ere as a vapor. Transpiration is the uptake of water by the roots of a plant and the subsequent passing of water from the leaves into the air as the plant breathes. By these two processes, wastewater is removed from the sand-filled ET bed. As the treated wastewater enters the bed, it is drawn up into the spaces between the sand grains by capillary action, just as water is drawn up into a sponge sitting in a puddle of water. It is very important that the sand grains be of the proper size. If the grains are too large, there will be no capillary force pulling the wastewater up, Homes rates. 178 ONSITE WASTEWATER DISPOSAL and if the grains are too small, the water will be held SO tightly in the small spaces between the grains that it will not move. As the water nears the bed surface and the sun hits the thin film of water around each grain of sand, some of the energy of the sun is absorbed by that water. The water evaporates into the air, increasing the humidity near the sand surface. The humid air is then carried away by the wind. Plant roots can absorb very large quantities of water. For example, a square foot of salt cedar bush can use 60 gal of water per year, although most of this consumption occurs during the growing sea- son. This may or may not be the time of year when you need it the most. Dormant or dead plants may actually reduce water loss from the ET bed by shading the soil from the sun and by reducing wind movement across its surface. ETA systems use a combination of evapotranspiration and soil absorption to dispose of the sewage effluent. These systems have a much better reputation than the ET systems because they are gener- ally cheaper and because they allow a greater margin of safety in terms of their ability to handle temporary overloads without failure. A layer of salt will build up on the surface of the bed during normal operation of an ET system as sewage effluent evaporates and leaves behind its dissolved salts. This presents two concerns: that the crust will impede evaporation, and that high s