ID-164-W

Cooperative Extension Service
Purdue University
West Lafayette, IN 47907

Because many of the soils in Indiana are not well suited for disposal of residential wastewater, innovative septic system designs often are required for satisfactory performance. A pressure distribution system is one such design.

Pressure distribution systems are used primarily when the soil disposal area is located upslope, or too far away from the septic tank to obtain gravity movement of effluent. They also are recommended for use in sandy soils to ensure adequate treatment of wastewater.

This publication describes the construction procedure for the installation of a pressure distribution system following the final design approval by the health department. The following items are discussed: system location, trench construction, and the construction and installation of the pressure distribution network. Septic tank and pump chamber installation also are discussed.

These guidelines should aid contractors and do-it-yourself homeowners in applying the design information supplied by a consulting engineer and the Fast Agricultural Communications Terminal System (FACTS) computer program, RWASTEII (FX-89), available at the Cooperative Extension Service office. The RWASTEII program also is available in C-Language for use on personal computers that are IBM compatible. RWASTEII recommends a specific type of system best suited to a particular set of site and soil conditions, and to provide specific design information. The program estimates the amount of gravel needed for construction as well as suggesting system layouts which will best fit the shape and size of disposal area available. The program will also size the pump and piping needs of the pressure distribution network. The design recommendations must then be reviewed and altered to fit the actual features of the site by a consulting engineer.

Pressure Distribution

Nonconventional types of septic systems require more care than conventional ones in site selection, design, and construction. This is due not only because the contractor is apt to be less experienced with pressure distribution installation but, also because soil and site conditions may be difficult.

These systems differ from traditional gravity systems in that a small pump and a relatively small diameter pipe (usually 1-2.5 inches) is used to transfer the septic tank effluent to the disposal area. Carefully sized and spaced holes in the pipes laid in soil trenches result in a uniform application of wastewater to the soil.

Good construction techniques are essential if pressure distribution is to function properly. If followed carefully, they will help ensure many years of trouble-free operation. Consider the following step-by-step procedure:

A. Site Selection and Layout

Step 1: The selected site must be one that sheds water. The long axis of the soil disposal area must be oriented parallel to the contour of the slope (i.e., lines of equal elevation). Try to avoid areas where the long axis would run up or down the slope or where wastewater movement will converge (Figure 1). Because a pump generally is used to pressurize the system, the soil disposal area can be located upslope from the home. (Of course, this would not be an option if a siphon is used to pressurize the system). A contour or topographic map is rquired to determine the best layout (Figure 1).

Figure 1. Proper orientation of trenches on complex slopes.

Step 2: Lay out and stake the location of the feedline trench, septic tank, and pump chamber (Figure 2). Their exact locations may be dictated by minimum distance requirements from water supplies, structures, property lines, and bodies of water as outlined by Indiana State Board of Health regulations. These setback distances are also part of the FACTS program.

Figure 2. The entire system, including the location of the septic tank and pump chamber, should be layed out and staked before construction.

Step 3: Stake the center line of all soil trenches. Trenches should be spaced a minimum of 7.5 feet center-to-center, or as specified by the computer program. The greater the slope or the more impermeable the subsoil, the greater the spacing needed to keep from overloading the soil of the lower trenches since wastewater will tend to move more horizontally than vertically.

Step 4: The area required for the soil absorption system, and an additional distance of at least 50 feet downslope from the field should be fenced. This will prevent disturbance, scalping, or compaction of the soil absorption area by vehicular traffic or construction equipment. All traffic should be prohibited from the area before, during, and after the construction of the home and installation of the septic system.

Step 5: If the soil is classified as poorly or very poorly drained, or if the trenches are to be constructed on the side or at the base of a slope, install a diversion ditch and/or subsurface curtain drain (one backfilled with gravel and extending to the limiting soil layer or to at least 2 feet deeper than the elevation of the planned trench bottom, Figure 3) above the system to keep upslope runoff and seepage water away from the system. Subsurface drains generally are necessary to help control the water table below the field. Keep the subsurface drains at the distance specified by the health department from the outside edge of the field.

