Flow and Spill reporting

The amount of wastewater leaving our treatment works is measured and often referred to as ‘flow’. If there’s heavy rain, the sewer network may struggle to cope with the amount of wastewater and rainwater in it. The screened wastewater, which can be around 95% rainwater, is released through storm overflows. The storm overflows act as a pressure valve to release excess water through outfalls into rivers and the sea. This protects homes and communities from flooding. Releases from storm overflows are called spills.

We publish our annual flow and spill figures here (last updated March 2023).

We want to reduce storm overflow releases, so we have set up a Storm Overflow Taskforce to look at ways to do this. This includes looking at ways to slow the flow of water entering the sewers, making better use of existing infrastructure or building bigger infrastructure to cope with the amount of water.


Our flow data

Our flow data is reported every year to the Environment Agency. The graph below shows the total amount of treated water that left our sites each year in cubic metres, divided by county.

Wastewater volumes for 2022 have seen a decrease compared to previous years, in all areas Southern Water operate. This correlates with the low annual rainfall for the South of England in 2022, with periods of drought declared, which would have resulted in less surface water. The lower wastewater figures, may also have been influenced by the Temporary Use Ban, brought in 5th August 2022 to 4th November 2022 to reduce water use, due to the drought conditions.

As the population of our region continues to grow, the volume of wastewater received by our sites increases too. As a result, our flow figures fluctuate year-on-year based on the number of people who choose to live in the South East.

Although this would suggest our flow should increase year-on-year, other factors impact our flow too. For instance, some of our sewers carry rainwater away from gutters and street gullies to our treatment works to prevent flooding. As a result, our flow is also higher during years with a wet winter.

While the impact of the pandemic – including restrictions on leaving home, travel and businesses – affected the flow patterns at several of our treatment sites during 2020, activity at these sites returned to normal in 2021. However, the rainfall pattern has been more extreme with longer drier periods, with days of extremely heavy rainfall. As a result, sites surrounded by areas of high groundwater at the start of 2021 were affected for longer than the previous year.

For more detailed daily flow information, divided by site, please download the information below (in the format of .xlsx spreadsheets).

Flow data 2022

CSV data files for 2022 are also available for each county: Kent, Sussex, Hampshire, Isle of Wight.

Flow data 2021

CSV data files for 2021 are also available for each county: Kent, Sussex, Hampshire, Isle of Wight.

Flow data 2020

CSV data files for 2020 are also available for each county: Kent, Sussex, Hampshire, Isle of Wight.

Flow data 2019

CSV data files for 2019 are also available for each county: Kent, Sussex, Hampshire, Isle of Wight.


Our spill data 

What is a spill?

In some areas, sewers are ‘combined’ meaning rainwater from gutters and street gullies is also channelled to our wastewater treatment works during a storm, to protect properties and streets from flooding.

If our wastewater treatment works receives more rainwater and wastewater than we're able to process or store, our network could back up which could cause flooding via manholes, drains and toilets. To prevent this, where we can, we store and filter the surplus through a screen before releasing it through outfalls into rivers and the sea – This is called a spill.

Water companies are permitted by the Environment Agency to spill only under strict conditions, to protect homes, protect the environment and manage storms.

Sometimes, we also have to spill if wastewater stops moving through our network – for example, due to a pump failure. Again, wastewater with nowhere else to go can flood homes and communities, so we release it to prevent this happening.


Total number of spills per county, per year

We report our spill data every year to the Environment Agency. The graph below shows the total number of spills from our wastewater treatment sites each year, divided by county.

The latest figures published demonstrate our continued progress on supplying more – and more accurate – data on storm overflow releases to regulators and the public.

Storm releases are made for one reason – to protect homes and businesses from flooding. For instance, in heavy rain storm releases from our Budds Farm wastewater treatment works (and the related storm overflows) protect more than 2000 properties including schools and hospitals.

For more detailed information about spills that occurred during each calendar year, including the location and trigger, please download the .xlsx spreadsheets or CSV data files below:

Individual spill data 2022 – Excel

Summary spill data 2022 – Excel

Summary spill data 2021 – Excel

Summary spill data 2021 – CSV (Part 1)

Summary spill data 2021 – CSV (Part 2)

Individual spill data 2021 – Excel

Individual spill data 2021 – CSV

Summary spill data 2020 – Excel

Summary spill data 2020 – CSV

Individual spill data 2020 – Excel

Individual spill data 2020 – CSV

Spill data 2017–19 – Excel

Spill data 2017–19 – CSV


The acronyms used in the data files have the following definitions:

  • CEO – Combined Emergency Overflow: A storm overflow and an emergency overflow from a wastewater pumping station that discharge via the same outlet.
  • SSO – Settled Storm Overflow: An overflow designed to discharge heavily diluted and settled wastewater via an outfall pipe directly to controlled waters, when flows to a wastewater treatment works exceed the pass forward rate capacity for the site and the storm tanks are full, due to rainfall and/or snowmelt.
  • EMO – Emergency Overflow: An overflow at a wastewater pumping station which allows spillage of foul sewage to a watercourse or other waters in an emergency, in the event of mechanical or electrical failure of the pumping station, or due to failure of the downstream rising main.


Why spills happen 

You can find more information about Storm overflows.  


See what's happening near you

Beachbuoy gives you near real-time information about releases of stormwater or wastewater at each of our region’s designated bathing waters or recreational harbours. Check the interactive map to see what's happening at your favourite spot.