How sewage spills are counted
An annual spill count uses a defined counting method. It is not a simple count of each start signal shown in a live feed.
What to do
Compare spill counts only for the same reporting year. Read duration and monitor coverage beside the count. Do not add live start alerts and call the result an annual spill count.
The 12/24-hour method
The reporting method starts with a 12-hour block when a discharge begins. Discharges in that first block count as one spill. Further 24-hour blocks with discharges add to the count. After a full 24-hour block with no discharge, the sequence resets. Use the official guidance for the full method.
Why the live list may look different
A monitor can start and stop more than once within a reporting block. The annual return also goes through checks. Adding live start signals together will therefore not necessarily reproduce the annual spill count.
Compare like with like
Check the year, the number of outlets included and monitor coverage before comparing places. A sum of outlet hours can include events happening at the same time. It is not the number of hours when a whole river was polluted.
When the source changes
Each annual record links to the official data. Corrections can arrive after the first release, so check that source if a figure differs from an older news report. We review corrected returns before updating the site.