A reader recently asked, “I’m relatively new to this and learning more and more on each (septic system) design. What is the difference between a pressure trench system and a drip dispersal system?” While I have discussed the use of pressure systems in the past, new people are always coming into the industry and technicians are expanding their knowledge and skills. So since the question was asked, here is a brief overview of pressure distribution systems.

The primary reason we move to pressure distribution systems is to overcome a variety of site and soil characteristics where conventional gravity flow systems are not feasible or the environmental conditions so sensitive, they should not be used. These conditions include shallow soils over bedrock, soils and sites with high water tables, slowly permeable soils and very rapid permeability soils. Basically, any condition where treatment capacity of the soil is limited.

ENHANCING TREATMENT 

Pressure distribution allows us to apply effluent more uniformly than gravity over the entire soil dispersal area and over the day, not just when there is a water-using event in the dwelling. Pressure distribution networks are also used within or after advanced pretreatment components, allowing enhanced treatment for nutrients such as nitrogen.

Pressure distribution includes pressure dosed to a gravity soil treatment unit, low pressure distribution, drip distribution and spray dispersal. Pressure dosed to gravity involves delivering effluent under pressure to a bed or series of trenches, then using gravity to distribute the effluent. Low-pressure systems are used with a manifold in a trench or bed configuration, with the entire distribution network pressurized while the pump is running. Similarly, pressure distribution networks in media filters are also fully pressurized. Both result in effluent being uniformly distributed across the treatment area. 

Drip distribution is another form of pressure distribution, with same objectives of distributing effluent uniformly over the treatment area in time and space. It uses tubing with flow-regulating emitters installed at varying depths in the soil, usually 6 to 12 inches.

Pressure-to-gravity is used where soils are more suitable at an elevation higher than the house, or the house sewer exits too deep to allow for gravity and maintain separation from a limiting soil condition. A pump and dosing tank are added following the septic tank. Effluent is then pumped to a distribution or drop box and distributed by gravity in parallel or sequentially. My choice in these situations is to use sequential distribution allowing for more management.

Low pressure distribution uses small diameter (1- to 2- inch diameter PVC) for the manifold and laterals. The distribution laterals have small diameter orifices (1/8- to 3/16- inch). A low-pressure effluent pump is used to pressurize a manifold, which splits to flow to the network of laterals in the soil treatment area. 

It is important to recognize the pump and distribution network are designed together to deliver the desired dose at a determined hydraulic head. If the pump needs to be replaced at some point, it needs to be with one that will deliver the same flow in gallons per minute and operate at the same hydraulic head. If not, effluent will not be distributed uniformly over the area. When working on pressure distribution systems, a service provider needs the initial design specifications to properly evaluate and maintain the system.

ORIFICE SIZE 

Low pressure systems can operate with septic tank effluent or following advanced pretreatment in an aerobic treatment unit or media filter. The type of effluent distributed will help determine the size of orifices. With more highly treated effluent, orifice size can be smaller, since there will be fewer problems with orifices plugging.

Distribution laterals can be installed in separate soil treatment trenches fed by the manifold or in bed configurations. Bed configurations are generally used in soil treatment mound and at-grade systems. When pressure is used in media filters it is essentially the same, just it is usually done in some type of container.

While drip is also considered a pressure system, it is very specialized. As indicated above, the goals are the same in terms of delivery and treatment, but it is a unique system. Some drip systems are marketed to operate with septic tank effluent and others require aerobic treatment in front of the system. The system consists of the pretreatment device, a dosing tank, pump and controls, flow metering device, filtration headworks and the dripfield.

Effluent is stored in the dosing tank until it is delivered to the soil treatment area. Typically, a high head multistage turbine effluent pump delivers the water through the filtration headworks to the drip tubing with emitters.

A control system regulates the dosing to the field. This allows even distribution across the area and throughout the day. Some systems have built-in automatic filter backwashing and field-flushing capabilities, which operate periodically to flush the lines reducing emitter plugging and maintaining equal distribution. Since the emitters have small openings, the ability to flush the lines periodically is an important maintenance feature.

The dripfield is constructed of drip tubing approximately ½-inch in diameter with an emitter welded into the tubing wall. Pressure inside the tubing is operated at 15 to 45 psi, while the water exits the emitter at 0 psi. The tubing is placed along the contour to form a run. They can be connected directly to the supply and return manifold as ladder-shaped zone or be looped together and then connected to the manifold on the same or opposite end of the zones. 

The type of pressure system selected will be determined by state and local requirements and the specific site and soil conditions encountered.