JDP uses cookies to make this site simpler. Find out more about cookies

How to install a septic tank

Replacing an old septic tank? Installing a new one? Think you can do it by yourself? This overview of how to install a septic tank is just what you need!

Off mains drainage is a serious business. Those of us living in towns and cities often take it for granted that our wastewater simply disappears when we flush a chain or pull a plug. However, in more rural areas, where connections to the main sewer system can often be impractical, costly, or straight up impossible, private off-mains systems are often necessary. One of the most common options in such situations comes in the form of septic tanks.

If you are buying a new build house that requires an off-mains system, this will most likely have been taken care of by the builders already, and you need only concern yourself with the running and maintenance of it. But what if you are moving into an existing property that requires one, or you are the owner of an older off-mains system that needs replacing, can you install a septic tank yourself? These are the questions that this guide will attempt to answer.

Note: This article assumes that you are fully aware of all the off-mains solutions available to you and that you have chosen a septic tank to be the most suitable. If you are unsure about any of this, we recommend first taking a look at our guide to off-mains drainage article.

When it comes to installing a septic tank going down the DIY route is a popular choice, due in no small part to the potential cost savings. Hiring a specialist contractor will likely cost you somewhere between £150 - £250 per day (sometimes more depending on the location and conditions of the site), and it can take up to a week of work to install some of the larger systems. However, unless you have a mini excavator to hand, and have at least some plumbing skills and experience, we would highly recommend factoring this cost into your plans.

If incorrectly installed, an off-mains sewage system can leave you in a nasty mess; both physically and legally. Hiring a professional contractor to do the installation for you is the best way to give peace of mind that your system is suitable for your needs, reliable and legally compliant. Wrong installation is often the cause for the majority of septic tank drainage issues.

However, if you have the skills and experience to install one yourself, the following guide will take you through various steps and stages involved in planning and preparing for your installation.

Note: This guide is intended as a recommendation of points to consider when planning to install a septic tank system. It is not intended to be taken as legal advice, nor does it cover all aspects of installation for all types of septic tanks. Professional advice should always be sought to better understand the legalities of installing your system, and manufacturers recommendations and guidance should always be taken over those mentioned in this article, should they conflict in any way.

Step 1: Understanding the legislation

The legal aspects of installing an off mains drainage system can be quite daunting to deal with. There are rules and regulations for everything from the type of tank that is appropriate for a property, to where it can be installed, where it can discharge to, and what kind of ground conditions are acceptable.

This article focuses on the rules and regulations for installing septic tanks in England. Different rules exist for installations in Northern Ireland, Scotland and Wales. Consultant the respective national environment authorities for further details.

For septic tanks, it is especially important to be aware of the most current legislation, as there have been significant changes to where they can discharge too. As of 1st January 2020 septic tanks are no longer allowed to discharge to a watercourse as they provide very little treatment to the waste water. Your only options now are as follows:

  • Connect to a mains sewer, if at all possible
  • Install a sewage treatment plant instead if you still want to discharge to a watercourse
  • Install a suitable drainage field for the septic tank to discharge into

All references in this article to either “drainage fields” or “soakaways” refers specifically to those designed for wastewater only. Surface water soakaways, soakaway pits, soakaway crates or tunnels must NEVER be used for the discharge of septic tank effluent.

Complying with all of the regulations is of paramount importance, both for the environment and for you as the property owner. Local authorities and/or relevant environment agencies have the power to test any off-mains/private drainage system in their jurisdiction and take legal action against the owner if found to be non-compliant.

Do not be tempted to install a septic tank as a cheap option without the necessary consent. It could prove a very costly mistake. Make sure you are fully aware of all the regulations that could affect your installation, and operation, of a septic tank system.

