Sewage Treatment

Major Sewage Treatment Works

The major sewage treatment done in the Thame catchment is managed by Thames Water. There are 31 major works in our catchment which all discharge into our river. stw


Everything that goes through these Sewage Treatment Works (STWs) is treated so that it is at a level that has been deemed as suitable by the Environment Agency, this is known as consenting. The outfalls at the works are tested on a regular basis to ensure that they meet the consented levels, this includes both in-house testing and spot checks by the Environment Agency (EA). If the levels are breached during EA checks, then the works must work quickly to make necessary improvements in order to avoid becoming a ‘failing works’.

Unfortunately, even with the limits enforced, sewage treatment still has a major impact on our rivers as STWs are allowed to discharge elevated levels of phosphate, nitrate and ammonia into the Thame; this is quite common in rural river catchments. On top of this, Thames Water is placed under increasing pressure because of the extensive development happening in areas such as Aylesbury. Although the water company must be informed of dwellings of 10 or more, there is no requirement to tell them of anything less than this. Could you imagine if a 5 bedroom house was converted in to five small apartments, each holding 2-3 people, resulting in the sewage from 15 people entering the works, rather than the expected 6 people? If just 10 people chose to do this then that results in 70 extra people, increasing the amount of sewage that Thames Water has to deal with significantly, yet they have no way of preparing for this increase. It is no surprise that over time there have been some significant breaches, resulting in major pollution incidents along the Thame.

During a storm event, the process of sewage treatment can be slightly different, you can find out about this process here.

Sewage Treatment

The process of sewage treatment, credit: Thames Water



We have great dialogue with Thames Water and we are proud of the open and honest relationship that we maintain. We are always informed of incidents and this allows us to support improvements and also help them to look for natural and effective solutions. Keep an eye on our news pages for up to date information about any incidents that occur.

Small Sewage Treatment Works

In some areas, there are much smaller sewage treatment works which are council run, these are also referred to as Housing Acts Works (HAWS). These works are also monitored and consented by the Environment Agency.

For information on HAWS in South Oxfordshire click here.

Septic Tanks

Many rural homes are not connected to a major or small sewage treatment works. When this happens, each home will have its own treatment system, known as a septic tank or a cesspit. These will discharge directly into our waterways, making it difficult to monitor them regularly. There are many regulations that work to try and prevent pollution entering our rivers from these systems. You can help with this by monitoring your local river for potential signs of pollution from septic tanks and reporting any signs to the Environment Agency.

If you have your own septic tank, then you can find advice on maintenance and regulations in our section on ‘advice for households‘. If you are planning on installing your own sewage system then you can get advice on permits and compliance here.

Surface Water

It comes to a surprise to many people, that every surface drain you see on the roads and pathways leads directly to one place, the river! This means that anything placed down a surface drain does not go through treatment; have you ever washed your car? Did the soap go down a surface drain?

Another big problem all across the UK is misconnections. Misconnections are caused when a waste water pipe from a property is connected directly to a surface drain, resulting in waste water draining directly in to the river. The image below explains this further and you can get more information on what to do if you think that your home might be misconnected here.


How your home should be connected. Misconnections occur when a foul water sewer is connected to the surface water sewer, credit: Thames Water.

There are many campaigns to get people thinking about this, and you can find a lot more information on our ‘advice for households‘.