The Benefits of Pressure Distribution

The Benefits of Pressure Distribution

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Pressure or uniform distribution of effluent is the optimum method to spread wastewater out to gain the best treatment from soil treatment systems.

With pressure distribution, effluent is delivered either to taps in a pressure manifold (pressure-dosed gravity systems) or to multiple points across an infiltrative surface (low pressure or drip distribution). By definition, pressure distribution systems incorporate a pump or siphon to provide the energy needed to overcome the forces of gravity. Pressure distribution systems using pumps can be dosed either on demand or at specific times. Those with siphons are inherently demand-dosed. Effluent can thus be distributed along the entire length of an infiltrative surface without using electricity.

Pressure distribution uses a network of distribution pipes in which effluent is forced through orifices under pressure. Pressure distribution has been used for more than 40 years to apply septic tank effluent to soil treatment systems. In 1974, Converse et al. published a paper documenting the application and performance of pressure distribution systems, particularly in sandy soils. Today, media filters and many other advanced treatment units rely on pressure distribution. Because this trend is sure to grow, professionals need to understand how pressure distribution works.

Pressure distribution substitutes for the biomat in gravity systems to distribute effluent across the infiltrative surface. It provides increased treatment efficiency on sites with soil and size limitations. A pump typically controls the application, which proceeds at a rate determined by the long-term acceptance rate of the soil or media. Pressure distribution at the proper rate maintains an aerobic (oxygen-rich) environment that allows for effective treatment. Any pressure distribution system has four interdependent components:

  • Pump tank and pump that pressurizes the system
  • Pump controls
  • Pipes that deliver the effluent
  • Orifices that discharge effluent into the treatment area.

Each has important design and installation requirements. Pressure distribution applies effluent uniformly over the entire infiltrative surface such that each square foot of bottom area receives approximately the same amount per dose at a rate less than the saturated hydraulic conductivity of the soil. This application promotes soil treatment performance by maintaining vertical unsaturated flow and also may reduce the degree of clogging in finer-textured soils. Pressure distribution closely approaches uniform distribution. 

In a field study, the more uniform distribution of effluent offered by pressure distribution resulted in better aeration and nitrification during system startup. Fecal coliform removal was significantly lower in pressure-dosed systems over the course of the study (Bomblat et al., 1994). Uneven distribution of effluent can result in localized overloading and system failure.

One often-overlooked benefit of pressure distribution is that it ensures resting periods between applications, allowing time for the soil or media to recover. Designs should allow for a number of resting periods. A typical pressure distribution system is designed with four dosing and resting periods per day. Equally spaced applications allow resting between doses and more uniform application, resulting in more consistent oxygen transfer and better treatment potential. A timer can further assist in spreading out the application of effluent. Typical demand dosing, by comparison, turns the pump on whenever sufficient effluent is available.

Pressure distribution is usually used in locations where it is either desirable or required to:

  • Achieve uniform application of effluent throughout the soil treatment area
  • Disperse effluent higher in the soil profile
  • Avoid potential contamination of groundwater beneath excessively permeable soils
  • Improve the treatment performance and extend the life expectancy of a drainfield or other component
  • Reduce the potential for breakout or seepage on slopes
  • Avoid potential contamination on sites in aquifer-sensitive areas
  • Avoid potential contamination on sites with limited soil depth
  • Disperse effluent evenly for larger drainfield systems
  • Disperse effluent evenly in conditions where pumping is needed due to elevation problems.

One disadvantage of pressure distribution is that it needs a pump tank, pump, controls and an alarm. This means the owner has to pay for these components along with electrical service and usage. Another issue with pressure distribution is that the orifices must be kept clear for the system to work properly. Septic tank effluent inevitably contains some solids, which can plug the system. Effluent screens, which are increasingly popular, further limit the size of suspended solids leaving the pump tank. Maintenance of the septic tank and distribution system is critical for long-term performance. 

About the author: Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation, and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association and the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system maintenance and operation by sending an email to


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