The best place to start is in fully reviewing the General Binding Rules. This should govern the planning of your system and will inform whether a septic tank will be suitable for your property. The following points offer a quick overview of the main rules for septic tanks:

  • Tank must be rated to BS EN 12566-1 standards
  • It must be installed at least 7 metres away from any habitable building
  • It must be installed within 30 metres of an access point for desludging services
  • A septic tank can only service the sewage and wastewater needs of a limited number of people – please check with your national environment authority for advice
  • Installation is not permitted in Zone 1 of a Groundwater Source Protection Zone
  • They can only discharge into a soakaway/drainage field which complies with Building Regulations or BS 6297
  • Discharge must not be directly into a watercourse (stream, lake, etc.)
  • Consent to discharge must be applied for from the Environment Agency (England & Wales), SEPA (Scotland) or DAERA (Northern Ireland)

Ideally, if the conditions of the General Binding Rules are met, your septic tank should be able to discharge into a drainage field/soakaway. A drainage field (or soakaway system) is made up of a series of perforated/slotted pipes that provide the main stage of treatment for the discharged waste from a septic tank. However, installing a drainage field does come with its own set of additional rules.

You can do this by digging a 1m2 trial hole to a depth of at least 1.5m below the invert level of the proposed drainage field pipework. The groundwater table should not rise to within 1m of the invert level. Don’t forget that the groundwater levels will be different at different times of the year, so be sure to take that into account. The following additional rules will apply for the install of any drainage field:

  • No part of the drainage field should be within 2 metres of a neighbouring boundary
  • It should be at least 10 metres from any watercourse
  • It should be at least 15 metres from any building
  • It should be at least 50 metres from a water supply (e.g. well or borehole)
  • No access roads, driveways or paved areas should be located within the area

If your property does not meet the requirements for a drainage field, either due to poor ground results (see site survey section) or not enough space, you may want to consider installing a sewage treatment plant instead as this could give you the alternative option of discharging to a watercourse. Check out our sewage treatment plant installation guide to compare the steps involved.

In summary, these are just the main points and we stress that you should fully understand the legislation and implications of installing a septic tank, and always consult professional advice if you're unsure. If, however, you've absorbed relevant information from the General Binding Rules like good soil absorbs discharged effluent, and you're confident your system is within compliance, the next step is to perform a site survey.

Step 2: Site Survey

In order to buy and install the most suitable septic tank system for your property, you need to first evaluate the area of installation, in order to determine the conditions that will affect your choice. The best way to do this is to ask the following questions.

Can my property connect to the mains sewerage system?

The first thing to check is whether any part of your property is within 30 metres of a public sewer. If it is, you will not be allowed to install your septic tank or drainage field unless you obtain permission to discharge to an alternate system from the Environment Agency. Typically, however, they will not permit the use of a private system if a public one is available, unless a very good reason is given - such as the connection route to the main sewer being obstructed. To find out if there is a public sewer near your property, contact your local water authority.

Will my septic tank system comply with the General Binding Rules?

The next thing to do is to check that your proposed system will comply with the General Binding Rules, as mentioned in step 1. If, for any reason, it doesn’t, you’ll again need to apply to your national environment agency for consent to discharge. This could be the case in England if, for example, your drainage field would be within 500 metres of any:

You would also need a permit if the new discharge would be in or within:

  • 200 metres of an aquatic local nature reserve
  • 50 metres of a chalk river or aquatic local wildlife site

Unless you are 100% certain that your system would meet these criteria it is highly recommended you contact either JDP’s technical team or your national environment agency directly, to check if you’re in or near a designated sensitive area and to find out if you need a permit.

You must always get planning permission and building regulations approval for all septic tank installations. Make sure you do this before you go any further, as failure to comply with, or acquire any one of these may prevent your installation all together. Assuming that you have done all of these legal checks, and that your property is indeed suitable for a private sewage system, you can then move on to conducting the actual site survey

How do I perform a septic tank site survey?

Before you can start planning your system, you’ll need to do all of the following:

  • Conduct a Groundwater Source Protection Zone Search - to check your property isn’t in a restricted groundwater protection area
  • Conduct Trial Site Assessment Hole Test - to determine the water table level beneath your property
  • Conduct Percolation Tests - to check that the ground is suitable for a drainage field to be installed

View our dedicated article for how to perform a percolation test for a more detailed description of how to perform these tests and checks.

The last item on your site survey “to-do list”, is to check that the proposed location of your tank will comply with the general binding rules. Assuming you have read the rules and understand the restrictions, this should be as easy as measuring up your garden to make sure you can fit the tank on your property without it breaking the rules.

