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Pressure Distribution Septic Systems: How They Work, The Design & Cost

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Why Use Pressure Distribution Septic Systems?

Often pressure distribution septic systems are used when permeable soils are relatively shallow and a uniform distribution of the wastewater is required for better treatment.  Not only do pressure septic systems provide a better effluent treatment but sometimes drain fields are not always suitable to be placed in close proximity to a home.  When long distances of travel are required or high elevations provide gravitational challenges, a pressurized system is an answer.

In many septic systems, gravity is used to move the wastewater through the system and into the drain field. In a gravity-fed septic system, wastewater is partially treated within the septic tank before it flows out the tank and down to the drain field through a network of pipes. However, not all situations are conducive to making use of gravity to distribute the wastewater effluent leaving the tank. In cases where gravity-fed systems will not do the job effectively, alternative systems need to be employed. Pressure distribution septic systems are one such solution.

pressure septic system

A gravity septic system vs a pressure distribution septic system

While gravity can be a very effective method of distributing wastewater effluent, in some cases it is just not practical, and in others not ideal.

Gravity can only be used to move wastewater downslope from the septic tank. If the drain field is situated upslope from the septic tank, gravity cannot be used as a method to distribute the wastewater effluent to the drainage area for dispersal.

Since gravity determines where the wastewater will go and how efficiently it will get there, in gravity-fed septic systems the distribution of wastewater effluent is typically localized to a small area as system designers are limited in the area they have available to distribute the wastewater too.

Consequently, soils within the drain field can become oversaturated, resulting in clogging and pooling, as well as groundwater contamination, and which will ultimately reduce the lifespan of the drain field. Pressure distribution septic systems offer a solution to these common problems often associated with gravity-fed distribution systems.

 

How Does a Pressure Septic System Work?

As the name implies, a pressure distribution septic system uses pressure to move wastewater effluent to the drain field and to distribute it evenly throughout the soil in the drainage area.

Should the drain field be located in an area upslope of the septic tank, a pressure septic system can transport wastewater uphill via pipes, dispersing it into the soil in the designated distribution area. Because pressure septic systems promote a more even distribution of effluent throughout the drain field, they can help prevent soils in the drain field from becoming oversaturated.

 

So how does a pressure septic system work? Just like any conventional septic system, a pressure distribution septic system will also have a septic tank where solids and fats are separated from the wastewater before it flows out to a dosing tank.

Once in the dosing tank, the wastewater effluent is pumped to the drain field consisting of a series of perforated PVC pipes fitted with balancing valves that ensure the wastewater effluent is evenly distributed throughout the distribution area.

pressure distribution septic system

The wastewater effluent drains out of the drainage pipes and is disposed of in the surrounding soils where it undergoes further treatment by naturally occurring soil bacteria.

The dosing tank pumps a specified dose of effluent into the distribution network of the drain field, ensuring that the drain field is never inundated with wastewater to the point that it becomes too saturated. The consistent dosing delivered by the pressure septic system ensures that no part of the drain field receives more effluent that what it can drain, which extends the lifespan of the drain field.

Pressure Dose Septic System

A pressure dose septic system is quite similar to the pressure systems described already in this article.  The difference is instead of the effluent travelling directly to the piping network with uniform distribution through evenly spaced orifices, the effluent instead is pressurized to a distribution box. 

The distribution box then delivers the dose of effluent evenly throughout the drain field lateral sections.  Often these lateral sections from the distribution box are much larger in diameter than the lateral sections for uniform distribution.  The effluent travelling from the distribution box is then delivered to the field by gravity.

When is a Pressure Septic System a Good Option?

Besides being an effective solution for distributing effluent to drain fields located upslope from a septic tank, there are other instances where a pressure septic system would be the best choice.

For example, on sites that have a restricted layer consisting of subsoils with limited permeability, a pressure distribution system can prevent the underlying soils from becoming overloaded and the entire drainage system from becoming inundated. A pressure distribution system can accommodate a larger drain field and will ensure the effluent is distributed more evenly across the drain field rather than concentrated in a smaller area.

A pressure distribution septic system is also suitable for drain fields that are located on a slope, where drainage downslope may result in accumulation of effluent that can lead to flooding, particularly if wastewater is being distributed to small concentrated areas around the drain field.

A pressure distribution septic system will ensure that the wastewater effluent is distributed evenly throughout the dispersal field, improving the rate of absorption and reducing the likelihood of pooling downslope.

Finally, in cases where the drain field is going to be particularly large, due to either the size and design of the septic system or to onsite limitations (e.g. soil), a pressure distribution system is recommended.