Figure 3. Locations of subsurface and surface drains.

B. Trench Excavation and Preparation

Step 1: The trenches should only be excavated when the soil is dry and friable. Smearing and compaction due to construction in a wet soil decrease the soil's ability to absorb wastewater. If a sample of the soil at trench bottom depth forms a ribbon (e.g., 1/8-inch diameter) when rolled between the palms of the hands, the soil is too wet to excavate. If the soil crumbles, excavation may proceed (Figure 4). This preexcavation investigation is essential to help ensure proper operation of the system.

Figure 4. Construction must not take place if soil is too wet.

Step 2: Trenches should be kept as shallow as possible to take advantage of the more permeable horizons near the soil surface. The FACTS program will automatically do this by specifying the trench bottom depth for the site.

Excavate lateral trenches to design depth and width while frequently checking their bottom elevations with an engineer's level to ensure the trench bottoms are level over the length of each trench. After most of the soil is removed, use an excavating bucket with teeth to rake each trench bottom to final bottom elevation (Figure 5).

Figure 5. Keep backhoe bucket perpendicular to trench bottom to minimize compaction.

Sides of lateral trenches should also be raked to a depth of 1 inch to expose the natural soil structure and to remove any smeared and compacted soil surfaces caused by the excavating bucket. This can be accomplished by attaching fabricated raker teeth to each side of the bucket (Figure 6). Foot traffic on the excavated trench bottoms should be minimized to prevent further compaction. If foot traffic is necessary, use planks to spread out the workers' weight.

Figure 6. Fabricated raker teeth to reduce compaction and smearing of trench side walls.

Step 3: Excavate a narrow trench for the manifold pipe between the lateral trenches either at one end or in the center of the soil disposal area, depending on the lateral pipe layout recommended by FACTS program. To prevent plugging or freezing, the manifold must be placed so that it will drain between doses. It should drain into the lateral distribution pipes (Figure 7) or back to the pump chamber if connected to the lateral distribution pipes from below (Figure 8). Then excavate another trench for the feedline, either from the center or from one end of the manifold trench to the pump chamber, along the path staked out in Step 2 of Part A. The feedline should also be laid at a slope that permits drainage between doses to the pump chamber. Otherwise, the feedline must be placed below the frostline (about 40 inches in most of Indiana).

Figure 7. Sanitary cross connection. Drill 1/4-inch hole on underside of manifold downslope end with feedline drainage to pump chamber.

Figure 8. Tee-to-tee connection. Feedline and manifold must drain back to the pump chamber.

Once the properly sized manifold and feedline are laid to the desired grade and connected to the lateral piping, place soil around the manifold pipe and compact it well to prevent wastewater from seeping along the manifold from one lateral trench to the next. This is especially important on sloping sites where successive lateral pipes and trench bottoms are placed at lower elevations.

Step 4: Monitoring wells, which allow evaluation of the performance of the absorption field once it is in operation, should be installed at this point in construction of the system. These wells are usually 4-inch diameter PVC pipe. Each monitoring pipe is perforated with 1/2-inch holes over the lower 6-inch length. It extends to the ground surface and is covered with a friction fit cap or screw cap. Monitoring wells should be located in several trenches, extending from the trench bottom to the final surface grade (Figure 9). This will provide a means of evaluating the depth of ponding in a trench, a measure of the system's performance.

Figure 9. Monitoring wells shown in completed trench and outside of absorption field.

After completion of the system monitoring wells also can be installed outside the absorption field to evaluate seasonal groundwater levels which will affect the system's performance. These wells should extend to a depth of 40 inches or at least 2 feet below the elevation of the nearest trench bottom, whichever is greater.

