How to install a sewage treatment plant

Got a need for a sewage treatment plant? Reckon you can DIY the install? Double check this guide for everything you need to know about treatment plant installations.

We all want our wastewater to simply disappear when we flush a chain or pull a plug without needing to give it another thought. For most of us, this is exactly how it works, however it is becoming more and more common for properties to require an off-mains drainage system often because they are too far from a mains connection, or perhaps the mains system in the area has reached capacity. One solution to this problem is the installation of a sewage treatment plant.

What if you own or are moving into an existing property with an older off-mains system that needs replacing? What if you need to have a sewage treatment plant installed at your new build? Can you install a sewage treatment plant 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.

Table of Contents

Installing a Domestic Sewage Treatment Plant

Being upfront and honest, we do not recommend installing a sewage treatment plant yourself unless you are 100% confident in your ability to do so. 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.

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 sewage treatment plant. It is not intended to be taken as legal advice, nor does it cover all aspects of installation for all types of treatment plants. 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 installation of an off mains drainage system can be a daunting prospect when you start looking into all the legal aspects. 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.

Complying with all of these regulations is of paramount importance, as failure to do so can be disastrous for the environment and leave you, the property owner, open to prosecution. Your Local Authority and/or relevant environment agency have the power to test any off-mains/private drainage system that they feel isn’t complying with the regulations and can ultimately take legal action if that is/continues to be the case.

It is therefore imperative that you ensure full understanding of, and adherence to, all the regulations that will affect your installation, and running, of a sewage treatment plant. This is the sole responsibility of the property owner and should be taken very seriously.

For this reason, reviewing the General Binding Rules should always be the starting point for any non-mains foul drainage system in England. This should govern the planning of your system and will inform whether a permit from the Environment Agency would be required.

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

The following points offer a quick overview of the main rules for sewage treatment plants:

  • All sewage treatment plants must be EN12566-3 tested and approved for sale and installation to be deemed legal in the UK
  • The discharge (or outlet) from the tank must be at least 10 metres away from any water courses or buildings
  • If the plant needs electricity to operate it must be able to function without power for up to 6 hours or have an uninterruptible power supply fitted to it

Ideally, if the conditions of the General Binding Rules are met, your sewage treatment plant should be able to discharge into a local watercourse (if conditions aren’t met, you can still apply to the national environment agency for consent to discharge). However, if the conditions aren’t met (and/or consent to discharge has been denied), then your sewage treatment plant will need to discharge into a drainage field/soakaway instead. A drainage field (or soakaway system) is made up of a series of perforated/slotted pipes that provide an additional stage of treatment for the discharged waste. The following additional rules will apply if you do need to install one:

  • No part of the drainage field should be within 2 metres of a neighbouring boundary
  • 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 you find that a drainage field is your only discharge option, you may also be able to look at installing a septic tank as a more cost effective alternative to a sewage treatment plant. Check out our septic tank installation guide to compare the steps involved.

We stress that these are just some of the main points we have picked out here. A full reading of the General Binding Rules should be conducted before going any further. If you don’t understand the rules, you shouldn’t be installing a sewage treatment plant yourself. If you are intending to discharge into a watercourse we would highly recommend speaking with a member of our technical team regardlessly, as they can offer expert advice on the complex rules that surround the ability to do so.

If, however, you have absorbed all the relevant information from the General Binding Rules, like good soil absorbs treated effluent, and are confident you understand how to best ensure your system will comply with all the regulations, the next stage of the process is to perform a site survey.

Step 2: Site Survey

Before you can even think about buying and installing your sewage treatment plant you need to first evaluate the area of install to know what type of system you will need.

Important note: If any part of the building your treatment plant will serve is within 30 metres of a public sewer, you will need to apply to the national environment agency for permission to discharge to an alternate system, and have a good reason for wanting to do so. Typically, they will not permit the use of a private system if a public one is available, as per the General Binding Rules. To find out if there is a public sewer near your property contact your local water authority.

Unfortunately, when it comes to sewage treatment, it’s not always a case of just picking a product “off the shelf” and having it installed. There are many factors that will affect the suitability of any tank to your property including the type of wastewater to be treated, the effluent output quality needed, the proximity of the intended install point to any buildings, the location of the discharge point, the porosity of the ground if a drainage field is needed and the availability of site access for maintenance.

But how do you find all this out? Well, the first thing to do is check your proposed system would comply with the General Binding Rules, as mentioned above. If, for any reason, it doesn’t, you’ll need to apply for consent to discharge. You'll also need to consent if your planned discharge point would be within 500 metres of any:

  • Special areas of conservation
  • Special protection areas
  • Ramsar sites
  • Biological sites of special scientific interest
  • Freshwater pearl mussel population
  • Protected shellfish water
  • Designated bathing water

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.

It is also important to check whether you would need building regulations approval and/or planning permission for your sewage treatment plant before you go any further. Failure to comply with, or acquire any one of these may prevent your installation altogether.

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 even more checks - nobody said installing an off-mains system was easy!

