urface or shallow subsurface application, more clearance can be gained from the point of release of the wastewater to shallow ground water or bedrock. Further, evaporation of water to the atmosphere is enhanced by these application techniques, overcoming a limitation in slowly permeable soils. As indicated in the land application sec- tion of Part II of this book, there are definite limitations to each of these methods, including health and climatic considerations. Evapotranspiration is the process whereby water is drawn to the ground surface by plant roots and by soil capillarity, where it is transpired into the atmosphere by plant leaves and is evaporated from the soil surface aided by the sun and wind. The evapotranspi- ration bed can be designed to use what little soil absorptive capacity may be available, or it can be designed as an enclosed system where all the liquid introduced into the bed is transferred to the atmos- phere. Where the bottom and sides of the bed are sealed, there is SELECTION OF SITE AND SYSTEM little potential for polluting ground water or discharging too near a layer of bedrock or other restrictive soil stratum. Aerobic treatment units produce a reasonably high quality effluent with a reduced potential for offensive odors. The effluent is suitable for discharge into aboveground sand filters and for surface land application. Some states still grant credit for ground water clearance or drain field size through the use of an aerobic treatment unit. These units require a high degree of maintenance and are suscepti- ble to upsets which can result in very low quality effluent. Since by far the most common method of onsite treatment of domestic wastewater is by means of a septic tank, since the design and construction of aerobic treatment units is beyond the capacity of most homeowners and contractors, and since these units come com- plete from the manufacturer with operation and maintenance instructions, a detailed discussion of aerobic units will not be presented. Sealed vaults or holding ta