ating an accumulation of effluent in the bottom of the bed. The next task is to find out the amount of storage (S) being used as the effluent accumulates over the period of time when the influent volumes exceed the evaporation volumes. If you are using the proper grade of sand as fill, the volume of space between the sand grains will be about 40% of the total sand fill volume. That means that if we pour a 1 in.° volume of water onto a column of sand with a surface area of 1 in.2, the saturated water level in that sand column would rise 2.5 in. (1 in. divided by 0.40). Therefore, for every inch of accumulating effluent, the saturated level in the ET bed will rise 2.5 in. and for every inch of evaporation over the inch of input, the saturated level will drop 2.5 in. Naturally, the effluent level in the bed cannot drop below the bottom of the bed, SO the amount of storage used (S) can be zero, but never negative. To start the calculations to determine the amount of storage used from month to month, assume that in July the bed will be essentially dry, and all the storage volume will be available. S is zero. Then go to August. In our example, evaporation exceeds input by 4.03 in., SO the bed remains dry. In fact, the bed remains dry until November, when the input exceeds the evaporation by 1.32 in. Multiplying the ONSITE WASTEWATER DISPOSAL Bed Surface 0.05 gpd/sq " gpd/sq ft Figure 7.4 Change in water levels in ET bed. 1.32 in. accumulation by 2.5 gives us the inches of sand reservoir used up by this accumulation. So 1.32 in. X 2.5 = 3.3 in. - storage used In December, input still exceeds evaporation, SO another 1.85 in. of excess effluent accumulates 1.85 in. X 2.5 = 4.t n. of storage used Adding the December accumulation to the November accumulation, 3.3 in. + 4.6 in. = 7.9 in. yields the effluent level in the bed at the end of December. In our example, input exceeds evaporation through March, and the efflu- ent level rises to 18 in. above the bottom before excess evaporation begins to lower this level. Assuming th