• Home
  • DIY
    • Self build
    • Projects and advice
      • Bathroom
      • Building, brickwork and masonry
      • Ceilings
      • Cleaning
      • Concrete work
      • Damp, rot and infestation
      • Drains
      • Electrical
      • Fireplaces
      • Floors
      • Garden
      • Heating
      • Insulation
      • Kitchen
      • Lighting
      • Lofts
      • Painting and decorating
      • Plasterwork
      • Plumbing
      • Roof and guttering
      • Safety
      • Security
      • Tiling
      • Walls
      • Windows
      • Woodwork
    • Glossary
    • Know your tools
    • Know your materials
  • Cost guides
  • News
  • Community
Facebook Twitter Instagram
lets-do-diy.com
  • Home
  • DIY
    • Self build
    • Projects and advice
      • Bathroom
      • Building, brickwork and masonry
      • Ceilings
      • Cleaning
      • Concrete work
      • Damp, rot and infestation
      • Drains
      • Electrical
      • Fireplaces
      • Floors
      • Garden
      • Heating
      • Insulation
      • Kitchen
      • Lighting
      • Lofts
      • Painting and decorating
      • Plasterwork
      • Plumbing
      • Roof and guttering
      • Safety
      • Security
      • Tiling
      • Walls
      • Windows
      • Woodwork
    • Glossary
    • Know your tools
    • Know your materials
  • Cost guides
  • News
  • Community
lets-do-diy.com
You are at:Home»Projects and advice»Soak away for sewage effluent

Soak away for sewage effluent

LAST UPDATED: 9th January 2023 by The Editorial Team

Digging a septic soak away

Summary: Step by step guide and video showing how to build a septic soak away, calculate the size and construct a within regulations and complete a percolation test.

Contents

  • How to construct a septic tank soakaway
    • Regulations
    • Sizing of a soakaway
    • Minimum soakaway site distances
    • Construction of soakaways and drain fields
    • Percolation tests for a soakaway design
    • How to carry out a percolation test
    • Calculating the size of the soakaway
    • Safety advice
  • How to construct a septic tank soakaway pit video

    How to construct a septic tank soakaway

    A sewage effluent soak away is not the same as a soak away used for surface water dispersal. The only function of a surface water soak away is to disperse the water into the ground, but a sewage effluent soak away has two functions.

    The first is to soak the septic tank liquid away. The second function is to further treat the effluent to remove suspended solids, bacteria and viruses, nitrogen and phosphorous using natural aerobic bacteria which grow within the soak away. Only then can the waste be soaked away into the ground. This is to avoid groundwater contamination, as much of our drinking water is sourced from it.


    Regulations

    Sewage effluent soak aways must conform with Section H of the Building Regulations – Sewage and Drainage issues and BS 6297 : 2007


    Sizing of a soakaway

    The size of the soak away, (i.e. the area in square metres), is determined by the number of bedrooms in the house and the porosity of the soil. Currently, The Environment Agency requires this to be one person per bedroom plus 0.5 persons per house. The porosity of the soil can only be determined by a percolation test.


    Minimum soakaway site distances

    • 10m (34ft) from a watercourse or ditch.
    • 50m (164ft) from water abstraction points.
    • 15m (50ft) from any building, and sufficiently distant from any other soak away, including roof water soak aways.
    • 2m (7ft) from a boundary.
    • The soak away or drain field area should be downslope of any groundwater source.
    • No water supply pipes or underground services should be within the soak away area.
    • No access roads, driveways or paved areas should be within the soak away area. This includes fields with tractor and agricultural traffic.
    • The water table or bedrock must not, at any time, be within 1.2m (4ft) of the bottom of the soak away trench.

    Construction of soakaways and drain fields

    Sewage effluent soak aways are carefully designed constructions consisting of trenches containing pipes laid on gravel or stone beds, covered with more stone and geotextile membrane and backfilled with topsoil.

