Dwr Cymru Welsh Water

was responsible for at least 113,538 sewage overflows in 2023
On average 311.1 times a day

These lasted for 971,403 hours (That's 112.43 years!) - in 1,982 different locations across their service area

2023 - By Water Company

How does each water company compare? These are the totals by company for each. The totals are really high, counting things in hours doesn't make it very easy to think about. Here we show how many years of continuous sewage dumps happened in a single year, by water company.

2023 - Asset Spill Duration Comparison

It isn't like there are just a few outliers... the problem exists across the entire estate of water assets. Here we show a 'cumulative frequency diagram' - where the number of CSOs that dumped sewage for a time, or longer, is shown. You can see that many CSOs dumped sewage into a river for months and months.

Dwr Cymru Welsh Water is highlighted in the plot below

2023 - Worst Locations Dwr Cymru Welsh Water

1. CSO at Llanbrynmair STW Llanbrynmair

Montgomeryshire polluted for 5,811 hours, into River Laen

291 incidents

That's 8.07 months.

5.6 times a week, on average

2. Pwllmeyric SPS (CSO)

Monmouth polluted for 5,755 hours, into Mounton Brook

262 incidents

That's 7.99 months.

5 times a week, on average


Ogmore polluted for 5,344 hours, into River Garw

271 incidents

That's 7.42 months.

5.2 times a week, on average


Ceredigion polluted for 5,012 hours, into Nant Cledlyn

353 incidents

That's 6.96 months.

6.8 times a week, on average

5. Abererch WwTW Storm Overflow

Dwyfor Meirionnydd polluted for 4,697 hours, into River Erch

238 incidents

That's 6.52 months.

4.6 times a week, on average

6. Capel Curig Sd.Scheme CSO

Aberconwy polluted for 4,677 hours, into River Llugwy

237 incidents

That's 6.5 months.

4.6 times a week, on average

Water Companies

Data Sources & Accuracy

EDM (Event Duration Monitoring) data and Consented Discharges to Controlled Waters with Conditions from Environment Agency & Data Map Wales, and Scottish Water

Experimental derived data from Thames Water API, Some data may be missing or inaccurate. This uses Environment Agency rainfall data from the real-time data API (Beta) - Rainfall data is delayed by up to two days.

This website is intended to provide an accurate representation of the Environment Agency data. The data is hard to use, and thus some errors may have been made. If you find something that is incorrect, please raise an issue at the GitHub issues page and we'll endeavour to fix it quickly.

This project is not associated, approved or endorsed by any of the companies whose logos you see here.