Laserfiche WebLink
1 <br />1 <br />' There are no standards governing septic tank effluent quality, and sources <br />differ on their methods of determining treatment efficiency. EPA's concept <br />' is to assume that the tank-drainfield-soil combination removes all <br />pollutants, so that the only loading on the environment is from the septage <br />' extracted periodically from the tank. On this basis, septic tanks get an <br />overall removal efficiency rating of 92.4 percent. The second concept <br />' views the effluent from a tank as a loading on the soil and groundwater <br />sectors of the environment. Pollutant removal efficiency from this point <br />of view is 17.2 percent due to waste digestion by the anaerobic bacteria in <br />' the septic tank. <br />' A realistic assessment of treatment efficiency lies somewhere between these <br />two extremes, depending upon site-specific variables. If soil and <br />' groundwater conditions are optimal, and the septic tank is designed, <br />installed, and maintained correctly, pollutant removal rates as high as <br />' secondary treatment plants can be achieved. Under these ideal conditions, <br />EPA's view of septic tank efficiency may be more correct. <br />' <br />Unfortunately, septic tanks are often used inappropriately. If they are <br />installed in clay, muck, or other impervious soils, the effluent cannot <br />percolate properly, and the system may back up. Where shallow bedrock or <br />hardpan exists, effluent may travel laterally for much longer distances <br />' <br />than expected. If the percolation rate is high and the groundwater table <br />is close to the surface, effluent will reach the groundwater before it has <br />' <br />had the full benefit of treatment by the soil. An unsaturated zone under <br />the drainfield at least 1 to 2 meters (40 to 80 inches) thick is desired to <br />' <br />allow aerobic decomposition and other attenuation, although the state only <br />requires that the <br />water table be more than 24 inches below the bottom of <br />the trench. Undersizing of a septic tank and infrequent pumping of it <br />' <br />result in sludge and scum storage capacity problems. Under these <br />conditions, solids are washed out into the drainfield, causing clogging and <br />' <br />eventual failure of the drainfield. <br />IRC.VB4 <br />6/4/87 <br />