    • Use solid perforated foul drainage pipe, with the slots/holes at the bottom, lay in trenches of a uniform gradient not steeper than 1:200. Perforated ‘Flexicoil’ type pipe is not allowed.
    • Pipes should be laid at a minimum depth of 200mm (8in) and a maximum depth of 700mm (28in) to enable aerobic contact between the effluent and the soil particles.
    • The length of a single trench should not exceed 30m (98ft).
    • Pipes should be laid on 300mm (12in) of clean shingle, gravel or broken grade 1 stone (20mm– 50mm).
    • Soak away trenches should be filled to a level of 50mm (2in) above the pipe and covered with a layer of geotextile membrane to prevent silt entry.
    • The remainder of the soak away trench can be filled with topsoil.
    • Drainage trenches should be between 300mm (12in) and 900mm (3ft) wide with areas of undisturbed ground of minimum 1m wide between parallel trenches.
    • An inspection/distribution chamber should be installed between the septic tank and the drain field.
    • The soak away layout should be set out as a continuous loop or grid fed from the inspection chamber, not a straight pipe run with an ‘end’.
    • The soak away pipes should be fed by a distribution chamber.
    • The area of the soak away is calculated after carrying out a minimum of three percolation tests in the proposed drain field area, over three consecutive days and applying a formula as detailed on the percolation test information below.

    Percolation tests for a soakaway design

    A soil percolation test is required to be carried out on the land where the septic tank or treatment plant soak away is intended to be built. It is a test that determines the rate in which soil absorbs a known volume of water. This is to ensure that the land (soil) is suitable for a soak away, and to ensure that the soak away is designed properly.

    A percolation test is always required for septic tanks.


    How to carry out a percolation test

    A percolation test should not be carried out during abnormal weather conditions and if done in summer, then the times should be increased by 30% to 50%, depending on the recent weather.

    The percolation test method

    • Excavate a hole 300mm (12in) square and 300mm (12in) below the proposed outlet from the septic tank or sewage treatment plant.
    • Dig the test hole vertically to the appropriate depth. Remove all loose debris.
    • Mark the hole 75mm (3in) from the bottom and 75mm (3in) from the top by pushing 6 inch nails into the sides of the of the percolation test hole.
    • Fill the test hole with water to a depth of at least 300mm (12in). Allow to seep away overnight.
    • Next day, refill the test section with water to a depth at least 300mm (12in). Observe the time, in seconds, for the water to seep away from 75% full to 25% full.
    • Divide this time by 150mm (6in), (i.e. 50% of the 300mm (12in) depth). The answer gives the average time in seconds (V) required for the water to drop 1mm.
    • Carry out the test at least 3 times, with at least 2 trial holes. The average figure from the tests should be taken.

    Drainage field soak away disposal should only be used when percolation tests indicate average values of V of between 15 and 100 and the preliminary site assessment report and trial hole tests have been favourable. A percolation test is deemed to have failed if the results are less than 12 or more than 100.

    This minimum value ensures that untreated effluent cannot percolate too rapidly into ground water. Where V is outside these limits effective disposal is unlikely to take place in a soak away drainage field.

    * Some Councils insist that percolation tests are carried out by ‘qualified persons’ and do not accept your own test results. You may be required to contact a contractor to carry out the percolation test. Please check with your Local Authority.


    Calculating the size of the soakaway

    The calculation gives the area (A), in square metres, required for the soak away trench.

    V = The time in seconds for the water in the test hole to drop by 1mm.
    p = The maximum number of persons that the unit is designed to serve.

    For sewage treatment units
    Area (A) = Vp X 0.20

    For septic tanks
    Area (A) = Vp X 0.25

    Further calculations are required to give the length of pipes required depending on the width of the trench, e.g. for a 600mm (2ft) wide trench, the area would be divided by 0.6.


    Safety advice

    Never breathe septic tank gases as they can overcome you very quickly.


