OTHER RECOMMENDATIONS:

1. Sleeve lateral in 3- or 4-inch conventional nitrification line perforated tubing. Reduces the effects of rock shadowing and root intrusion and enables easy removal or replacement of the LPP laterals, if necessary. (See figures 1 and 3)
2. Following item 1, drill all orifices facing upwards, except for a hole at the beginning and end of each line which should face down. Having the orifices face up: helps bleed air out of the system better during pressurization; ensures no shadowing of these orifices by rock, roots, or perforated pipe corrugations; helps breaks the air block better after pump shut off, reducing siphonage from laterals to the supply manifold, which, in turn, may reduce the overloading of teh lower laterals; and allows less opportunity for hole clogging by solids and bioslime. Having the two holes on each line face down allows effluent to drain after pumps shut off which: prevents anaerobic conditions inside the laterals; prevents standing water inside the laterals which could possible attract plant, grass, and tree roots; and prevents freezing of the laterals (though typically not a problem in southeastern climates). (See figure 3)
3. Use one 90-degree bend for lateral turn-ups, not one or two 45-degree bends. This will facilitate accurate measurement of operating pressure head in the laterals. Using one or two 45-degree bends for constructing the manifold cleanouts, however, would be warrented. (See figure 3)
4. Use geotextile fabric over the trench, at the grave - backfill interface. Prevents the infiltration of small soil particles into the trench voids. (See figures 1 and 3)
   

   FIGURE 1: Cross-Sectional, Profile of Recommended LPP Trench.

5. For LPP lateras, tee off the manifold directly using, for example, a 3" x 3" x 1-1/4" tee, rather than "bushing-down", (i.e. using a 3" x 3" x 3" tee with reducing fittings to a 1- 1/4"). This results in a hydraulically smoother transition and allows less opportunity for irreversible solids accumulation. (See figure 2)
   

   FIGURE 2: Side, Profile Detail of Recommended Supply Manifold-to-Lateral Connection.

6. Elbow the manifold-to-lateral connection over the earthen dam, particularly on sloping sites. This reduces effluent drain-back through the manifold or along the manifold trench. (See figure 3)
   

   FIGURE 3: Side, Profile Detail of Recommended LPP Trench, including Cross-Section of Recommended Manifold-to-Lateral Connection.

7. Provide two cleanouts on each supply manifold for LPP systems on sloping sites; oone at the beginning (typically, the bottom) and one at the end (typically, the top) of the line. Facilitate cleaning out the manifold after blowing out the LPP laterals, without reintroducing solids into the LPP laterals.
8. Use the smalles pressure-adjustment gate valves which do not cause total dynamic head of the system to exceed that which can be adequately handled by a reasonably-sized pump. This is typically achieved using gate valves which are one nominal size smaller than the effluent supply line or manifold (whichever is smaller). The use of smaller valves allows for more precise pressure head adjustment. There will typically also be a net cost savings assocated with the use of a smaller valve.

   In fields served by multiple manifolds, place all pressure- adjustment gate valves in a common area for easy adjustment by one individual.

9. Design/configure system to prevent or offset the hydraulic overloading of the lower laterals. Overloading of lower LPP lateral is anticipated because: the lowest laterals fill up with effluent first and drain last, and they will receive effluent stored in the manifold and, perhaps, upper LPP lateral after dosing. Additionally, lateral, subsurface movement of water through the drainfield may occur, potentially contributing to the overloading of the olower laterals. There are several ways to alleviate these problem, including item number 6, listed above, as well as the following:

A. Design the system to "overload" the upper laterals during dosing. This can be accomplished by varying the orifice spacing and/or orifice size among the LPP laterals.

B. Incorporate a drainback mechanism to return effluent to the pump tank between doses.

C. configure fields to make maximum use of slope width (along contour), thereby reducing the number of lines installed at different elevations.

D. Use check valves to keep effluent dosed to the upper subfields from draining to the lower subfield laterals.

E. Split flow to the subfields or laterals using a pressure manifold above the highest lateral, to prevent redistribution of effluent to the lower subfields or laterals.

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Low Pressure Pipe Drainfield Design NCSU Wastewater Demo Site

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Podunk County Health Department Environmental Health Division Operations Permit

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Please address any questions to Dr. David Lindbo.

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This page (http://www.ces.ncsu.edu/plymouth/septic/95lpp.html) created by
Vera MacConnell, Research Technician, I on March 16, 1999.
Last Updated on June 3, 1999.

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