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Alternating Bed Septic System Designs InspectAPedia®  -     Alternating bed septic systems with parallel or interspersed drainfields Designs for various types of septic effluent dosing systems Septic system designs using gravity dosing, effluent tipping buckets, drip 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. Our site offers impartial, unbiased advice without conflicts of interest. We will block advertisements which we discover or readers inform us are associated with bad business practices, false-advertising, or junk science. Our contact info is at InspectAPedia.com/appointment.htm. This document discusses alternate bed septic systems, a variation on dosing systems for septic system effluent final treatment and disposal. 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. Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at "References." © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Alternating Bed Septic Systems Explained

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 avaliable
• 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 INSPECTION)

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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• Additional technical contributors & reference sources for this article are listed below.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

• Septic Tank Soil Absorption Systems - Decentralized Sysetms Technology Fact Sheet, US EPA, EPA 932-F-99-075, September 1999 - original citation www.epa.gov
• Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. (727) 595-4211 mark@BestTampaInspector.com
• Victor Faggella, is a senior home inspector in New York and can be reached at Centurion Home Inspections, Inc. Mahopac, NY 10541. 845-628-0941 vjf@centurion-inspections.com The company has offices in Mahopac, NY, Woodbury CT., and Mansfield Center, CT.
• Hankey and Brown home inspectors, Eden Prairie, MN, technical review by Roger Hankey, prior chairman, Standards Committee, American Society of Home Inspectors - ASHI. 952 829-0044 - hankeyandbrown.com
• Rissy Plastics - Matt Cauthorn, Flout@engineer.com for text describing the Flout(TM) floating outlet valve dosing system control (see above).
• Daniel Friedman - principal author/editor of the InspectAPedia ^TM Website
• Construction Guidelines for Gravity and Flood-Dose Trench Onsite (Septic) Systems, Indiana state health department
• Maintenance of Low Pressure Distribution Septic Systems, Vermont Cooperative Extension
• Dosing Gravity Drainfield Systems, Recommended Standards and Guidance for Performance, Application, Design, and Operation & Maintenance, Washington State Department of Health, July 1, 2007

Septic Effluent Dosing System Products and Suppliers

Rissy Plastics FLOUT floating outlet for septic effluent dispersal. Contact Rissy at 518-834-7940 or Flout@engineer.com - Keeseville NY.

Please also see ALTERNATIVE SEPTIC PRODUCTS and also review the suppliers listed at ATU Suppliers

Some basic information about handling septic effluent follows.

How and When Septic Effluent is Moved Through a Septic System

Septic effluent is distributed to a system final treatment and disposal using either gravity methods (which depend on terrain slope) or pressure methods (which use a pump to move effluent to its destination treatment and disposal area).

Methods For Septic Effluent Distribution Using Gravity Systems

• Single Effluent Line: A 4" perforated PVC pipe receives effluent by gravity from the septic tank. The pipe is buried in a gravel trench and may be run in a straight line or a loop.
• Distibution Box/Network of Lines: A distribution box receives effluent by gravity from the septic tank and routes it to a network of perforated pipes. The network is made of mulitple independent trenches which maybe on a flat or sloped site.
• Serial relief line: multiple, serially connected trenches are built on a sloping site and used serially.
• Drop box: multiple independent trenches are built on a sloping site, connected from drop boxes.
• Gravity Dosing, Bell Siphon Dosing, Float Dosing (discussed in this document): 4" perforated pipe, with or without a distribution box, are installed all at a single elevation. A hinged "bucket" chamber receives effluent and periodically, as it fills, the bucket tips to spill effluent into the piping system (A "dipping" or "tipping" system).
  
  Bell siphon dosing systems (a bell and siphon method of moving effluent to the drainfield) or float-controlled (a floating valve opens or closes) septic effluent dosing system designs are also available and are discussed in this document. Gravity dosing systems distribute effluent periodically rather than continuously to the absorption field, letting the field rest between doses and extending its life and capacity. However because the effluent dose is "poured" suddenly into the drainfield, local spot or point overloading may still occur.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
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More Information on Building Diagnostic Inspections and Repairs

• The Septic System Information Website home page for this topic
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Home & Outdoor Living Water Requirements
• Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
• Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.

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