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Table 4-7. Pressure manifold sizing Tap spacing ' Manifold size Single -sided manifold Lateral tap diameter (inches) Double -sided manifold Lateral tap diameter (inches) (feet) (inches) 0.50 i 0.75 , 1.001 1.25 1.50 2.00 0.50 0.75 1.00 1.25 1.50 2.00 Maximum number of lateral taps Maximum number of lateral taps 0.5 2 4 2 2 3 9 5 3 2 4 2 4 16 9 5 3 2 7 4 2 6 >40 21 12 7 5 3 18 10 6 3 2 8 38 22 12 9 5 17 10 6 4 2 3.0 2 8 2 2 3 14 12 3 2 6 2 4 21 18 6 3 2 16 5 3 6 38 30 26 8 5 3 >20 19 7 3 2 6.0 2 5 4 4 3 9 7 6 2 7 3 2 4 14 11 9 4 2 10 9 3 6 27 20 17 14 7 3 19 15 1 13 4 3 Source: Adapted from Berkowitz, 1985. Figure 4-15. Horizontal design for pressure distribution Source: Washington Department of Health, 1998. wastewater effluent over the entire infiltration surface simultaneously during each dose. They are well suited for all dosed systems. Because they deliver the same volume of wastewater effluent per linear length of lateral, they can be used to dose multiple trenches of unequal length. Although rigid pipe pressure networks can be designed to deliver equal volumes to trenches at different elevations (Mote, 1984; Mote et al., 1981; Otis, 1982), these situations should be avoided. Uniform distribution is achieved only when the network is fully pressur- ized. During filling and draining of the network, the distribution lateral at the lowest elevation receives more water. This disparity increases with increasing dosing frequency. As an alternative on sloping sites, the SWIS could be divided into multiple cells, with the laterals in each cell at the same elevation. If this is not possible, other distribution designs should be considered. The networks consist of solid PVC pipe manifolds that supply water to a series of smaller perforated PVC laterals (figure 4-16). The laterals are de- signed to discharge nearly equal volumes of USEPA Onsite Wastewater Treatment Systems Manual 4-25