Spherical septic tank for sewage treatment with manhole cover and frame
Calendar 18 March, 2024

Sewage Treatment Plants: Everything You Need To Know

Sewage treatment plants provide efficient and environmentally friendly sewage management for properties that don’t or cannot be connected to a main sewer.

If your property is in a rural, off-grid or temporary location, it’s likely you won’t be connected to a main sewage system; there may even be no nearby system you can connect to. Older properties may be connected to a cesspit or septic tank, but the modern, more efficient and more environmentally friendly option is a sewage treatment plant.

In this blog post, we look at:

  • how a sewage treatment plant works
  • the advantages of a sewage treatment plant
  • choosing the right size sewage treatment plant for your property 
  • the regulations concerning sewage treatment plants
  • a few points to consider before installing a sewage treatment plant
  • how to maintain a sewage treatment plant

How A Sewage Treatment Plant Works

In many ways, a sewage treatment plant is not dissimilar to a traditional cesspit or septic tank; it is an underground storage chamber for effluent and domestic wastewater which requires periodic emptying. However, unlike those traditional systems, a sewage treatment plant uses various processes that clean the water discharged from the plant to be released safely into the environment. 

These processes are as follows:


Wastewater begins its journey through the sewage treatment plant in a primary chamber, where gravity separates the solid waste – also known as sludge – which sinks to the bottom of the chamber. This sludge is periodically removed from the primary chamber, although not as frequently as the sludge in a septic tank.

The liquid waste remaining at the top of the chamber then moves forward to the secondary chamber and the next stage of the treatment process.


In the secondary chamber, compressed air is blown through the liquid waste to increase its oxygen content. This encourages the growth of bacteria that are fundamental to the treatment process.


Once this process has decontaminated the liquid waste sufficiently for it to be discharged into the environment, the bacteria are first allowed to settle and then returned to the primary tank for eventual removal. This is called the disinfection process.


How and where the treated wastewater can be released will depend on the location and the permits that have been granted. This may be to a nearby watercourse such as a ditch, stream or river, a pond or lake, or an installed soakaway system.

The Sewage Treatment Plant: Its Advantages

While installing a sewage treatment plant may be a heavier financial investment, its benefits are increasingly outweighing the benefits and downsides of a septic tank:

A sewage treatment plant produces less sludge

Compared with a septic tank, which is simply designed to hold effluent, by separating and treating liquid waste a sewage treatment plant produces significantly less sludge. This means it requires emptying less frequently than a septic tank.

A sewage treatment plant is better for the environment

Untreated sewage produces ammonia, a noxious chemical which is harmful to natural habitats and wildlife and can even present a human health hazard. In contrast, by the time it is ready for discharge the treated effluent in a sewage treatment plant is 95% clean – higher still with some more advanced systems.

Less frequent emptying of a sewage treatment plant also reduces the carbon footprint and energy consumption of the tanker vehicles used to empty the effluent and carry it to a disposal point.

A sewage treatment plant costs less to run

Despite its higher purchase and installation cost and the cost of electricity to run the compressed air pump, over the long term, the operational costs of a sewage treatment plant are lower than those of a septic tank as it requires emptying less frequently.

A sewage treatment plant may now be your only option

While regulations covering sewage discharge may once have been more lenient, new legislation stipulates that septic tanks can only be discharged into the ground via a soakaway system or drainage field. While an older septic tank may currently discharge into a watercourse, if you replace this it must be replaced with a system that discharges into a soakaway system or drainage field. 

These systems both require space away from buildings and trees and cannot be built over. If space is an issue, a sewage treatment system may be the only option.

What Size of Sewage Treatment Plant Do I Need?

When calculating the correct size sewage treatment plant for your building you need to consider what facilities the building will contain (number of toilets, baths, showers, washing machines, dishwashers) and the number of people who will be using these facilities. 

To be safe, allow for maximum occupancy to avoid problems later on; that is, if you only have two people living in a five-bedroom property, calculate as if all the bedrooms are occupied. It’s also a good idea to check the manufacturer’s specifications as these can vary from one manufacturer to the next.

The following calculation methods are a good guide to what you will require for average residential properties.

  • For a single property with up to 3 bedrooms, allow for a minimum occupancy of five persons – you will require a 5P sewage treatment plant.
  • For a single property with more than 3 bedrooms, start with your 5-person/3-bedroom baseline and add one person for each additional bedroom. So if you have five bedrooms, you have your original 5 plus 2 extra – you will require a 7P sewage treatment plant.
  • For two properties, one with 3 bedrooms and the other having 5 bedrooms, allow for a minimum occupancy of 5 for each house plus two additional bedrooms for the larger property (12 occupants in total)  – you will require a 12P sewage treatment plant
  • For a property or group of properties with between 13 and 25 occupants, you still need to take into account the number of buildings and bedrooms, but a more accurate calculation can be achieved by multiplying this result by 0.9. so if you have 4 buildings, each containing 4 bedrooms, the standard calculation would be 4 x 6P, which totals 24P. multiply this by 0.9 and you get 21.6P. Round up this figure to get your calculation of 22P.
  • If the total number of residents is between 26 and 50 people, use the same formula but multiply it by 0.8 instead of 0.9. In this case, 5 houses each with 4 bedrooms would return an initial calculation of 30P which then is adjusted to 24P.

