Evapotranspiration beds.
El Paso City Code § 18.21.250
in effect18.21.250 - Evapotranspiration beds.
A. General. Evapotranspiration bed systems may be used in locations where soil conditions are not suitable for any type of soil absorption system. For very porous soils, solutioned limestone, fractured bedrock, and situations that would allow excessively rapid migration of sewage toward groundwater, lined evapotranspiration beds must be designed and certified by a registered professional engineer or registered professional sanitarian. The beds must be located outside the flood-prone area and not within areas subject to inundation or erosion by flood waters or rainfall runoff. An applicant for a permit to install a sewerage system shall consult with the local floodplain ordinances administrator, county engineer, State Highways and Public Transportation Department, nearest river authority, Farmers Home Administration, Federal Emergency Management Agency, and any other officials who may have information regarding the potential for flooding at the site of the evapotranspiration beds.
B. Construction Features. The following factors must be considered in the design of evapotranspiration beds: annual mean rainfall and wettest month of the year, annual mean evaporation rate and monthly minimal rate, growing season variations, native grasses and shrubs available for cover, absorptive capacity of the soil surrounding an unlined bed, and site conditions, including varying sunlight and air movement.
1. Evapotranspiration beds are constructed in impervious soil or soil with very high absorptive capacity. When the soils have a very high percolation rate, less than five minutes per inch, impervious liners must be constructed to guard against the possibility of wastewater discharging through the soil (fissured rock or gravel) and contaminating streams, lakes, or shallow groundwater. Impervious liners may consist of reinforced concrete, twenty mil minimum single layer thickness plastic, or rubber liners. All must be repairable in the field. Liners are not required in slowly permeable soils (having permeability of less than ten centimeters per second) and should not be used since some of the wastewater may be absorbed into the soil and will help to reduce the overall evapotranspiration load. An evapotranspiration system shall be designed using the following parameters:
a. Beds may be designed in any configuration, but the total number of square feet of bed area must be determined by the formulas in subsection C of this section.
b. At least two beds must be constructed with valving arranged to allow the effluent from a sewerage unit to alternate between each bed. When one bed becomes saturated (top of bed remains moist) the valving must be operated to allow effluent to flow into the alternate underloaded bedding. In order to determine the water level in the beds during use, an inspection port shall be installed in each bed. Inspection ports shall be designed to prohibit access to the bed bottom by insects, small animals, and unauthorized persons.
c. The beds shall be constructed as shallow as possible with a depth ranging from eighteen inches to a maximum of thirty-six inches. This is necessary to keep the beds aerobic and prevent clogging. Treatment tankage should be installed as high as practical to permit shallow bed construction.
d. Evapotranspiration Bed. Its design features, as illustrated in Figure 18.21.250(A), are as follows:
i. All piping must be three to four inches in diameter. All perforated collection lines of an evapotranspiration bed monitor system must have a minimum crush strength of one thousand five hundred pounds per linear foot. The standpipe and sample sump must be of Schedule 40 polyvinylchloride or stronger. All connections shall be glued or rubber gasketed joints with the exception of the intruder resistant standpipe cap located at finished topsoil grade. Perforations in the collection lines must face downward, with the two rows of perforations equally offset perpendicular to the ground.
ii. The end(s) of the perforated collection line(s) shall extend to within two feet of both opposite ends of the bed, measured at the bed bottom.
iii. An evapotranspiration bed which is twenty feet wide or less shall have a minimum of one collection line which must be located centrally, down the length of the bed. An evapotranspiration bed which is greater than twenty feet but equal to or less than forty feet in width, shall have a minimum of two collection lines. These lines must be equally spaced (within approximately one foot) from the edge of the bed to pipe, and from pipe to pipe, installed parallel down the length of the bed. The two collection lines must be jointed by a perforated header line. As indicated in item (ii) of this subparagraph (B)(1)(c), the header line shall be within two feet, and parallel, to the edge of the bed measured at the bed bottom.
4. The collection line system must maintain a minimum fall of one-sixteenth inch per foot toward the standpipe/sump assembly.
5. The collection line(s) shall tee into a solid wall sandpipe which must have a sampling sump below the bottom of the tee. This sump shall be eight to twelve inches in depth, measured from the bottom of the tee. A cap or plug must be glued to the bottom of the sump to provide a watertight connection. The top of the standpipe shall be flush with the finished grade of topsoil, and shall be covered with an intruder-resistant, removable access cap. A minimum three inch wide by three inch deep dry moat shall surround the standpipe cap to facilitate ease of cap removal and replacement.
6. The pit bottom on which the collection line(s) shall be placed must be an impermeable surface, graded to provide a minimum fall of one-eighth inch per foot toward the collection line from either side. This will result in an impermeable shallow vee trench for the collection line to rest in.
