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Buying a house with a septic tank

A Guide to Drainage Field Design and Size Calculation

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A Guide to Drainage Field Design and Size Calculation

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What is a Drainage Field?

A Drainage Field is a system of infiltration pipes placed in trenches and arranged so that effluent can be discharged to the ground. The field must be constructed in a certain way that allows for the effluent to be treated in the soil through aerobic digestion. This means it must be constructed in the top 700-800mm of the soil to ensure aerobic contact.

The Field must be big enough to treat and disperse the maximum amount of discharge the system will discharge to it. This is calculated by a formula that we will examine below in this article. The two main variables needed to calculate this are first, Vp, a measure of soil porosity gained from a percolation test. Secondly, the population of the household or population rating of the tank present in the system.

If a company comes to quote for a system, with a discharge to ground and do not require to undertake a percolation test then we would strongly advise you to discount their advice and quote. Without a percolation test, you cannot design a system that adheres to the Building Regulations or the General Binding Rules. Further, the system is unlikely to work properly.

What is the regulation concerning the design and build of a Drainage Field?

There are there key pieces of legislation that concern the design and implementation of a Drainage field. Firstly, BS 6297: 2007, HM Governments Building regulations 2010, Drainage and waste disposal section H2 and the General Binding Rules.

Summary of the Key Features of BS 6297: 2007, HM Governments Building regulations 2010, Drainage and waste disposal section H2.

We will treat HM Governments Building regulations 2010, Drainage and waste disposal section H2 as the primary reference as it was issued after BS 6297: 2007. Just to be clear this is a summary and the full documents should be read and understood if you intend to carry out this work yourself.

  • A drainage field must be at least 10m from any watercourse or permeable drain.
  • It must be at least 50m from the point of abstraction of any groundwater supply and not in any Zone 1 groundwater protection zone.
  • At least 15m from any building.
  • Sufficiently far from any other discharge to ground to ensure the overall capacity of the ground is not exceeded.
  • The field must be down water/slope of any groundwater source.
  • No underground services or water pipes are allowed to be located within the dispersal area.
  • No access roads, driveways or paved areas should be located within the disposal area.
  • A percolation test must be carried out, please see our guide.
  • Drainage fields or mounds must ensure aerobic contact between liquid effluent and the subsoil. The minimum depth of the pipes should be 500mm below the surface.
  • Drainage fields should be constructed using perforated pipe laid in trenches of uniform gradient that is not steeper than 1:200.
  • The perforated pipes must be laid on a 300mm layer of clean shingle or broken stone graded between 20mm and 50mm.
  • Trenches should be filled to a level 50mm above the pipe and covered with a layer of geotextile to prevent the entry of silt.
  • Drainage trenches should be from 300mm to 900mm wide, with areas of undisturbed ground 2m wide being maintained between parallel trenches.
  • An inspection chamber should be installed between the Septic Tank and Drainage Field.

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We have a large collection of articles containing the answer to all of your questions. Check out our Blog Page 

Drainage field design

Is a Drainage field different from a Soakaway?

Technically yes they are very different. In the industry, however, the words are commonly used interchangeably. A Soakaway is an underground area where a liquid dissipates into the surrounding ground. There is no specific guidance on how a Soakaway is constructed or calculations to support its capacity or ability to treat effluent.  

How is a Drainage Field different to a Drainage Mound?

Drainage mounds are essentially drainage fields constructed above ground, under a mound of soil. This allows for the aeration and treatment of the effluent in areas that are occasionally waterlogged. Conditions for an underground drainage field must, when not waterlogged, be suitable. We are finding that with the impact of Climate Change, increasingly wet winters and flooding, Drainage Mounds are increasingly needed.   


Drainage Field, size calculation and design

What is an Infiltration system?

Currently, written legislation has fallen behind the technology available, notably in Sewage Treatment Plants. Sewage Treatment Plants produce near clean water, thus the need for secondary treatment is redundant.  A Septic Tank with a Drainage field is nearly always cheaper than a Sewage Treatment Plant with a Drainage Field.  Written legislation allows you to build a slightly smaller drainage field but the impact is minimal. Essentially a rational economic person would not install a Sewage Treatment Plant with a discharge to ground and a Drainage Field.

