 natural soil has a very low absorption capacity and when the depth of fill is not very great, this area can be inadequate. Therefore, the minimum natural soil absorption area required should be calcu- lated SO that if the initial design you develop does not meet the minimum requirement, it can be modified. The minimum natural soil absorption area is found by dividing the design flow by the natural soil infiltration rate. For example, if the flow is 300 gal per day and the infiltration rate is 0.25 gal per day per ft2, then minimum basal area = 300 gpd 0.25 gpd/ft2 = 1200 ft2 The finished design should have a mound base of at least 1200 ft2. If it did not, the mound sidewall slope should be reduced to provide the additional area necessary. Or, additional area and additional treatment can be obtained by increasing the depth of fill and main- taining a three to one slope. ONSITE WASTEWATER DISPOSAL No slope Slope Figure 6.7 Available absorption areas on flat and sloping ground. Example 1 To design a mound on level ground, we need to know only four things: 1. the design flow 2. the depth of fill required below the excavation in the mound 3. the fill infiltration rate 4. the natural soil infiltration rate Design assumptions: design flow = 450 gpd fill depth below excavation = 2 ft fill infiltration rate = 1.2 gpd/ft2 natural soil infiltration rate = 0.25 gpd/ft2 CONVENTIONAL SYSTEM MODIFICATIONS The absorption area required within the mound is: 450 gpd 1.2 gpd/ft2 = 375 ft2 Since there is no slope, let's use a bed 15 ft wide. The bed length therefore, will be 375 ft2 15 ft = 25 The total height of the mound at the bed edges will be: 2 ft below the excavation plus 9 in. of bed depth plus 1 ft of cover, or a total of 3 ft 9 in. (3.75 ft). With a three to one slope, the sides of the mound would extend 3.75 ft X 3/1 = 11.25 ft So the overall width of the mound would be: 15 ft of bed plus two sides of 11.25 ft each, or a total of 37.5 ft. The overall length would be 25 ft of bed plus two sides of 11.25 ft each, or a total