This article explains the design and construction specifications of an absorption bed septic leachfield or soakaway bed or drainfield.

Septic drainfields, also called leach fields, absorption beds, soil absorption systems, soakaway beds, and leaching beds, perform the functions of septic effluent treatment and disposal in onsite wastewater treatment systems, conventionally called "septic systems".

Page top sketch: shown again below as a larger image, an absorption bed septic field combines multiple effluent lines.

Absorption Bed Septic Drainfield Systems

Here we summarize the basics of an absorption bed septic drainfield design.

So what's the difference between a septic drainfield trench and a septic absorption bed?

An absorption bed system operates on a principal similar to the absorption trench except that several laterals, rather than just one, are installed in a single excavation.

This reduces the effective sidewall infiltration area per linear foot of lateral or leach line.

In the most common design of drainfield, perforated pipes are buried in gravel-filled trenches to form the drainfield.

[Click to enlarge any image]

This sketch shows a cross section of a typical drainfield trench, and places below the trench the critical biomat as well as other septic field design areas and considerations. (Source US EPA who in turn obtained the drawing from Ayres Associates, in turn modified by InspectApedia.)

a. Site Requirements for absorption bed septic system

(i) Percolation rate limits for absorption bed septic systems:

A bed system may be built in soils with a percolation rate between one and 30 minutes per inch.

A n absorption bed septic system shall not be built where the soil evaluation indicates silty loam, clay loam, or clay.

(ii) Slope of the site of an absorption bed septic system shall not exceed eight percent.

(iii) Absoption bed septic systenm site shape:

Septic absorption bed systems are more practical on sites that are long and narrow with a minimal slope.

(iv) All absorption bed septic vertical and horizontal separation distance requirements shall be met.

b. Design Criteria for absorption bed septic systems

(i) Pressure distribution is required

for the installation of an absorption bed system.

The local health department having jurisdiction may allow the use of siphon dosing on specific sites.

(ii) The maximum width of the bed of an absorption bed septic design shall be 20 feet.

The maximum length of each lateral from a pressure manifold shall be 100 feet.

Utilizing a center manifold system, a bed may then have a maximum length of 200 feet.

Laterals for siphon dosing systems in beds are limited to 75 feet.

(iii) The depth of the septic effluent absorption bed

shall be between 18 and 30 inches below original ground level.

(iv) Absorption bed septic system laterals shall be spaced five (5) feet apart.

Two and one-half feet (2 1/2') must be provided between the laterals and the sidewalls. In the maximum width of 20 feet, only four laterals may be installed.

(v) Maximum dimensions for absorption bed septic effouent pressure distribution:

Using pressure distribution with a center manifold, a bed system shall have maximum dimensions of 205 feet by 20 feet.

(vi) The required septic effluent absorption bed bottom area

shall be calculated from the application rates shown in Table 5 - below

Table 5 - Absorption Bed Septic System Required Bottom Area

Notes to the table above

Adapted from this original source: NYS Septic Design Code, Table 5, Absorption Bed Septic Systems -- Required Bottom Area

c. Materials Used for absorption bed systems

Absorption bed septic systems use essentially the same materials as found

at CONVENTIONAL SEPTIC LEACH FIELDS

d. Construction of absorption bed septic systems

(i) Heavy construction equipment shall be kept outside the proposed bottom area of the bed.

Watch out: See warnings

at SHALLOW BED SEPTIC ABSORPTION TRENCH CONSTRUCTION

(ii) The required bed bottom area is excavated as level as practical.

(iii) The bottom and sides of the excavation are hand raked to reduce soil smearing.

(iv) After excavation, a six inch layer of aggregate is placed across the bottom of the bed.

(v) The laterals are laid level on the aggregate and covered with aggregate to a level two inches above the top of the pipe.

(vi) The entire bed area is covered with a permeable geotextile. Untreated building paper or a four inch layer of loose hay or straw may be substituted if a permeable geotextile is unavailable.

...

Continue reading  at SEPTIC DRAINFIELD SHAPE or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see SEPTIC DRAINFIELD SIZE FAQs - reader questions & answers.

Or see these

Recommended Articles

• NEW YORK STATE 75-A.8 SUBSURFACE TREATMENT of EFFLUENT
• SEPTIC DRAINFIELD INSPECTION & TEST - home
  • SEPTIC DRAINFIELD SIZE
  • SEPTIC DRAINFIELD SHAPE
• SEPTIC DRAWINGS
• SEPTIC SOIL & PERC TESTS
• SEPTIC SYSTEM DESIGN ALTERNATIVES - home
• SEPTIC SYSTEM DESIGN BASICS
• SEPTIC TANKS - home
• U.K. OFF-GRID SEPTIC REGULATIONS & SEWAGE SYSTEMS
• U.S. SEPTIC AUTHORITIES & DESIGN SPECIFICATIONS - Septic codes & regulations for each US State or Territory

Suggested citation for this web page

SEPTIC DRAINFIELD DESIGN - ABSORPTION BED 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 DRAINFIELDS & DBOXES

Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Ask a Question or Search InspectApedia

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Note: appearance of your Comment below may be delayed: if your comment contains an image, photograph, web link, or text that looks to the software as if it might be a web link, your posting will appear after it has been approved by a moderator. Apologies for the delay.

Only one image can be added per comment but you can post as many comments, and therefore images, as you like.
You will not receive a notification when a response to your question has been posted.
Please bookmark this page to make it easy for you to check back for our response.
Our Comment Box is provided by Countable Web Productions countable.ca

Citations & References

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

---

ADVERTISEMENT

---