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

  • Conduct a Groundwater Source Protection Zone Search
  • Conduct Trial Site Assessment Hole Test (if you need to install a drainage field)
  • Conduct Percolation Tests (if you need to install a drainage field)
  • Check that a tank, and a drainage field (if using one), can be situated far enough from habitable dwellings, as per the General Binding Rules

View our dedicated article for how to perform a percolation test for a more detailed description of how to perform these tests and checks, or have a watch of our companion video to see how it's done.

With all these tests and searches complete, and you've checked that the proposed location of your tank will comply with the General Binding Rules, all you will need to do is grab a tape measure and make sure you can fit a sewage treatment plant on your property without it breaking the distancing rules. The following is a summary of recommended guidelines, but please make sure that you check the regulations thoroughly yourself:

  • At least 3 metres away from any boundary line of the property
  • At least 3 meters away from any significant root systems (i.e. from trees and bushes)
  • At least 10 meters away from any habitable building
  • No greater than 30 meters away from a hardstanding area suitable for desludging tanker access

Assuming green lights across the board for all of the above, you can now 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 what type of discharge method you can choose, to how deep your tank can be installed and where it can be located, your available options should already be zeroing in on finding the most suitable system for you. Now comes the time to start looking for a sewage treatment plant that will fit your criteria.

But before you disappear to peruse our comprehensive catalogue of sewage treatment plants 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 sewage treatment plant?

Given that most sewage treatment plants are catagorised by the population size they can service, you might think it a simple case of matching the category to the number of people lining in the property. Unfortunately, calculating what size sewage treatment plant you need is a little bit more complicated. You need to consider the full potential population of the property, based on its size and the number of bedrooms. You may be only two people in a 4 bedroom house, but if you install an STP based only on your current needs, you may find yourself in a mess if you have family come and visit for a week or two over the holidays.

You might also find it difficult to later sell your property without first upgrading the STP to a more appropriate size. 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 domestic sewage treatment plant?

Unfortunately, there is no straightforward, easy answer to this question. The best sewage treatment plant for you will depend on a multitude of factors, ranging from those born by restrictions (such as how deep you can install a tank and your discharge options) to those of a more personal choice (such as a desire for low running costs or minimum aesthetic disruption to your garden). Due to this, it is very difficult to offer advice on how to choose the best sewage treatment plant, without knowing specific details of the project.

Do I need a gravity fed or a pumped sewage treatment plant?

Most sewage treatment plants are designed to work with the gradual feed of a gravity only system. However, if the tank is to be installed in a location where there is no natural fall of gravity from the outlet pipe, the aid of a pump may be needed to achieve the required discharge level. This can sometimes be the case when discharging to a watercourse. A gravity system is the preferable option if at all possible, as adding a pump to the system adds an extra moving part that needs servicing to prevent the system from failing. Relying on a pump also makes the system even more dependent on an uninterruptible power source, otherwise the tank could quickly fill up in the event of a power cut. If you are unsure as to whether your system would need a pump or not, it is highly recommended that you seek professional advice.

Do I need a gravity fed or a pumped sewage treatment plant?

Septic drainage fields (sometimes referred to as septic drain fields, leach fields or foul water soakaways) are subsurface wastewater treatment facilities which are used to remove contaminants and impurities from the effluent that emerges from a septic tank or sewage treatment plant. They usually consist of an arrangement of trenches, which contain perforated pipes and gravel, sited beneath a soft landscaped area such as a lawn. If you need a drainage field for your sewage treatment plant (presumably because you can’t discharge to a watercourse), 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 sewage treatment plant system. You will have designed said system to perfectly fit your property, while remaining fully compliant with said rules and regulations, and have placed an order for the most suitable sewage treatment plant. Now comes the hard work... installing it all!

This is where we need to reiterate our recommendation that the installation of a sewage treatment plant should always be carried out by a suitably trained and qualified professional. If any part of your private sewage system is installed incorrectly, you could be faced with repair costs, and potentially hefty fines, that will be far greater than the original cost of hiring a professional installer. However, if you still 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, as well as advising you of all the health and safety measures that should be taken. The following is a list of the general steps involved, but please make sure you adhere to the manufacturer’s guidance during the actual install, as some steps may vary and you will need more detailed instructions than what is provided here.

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

Stages of installing a sewage treatment plant

  • 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.

  • 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 the inlet and outlet

    Assuming that everything has gone to plan thus far, connecting up the pipe work should be fairly easy. It is worth noting though that some manufacturers advise the added install of inspection chambers before and after the treatment plant for easier maintenance access should problems arise in the future.

  • Wire up the electrical connection

    Unless you are a qualified electrician yourself, please leave all electrical work to a professional. The installation manual will detail the steps required but we strongly advise against trying to do this yourself.

  • Fit correct cover and frame

    Assuming you provided the sewage treatment plant supplier/manufacturer with the correct details of your site, the most suitable cover and frame should already be provided. However, it is worth noting that manufacturers will always assume the plant 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. Ideally, there should be a vent on the treatment plant and, if you have a drainage field, venting at the ends of the percolation trenches too. Contact our technical team for advice if you are unsure of the best arrangements for your site.

