The Nuts and Bolts of Pressure Distribution

For more than 40 years, system designers have sought to perfect effluent dispersal by controlling dosing to the soil treatment area.

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In the past year we have discussed installation of pressure distribution systems and there have been a few comments and questions, so we will briefly revisit the topic. If you came in late on some of the discussion, here is a review of some attributes of pressure distribution:

Pressure distribution has been a part of the onsite sewage treatment system picture for over 40 years. It was used in mound systems installed in 1971 in Wisconsin and it was taught in classes for onsite professionals in Minnesota as early as 1972. It was somewhat slow to catch on but now it is a part of final soil dispersal areas and a number of media filters as well.

Anyone working in the profession now needs to know about and understand pressure distribution.

True pressure distribution is where effluent is delivered uniformly to multiple points through a distribution network of pipes; it can be low pressure as in mound or at-grade systems, or high pressure as is the case with drip systems.


Pressure distribution systems substitute for the biomat to deliver effluent across the entire soil treatment area. A pump is used to control the application on demand, where effluent is pumped as water is used in the residence, or using a timer to spread the flow out more evenly during the day. If properly designed and operated, pressure distribution provides an aerobic environment in the soil, avoiding development of the biomat and allowing for efficient treatment.

By applying effluent uniformly over the area and maintaining oxygen, the vertical flow through the soil remains unsaturated and reduces the development of the biomat as aerobic organisms are present to break down the organic component of the waste. The more even the effluent is distributed, the better the treatment, with less chance of certain areas of the system being overloaded leading to less efficient treatment.

The designer and installer are balancing the desire for equal distribution across the area as well as distributing the effluent throughout the day. Designs should allow for resting periods between applications. This gives the soil time to recover and operate efficiently in terms of treatment.

The original mounds we designed and worked on in Minnesota called for four equal applications during the day. That way any single dose to the system would be no more than 25 percent of the average daily flow. So for a 600 gpd system, the maximum dose volume to the mound bed would be 150 gallons. This would theoretically mean the doses would be spread out during the day with more uniform application, resulting in more consistent oxygen levels and better treatment.


However, these were on-demand systems. So when does the majority of water get used in a house? Of course, during the week it’s in the morning when everyone gets up and goes off to school or work, and then again in the evening when everyone is home. Laundry typically gets done all in one day whenever there is time off. So the idea that effluent is being evenly distributed over time is not quite accurate in on-demand systems.

Timers are now often added to the system, and tank storage capacity is increased so flow during high-use periods can be collected and sent out in even doses across the 24-hour time period. This strategy has proven to improve treatment.

Work done on sand filters in California showed marked improvement of treatment if effluent was applied to the sand filter by pressure in small doses. By extension, this has been applied to soil treatment areas as well. Using a timer, effluent can be sent to the field in small doses over the entire day and as long as the daily flow estimate is not routinely exceeded, treatment is handled more efficiently.

There are a couple of things to be aware of, however, with using small doses.

There are limitations to the size of the dose. In order for pressure systems to operate, all of the piping needs to be filled before effluent will be applied to the soil. This means the supply pipe, the manifold piping and the distribution laterals are full. So the volume of the piping system must be calculated to know the appropriate minimum dose to start the distribution. We have seen pumps burn out due to continuously pumping effluent because they are delivering such low dose volumes.


This is addressed on the design side by setting the minimum flow requirement for system operation at four to five times the volume of the piping. This ensures the pump operates efficiently and will last longer. Many state codes currently require this operational volume but it sometimes gets lost in the drive to have more efficient systems. The point is not that five times the volume is a magic number; it is to make everyone aware there is a limitation and to be careful when setting dose volumes, thinking you are helping with treatment while damaging another part of the system.

Another benefit of proper dosing is that enough water moves through the piping and orifices to prevent solids growth and plugging. This does not mean the piping never needs cleaning but it reduces problems due to plugging.


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