Alternating Bed Septic System Designs

Septic Diverter Valve & 2 or more effluent destinations

Alternating bed septic systems:

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.

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.

Page top image, given in larger form below, 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.

Discussed here: 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.

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, LOCATE, 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 or longer.

We recommend that the alternation dates be shifted so as to avoid always using the same field during the wettest months of the year.

Watch out: if your drainfield is in failure, for example because of a mature, soil-clogging biomat, the field will need considerably longer than a few months to "recover".

Diverter Valves for Septic Systems

See SEPTIC D-BOX SPLITTERS - Using a D-box control or a Splitter Valve for drainfield resting & restoration

...

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Alternating Septic Drainfields: will unused field be "bad"?

Moved into a house with 1250 gallon Septic system and a 2 field diverter.

The house was empty over a year so we were advised to wait a couple years before pumping out the tank.

rMost of the time it is only my wife and I in the house. I was never told about using the diverter valve so when they came to clean the tank a few weeks ago they said I should switch the valve.

I know the valve has not been switched in at least 7 or 8 years and we have never had an issue.

Is there a chance that by switching fields now they may be bad after being unused for so long? If I do switch the valve how can I tell if the second field is working properly? Any advice would be appreciated. On 2020-03-16 by Daryl

by (mod) - chances that my alternating bed septic system is still ok?

Dary

I cannot of course know the condition of the soil or drainfield by e-text; it's possible that the second field is usable and that by having rested for 7 or 8 years it will have some remaining usable life.

You will know if the fields are not working because you'll either see an effluent back-up at the septic tank or you'll see effluent leakage or breakout on the soil or surrounding area.

But beware that any septic installation that is using alternating fields or beds is a warning that the soil's ability to absorb effluent is limited and you may be facing field replacement before long.

See details at ALTERNATING BED SEPTIC SYSTEMS

Can a homeowner be trained to do the maintenance on the ATT systems

Can a homeowner be trained to do the maintenance on the ATT systems so not to be spending thousands every 2 years on a required maintenance agreement to some septic company who gets warnings over the phone line? (Feb 8, 2015) Diane

Reply:

Diane:

Certainly there are septic system maintenance procedures that a homeowner can do such as changing filters and - WITH CAUTION as you could die - if there is an access port to inspect the septic tank level a homeowner can certainly look therein to see if levels are normal. And for systems that use pumps or areators an owner can and should learn to know if the equipment is running.

The hazards that can be fatal are leaning over a septic tank opening (overcome by fumes), working alone, or entering a septic tank (NEVER do that).

If you know the design, equipment brands, etc. of your system together we should be able to undertand what's installed and which tasks an owner can and should perform.

Diane said:
Thank you Dan. Understand the cautions you mention and appreciate the list. No plan on entering the tank...ever. Planning for future needs and some requirements I was told in maintenance agreements cost a fortune and seemed easy for a home owner to do and assure a good working system....that is affordable.

Reply:

Don't hesitate to ask us if specific septic system maintenance or care questions arise. Indeed studies by Small Flows and other expert sources have repeatedly indicated that the number one factor in early failure of septic systems is that owners ignore the system maintenance requirements.

...

Continue reading  at GRAVITY/SIPHON DOSING SYSTEMS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Septic System Articles

• ALTERNATING BED SEPTIC SYSTEMS
• SEPTIC CONSULTANTS, DESIGNERS, ENGINEERS
• GRAVITY/SIPHON DOSING SYSTEMS
• PRESSURE DOSING SYSTEMS
• SEPTIC D-BOX SPLITTERS - diverter valves or splitters to route effluent between alternating fields
• SEPTIC SYSTEM DESIGN ALTERNATIVES - home
• SEPTIC SYSTEM DESIGN BASICS - home
• TYPES OF SEPTIC SYSTEMS - master list

Suggested citation for this web page

ALTERNATING BED SEPTIC SYSTEMS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to SEPTIC SYSTEMS

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