Do Necessary Soils and Site Homework to Choose the Right Dispersal Plan

Homeowners never want to hear they’ll need a pressure distribution system, but sometimes that will be the only path to treatment success.

Do Necessary Soils and Site Homework to Choose the Right Dispersal Plan

Jim Anderson, Ph.D., is an emeritus professor at the University of Minnesota Department of Soil, Water and Climate and recipient of the pumping industry’s Ralph Macchio Lifetime Achievement Award. Email Jim questions about septic system maintenance and operation at

A few months ago, I had an inquiry from a homeowner indicating the need to replace a failing drainfield and being told by the local regulator that a replacement system using pressure distribution would probably be required. The homeowner noted the extra expense involved with adding a pump tank and a pump and wanted to know how the need for pressure distribution was determined.

If you have followed this column over the past few years, I hope you recognize that I am all for using gravity distribution over pressure when site conditions permit. Gravity is the simplest (not necessarily as simple as some think though!) and least expensive solution, so it is the most popular choice. The fact it is “simple” and “inexpensive” has resulted in gravity distribution being used on many sites unsuitable for this distribution method. The result is often poorly functioning systems that cost the homeowner more through either increased maintenance or premature replacement, or both.

Gravity distribution can be used to effectively treat wastewater on sites that have deep, well-drained soils with no limiting conditions such as high bedrock, regional or seasonal water tables, or dense, slowly permeable soil layers. To maximize treatment using gravity distribution, the installer should make use of sequential distribution techniques and install system components so they can be readily evaluated and serviced (this is the not necessarily simple part).


As the homeowner was already aware, additional system components are associated with pressure distribution. Increased costs are due to the fact that pressure distribution should be used to overcome site and soil conditions where gravity systems will not provide adequate treatment or acceptance. The site is more of a risk to provide long-term sewage treatment. To provide better and consistent treatment, pressure distribution is used to more uniformly spread the effluent over the soil treatment area and to spread the applications throughout the day. While this does involve some additional upfront and maintenance costs, it ensures consistent treatment of effluent throughout the year and the system will accept the water generated from the house without surfacing problems.

A gravity system will not perform consistently throughout the year when the treatment trenches are installed at a depth in contact with a seasonal water table or saturated zone. During certain times of the year, usually spring, the system is in contact with the water and the ability of the soil to accept wastewater is reduced, which often results in sewage coming to the surface. This system is not meeting the requirement to accept and treat effluent 365 days a year, year in and year out. Further, this periodic saturation, over time, can lead to the development of a more resistant biomat and ultimate system failure — not just during wet periods.

The direct answer to the homeowner is that there is a limiting soil or site condition that precludes the use of direct gravity distribution due to treatment and acceptance concerns. The configuration of the pressure distribution system, though, could take many forms, again depending on nearby soil and site conditions.

With deep, well-drained soil at an elevation higher than the septic tank outlet, the system could consist of septic tank, pump tank, pressure to a dropbox or distribution box, to gravity trenches. The system becomes a combination of pressure and gravity. I have had colleagues question why this would be an option when a pump tank and pump are needed anyway.

Why not just use small-diameter low-pressure pipes in trenches and have an entire pressurized system? My answer is that when working with the client, we should look at the lowest-cost, least-complicated system that will do the job of long-term treatment. By using this approach, long-term maintenance costs are reduced for the homeowner and treatment is provided.


As indicated earlier, the pressure system can also look like this: septic tank, pump tank, to low-pressure pipe in a shallow trench. Use of this configuration is dependent on the depths to limiting layers in the soil. For example, if the depth to water table is 4 feet and to install gravity trenches would require a 2-foot excavation depth, the required 3 feet of good soil is not available for treatment; pressure could be used in a 1-foot excavated trench.

Limiting layers closer to the surface may require installation of a mound system with an elevated clean sand bed with a pressure distribution network, which allows effluent to be distributed over the sand bed and then through the sand to the original soil surface. The treatment train here is septic tank, pump tank, mound bed. A variation would be an at-grade system where effluent is distributed through drainfield media to the original soil surface.

There are additional options where media filters can be used either as pretreatment devices or as final soil dispersal components incorporating pressure distribution as a part of the treatment process. With each additional component to the system, specific operation and maintenance factors need to be considered when deciding which set of components or approaches are best for that site.


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