How does a sewage treatment plant work?

Posted by Callum Vallance-Poole, on May 23, 2022.

Graf UK One2clean sewage treatment plant

Around 5% of UK homes are too remote to connect to the main sewer system and instead rely on septic tanks and sewage treatment plants to dispose of their household waste. These systems are essential for off-grid houses, farms and businesses, and here we explain how a sewage treatment plant works, what size you may need, legal requirements and more.

What is a sewage treatment plant?

A sewage treatment plant is a system attached to a property that is used to dispose of grey water (baths, sinks, showers, dishwashers etc.) and blackwater (toilets). It  features a series of chambers and air pumps that are used to create bacteria to naturally break down wastage, before being safely discharged into the local environment. 

In domestic situations, a sewage treatment plant is primarily used by houses in rural or coastal areas that are too isolated to be connected to a main sewer network. It bears some resemblance to a septic tank, although there are some key differences in the way that they work.

How does a sewage treatment plant work?

The idea of a sewage treatment plant appears complicated from the outside, although the way it works is relatively simple.

A sewage treatment plant works in a similar way to a septic tank. Wastewater is sent from the connected property (which is referred to as influent). This is the first treatment chamber that stores the water until it settles, with solids settling on the bottom of the tank and oil, grease and scum floating to the top. The separation process plays an important role in ensuring the wastewater can move into the secondary chamber so it can be treated efficiently.

Once the waste arrives in the second chamber, this is where the treatment process differs from a septic tank. The chamber features an air pump that circulates compressed air around the chamber, encouraging the growth of aerobic bacteria. This is intended to break down any contaminants that may be present in the water, which serves to clean it.

In the last settlement tank, any solids that may be remaining sink to the bottom before the effluent is discharged. At this point, the wastewater has been cleaned well enough so it will not cause any harm to the surrounding environment. Unlike a septic tank that requires the wastewater to be discharged into a drainage field, with a sewage treatment plant it can be sent to a local watercourse, ditch, or land drainage system.

What is wastewater?

Wastewater is the flow of what is referred to as foul water from the home that is managed by either a sewage network or another disposal system. This flow includes water from sinks, showers, dishwashers, washing machines and similar.

While wastewater is mostly comprised of water – averaging around 99.94% water by weight – as little as 0.06% is actual waste. The waste material is either suspended or dissolved in the water and comes in a variety of forms. Apart from human waste, wastewater can also contain food particles, oil, dirt, grease, proteins and organic materials (such as sugar) and inorganic materials (such as personal care products, salts, cleaning chemicals and pharmaceuticals).

What size sewage treatment plant do I need?

The size of sewage treatment plant will depend on the number of bedrooms in your property and the amount of people who live there. More people living in the property will create a larger amount of wastage, so this needs to be accounted for to ensure you get the right size.

That also means that the size is based on the potential occupancy of the property and not the number of people currently living there. While it may add a little more to the cost, it is better to over-estimate the size of the system rather than going too small, because if the system is overloaded it can break down and cause issues for the local environment.

For example, if you live in a 3-bedroom property, you should size the treatment plant for five people. If you are unsure about what size system to buy, this is something that the system provider should be able to advise you on before installation.

What are the legal requirements?

In January 2017, the Environmental Agency introduced new laws regarding sewage treatment plants. It stated that if you are discharging to a surface water, such as a stream or rive, you must use a small sewage treatment plant.

If you already own a septic tank that discharges to water, you will need to either change to a sewage treatment plant or discharge to a drainage field, as discharging to water using this type of system has been outlawed since the start of 2020.

What are the benefits of a sewage treatment plant?

If you are thinking of installing a sewage treatment plant, you should consider the following benefits:

Remove potential diseases

Harmful organisms and disease-causing bacteria are eliminated by a sewage treatment system. It ensures contaminants are filtered out before the wastewater leaves the tank and is discharged into the local environment. This means water sources, plants and farm animals are protected from diseases due to the cleaning process involved.

Minimal odour emissions

Modern sewage treatment systems release minimal odours, especially in comparison to older models. The emission of strong odours is something that used to put people off investing in these types of systems, but modern technology ensures this is no longer a concern, providing the system is well maintained.

Low maintenance

Modern systems last much longer than older models and do not require anywhere near as much maintenance. Depending on usage levels, you may need to de-sludge the tank every 2-3 years and carry out maintenance inspections at these intervals too. You should be able to carry out your inspections by checking for some tell-tale signs, although the de-sludging may be best carried out by a professional firm.

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!