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 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?

[Click to enlarge any image]

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

Question:

I was interested in a piece of property which I understand that cannot accommodate sumping. I am under the impression (because I have not gotten to the end of figuring out why just yet) that this is because it is a waterfront property and/or the water table is too close to the surface thus reducing the availability of sufficient drainfield without causing water contamination. So my question is; is there another approved alternative to the absorption field to effectively control liquid effluent retention? - D.M. 8/29/2013

Reply:

From your email I think you need an onsite septic design engineer - there sounds like a confusion of terms, codes, and requirements, and in my own case I'm not sure what you mean by "sumping" nor "liquid effluent retention" - those terms are not ones I use for onsite wastewater disposal.

If you are asking about a holding tank (sewage is retained and periodically pumped and removed by a waste hauler) some communities permit that design along a waterway but many do not.

If you are asking about how sewage and (separated) effluent are handled at wet sites, there are some designs that can handle that case, sometimes combining treatment above ground with disinfection; but again, not all communities will approve them.

That's why you need an onsite expert who also knows local codes & officials.

Some options you might want to discuss can be found in the article link I give just below. Also take a look at Anish Jantrania's book listed in the references section of that article. Dr. Jantrania has described wastewater treatment systems that can function effectively entirely above ground, producing sanitary wastewater discharge.

Question: explanation of septic drainfield soakbed layouts

(Mar 23, 2014) Rocky said:
i noticed that after my septic tank i come across a four way of orangeburg piping. it looks to me that the d box is layed out after this fourway. What would be the most logical explanation for this layout?

Reply:

Rocky,

Indeed in a typical septic tank and drainfield or soakaway bed installation, a single line exits the septic tank and connects to a distribution box or D-box that in turn feeds two or more outlets of piping that are routed into drainage trenches, galleys, or whatever.

But I cannot guess at how your piping is laid out. If that's what you're asking you'd start by guessing by taking a look at the size and shape of the available drainfield area.

In a companion article

inspectapedia.com/septic/Septic_Soakaway_Bed_Shape.php

we show several typical drainfield layouts.

Question: find information about the Clivus Multrum composting toilet

(May 29, 2014) Anonymous said:
Went to Hawk Mnt,Allentown,Pa. Used an outdoor facility called, I belive clumus moltrom. Can you tell me about this and the correct spelling

Reply:

Sure Anon, you're talking about a Clivus Multrum composting toilet discussed here at

inspectapedia.com/septic/Composting_Toilets.php

Question: soil perc test results

(Oct 1, 2014) perne construction said:
We had a test boring done in the only place we can put a new cesspool and the results were Perched Water: 6'1" comment possible stream. Estimated Actual Ground Water Level 18' O +/-

My question is how and what kind of cesspool can be put in under these conditions

Reply:

Perne

I would not install any kind of cesspool - as that approach to wastewater disposal does not effectively treat the effluent (not enough aerobic bacteria) and as cesspools are not permitted in new construction in most jurisdictions.

If your lot space is very small you may need an advanced wastewater treatment system, even an aboveground one such as Jantrania discusses. It's time to ask for help from a septic design engineer.

Question: design for septic systems over soil with high water table

20 January 2015 Susie said:
Im trying to get a septic permit for a property I gave an offer to. In 2002 it was denied due to "too shallow to water table". Is there any solution to this?

Reply:

Susie you need

1. to find out what septic designs your local health department will approve, perhaps a raised bed septic or a mound septic design or another alternative design - see the designs including the two I cite In the ARTICLE INDEX found among the Continue Reading links at the end of this article listed under SEPTIC SYSTEM DESIGN ALTERNATIVES

2. to find a local septic design engineer who understands local soil conditions as well as what the building department finds comfortable among various designs

Keep us posted

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

(Feb 8, 2015) Diane said:
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?

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.

(Feb 12, 2015) 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.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to SEPTIC SYSTEMS

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Citations & References

In addition to any citations in the article above, a full list is available on request.

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