedt, "Sewage Disposal" by Evaporation-Transpiration, Municipal Environmental Research Lab, USEPA-600/2-78-163.
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8. Ingham, Alan, "Guidelines for Evapotranspiration," [California] State Water Resources Control Board, January 1980.
9. Kittredge, Joseph, "Influences of Forest on Snow in the Ponderosa-Sugar Pine-Fir Zone of the Central Sierra Nevada," Hilgardia, Vol. 22, No. 1, March 1953.
10. Maurer, Glenn, "Field Application: Sand Mound and Evapotranspiration Systems" presented at NSF Third National Conference, November 1976, pp. 93-101.
11. Pence, Harry and Robert Pusey, "Performance of Evapotranspiration Systems for On-Site Wastewater Disposal," presented at 1985 Winter Meeting of the American Society of Agricultural Engineers.
12. Penman, H. L., "Vegetation and Hydrology," Technical Communication No. 53, Commonwealth Bureau of Soils, pp. 30-65.
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15. Ward, John, "Evaporation of Wastewater from Mountain Cabins," Office of Water Research and Technology, Washington, D.C., PB-266 712.
About the Author:
Wanye L. Frank, P.E., is a project engineer with Geraghty & Miller, Inc. in Denver, Colorado.