That’s a basic look of the things you need to do when installing your sewage treatment plant. It is not a comprehensive guide by any means, that is what the manufacturer’s installation manual is for, but it should give you an idea of the general steps involved. In addition to these steps, you may also need to install a drainage field/soakaway to complete your system (if discharge to a watercourse isn’t possible or permissible). For that stage, however, we have a separate, dedicated guide to installing a septic drainage field you should read.

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


  • 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 sewage treatment plant. Hopefully you can see that it is a task no sane person should want to undertake themselves without professional help. We cannot stress enough the importance of at least seeking expert advice to help with your planning.

When you consider that the cost of having a sewage treatment plant installed professionally can sometimes be equal to the cost of the plant itself, you may think it a better option to do it yourself and save the money, but it would only be a saving in the short term. Unless the system is installed by a registered professional you may find that offers of warranties and guarantees are automatically void. If the system was installed incorrectly, or fails due to undetected damage or non-manufacturing fault, you are unlikely to be covered and will have to incur the full cost of fixing the system yourself.

Our recommendation is to do it right, plan for the extra cost in your budget, and hire a reputable sewage treatment plant installer.

Whatever you choose to do though, 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 biological treatment process of a sewage treatment plant is generally self-regulating and will require no specialised knowledge to operate. However, 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.

Sewage treatment plants use colonies of live natural micro-organisms (also known as biomass) to break down the pollutants in the sewage that you flush, wash and drain away. This biomass can be damaged or destroyed by certain chemicals found in household cleaning products, especially if they are used in excessive amounts. This can lead to unpleasant smells occurring and, in extreme cases, the ultimate failure of the sewage treatment plant. 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:

  • Detergents (washing machine & dishwasher) and washing up liquids. These are generally okay to use in the normal concentrations for typical household applications. Problems can occur, however, if you find yourself having to do unusual amounts of clothes washing, for example. Should this be the case, it is advisable to spread the extra cleaning out over a few days just to be safe. Excessive use of Biological washing powders can cause degradation of the biomass. Other ways you can help protect your tank's biomass are by switching to non-biological detergents, without enzymes, or using liquid detergents instead of powder. Always avoid excess use.
  • Floor cleaners, disinfectants, and bleaches. Generally safe to use, in accordance with makers recommendations, but it is advisable to do so with the minimum necessary concentrations. However, you should never pour neat disinfectant or bleach down sinks or outside gullies, as this would be damaging to the biomass. If there is a smell arising from the drains it will usually indicate a problem in the system that will need fixing by a professional.
  • Nappy disinfectants and bottle sterilising fluids. These can still be used, but you should ensure that the used fluid is well diluted with water before pouring it down the drain.
  • Waste disposal units. Using a waste disposal unit won’t cause damage to the biomass, but it can put a considerable extra load on the sewage treatment plant, both in terms of organic load and liquid, as the ground up material is flushed into the unit. If you intend to use one of these with your sewage treatment plant system, please speak to the manufacturer of your chosen tank to find out if their product will be able to deal with the extra loads.

Some things must NEVER be discharged into the drains if you have a sewage treatment plant. These include:

  • Motor oil, grease, anti-freeze, brake fluid etc.
  • Cooking oil and fat
  • Weed-killers, insecticides, fungicides and other gardening chemicals
  • Paint, thinners, white spirit, turpentine, creosote etc.
  • Chemical drain cleaners/commercial cleaning products
  • Acid based brick/stone floor cleaners
  • Medicines (return unused medicines to a pharmacist for safe disposal)
  • Photographic developing fluids
  • Nappies, sanitary towels, toilet wipes etc. (even biodegradable ones as they do not fully degrade in a treatment plant and can lead to blockages and malfunctions)

Step 6: Maintenance

Before installing your sewage treatment plant, 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, what can you do to help keep it in good working order and problem free?

First and foremost, you are going to want to take out some kind of maintenance agreement and service plan with either the manufacturer, if available, or a suitably qualified maintenance company. Annual desludging and servicing is critical to operation of your system, but it can be costly. Taking out a service plan at the time of purchasing your tank though, could save you money in the long run, and will certainly save you the hassle of remembering to get it done yourself.

The annual servicing, which should be conducted by a qualified professional, will usually check things like:

  • Sludge return
  • Functionality of blower and/or pump
  • Pump pressures
  • Replacement of pump filters
  • Pump diaphragm
  • Alarm functionality
  • Ventilation functionality
  • Covers and locks

Between these services though, there are some checks you can make yourself to ensure the system is operating correctly. These include:

  • Checking all vents to make sure they are not blocked or obscured
  • Checking the air blower is working by listening for a gentle hum when standing beside the plant

The installation manual provided by the manufacturer will most likely have other recommendations of checks you can make yourself. 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 sewage treatment plant, contact us today. Our technical team is always on hand to help with all of your drainage needs.

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