 hole spacing is calculated as follows for the upper lateral with only 2 ft of head. ONSITE WASTEWATER DISPOSAL 5 ft X 0.59 gpm 0.72 gpm If the third lateral is 1 ft lower than the center lateral, then its hole spacing would be based on a 4-ft head: 5 ft 0.72 X 0.83 gpm = 5.8 ft = 5 ft 9 in. To figure the number of holes required, simply divide the total lat- eral length by the hole spacing. If, as in our example, we need 240 ft of lateral, and we assume an average 5-ft hole spacing, then 240 ft 5 ft = 48 holes Three-sixteenth inch holes will deliver, according to Table 6.2, 0.72 gal per min with an average of a 3-ft head pushing the water out. 48 holes X 0.72 gpm = 34.6 gpm, the total flow rate. At this point, it is a good idea to check the flow rates in each lateral to make sure they are about equal. This is very important for a sloping site. If the elevation difference between the highest and lowest lateral is greater than 4 ft, making the irrigated area longer and narrower may reduce elevation and head differences between laterals. If two manifolds and two sets of laterals are necessary, the hydraulics for each system are calculated separately and final pres- sures equalized between the two manifolds by adjusting pressure control valves placed in front of each manifold. Now calculate the friction loss from the distribution system pipes. If we have six 40-ft laterals and put a manifold at one end (Figure 6.18), and we use three-sixteenth inch holes, we can see from Table 6.6 that 1-in. diameter laterals can be used. We'll use a 2-in. mani- fold and supply line to minimize additional friction (and additional pump motor capacity). If we have six laterals 5 ft apart, we will need a 25-ft manifold. Assuming the absorption field area is 50 ft from the dosing chamber, CONVENTIONAL SYSTEM MODIFICATIONS the pump must overcome friction from 75 ft of 2-in. solid pipe and 240 ft of 1-in. pipe with holes in it. Friction loss from the 2-in. PVC pipe will be, according to Table 4.4, 2.05 ft per 100 ft at 35 gpm, or a total 