If all the above come back with good results you can then move on to the design stage of your system.

Step 3: Design the system

The results from your site survey will all have an impact on the design of your system. From how deep your tank can be installed and where it can be located, to the size of the drainage field you will require. Now comes the time to start looking for a septic tank that fits all the criteria.

Before you go and check out our comprehensive product range of septic tanks there are a few more calculations to make and things to consider to help you find the right one. These include:

  • Sizing the tank to your needs
  • Choosing the right tank
  • Sizing the drainage field

How do I size a septic tank?

Sizing a septic tank can be a bit tricky, as they are usually categorised firstly by their capacity and then by the number of people they can service. You need to be able to determine the minimum population size of your property (which calculates the property’s potential population rather than just the number of people currently living there) as well as the volume of sewage and wastewater that your property is likely to produce, as this will help you work out the capacity needed.

Undersizing or oversizing the system you need can both cause costly problems in the future (especially if you ever come to sell your property). For these reasons, we would strongly recommend that you read our dedicated how to size a septic tank or sewage treatment plant article to ensure you are selecting the right capacity for your property.

How do you choose the right septic tank?

It's quite difficult to offer advice on how to choose the best septic tank without knowing specific details of the project. Remember to choose the tank that not only best suits the size of tank, but also consider the installation ease and costs, the aesthetic aspects of after installation, and operation and maintenance.

How do you size a drainage field/soakaway?

Septic drainage fields usually consist of an arrangement of trenches, which contain perforated pipes and gravel, sited beneath a soft landscaped area such as a lawn. Determining the size required for your system will depend on both the size of your property/number occupants and the results of your percolation tests. Check out our dedicated article sizing a septic drainage field for a guide to the calculations involved.

Step 4: Installation

By now, you’ll have become an expert on all the legalities, requirements and rules surrounding the installation of a domestic septic tank system. You will have designed your system to perfectly fit your property, while remaining fully compliant with rules and regulations, and have placed an order for the most suitable tank. Now comes the hard work... installing it all!

We always recommend that the installation of any septic tank should always be carried out by a suitably trained and qualified professional. However, if you feel you are up to the task of a DIY install, the manufacturer of your chosen tank will provide you with a detailed manual that will give step by step instructions on how to do it. This will also advise you of all the health and safety measures that should be taken throughout - please make sure you follow these in full.

As a brief guide, the following list of the general steps should give you an idea of what will be involved.

Before installing your septic tank

  • Read the full installation guide provided with delivery of goods
  • Ensure Building Regulation approval
  • Ensure consent of discharge is approved from your environment agency
  • Use a pump to keep excavation clean and free from rising ground water during installation

Stages of installing a septic tank

  • Inspect tank for damage

    Tanks will normally have been fully tested and checked before being dispatched to you, but it isn’t uncommon for them to become damaged during transportation and offloading. Make sure to thoroughly inspect the tank when it arrives as, once the tank is installed, most manufacturers won’t accept claims for damage.

  • Check you have correct invert depths

    You should have already checked the depth of your incoming pipework during the site survey and design stage of your project, and ordered a tank with the appropriate invert levels. However, It is worth checking this again to ensure that the physical tank delivered will still connect to your existing pipework and remain within the maximum permissible depth.

  • Siting the tank

    Double check your planned install location against the guidelines in the install manual, as they may recommend extra conditions and minimum distances specific to their tank. British Standard BS: 6297-1983 recommends the following

  • Excavate the hole

    Make sure it is large enough for the tank and any recommended backfill, then install the right type of base (often a concrete one) for the tank to sit on. Please ensure that you follow the manufactures detailed health and safety guidelines that will be provided in the manual when excavating your hole.

  • Consider drainage falls

    This should have been part of your planning already, but it is worth double checking, especially for a gravity system. Typically these will be 1 in 60/70 between the house and tank, and a maximum of 1 in 200 for a drainage field.

  • Place the tank

    Carefully lower the tank into the hole, using the recommended lifting system, while checking that the inlet and outlet orientation is correct and ensuring the tank is level.