7. The collection line(s) shall be located within the sand cushion required under lined beds. The minimum four inch sand cushion depth will increase as it follows the grade fall of the pit bottom toward the collection line. The backfill material around the standpipe should consist of sand or sandy loam.
[image omitted; see the official code]
8. Rock media that is generally one uniform size from 0.75 to two inches shall be placed on the bed bottom to a depth of twelve inches or less depending on the overall bed depth, after the liner and sand cushion are placed over the monitor system.
9. The top of the distribution pipe must be flush with the rock media and adequate to provide for uniform distribution of effluent. A twelve-foot maximum separation between pipes and no less than three feet separation between bed walls and the pipe is permissible. The bed bottom and the pipe must be level.
10. A water permeable sand barrier (butcher paper, newspaper, or geotextile filter fabric) is then placed over the rock. Sand is then added to fill the bed to within two inches from the top.
11. In order to provide continuous capillary action in the sand, wicks shall be incorporated in the rock media. Wicks are simply sand structures which penetrate through the rock media to the bottom of the bed (See Figure 18.21.250(B)). The total wick area shall be ten to fifteen percent of the bed surface area and shall be uniformly spaced throughout the bed. Wicks may also be constructed by simply grading furrows in the rock media in between the distribution pipe. In areas of the state where rock media is difficult to obtain, the total amount of rock may be reduced by filling the initial twelve inches of the bed with coarse sand (two millimeters) and placing rock media only around the top, bottom, and sides of the distribution pipe to form an eighteen-inch enclosure, as indicated in Figure 18.21.250(B).
12. After the sand is in place, the final two inches of bed volume are filled with sandy loam and mounded with a downward slope of two to four percent.
13. Final bed construction consists of covering the surface of the bed with vegetation having good transpiration properties and providing for the most stormwater diversion that is practical.
C. Bed Sizing. The bed area may be approximated by using the following formulas:
A = 31,000 (1 + B) / EA—1/2 RFRA
Where:
A = Total area of both beds (to find one bed area divide A by 2)
B = Total number of bedrooms (B = 2 for minimum residence)
EA = Mean evaporation rate in inches per year
(See Table 18.21.250)
RFRA = Mean rainfall rate in inches per year
(See Table 18.21.250)
For non-single family residential situations:
A = 310 Q / EA—1/2 RFRA
Where: ;b10; Q = ;hg;Average daily flow into the system, expressed in gallons per day (see Table 18.21.190).
Evaporation and rainfall data for various areas of the state are listed in Table 18.21.250. Additional data may be found in "Report 192" and "Report LP192" published by the commission.
D. Plants and Grasses for Transpiration. The bed surface shall be covered with vegetation designed to take maximum advantage of transpiration, depending on the season and site's location. Evergreen bushes having shallow root systems can be planted in the bed to assist in water uptake. If grasses are used which have dormant periods, steps shall be taken to provide appropriate vegetation on the beds during these periods. Overseeding with winter grasses is commonly used to provide year-round transpiration.
[image omitted; see the official code]
Table 18.21.250
Mean Pan Evaporation and Rainfall
Station | E A Mean Evaporation Rate (Inches/Year) | RFR A Mean Rainfall (Inches/ Year)
Abilene | 75.0* | 23.8**
Amarillo | 87.9 | 20.28
Austin | 70.0 | 32.49
Beaumont | 47.8 | 55.07
Brownsville | 56.0 | 25.13
Canyon Lake | 80.8 | 33.19
Corpus Christi | 67.0* | 32.0**
Daingerfield | 74.2 | 46.12
Dallas | 85.0 | 35.94
El Paso | 106.5 | 7.77
Fort Stockton | 105.6 | 11.85
Houston | 48.7 | 48.19
Lake Somerville | 71.7 | 37.45
Laredo | 87.0* | 21.0**
Lubbock | 88.7 | 18.41
Lufkin | 51.0* | 43.00**
Midland-Odessa | 85.0* | 13.0**
San Antonio | 70.0* | 30.0**
San Angelo | 84.0* | 19.0**
Temple | 68.2 | 34.00
Tyler | 55.0* | 42.0**
Uvalde | 72.0* | 25.0**
Wichita Falls | 73.0* | 27.0**
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* Interpolated from map, p. 66 "Climatic Atlas of Texas" ** Interpolated from map, p. 18 "Climatic Atlas of Texas"
(Ord. 9448 (part), 1988)
Source: City of El Paso, City Code and City Charter (Municode Library, library.municode.com)URL: https://library.municode.com/tx/el_paso/codes/code_of_ordinances?nodeId=TIT18BUCO_CH18.21PRSEFA_ARTVIDESTEFDISY_18.21.250EVBEFetched: 10/28/2026