Calculation of minimum absorption trench length. Design Flow (gallons per day). Residential flow must be designed in accordance to Table 5 (Section 22-156). Nonresidential design sewage flow shall be equal to twice the daily average sewage flow. Loading Rate (gallons per day per square foot). The loading rate is determined by Figure 3 (Section 22-160) using the State equivalent percolation rate. Available Absorptive Area (square feet per linear feet). Deep trenches. The available absorptive areas shall be equal to the height of the absorptive layer found on the percolation test report. Shallow trenches. The available absorptive area shall equal the bottom area per linear foot of drainfield. Dry wells. The available absorptive area shall be equal to the height of the absorptive layer found on the percolation test report multiplied by the circumference. Required Absorptive Area. Design Flow (gpd) Minimum Length of Trench = Required Absorptive Area (ft 2 ) = feet of Trench Available Absorptive Area (ft 2 ) / ft