SU0001451 - Laserfiche WebLink

Home Browse Search

Evapotranspiration-Absorption (ETA) Septic System — General Design Approach & Construction Specifications — Design Approach: We propose utilizing an evapotranspiration absorption (ETA) septic system for septic effluent disposal. The ETA septic system can be used when the native soil absorption rate is not satisfactory for a conventional septic system, when shallow ground water is present, or where bedrock conditions are present. The ETA septic system design approach is described by Otis and Boyle(1980) for the U.S. Environmental Protection Agency. The ETA sewage loading rate for the subject project is the most important consideration for the design of the system. The ETA sewage loading rate was determined, in part, from information from a local agricultural weather station and from a percolation test performed at the site. The average daily pan evaporation and the average daily precipitation was gathered at Lodi weather station LODI.0 (NCDC #5032,Lodi) (http://www.ipm.ucdavis.edu/WEATHER/ AVERAGES/lodi.html). Daily pan evaporation was not available from the Stockton Station. The weather station data gathered for this design is from a period beginning in 1951 to several months ago. Reference Evapotranspiration(ETo) is a term used to estimate the evapotranspiration rate of a reference crop. The reference crop, in this case, is grass which is closely clipped, actively growing, completely shading the soil, and well watered. The ETo is expressed in inches/day (in/day) similar to pan evaporation however it is a slightly lesser value. The daily ETo has not been measured over the long term like pan evaporation; however, it can be extrapolated from pan evaporation by taking the pan evaporation value and multiplying it by a local conversion factor of 0.75 to obtain ETo(oral communication with CIMS help line, 2001). See Equation no. 1. Equation No. 1 Pan Evaporation (in/day) x 0.75 = ETo in/day A deficit curve was found by subtracting average daily precipitation from average daily ETo(see Equation No. 2 and Plate 1). The term "deficit"refers to a period occurring between November 15 to April 1 where average daily precipitation exceeds average daily ETo. The highest deficit rate was found to occur on January 22 when using a moving average filter with a two (2) day filter window. The highest deficit for the average year is 0.14 in/day. The deficit rate may be exceeded approximately five (5) isolated days per average year. Equation No. 2 (Also see Plate 2, December 24) Precipitation (in/day)- ETo (in/day) = Deficit Rate (in/day) The deficit rate i5 the condition where precipitation exceeds ET, —Calculations, Page I—