Gravity septic systems struggle to distribute wastewater consistently, even in smaller drain fields and are inadequate for dispersing effluent across a larger drain field.

 

Pressure Septic System Design

A pressure distribution system that works on a timer allows the daily effluent outflow to be spread out more evenly, both in terms of time and space, throughout the day.

This takes pressure off those peak times of the day when households typically use more water (early morning and evening), spreading the flow more evenly during the course of the day and allowing sufficient time between batch outflows to ensure that the system is not inundated or overloaded and that efficient treatment occurs.

In a pressure distribution system, wastewater is evenly distributed throughout the entire dispersal network each time the pump is switched on.

This alleviates preferential flow that sometimes occurs in gravity systems until the biomat forms within the soil, which limits the rate of flow and aids the treatment process. When the wastewater passes too quickly through soils (e.g. loose sandy soil), treatment will be compromised.

A pressure distribution septic system allows the flow rate of the wastewater to be controlled, ensuring that water moves slowly through the soil so that it can be adequately treated as it passes through the soil.

 

In order to achieve a uniform distribution across the drain field, the system is designed in such a way that the volume of wastewater from the septic tank flowing out of each perforation in the distribution pipeline is equal. To achieve this, the diameters of both the pipes in the distribution network and the holes in those pipes need to be carefully sized — a minimum orifice size of 3.2mm (⅛”) diameter is standard.

 

A conventional pressure septic system, such as that used in a sand mound or shallow trench septic system, consists of the following key components:

pump tank

  • Septic tank or other pretreatment chambers where solids are separated, stored and removed.
  • The screen (effluent filter) between septic tank outlet and the pump chamber to protect both the pump and the holes in the distribution pipes from becoming clogged with solids.
  • Pump or syphon tank which collects and stores wastewater effluent from the septic tank between pump cycles and in the event of a malfunction.
  • Pump to pump wastewater under pressure throughout the distribution network.
  • Pump control box and power supply to facilitate pump operation and timing of distribution.
  • Main pipeline (force main) (connecting the pump and manifold) and manifold that connects to lateral pipes in the drain field.
  • Lateral pipes that have holes drilled at equally spaced intervals (orifices) on the inverted side of the pipe, and cleanouts and monitoring ports fitted at the distal ends of each pipe.
  • Drain field consisting of natural soil or other media that facilitates the biological and physical processes necessary to ensure adequate treatment of effluent.
  • Washed drain rock or utilizing an infiltrator system.

For an in-depth review of the British Columbia’s septic design criteria, have a look at our article.

 

Pressure Septic System Cost

Pressure distribution septic systems offer many benefits over gravity septic systems, they are, however, associated with a higher cost. While the initial cost is only slightly more expensive than a gravity distribution system, typically $5,000-$7,000 more, but cheaper than many other alternative types of septic systems. Pressure distribution systems require regular maintenance, which can ramp up their cost over time as well.

For an evaluation of your septic system criteria, try our free assessment.

Pressure septic system maintenance

A pressurized distribution system is designed to extend the lifespan of a drain field, however, unless they are regularly maintained, they won’t necessarily last long themselves.

One of the biggest problems is clogging due to mineral buildup or accumulation of other contaminants in the distribution network over time.

If the holes (orifices) in the dispersal pipes are clogged, the pressurized distribution system won’t work as efficiently as it should, and may eventually malfunction.

 

Consequently, the lateral distribution pipelines need to be cleaned and flushed out regularly — typically twice a year but minimum once a year is required. This maintenance work is a minor inconvenience to the overall  health of your system, but because it is somewhat labour intensive, it comes at a cost, conctact your local maintenance provider for rates.

If you don’t maintain your dispersal pipelines regularly, system performance will decline as the pressurized distribution system essentially becomes ineffective, and will eventually reach a point where the entire distribution pipe network will need to be replaced.

 

Summary

As we can see from the above, pressure distribution septic systems offer a great alternative septic solution for scenarios where gravity distribution systems simply would not suffice.

They are specifically designed to deliver a predetermined volume of effluent to the drain field per cycle, ensuring even distribution of effluent across the dispersal area.

However, to be effective, the pipes that make up the distribution network need to be regularly maintained to ensure they remain unclogged so that they can continue to distribute the effluent across the drain field evenly.

Pressure systems are more costly than a conventional system but offer the ideal solutions to problematic challenges.  Sometimes permeable soils can be located in far-reaching locations where a conventional system would not be able to accommodate, this can also mean upslope travel.

Work closely with your septic service technician and ask lots of questions for a clear understanding of the best solution for your home and lot.

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