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component to an otherwise "passive" system and add cost because of the dosing equipment. The improved performance of dosed -flow systems over gravity flow systems should outweigh these perceived disadvantages, especially when a management entity is in place. It must be noted, however, that if dosed infiltration systems are allowed to pond, the advantages of dosing are lost because the bottom infiltration surface is continuously inundated and no longer allowed to rest and reaerate. Therefore, there is no value in using dosed -flow distribution in SWISs designed to operate ponded, such as systems that include sidewall area as an active infiltration surface or those using serial relief lines. Perforated pipe Four -inch perforated pipe networks (with or without d -boxes or pressure manifolds) that receive dosed -flow applications are designed no differently than gravity -flow systems. Many of the advantages of dosing are lost in such networks, however, because the distribution is only slightly better than that of gravity -flow systems (Converse, 1974). Pressure manifold A pressure manifold consists of a large -diameter pipe tapped with small outlet pipes that discharge to gravity laterals (figure 4-14). A pump pressur- izes the manifold, which has a selected diameter to ensure that pressure inside the manifold is the same at each outlet. This method of flow division is more accurate and consistent than a distribution box, but it has the same shortcoming since flow after the manifold is by gravity along each distribu- Figure 4-14. Pressure manifold detail Inlet pipe tion lateral. Its most common application is to divide flow among multiple trenches constructed at different elevations on a sloping site. Table 4-7 can be used to size a pressure manifold for different applications (see sidebar). This table was developed by Berkowitz (1985) to size the manifold diameter based on the spacing between pressure lateral taps, the lateral tap diameter, and the number of lateral taps. The hydraulic computations made to develop the table set a maximum flow differential between laterals of 5 percent. The dosing rate is determined by calculating the flow in a single lateral tap assuming 1 to 4 feet of head at the manifold outlets and multiplying the result by the number of lateral taps. The Hazen -Williams equation for pipe flow can be used to make this calculation. Pressure distribution is typically constructed of Schedule 40 PVC pipe (figure 4-15). The lateral taps are joined by tees. They also can be attached by tapping (threading) the manifold pipe, but the manifold pipe must be Schedule 80 to provide a thicker pipe wall for successful tapping. Valves on each pressure tap are recommended to enable each line to be taken out of service as needed by closing the appropriate valve. This allows an opportunity to manage, rest, or repair individual lines. To prevent freezing, the manifold can be drained back to the dose tank after each dose. If this is done, the volume of water that will drain from the manifold and forcemain must be added to the dose volume to achieve the desired dose. Rigid pipe pressure network Rigid pipe pressure distribution networks are used to provide relatively uniform distribution of Enlargers to increase pipe Distribution laterals to size of trench pipe 4-24 USEPA Onsite Wastewater Treatment Systems Manual