LARGER SKETCH of a typical septic effluent gravity dosing system - Alternating Bed Septic System Designs
     

  • ALTERNATING BED SEPTIC SYSTEMS - CONTENTS: Alternating bed septic systems with parallel or interspersed drainfields. What is an alternating bed septic system?
    • How Far Apart Are the Alternating Septic Bed Drainfield Trenches and Leach Lines?
    • How is Effluent Dispersal Switched Between Alternating Septic System Beds
    • How Often is Effluent Distribution Switched Between Alternating Septic Beds
  • POST a QUESTION or READ FAQs about septic dosing systems and alternating bed septic system designs
  • REFERENCES

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This article defines, illustrates, and explains the operation of alternate bed (or alternating bed) septic systems, a variation on dosing systems for septic system effluent final treatment and disposal. We explain how alternating bed septic drainfields work, how the fields are switched, and how often they are alternated or switched.

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Alternating Bed Septic Systems Explained

While alternating bed septic systems are not complicated and are discussed here as a septic design alternative, the concept - manually or automatically switching effluent disposal between two or more collections of drainfield trenches - is similar to septic dosing systems in concept. Readers of this article should also see GRAVITY/SIPHON DOSING SYSTEMS and also PRESSURE DOSING SYSTEMS. Page top image shows a sketch of a basic septic system design using gravity dosing, with effluent flowing from a septic tank to a dosing chamber and from there to a drainfield. Image: Indiana state health department.

What is an alternating bed septic system?

Alternating bed septic system designs are intended to decrease hydraulic overloading of septic drainfield soils by switching effluent distribution between two or more sets of septic drainfield trenches.

The alternating drainfield areas may be uniformly interspersed (alternating individual septic drainfield lines) or they may be parallel in the same area, or they may even be in completely different physical areas on a property. The layout choice for alternating septic bed systems depends on the space available as well as soil characteristics.

In the U.S., some states such as Massachusetts, define alternating bed systems as a design intended to provide a backup septic capacity: " Alternating Bed Systems - Also known as an alternating leachfield. An absorption system designed with a backup absorption field for use while the primary absorption field rests."

How Far Apart Are the Alternating Septic Bed Drainfield Trenches and Leach Lines?

The answer is, it depends: on

  • The amount of site space available
  • The percolation rate and other characteristics of the drainfield soils
  • State and local building and health codes
  • Common local practices: often traditional drainfield trenches were separated by 6' ( 7' in some areas) and septic drainfield trenches are usually limited to 100' in length. Some installers place alternative septic trenches between the original drainfield lines. particularly where an alternating bed septic design is being considered, this seems to us quite reasonable: excavation of a slow or failing septic drainfield trench and examination of it in cross section usually discloses that the soil clogging that is occurring is in the first few inches of soil around the drainfield trench perimeter.
  • See SEPTIC DRAINFIELD SIZE and see SEPTIC DRAINFIELD SHAPE which includes septic trench specifications.

How is Effluent Dispersal Switched Between Alternating Septic System Beds

Alternating drainfield designs are used most often on flat sites and in combination with a programmable or timed automatic distributing valve, but they may be operated manually.

Our friend and associate Victor Faggella reports a long-standing and traditional alternating septic bed design at which the property owner simply opened the distribution box for his drainfield and periodically switched a plug from one set of drainfield pipes to the other. (See SEPTIC D-BOX INSTALL REPAIR)

An example of a floating outlet design for dosing systems and that can be adapted to alternating bed septic system designs to handle dosing type septic system effluent disposal can be seen in animation at Rissy Plastics FLOUT floating outlet for septic effluent dispersal.

How Often is Effluent Distribution Switched Between Alternating Septic Beds

Sometimes installed as a retrofit where an existing septic drainfield is failing, the contractor may make a backup absorption field, adding the ability to route septic tank effluent to either field.

The "backup" septic drainfield is used while the primary field is rested and allowed to recover through biological activity. In this design septic drainfields or leaching beds are often alternated every 6 months. We recommend that the alternation dates be shifted so as to avoid always using the same field during the wettest months of the year.

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