ever it is to be used, and (2) there is little agreement on the basic assumptions used for system design. The method for sizing an ET system presented here is a method which will work for most situations. It can be used for both seasonal and year-round residences, and in many different climates. Before we get into the calculations, though, some assumptions on which those calculations are based must be presented. A very fine sand and a medium sand show similar evaporation rates with the water level a few inches below the sand surface, but the capillary rise in fine sand is about three times that of coarse sand. The maximum practical distance for capillary rise is about 30 in., SO the maximum depth of the ET bed is 30 in. A greater depth could result in a permanent saturation of the sand at the bottom of the bed, SO the extra depth would not serve as usable reservoir capacity. The minimum depth of the bed is about 20 in. A shallower depth may not provide sufficient storage volume to accommodate seasonal variation in evapotranspiration. Fine to medium sand is best suited for use in the ET bed. All the sand grains should be between 0.06 mm and 2.0 mm in diameter. That is, the sand should pass through a number 10 sieve. Further, about half the sand by weight should be equal to or smaller than 0.1 mm. That is, about half the sand should pass a number 18 sieve. Naturally occurring sands such as alluvial sand deposits and river ONSITE WASTEWATER DISPOSAL bottom sand can be used as long as they generally correspond to the above specifications, and as long as they contain little or no silt and clay. Manufactured sands such as concrete and mortar sand may be too coarse. It is a good idea to check the sand you intend to use by performing an experiment. Find a clear tube about 1 to 6 in. in diameter and 36 in. long. The plastic tubes used to protect the handles of golf clubs work well. Put the tube in a bowl of dyed water and fill it with the sand you will be using in the ET bed. Support the tube SO it doesn't fall over. With