er square inch (psi). A pressure head of 1 ft of water equals a pressure of 0.43 psi. There should be from 1 to 4 ft of pressure at the lateral holes, with 3 ft of head being a good pressure to shoot for. The dosing rate depends on the total length of the laterals and supply pipe, the size and number of holes in the laterals, and the pressure head supplied by the pump. The dosing rate should be at least 25 gal per min for a three-bedroom house and 30 gal per min for a four-bedroom house. Rates lower than these can result in un- even distribution of effluent inside the mound. The design dosing rate can be calculated using the following three steps: 1. calculate the total number of holes needed 2. find the flow rate per hole 3. calculate the total dosing rate CONVENTIONAL SYSTEM MODIFICATIONS Table 6.5 Flow Rates Through Holes of Various Diameters (gpm) Pressure Hole Diameter (in.) Head (ft) Calculation of the number of holes was discussed earlier. Determining the flow rate per hole involves using Table 6.5. Begin by assuming a pressure head of 3 ft at the distribution laterals and a hole diameter of three-sixteenths inch. This gives a flow rate of 0.72 gal per min per hole. The total dosing rate is simply the dosing rate per hole times the number of holes. As an example, assume: length per lateral leg = 25 ft no. of lateral legs = 6 hole diameter = 3/16 in. hole spacing = 4 ft pressure head at laterals = 3 ft 1. No. of holes = length lateral X no. of laterals per = hole spacing 2. Flow rate per hole (from Table 6.5) = 0.72 gpm 3. Total dosing rate = no. of holes X flow rate per hole = 38 X 0.72 gpm = 27 gpm A dosing rate of 27 gal per min is about right. If flow rates more than double the minimum are projected, it may be desirable to reduce the rate somewhat in order to be able to use a smaller pump or smaller diameter pipe. The hole spacing can be increased or the hole diameter decreased. It is usually better to use wider hole spac- ing in order to be spared the plugging that occurs with small holes. Pipe diamete