What are the different types of sewage treatment plants available and the benefits of each system

Posted by Callum Vallance-Poole, on January 4, 2021.

Graf UK One2clean sewage treatment plant

If you are looking to build a property that cannot be connected to the main sewer network, then you will need to install some form of sewage treatment system to process the wastewater into a form that can be safely drained into your local water course or can be discharged into a soakaway The more common approach to this is to install a sewage treatment plant.

A sewage treatment plant is an off-mains drainage system that treats waste coming from a property, to a point where it can safely re-enter the water cycle or groundwater reserves without causing damage to the local environment or eco-system.

But as you will see when doing your research there are many different types of treatment plants available that work in different ways. Here is a list of the different types of systems available and the key benefits of each type.

Activate Sludge Process (ASP)

Activated sludge process is one of the most common methods for treating waste in the UK as this solution is not only used in domestic systems but the concept is used in large sewage treatment plants. These systems have two mains parts to them, a biozone chamber and a settlement chamber.

Once the waste from a property enters the tank it enters the ‘biozone’. Within this zone an air diffuser keeps the bacteria alive by providing them with oxygen. The bacteria that live inside the ‘biozone’ help to break down the solids/sludge inside the system.

Once the effluent has been treated it enters the settlement zone, the suspended solids inside the effluent will settle to the bottom of the tank before re-entering the ‘biozone’. The treated effluent towards the top of settlement zone is then discharged out of the tank.

These systems are very much a low-cost solution and have no mechanical or electrical parts inside the system making the system easy to maintain and the cost to maintain these systems are very low. However, the electric consumption for these systems can be higher than other systems on the market due to the air diffuser working most of the time and the installation costs of the system can be high compared to other systems as they are greater in size meaning you will need to excavate more.

Fixed bed reactor

A fixed bed reactor system combines both media and aeration to treat waste coming from a property. Unlike most plants these systems have three chambers; a primary settlement chamber, a secondary treatment chamber and a final settlement chamber.

Firstly, waste flows from a dwelling into the tank, into the primary settlement chamber, where the solids sink towards the bottom of the chamber. Wastewater then flows in the secondary chamber where it passes through the media in the chamber. This media houses that the bacteria in the tank help to clean the water, and the bacteria has a constant feed of oxygen via an air diffuser at the bottom of the chamber. The treated water then flows into the final settlement tank where any suspended solids sink to the bottom of the chamber and the cleaned effluent is discharged out of the tank. Finally, any solids or sludge at the bottom of the third chamber is transferred back to the primary chamber via a sludge return.

These systems are easy to install and only need power going to the compressor that powers the diffuser at the bottom of the secondary chamber, however this compressor is running all the time which in turn uses power from the property 24 hours a day.

Non-electric filter

Non-electric treatment plants as you could guess in the name don’t require any power to the unit in order to operate as everything is done via gravity. Again, with this system you have two chambers, the first is a primary settlement chamber and the second is the treatment chamber.

When the waste comes into the tank it flows in the settlement chamber in which the solids sink towards to bottom and the clearer water rises to the top, similar to a septic tank. The effluent then flows into the treatment chamber passing through media as it does so. The natural bacteria in the tank live on the media and help to clean the wastewater as it flows through it.

The best thing about this system is obviously that no power is required which means the running are relatively low compared to other systems on the market. However, as there is no power in the system the outlet is towards to bottom of the tank, which means the invert level of the outlet is incredibly low. Which means you either need a pumped outlet which would require power or you would need to install a soakaway deep into the ground which can be costly and if you live in an area with high groundwater then this is not an option.

Rotating Disc System / Rotating Biological Contractor (RBC)

Rotating discs systems are the most recognisable treatment plant in the UK and most homes that require an off-mains system will have one of these installed. These systems have what’s called a ‘biodisc’ inside the tank which pretty much one big media disc that the bacteria live on, similar to an non-electric system.

As the waste from a property enters the tank it will flow into a primary settlement, where similar to the above system, the solids inside the waste settle to the bottom of the chamber.

The wastewater then flows into the what’s known as the ‘biodisc’ where the bacteria living on it help the clean the effluent. The suspended solids are then returned to the primary settlement chamber and the effluent flows to another ‘biodisc’ for another round of treatment. After this second round of treatment the effluent is then discharged out of the tank.

This system has been around for many years in the UK and is preferred solution to many installers and builders. However, what is sometimes overlooked by the installers and builders in the potential high maintenance costs that fall onto the homeowner. There are a lot of mechanical and electrical parts inside these tanks that have a tendency to go wrong. The costs of purchasing spare parts and having someone replace the parts can be very expensive and the costs fall onto the homeowner.

Sequence Batch Reactor (SBR)

SBR sewage treatment plants are renowned for their high-quality effluent level after treatment using air diffusers to feed the natural bacteria in the tank with the oxygen they need to survive. Again, these systems have primary settlement chamber and a secondary treatment chamber and work in a similar way to an ASP system. 

Once the wastewater enters the tank it flows into the primary chamber where the solids settle to the bottom of the tank. The wastewater in then moved to the secondary chamber to be treated, depending on the type of system you install this is usually done via an airlift.

The wastewater is then aerated with air diffusers, which supply oxygen to the bacteria that help clean the water and break down the solids. After the initial aeration phase, the next step is a resting phase in which the air diffusers stop, and the solids or sludge start to sink to the bottom of the tank.

Once the resting phase has finished a clear water pool forms at the top of the water level and this clear water is then discharged out of the tank and depending on the type of system you install the sludge at the bottom of the secondary treatment chamber is returned to the primary settlement chamber via another airlift.

The operation of this system in controlled by a control panel and compressor which come with the systems and most of the controls panels are pre-set by the manufacturers making these types of systems a plug and play solution. However, the control panel and compressor will need to be situated near the tank, it is worth checking with the manufacturer how far this distance can be as you may require an external cabinet to house the controls and they can’t be fitted close enough to the tank in a garage or utility room.

Even though these systems tend to cost more than other system on the market, SBR treatment plants have a very high clean performance, there are no moving or electrical parts inside the tank itself and they have very low maintenance costs.

Submerged Aerated Filter

A submerged aerated filter system works in a similar way to a fixed bed reactor as it uses both media and aeration to treat wastewater and has three chambers. These systems comprise of; a primary settlement chamber, a secondary treatment chamber and a third settlement chamber.

Like every treatment plant above waste, from a property, enters the primary settlement chamber where the solids inside the waste settle at the bottom of the chamber. Then water flows from this chamber to the secondary chamber or ‘biozone’ and this chamber combines both media and aeration.

However, unlike a fixed bed reactor the media inside this chamber is loose and is floating in the water. Again, this media houses all the bacteria that help to clean the water and the bacteria again is fed with oxygen via an air diffuser.

Once the water has been treated in the secondary chamber it flows into the third settlement chamber and any suspended solids in the waste settles at the bottom of the tank. Again, similar to a fixed bed reactor system, any solids or sludge at the bottom of the chamber is transferred back to the primary chamber via a sludge return.

These systems have similar benefits and drawbacks to the fixed bed reactor as even though they don’t have any mechanical or electrical parts inside tank itself, they still need a constant supply of power to the compressor which is running constantly.

There are many different types of sewage treatment plants available in the UK and they all have different benefits compared to each other. But when it comes to installing one for your property the question you should always ask yourself is which systems suits my project the most. If you have any questions about any of these systems please call us on 01608 661500 or email us at info@grafuk.co.uk and we are more than happy to answer any questions that you may have.

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!