While explicit written standards have not caught up, building control and local authorities have. With consent, it is possible to install a series of Infiltration tunnels to discharge the treated effluent from a Sewage Treatment Plant. Again there are calculations to perform based on porosity etc to ensure an element of secondary treatment and adequate dispersal. A Sewage Treatment plant with an infiltration system is generally cheaper than a Septic Tank with Drainage Field.

Further, an infiltration system can be installed in a far smaller area increasing flexibility and applicability. For instance, the majority of Septic Tanks currently do not have a Drainage field that meets the general binding rules. The Environment Agency is unlikely to check on your Drainage Field. However, when you come to sell your house or replace a failed Soakaway a Drainage Field will need to be installed. Drainage fields cover a large area, in most cases four to five times the surface area of a traditional Soakaway. Many properties with Septic Tanks do not have sufficient space to install a Drainage Field constructed in line with legislation. A Sewage Treatment plant with an infiltration tunnel system could be the only viable option.         

How big a drainage field do I need and how is this calculated?

Firstly you need to undertake or have a Percolation test professionally undertaken. We generally undertake these for £250+VAT where we can hand dig, if a machine is required then there is an additional cost. Please see our guide on Percolation tests.

At = p X Vp X 0.25

At = Area of the drainage field in m2

p = Number of persons served by the tank

Vp = Percolation value.

BS 6297: 2007 states that for a Sewage Treatment Plant a factor of 0.2 can be used. However, this is not mentioned in the 2010 Building regulations guide. As we discussed earlier a Drainage field with a Sewage Treatment Plant would not be our optimal solution.  

Our Core Services: The Sewage Treatment expert Installers

On the 1st January 2020, all Septic Tanks that discharge to a Stream, Watercourse or Drainage Ditch will become illegal and will require replacing with a Sewage Treatment Plant. Please see here for more details.

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Expert wastewater treatment engineers who specialise in Septic Tank and Soakaway replacements as well as Sewage Treatment Installations. We are based in the New Forest, Hampshire but operate across the South Coast as well as Wiltshire, Somerset, Devon, Sussex, Surrey, Oxfordshire, and Berkshire.

The Home of Septic tank and private drainage system solutions. 

Based in the New Forest, Hampshire on the border with Dorset and Wiltshire we operate an expert team of off-mains drainage specialists. This encompasses Septic Tank, Soakaway and drainage system problems, installations, blockages, and servicing. In addition, any issues relating to cesspits and sewage treatment plants.

Homeseptic offers high quality, professional advice, repairs and installation for all aspects of effluent cleansing and disposal across the south of England. Whether you’re looking for septic tank installation, sewerage treatment plants or systems, piping or soakaway solutions, Homeseptic employs only the most competent engineers in the drainage and treatment industry and knows exactly which solutions suit each property.

We only install and offer quality systems we believe in, from suppliers we know and trust.

New Forest Philosophy

As we have said Homeseptic is based in the New Forest. A great location in between the cities of Southampton, Salisbury and Bournemouth. We are a group of good friends who grew up together in the area.  Individually we all have grown tired of the way other Drainage firms are run. As a result, this is why we set up Homeseptic. If you want to know more please check out our About Page

We want to actually put the customer first and turn up to do a great job that we can be proud of. Unfortunately, this is not the case in the market, with most companies pressuring their workers to sell unnecessary works on a visit. Due to this, we offer upfront quotations with any variations discussed before we start work.

We are honest hard working professionals, which we hope you will agree with. As a result, it will certainly not turn up in flash sales vehicles and give you a load of chat and then do a terrible job. Our team will come with the job in mind and the aim of giving you the best value we can in order to facilitate a quality installation. 

We cover all of Hampshire, West Sussex, Dorset, Somerset, and Wiltshire. Please get in touch and see what you think.

Drainage Field, size calculation and design a guide
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Drainage Field, size calculation and design a guide
A guide on the design and size calculation for drainage fields
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