Effluent Distribution Using Pressure Manifolds
Pressure manifold systems are dosed using a pump or, in some cases, a siphon. Siphons can only be used on sites with sufficient slope so that the drop in elevation from the siphon to the manifold can pressurize the manifold. Pressure manifold systems can overcome some of the limitations of distribution boxes and ensure equal distribution to each trench. In addition, pressure manifold systems can be used to proportion effluent to unequal length trenches. Two general types of pressure manifold systems are discussed here, level manifolds and sloped manifolds.
A sloped manifold can be used to distribute flow evenly to trenches on a sloping site. Typically a reducing tee is used to install a short branch off of the manifold and into whatever gravity lateral trench product or material is used. The branched line is capped and one or more holes (orifices) are drilled into it. Since, on a sloping site, the branch line into each lateral trench is at a different elevation, the pressure in the manifold line at each branch will vary. Valves are installed on each branch line and used to set each branch to operate at equal pressures.
A calculation tool is provided for registered installers to use in designing pump pressurized sloped manifolds. The designer should select the type of lateral system and its size based on the site/soil evaluation by a registered onsite soil evaluator. It is perfectly acceptable to exceed the minimum standards by rounding up or adding more absorption area. Once the number of equal length trenches is determined, the installer will need to measure the total length of the manifold line and the elevation difference from the low water level in the proposed pump tank to the highest point in the pressure system. Then using the calculation tool, the number and size of holes per branch line and the operating head can be selected to optimize the pump selection or to match a pump's performance curve. The tool includes detailed line-by-line discussion of the calculations and several related tables and formulas.
If unequal length trenches are needed to fit the soil treatment system into the available area, the calculation tool can be used to select the number of orifices per branch to control the flow to each trench in proportion to the trench length. Alternatively, the size of orifice(s) and operating pressure in branch lines can be varied to proportion the flow to each trench: Use the tables or table formulas from the calculation tool to assist with the design of a system with unequal trench lengths. Based on the initial choice of number and size of orifices and operating pressures, calculate the flow to each trench and total the flows. The flow to a trench divided by the total flow should equal the length of that trench divided by the total trench length:
Flow trench / Flowtotal = Lengthtrench / Lengthtotal (+ or – 5%)
Initial choices should be adjusted to set the proportion of flow to each trench within five percent (5%) of the proportion of that trench length to the total trench length.
On level, or nearly level sites, a level manifold would generally be used. Even on sloping sites, a level manifold can be used by locating the manifold above the field with the manifold outlets all at the same elevation. Effluent flows from the manifold outlet to each trench by gravity. It is easy to divide flows equally to equal length trenches with a level pressurized manifold. If the manifold is properly sized, outlets at the same elevation will have the same pressure; therefore, equal sized outlets will discharge equal volumes. It is important to ensure that the manifold diameter is not too small for the number of outlets and that the outlets are not spaced too closely. Valves are not needed on each line to adjust pressure; however, a valve should be installed on each discharge line to allow a line to be removed from service, if needed, for repairs or to rest.
If unequal length trenches are needed to fit the soil treatment system into the available area, a level manifold can be constructed to deliver different volumes based on the trench length so that the trench bottom areas are loaded equally. Since the manifold pressure is equal and there is usually one outlet per trench, this is accomplished by varying the size of the outlets. Use the tables or table formulas from the calculation tool to assist with the design as described for sloped manifolds above.
Pressure manifold system management: Manifold systems should include a valve on each line as previously mentioned. In addition, inspection ports should be installed in each lateral to allow periodic inspection and adjustment, if necessary, to ensure acceptable operation and extend the useful life of the system.