  • Use correct backfill

    Concrete, gravel or sand are the most common recommendations, but check the instruction manual to see what the manufacturer advises and follow their steps for adding backfill to the site.

  • Install the inlet and outlet

    Should be a straightforward task if everything has gone to plan, but a qualified plumber should be consulted if you are unsure.

  • Install septic drainage field

    See our dedicated guide to installing a septic drainage field.

  • Fit correct cover and frame

    Manufacturers will always assume the tank is going to be installed in a traffic-free area. If this is not the case, it is highly recommended that you contact a structural engineer before installing your system.

  • Consider ventilation arrangements

    This can often be overlooked, but it is important to ensure your system is well vented to prevent odour issues on site. Contact our technical team for advice if you are unsure of the best arrangements for your site.

Now, to round things off, here are a few key things that you should ensure you DON’T do when installing your system.

DO NOT

  • Subject the tank to impact or contact with sharp edges
  • Add neck extensions to the tank, nor build a brick manhole above the tank neck (as this increases burial depth of the tank) as it is not recommend extending the neck of the tank under any circumstances
  • Install the tank deeper than the depth that the fitted neck will allow
  • Install in trafficked areas without a suitable backfill design
  • Site the tank so that it is subjected to excess ground pressure (e.g. sloping ground) or applied loads such as may be generated by vehicular traffic
  • Fill an unsupported tank
  • Backfill an empty tank

So there it is - there’s our guide to all of the things you need to think about and do when installing a septic tank. However, the physical installation of the system is not the end of this journey. Once the tank is in the ground and everything is connected up, you need to turn your attention to its operation and maintenance.

Step 5: Operation

The treatment process of a septic tank system is generally quite simple and will require no specialised knowledge to operate. However, as with any other off-mains system, it is important to know how it works and to understand that there are some restrictions to use, when compared with a normal mains sewer connection.

Unlike a sewage treatment plant, a septic tank doesn’t provide any treatment to the waste water, and so must always discharge into a drainage field. However, what a septic tank does still do is to separate out the solids using gravity. The lighter solids, along with oil and grease, float to the surface, while the heavy solids sink to the bottom of the tank. There, some of the ‘sludge’ is broken down by natural bacteria, though much of it will still remain and build up over time. As such, your septic tank will still need emptying at regular intervals by a registered waste carrier. Whilst limited, this sludge breakdown is an important part of the process, and the bacteria in the tank that facilitates it needs to be protected.

For this reason, it is important to follow the manufacturer’s guidance on what should and shouldn’t be flushed into the tank. The following is a list of the things that could be affected:

  • Washing dishes - use a dishwasher if possible as they are much more efficient at converting fat into soap.
  • Whilst all active ingredients in soap and detergents should be biodegradable and, in normal use, safe for septic tanks, excessive amounts can upset the natural balance of your septic tank. Try to use mild detergents, toilet fresheners, fabric conditioners, washing powders and washing liquids in moderation to prevent this.
  • Use all bleaches and disinfectants sparingly as they can kill the essential bacteria within the septic tank.

There are some very important “Don’ts” when it comes to protecting your septic tank. Some of them are obvious, but others are things you might not think twice about if you have never had a septic tank system before. These include:

Don't use your toilet and sink as a rubbish bin

  • Never flush cat litter, disposable nappies, pantie liners, tampons, paper towels, facial tissues, coffee grounds, or cigarette ends down the toilet. They will clog your septic tank in less time than you might imagine.
  • Don't install a waste disposal unit as these can double the amount of solids going into the septic tank.

Be sensible with cleaning and disinfection products and what you pour down the sink

  • Overuse of anti-bacterials, disinfectants and heavy cleaners will kill the beneficial bacteria in the septic tank which digest your solids.
  • Don't use anti-bacterial hand wash products as you are poisoning your septic tank bacteria with every wash.
  • Do not pour grease or oil down the sink. Wipe greasy dishes with paper towels before washing.
  • Grease clogs the septic tank soakaway, and effectively 'waterproofs' it, making it impossible for soil to absorb liquids. If that happens you'll need a new soakaway.
  • Do not pour fats and cooking oil down the sink. They should be disposed of separately as they can prevent the soil surrounding your soakaway system from being able to absorb waste water effectively.
  • White spirit, varnish, paint thinners, motor oils, petrol and other similar chemicals will ruin your septic system and are not easily broken down by soil bacteria. They will pollute groundwater.
  • Condensation from condensing boilers is very acidic and must not go into the foul drains.