    How to construct a septic tank soakaway pit video


    Author

    The Editorial Team

    The Editorial Team at lets-do-diy.com are a team of experts led by Mark Wilson. We have been creating DIY projects, articles, costs guides and product reviews since 2008. For more info see our about page.

    Related projects

    • Drain unblocking
    • Driveway and patio drainage
    • Installing French drains
    • Building a soak away

    Related cost guides

    • Average cost of a septic tank
    • Average septic tank cleaning cost

    Recent Comments
    • Vic on Is Christmas a good time for DIY?
    • Anonymous on Average air source heat pump cost (2022)
    • Reggie on Prepare your home for moth season
    Latest News
    0

    Potential DIY home improvement trends for 2023

    1

    Is Christmas a good time for DIY?

    0

    Design trends for 2022 and 2023

    0

    Product Review: Handsaws

    Popular Guides

    Average cost to lay turf

    Average cost of water softener installation

    Average cost to hire a cherry picker

    Average solar panels cost

    Popular Projects

    Cutting a hedge

    Brick calculator

    Building a stud partition wall

    Re-filling central heating system

    DIY Categories
    Bathroom
    Building, brickwork and masonry
    Ceilings
    Cleaning
    Concrete work
    Damp, rot and infestation
    Drains
    Electrical
    Fireplaces
    Floors
    Garden
    Heating
    Insulation
    Kitchen
    Lighting
    Lofts
    Painting and decorating
    Plasterwork
    Plumbing
    Roof and guttering
    Safety
    Security
    Tiling
    Walls
    Windows
    Woodwork

    Self Build Guide
    How to find a building plot
    Evaluating a building plot (2023)
    How to get a self build mortgage
    Self build construction options
    Self build architect and design considerations (2023)
    Self build planning permission and legal consents
    © 2023 lets do diy ltd | ebuild
    • Home
    • About
    • Contact us

    Type above and press Enter to search. Press Esc to cancel.

    We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. By clicking “Accept All”, you consent to the use of ALL the cookies. However, you may visit "Cookie Settings" to provide a controlled consent.
    Cookie SettingsAccept All
    Manage consent

    Privacy Overview

    This website uses cookies to improve your experience while you navigate through the website. Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. We also use third-party cookies that help us analyze and understand how you use this website. These cookies will be stored in your browser only with your consent. You also have the option to opt-out of these cookies. But opting out of some of these cookies may affect your browsing experience.
    Necessary
    Always Enabled
    Necessary cookies are absolutely essential for the website to function properly. These cookies ensure basic functionalities and security features of the website, anonymously.
    CookieDurationDescription
    cookielawinfo-checkbox-analytics11 monthsThis cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Analytics".
    cookielawinfo-checkbox-functional11 monthsThe cookie is set by GDPR cookie consent to record the user consent for the cookies in the category "Functional".
    cookielawinfo-checkbox-necessary11 monthsThis cookie is set by GDPR Cookie Consent plugin. The cookies is used to store the user consent for the cookies in the category "Necessary".
    cookielawinfo-checkbox-others11 monthsThis cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Other.
    cookielawinfo-checkbox-performance11 monthsThis cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Performance".
    viewed_cookie_policy11 monthsThe cookie is set by the GDPR Cookie Consent plugin and is used to store whether or not user has consented to the use of cookies. It does not store any personal data.
    Functional
    Functional cookies help to perform certain functionalities like sharing the content of the website on social media platforms, collect feedbacks, and other third-party features.
    Performance
    Performance cookies are used to understand and analyze the key performance indexes of the website which helps in delivering a better user experience for the visitors.
    Analytics
    Analytical cookies are used to understand how visitors interact with the website. These cookies help provide information on metrics the number of visitors, bounce rate, traffic source, etc.
    Advertisement
    Advertisement cookies are used to provide visitors with relevant ads and marketing campaigns. These cookies track visitors across websites and collect information to provide customized ads.
    Others
    Other uncategorized cookies are those that are being analyzed and have not been classified into a category as yet.
    SAVE & ACCEPT