Sewage treatment plants are a great option for properties that are not connected to the main sewage network. Here we explain more about what a treatment plant does and discuss some of the other important factors you need to consider.

How does a sewage treatment plant work?

While the are some similarities between a sewage treatment plant and a septic tank, there are some key differences that make the processes very distinct.

The sewage treatment plant receives wastewater and sewage from the property, where is it fed into the main settlement tank. Here, the liquids and solids naturally separate from each other into layers based on their composition and weight, before flowing into the biozone chamber. This is referred to as the sedimentation process.

Once the solid and liquid waste has been separated, the treatment process can now start, eventually enabling it to be discharged without harming the environment. There is a high biological oxygen demand present in sewage, so the liquid waste is moved into the aeration chambers where oxygen levels are increased by pumping in compressed air, encouraging the growth of bacteria.

The final stage of the process is disinfection, although further treatment of the waste may be needed, depending on what it is and the type of system that has been installed. To achieve this, bacteria are allowed to settle and returned to the first tank where it can be removed. The wastewater left behind that has been decontaminated is around 95% clean, so it can then head towards the discharge point.

How to choose the right location for a sewage treatment plant

The location of the sewage treatment plant is important, as you need to ensure it follows Building Regulation guidelines. For example, the unit must be more than 7 metres away from the foundations of the property and a minimum of 10 metres away from the nearest watercourse (river or stream).

Everyday odours shouldn’t be a concern, although it is something to bear in mind when it is being emptied. Sewage treatment plants are also electricity powered, which means they will emit a low constant hum. While neither of these are major concerns, you should factor them into where you want to install the system.

Plant accessibility is another key point to factor into the installation position. The treatment plant will need to be emptied on occasion, so it may be a good idea to place it in a position that is easy to access by lorry. This will make the emptying process much easier to manage, and faster to complete.

The installation company will likely recommend that the plant is installed downhill from the waste source, if possible. This is simply because gravity will be able to naturally support the treatment process, which can reduce the need for additional pumps to be installed.

Other things to consider

Before any part of the system can be installed, the depth of the drainage pipes and the outfall from the treatment plant need to be calculated. Decisions also need to be made about whether pumps are needed for the system and how large the treatment plant will be (it is generally recommended to over-estimate the capacity, for example, a tank suited to 5 people is good for a household of 3 people).

It usually takes around 6-8 weeks for a sewage treatment plant to become fully operational, so bear this in mind after installation. A sample will need to be taken at this stage to confirm it is operating correctly, with further testing and sampling recommended to ensure all is well.

Do I need to worry about nutrient neutrality?

Nutrient neutrality refers to the level of nutrients (phosphorus and/or nitrogen) present in surface water run-off and the amount produced by wastewater. Neutrality is achieved when nutrient levels in nearby watercourses remain the same, even when exposed to wastewater. If there are too many nitrates or phosphates present in the water, it can reduce oxygen levels for fish and other forms of aquatic life.

This is something that should be considered and managed by the installation company, which is another reason why this part of the process is so important. Otherwise, it could lead to environmental damage which you could be held responsible for.

Why do I need to protect a sewage treatment plant from rainwater?

You should ensure that your sewage treatment plant is sealed and fully always protected from exposure to rainwater. While it doesn’t appear that rainwater will have any sort of negative effect, the reality is somewhat different.

If any additional sources of water find their way into the treatment system there is a high chance it will affect the growth of bacteria, and subsequently, the decontamination process. This compromises the entire system and could mean that polluted wastewater is being discharged into the watercourse. There are also instances where continued exposure to rainwater could cause the tank to flood, which can be a costly issue to deal with.

Why use a sewage treatment plant?

Some of main reasons to use a treatment plan include:

Lower sludge levels

The maintenance requirements for a sewage treatment plant are very low, largely because there is not much sludge in the system at any given time. Of course, it depends on usage levels, but generally, most treatment plants only need to be emptied once a year.


The low maintenance requirements also feed into the lower running costs of the system. Of course, there will be an initial financial outlay that you will have to account for, but in the long-term, running costs are low and because the system only usually needs to be emptied once a year, the additional costs are not high.


Installing a sewage treatment plant is one of the most eco-friendly ways to dispose of household waste. The waste is treated until it is around 95% clean before being discharged, and rather than creating harmful ammonia which can affect local wildlife and habitats, it instead produces a non-polluting effluent. There are also some models that can treat the waste until it more than 95% clean.

Callum Vallance-Poole

Marketing Coordinator - Based at our UK HQ in Banbury, Oxfordshire, Callum is responsible for promoting Water Management Systems, Attenuation Tanks, Treatment Plants, Rainwater Harvesting Systems and more!

Marketing Coordinator - Based at our UK HQ in Banbury, Oxfordshire, Callum is responsible for promoting Water Management Systems, Attenuation Tanks, Treatment Plants, Rainwater Harvesting Systems and more!