Step 6: Maintenance

Before installing your septic tank system, you should have already planned for future maintenance needs. Things like making sure there is suitable access for desludging to take place, and keeping covers clear of obstruction (i.e. don’t cover them with soil or grass to try and hide them away). But once the system is installed, there are ongoing maintenance needs that need to be considered. Septic tanks and drainage fields won’t last a lifetime. They will need replacing eventually. However, with a bit of care and good practice operation, you can help ensure that you get as many years out of them as possible.

It is recommended that you take out a service agreement (available through your local JDP branch and manufacturer), as annual desludging and servicing is critical to the longevity of your system. It is also a requirement of the new PPG4 guidelines that systems are regularly maintained.

Between these services though, there are many things that you can do yourself to help prolong the life of your septic tank and soakaway drainage field. These include:

Divert rainwater from the septic drain field

  • A saturated drainfield can't absorb enough septic effluent.
  • Plan landscaping, roof gutters and surface drains so that excess water is diverted away from the soakaway area.

Don't overload the septic tank and drain field

The amount of water used in the home has increased dramatically over the last 20 years. If your septic tank pre-dates the current regulations, it is probably too small and struggling to cope with current flow rates. There are things you can do to reduce this problem:

  • Repair leaking and dripping taps and toilets. Dual flush toilets often leak at the valve after a period of time and need replacing.
  • Use aerators on taps and low-flow showers to help lower water consumption. Power showers just increase the volume of water used.
  • Change to a low water consumption washing machine and reduce water levels for small laundry loads.
  • Don't run a half-empty dishwasher.
  • Toilets account for over 35% of all the water used in the home, water which overloads your soakaway. Fit dual flush toilets, use a 'Hippo' or, even better, convert your existing toilet to an 'eco' toilet with an INTERFLUSH conversion kit to reduce the amount of water needed to flush to the absolute minimum. The Interflush is a kit which fits on top of your WC siphon and connects to the front mounted flush handle. The toilet only flushes when the handle is held down, releasing the handle stops the flush (when pan is clear). It only uses the exact amount of water required, any less and the toilet would need flushing again. That is why nothing can flush a toilet with less water.

Keep trees away from the septic system

  • Discourage root damage by keeping trees at least 30 metres away from the soakaway.
  • Trees with very aggressive roots, such as willows and poplars, should be even farther away from the system.

Protect your soakaway

  • Do not drive over the soakaway, build a structure on top of it, or cover it with concrete or tarmac. Gravel is ok, but only for foot traffic
  • Sow grass over the soakaway area, if possible, as grass takes up a lot of water.

Be vigilant against signs of problems with system

If you notice any of the following, or anything unusual in general, it is best to get your system checked out as soon as possible.

  • Dirty water pooling above ground over your tank or drainage field.
  • Toilets flushing slowly or overflowing.
  • Your tank needing to be emptied more frequently than usual.

Maintain your septic system

  • Solids must be pumped from the septic tank on an annual basis. They only store 12 months worth of sludge, so extending the emptying interval beyond that will ruin your soakaway. Depending on usage it may require pumping more often.
  • Never lift the lid of a septic tank yourself, particularly if you are alone! The gases in the tank can overcome you very quickly and the bacteria are dangerous. Sewage workers must have regular vaccinations, including hepatitis, tetanus and diphtheria, to protect against them.

The installation manual provided by the manufacturer will most likely have other recommendations of checks you can make yourself too. However, please ensure that you always follow their health and safety guidance for any maintenance you plan to do.

If you have any questions, or need any further advice on how to install, operate or maintain a septic tank system, contact us today. Our technical team is always on hand to help with all of your drainage needs.

Questions? Ask the Experts!

Can we help with something else?