C. Pressure Distribution Network Installation

Step 1: Carefully place washed Indiana State Highway Specification (Spec) #5 aggregate, or other trench fill material which has been approved by the County Sanitation, to a depth of 6 inches over the bottom of the trenches. Finally, level the aggregate.

Step 2: The lengths and diameters of the distribution lateral pipes will be provided by the FACTS program. All pressure distribution piping and fittings should be schedule 40 PVC plastic (ASTM-D-1785). First, lay out the pipe in each trench. Clean all glue joints with a solvent. Apply glue to both male and female sections of the joint, then join. Twist slightly to create a leak-proof connection. Glue a cap to the lateral pipe end that is away from the manifold.

Step 3: Starting from the uncapped end of the lateral, the first hole is drilled at 1/2 the hole spacing distance. The following holes are then drilled at the specified hole spacing distance until the end is reached. These holes should be drilled in a line along the length of the lateral pipe. When finished all holes should face the same direction. On the top side of the end cap opposite from the row of dosing holes, drill a 1/4-inch diameter air release hole to ensure that all wastewater drains from the lateral pipe after each dose (Figure 10).

Figure 10. Air release hole ensures that all wastewater drains from the lateral pipe after each dose.

Remove all burrs around the dosing holes, both inside and outside of the pipe, taking care not to enlarge any hole beyond its design diameter. Be sure to remove any loose chips from inside the lateral pipes to prevent possible clogging of the dosing holes. If the various parts of the network have been carefully identified, the hole drilling and capping can be done in a shop or workroom and then taken to the site for assembly.

Step 4: Assemble the network on top of the aggregate in the lateral trenches. Make sure the dosing holes of each lateral pipe are facing downward (the invert side of the pipe) before connecting the laterals to the manifold. Also, make sure each lateral pipe is level over the entire length of the trench by using a carpenter's or engineer's level.

D. Backfilling the Trenches

Step 1: Carefully cover the pipe network with additional washed Spec #5 gravel or approved aggregate to a depth of at least 2 inches above the crown of the pipe.

Step 2: Next, place a backfill barrier such as a synthetic fabric filter, 4-6 inches of marsh hay or straw, or untreated building paper (red rosin) over the aggregate cover.

Step 3: Backfill the trenches with excavated soil and compact slightly. Mound the soil 4-6 inches over the top of the trench to allow for settlement (Figure 11). Individual trenches can be excavated and completed in sequence for ease of construction.

Figure 11. Crown backfill over tanks and trenches.

Step 4: Since settlement may take 6-12 months, the construction area should be resodded or reseeded immediately using grasses adapted to the area.

E. Septic Tank and Pump Chamber Installation

Step 1: Excavation depths for the septic tank and pump chamber are determined largely by what is necessary to obtain gravity flow in the sewer from the point where it leaves the house. A two percent slope is required for the house sewer to the septic tank while a one percent slope is sufficient for a pipe carrying septic tank effluent. Make sure both tanks are tightly sealed against groundwater seepage before installation. The pump chamber should be the same size or larger than the septic tank to allow for at least one day of reserve storage of effluent after the high water alarm is activated due to pump or float failure.

Step 2: Carefully level both the septic and pump tanks for proper operation after their installation. Access must be provided to all parts of both tanks to allow for inspection and maintenance. Use 4-inch diameter PVC sewer pipe (ASTM-D-2665, 3033, or 3034) with water tight connections between the house and the septic tank and between the septic tank and pump chamber. Footing and roofing drains must not be connected to the septic system. Crown backfill to a height of 6 inches over the tanks to allow for settling and to divert surface runoff (Figure 11).

