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3. Reserve Area/Dual System. A reserve area having suitable site conditions and <br /> sufficient area for full, 100% replacement of the primary PD dispersal field shall be <br /> provided, or a complete dual primary and secondary PD system shall be installed <br /> initially, with the two fields interconnected with an approved flow diversion device <br /> (pressure-rated), intended to allow alternate use of the two fields. <br /> D. DESIGN CRITERIA <br /> 1. Treatment. Pressure distribution may be used with either septic tank effluent or <br /> with supplemental treatment. <br /> 2. Design Sewage Flow. PD trench systems shall be designed on the basis of the <br /> projected sewage flow for the structure or facility being served, determined in <br /> accordance with sewage flow estimation requirements in this Manual. <br /> 3. Pressure Dosing. Septic tank effluent shall be applied to the PD trench system by <br /> pressure dosing, utilizing a pump system. The pressure distribution system shall <br /> be designed in accordance with accepted engineering practices to achieve, at a <br /> minimum: <br /> a. Uniform dosing of septic tank effluent throughout the system of PD trenches; <br /> b. Adequate flow rate, screening of effluent and suitable piping network to <br /> preclude solids accumulation in the pipes or clogging of discharge orifices; <br /> c. Suitable access provisions for inspection, testing and adjustment of the <br /> pressure distribution system; and <br /> d. Dosing volume to achieve minimum of 3 to 5 doses per day at design flow <br /> conditions. <br /> 4. Dispersal Trenches. PD trenches shall conform to the same design and <br /> construction requirements as standard trenches, as provided in this Manual, with <br /> the exception that the piping system shall consist of pressure piping rather than <br /> gravity piping. <br /> 5. Pressure Distribution Piping. <br /> a. Pressure-Rated Pipe Material. All pipe, fittings and valves shall be <br /> pressure-rated PVC pipe, minimum 150 psi. <br /> b. Solvent Welded. All joints in the pressure piping system shall be solvent <br /> welded. <br /> C. Pipe Sizing. All pressure distribution pipes and fittings, including transport <br /> lines, manifolds, laterals and valves, must be adequately sized for the <br /> design flow, and shall be designed to minimize frictional losses to the <br /> maximum extent practicable. <br /> d. Thrust Blocks. Concrete thrust blocks, or equivalent restraint, shall be <br /> provided at sharp changes in piping directions. <br /> 2 <br />