Understanding the data

•  How is data on sewage spills collected? 
•  Does this data include all the untreated sewage released into rivers? 
•  What can be done to make sure all spills are recorded?  
•  How are spills counted? 
•  Where does this data come from? 

The data on our Sewage Map comes from Event Duration Monitoring (EDM) equipment, which water companies have installed at sewer storm overflow points in the network. The equipment and the way it is installed varies considerably as it has to be retrofitted to sewage infrastructure which can be very constrained. EDMs are often water level sensors which pick up when a critical overflow point is reached on a weir. These sensors detect releases at designated points in the sewer network which allow untreated sewage to overflow directly into the environment. The sensors send a signal when untreated sewage and storm water starts to overflow, and again when it stops. These start and finish times are used to calculate the duration of the overflow event. EDMs only detect discharges, they do not measure the volume of a discharge or any resulting impacts.

Note, EDM sensors can be triggered in error, e.g. by a spider web or other debris blocking or trigger the sensor, rather than by a genuine rise in the level of sewage and storm water in the system.

Water companies undertake validation on these raw discharge event signals, such as checking upstream and downstream sensors to see if they correlate, and also checking rainfall and groundwater levels, but there is very little information available on how this validation is undertaken, so we would like to see much more transparency and audit trails available.

The near real-time data that we share comes direct from the EDMs, and has not been validated. The near real-time map just shows the current activation status of the EDM, not whether or not a validated discharge is happening. However, the annual EDM returns data only includes validated discharges. Once discharges are validated, they are categorised using the 12/24 spill counting method, which is explained in more detail below.

No, not all storm or emergency sewer overflows have EDM systems installed, so the data that we do have does not necessarily represent all the untreated sewage that has been discharged for the following reasons:

1. In England and Wales as of December 2023, all sewer storm overflows with sewage discharge consents (a type of formal permission to release sewage waste into a waterway) now have EDM systems installed and operating. However, not all of these EDM systems are operational for 100% of the year, with many monitors experiencing operational downtime due to sensor or maintenance issues. In Scotland and Northern Ireland the programme of EDM system installation is still underway, so it will be some years before these are on every storm overflow.
2. There are other overflows which can still spill, but for which data is not collected or reported. For example, emergency overflows can also spill (for example if a pump fails) but may not be monitored, and Environment Agency don't include these in the annual EDM summary dataset. Dwr Cymru/Welsh Water already publish their EDM data for emergency overflows which we welcome, and we will be working out how to include this in our map soon.
3. The data from sewer overflows which are monitored isn't always accurate due to unreliable, or poorly placed equipment.
4. There are thousands of private sewage facilities such as septic tanks or small package treatment works which are not monitored, and if not maintained or upgraded to keep pace with new development these can spill untreated sewage into our waterways.

1. We would like to see the pace of EDM installation escalated in Northern Ireland and Scotland so that river users have the same access to information as other parts of the UK.
2. We will be adding emergency overflows summary information to our map later this year, along with the live data feeds from Northern Ireland and Scotland once these are available.
3. If you spot an overflow spilling sewage (look for visible sewage 'rag' or strong sewage odour) which isn't on our map, or if you see any overflow spilling in dry weather then you can report this to the relevant agency: England - Environment Agency 0800 80 70 60, Wales- Natural Resources Wales 0300 065 3000, Scotland - Scottish Environmental Protection Agency 0800 807 060, Northern Ireland - Northern Ireland Environment Agency 0800 80 70 60.
4. To improve EDM data quality, water companies need to continue investing, undertaking regular data checks, audits and maintenance on the monitoring equipment fitted on sewer overflows to improve the reliability and to limit operational downtime.
5. Home owners and small businesses should ensure that private sewage treatment facilities are maintained and upgraded and that domestic appliances are connected to the correct drain to ensure they are not polluting waterways or groundwater.

Different types of sensors and equipment can be used to monitor storm overflows, so all raw overflow event data must be analysed using a spill counting tool to ensure that data handling is consistent between sites and organisations. Using this standardised approach allows spill performance to be compared between locations and against environmental regulations.

This tool transposes the data into the 12/24 spill counting method, as defined by  Defra :

• Spill counting starts when the first discharge occurs. A discharge occurs when the sewer level exceeds the threshold level.
• Any discharge(s) in the first 12-hour block is counted as 1 spill.
• Any discharge(s) in the next and subsequent 24-hour blocks are each counted as 1 additional spill per block.
• This counting continues until there is a 24-hour block with no discharge.
• For the next discharge after the 24-hour block with no discharge, the 12 hour and 24-hour block spill counting sequence begins again.

The chart above illustrates the application of 12/24 spill counting to real level data, with the start points of 12- and 24-hour blocks being determined by when recorded level exceeds an overflow threshold. The data in chart below shows a total of nine 12/24 spills counted.

England

Treated sewage discharges and sewer storm and emergency overflow locations were extracted from the Consented Discharges to Controlled Waters with Conditions database (Oct 2024), available under   Environment Agency Conditional License   via   data.gov.uk   (accessed 24/03/2025). 

Event Duration Monitoring - Storm Overflows - 2024 data were downloaded from   environment.data.gov.uk  (accessed 27/03/2025), available under Open Government License. Some locations in the data provided by the Environment Agency may be erroneous and therefore the point may appear in the wrong place on our map. 

The EDM returns 'Activity Reference' field does not always reliably distinguish between emergency overflows and storm overflows (especially for Southern Water who use the ambiguous category of ‘emergency/storm’) therefore we have included these alongside sewer storm overflows, but not EDM returns with the ‘Activity Reference’ of ‘Emergency Overflow’.

The near-real time data for England comes directly from the water companies, contributing to Water UK’s  National Storm Overflow Hub (NSOH) . You can access all data and find further information on the data on the NSOH. Details on the layers that are used in the near real-time map can be found in the  Web Map details . Water companies are required to publish updates within an hour of the discharge occurring, and layers are updated every 10-60 minutes depending on the water company. All data published by the individual water company, licensed under  CC BY 4.0 .

Wales  

Sewer Storm Overflow EDM Annual Returns Data for Wales were downloaded from the  Dŵr Cymru/Welsh Water  and  Hafren Dyfrdwy  websites. 

Treated sewage discharges were extracted from the Consented Discharges to Controlled Waters with Conditions database available under   Natural Resources Wales conditional license   via   datamap.gov.wales  (accessed 24/03/2025).