ure: you may remove the top 2 to 4 in. of sand when the filter clogs and replace it with clean sand; or you may overfill the sand initially, and remove 2-in. layers as necessary until the sand in the filter is only 24 in. deep, at which time you add another foot or SO of sand. The sand you remove should be buried; the local landfill is a potential disposal site. After removal of the top layer of sand, the period of rest gained while the other filter chamber is being used normally serves to restore the original hydraulic conductivity of the remaining sand. The maintenance required for both the intermittent filter unit and CONVENTIONAL SYSTEM MODIFICATIONS the recirculating filter unit are the same, except that with the recircu- lating filter, the solids should be removed from the recirculation unit when the septic tank and dosing chamber (if one is used) are pumped out. It is estimated that the maintenance required for a filter dosed with aerobic treatment unit effluent will be about half that required by septic tank effluent, all else being equal. Low-Impact Systems EVAPOTRANSPIRATION Application An evapotranspiration system (Figure 7.1) is a disposal system which is designed for disposal of septic tank effluent to the atmo- sphere rather than the soil. Because the system operates indepen- dently of the soil, any limiting characteristics of the soil (such as, there isn't any soil) do not affect the evapotranspiration bed's opera- tion. There are two basic types of evapotranspiration beds: the lined evapotranspiration (ET) and the evapotranspiration absorption (ETA) bed. If the limiting soil condition is that the soil is too tight to accept any type of absorption system, an unlined bed can be used. If the limiting condition is inadequate soil above bedrock or the ground water table, then a lined or ET bed can be used. The effectiveness of the ET and ETA systems is very dependent on the local climate. ET systems have been made to work in the north- ern United States, in the snow-clad Rocky Mountains and in Can- ada