Commercial properties are a little higher to calculate as their usage can vary so much. For example, an office building isn’t a residential property so the use of kitchen and bathroom facilities will be lower per capita. On the other hand, at a sports facility with multiple showers and toilets, this is likely to be considerably higher.

At best, we can only offer you an approximate guide and would strongly recommend speaking with your supplier before buying a sewage treatment plant.

What Regulations Cover Water Treatment Plants?

As with any wastewater treatment installation, stringent regulations apply that must be followed, including:

Legal Compliance

Any sewage treatment plant installed and operated within the UK must be certified to EN 12566-3: 2016 to be legally compliant. As the owner of a property, it is your legal duty to ensure any existing sewage treatment plant on a residential or commercial property does comply, even if it was installed before you purchased the property. And, of course, all new system installations must also comply.

Consent to Discharge

You are likely to require an environmental permit if your sewage treatment plant is discharging anywhere other than a soakaway system or drainage field. This includes natural water sources – rivers or streams, lakes or ponds, canals, estuaries or coastal waters. These permits verify that your sewage treatment plant has cleaned the liquid effluent or wastewater to a degree that it will pose no threat to wildlife when discharged: the UK Government’s guidance Discharges to surface water and groundwater: environmental permits is a good place to start if you want to learn more about permits.

Planning Permission

If you are incorporating a sewage treatment plant within the plans for a new-build property, a separate planning application will not be required. however, if you are planning to install a sewage treatment plant as a change to the building or its use, permission is likely to be required even if it isn’t required for the rest of your project.

Installing A Sewage Treatment Plant: A Few Tips

By giving a few hints on installing a sewage treatment plant we are not advocating this is something you can easily do for yourself; as with all specialised installations we would always recommend you use a professional who will already have extensive experience. These tips are simply intended to help you plan where your sewage treatment plant will be located.

Distances from foundations and watercourses

Building regulations stipulate that a sewage treatment plant must be located at least 7 metres from any building foundations and at least 10 metres from any watercourse.

Site access

Good access from the main roadway to the sewage treatment site will be required not only for construction vehicles to excavate the site and install the sewage treatment plant, but also for service vehicles to empty it. You will need to ensure there is not only sufficient space for large vehicles but also a good roadbed to prevent them from sinking into soft ground.

Letting gravity work for you

A sewage treatment plant should ideally be placed downhill from its source as this will reduce the need for intermittent pumps and help the treatment process.

If this isn’t possible, you may need to consider where additional pumps may be required. again, this is a subject best left to an expert.

Noise and smell

Because they use electric motors to power their aeration compressors, sewage treatment plants emit a constant sound. While in many cases this is inobtrusive, you should consult an expert to determine whether this might be a nuisance either on your property or to surrounding properties. If this is likely to be an issue, a noise reduction unit designed specifically for sewage treatment plants may be an acceptable solution.

While a sewage treatment unit rarely produces noticeable odours, this isn’t the case during emptying. If it is replacing an existing septic tank this may not be much of a problem, but where there hasn’t previously been an off-grid sewage system you should consider the distance between the sewage treatment plant and neighbouring properties.

Maintaining A Sewage Treatment Plant 

Your new sewage treatment plant is likely to come with a warranty, but to ensure this remains valid you will need to maintain the plant properly.

Use it properly

We are all aware that some items shouldn’t be poured down a sink – greases, fats and food waste – or flushed down a toilet – sanitary products, baby wipes and disposable nappies. With a sewage treatment plant, this is even more critical.

Household chemicals should also be used with caution as some of these can upset the delicate balance of bacterial activity and prevent the treatment plant from functioning correctly. 

Empty it regularly

The manufacturer should be able to guide you on how frequently your sewage treatment plant should be emptied. Once a year is generally sufficient, although cheaper systems may require emptying more frequently.

Service it annually

Regular annual servicing is essential to confirm that the plant is functioning correctly and that the right level of microorganisms is present to ensure treatment is complete and no pollutants are being released into the surrounding area.

Protect it from rainwater

While a sewage treatment plant is not itself damaged by water, if rain should find its way into the system can upset the treatment process by diluting the bacteria. Large amounts of rainwater can also result in flooding, which both disrupts the process and can be quite unpleasant.


We hope you have found this information helpful and interesting. If you have any further questions you are always welcome to call our friendly team of experts on 01420 555600 or email [email protected]

Also, look out for more articles in our ongoing series of blog posts, bringing you useful information, insights, guides and tips on all things drainage!

Mark Chambers Drainfast Team Portrait

Written by
Mark Chambers

Marketing Manager

As Marketing Manager, Mark plays an active role in running strategic projects to increase our brand profile.

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