Step 3: Install a submersible sewage effluent pump or siphon in the pump chamber that has the head and discharge characteristics recommended by the FACTS program. This recommendation is designed to balance pump characteristics against the hydraulic requirements of the piping network. Try to select a commercially available pump with a high confidence factor (a measure of the pump's ability to meet the head and discharge requirements of the piping network) from the list of pumps generated by FACTS. Do not select a pump that is different from those recommended unless the FACTS program was used to redesign the piping network for the characteristics of that particular pump. The proper pump or siphon should give an even discharge from all dosing holes in the soil disposal area. Set the pump 6-8 inches above the tank floor on a large platform of cement blocks to prevent settled sludge from interfering with the operation of the pump and piping network (Figure 12).

Figure 12. Cross sectional layout of pump chamber and associated components.

Step 4: Plumb the pump to the feedline with a riser pipe and quick-disconnect coupler so the pump can be removed easily for inspection and maintenance (Figure 12). The quick-disconnect coupler should be located near the access of the pump chamber to reduce the difficulty of disconnecting the pump. Common couplers include rubber or hose couplings anchored to the pipe ends by hose clamps or plastic PVC unions. One coupling which allows for quick disconnection or connection is the cam-lock type connector used on fire hoses (Figure 13). Where practical, connectors should be made of plastic instead of metal because the tank environment is quite corrosive. If an effluent pump is used, a 1/4-inch weep hole should be drilled on the underside of the feedline to allow drainage after each dose.

Figure 13. Three types of quick-disconnect couplers.

Step 5: Make the electrical connections from the pump to the control circuitry. The pump operation should be controlled by external mercury float switches (allowing easy adjustment of the dose volume) rather than a diaphragm controller built into the pump body. Install a high-water alarm float, approximately 3 inches above the high-water control float, on an independently fused A.C. circuit to alert the owner of a malfunction. All electrical leads should be strapped to the riser pipe of the pump, and should be long enough to allow the pump and floats to be removed from the chamber without electrical disconnection. Use nonmetallic, tamper and weatherproof electrical boxes for all electrical connections. The connection box may either be mounted in the basement of the house or near the access to the pump chamber. Be sure that the electrical circuitry is grounded properly for the safe operation of electrical equipment in a wet environment (according to the National Electrical Code).

F. Maintaining the System

Traffic and construction must be avoided over and immediately downslope from the soil disposal area to prevent compaction and to minimize frost penetration. You should maintain a good grass or vegetative cover over the area to maximize the uptake of water. You also should provide the homeowner with a layout diagram of the septic system, referenced to the house and lot boundaries. This will enable location of the tanks and absorption field for future maintenance.

Sludge should be removed from the septic tank and pump chamber every 3-5 years. This clean-out schedule is even more important with pressure distribution systems than with standard gravity fed systems to avoid carryover of solids to the disposal trenches. Carryover solids can plug the pressure distribution pipe openings in the trenches.

Homeowners should be encouraged to monitor the performance of the septic system by routinely checking for the presence and depth of water in the monitoring wells during spring, summer, and fall. Any progressive increase in ponding depth within the trenches over time may be indicative of future problems.

Water conservation measures in the home help to ensure the soil disposal area will not be overloaded. It would be wise to install faucet aerators and low-flow fixtures and appliances when possible.

Operating and Maintaining the Home Septic System (ID-142) is a helpful Cooperative Extension publication that is written for the homeowner. This publication and the above recommendations should help the homeowner better understand the operation and maintenance of their on-site septic system for many years of trouble-free operation.

Additional Assistance

In addition to the FACTS computer program located at each Indiana County Cooperative Extension office, assistance in selecting and designing an on-site system is available from your local county health department and your local Soil Conservation Service (SCS) office. In addition, some assistance may be available for design of innovative systems from the Purdue University On-Site Waste Disposal Project, Agronomy Department, West Lafayette, IN 47907.

Cooperative Extension Work in Agriculture and Home Economics, State of Indiana, Purdue University and U.S. Department of Agriculture cooperating. H.A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the Acts of May 8 and June 30, 1914. It is the policy of the Cooperative Extension Service of Purdue University that all persons shall have equal opportunity and access to our programs and facilities without regard to race, color, sex, religion, national origin, age, or handicap.