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Area of Outstanding Natural Beauty

Ynys Mon / Anglesey

Polluted by sewage 1,990 times in 2025 - lasting 19,257 hours - That's 2.23 years!

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Contains OS Data © Crown copyright & database right 2021-2024
© Top of the Poops 2021-2024

Totals for Ynys Mon / Anglesey in 2025

48 Sites polluted by Dwr Cymru Welsh Water
1,990 sewage dumps
19,257 hours duration
That's 2.23 years. On average 5.5 times a day

How does Ynys Mon / Anglesey compare?

Ynys Mon / Anglesey was the #5 most polluted Area of Outstanding Natural Beauty in England & Wales, by duration of sewage overflows, as reported by the water companies

Why the change?

Storm overflows spill when it rains, and 2025 was drier than 2024 - 90% of average annual rainfall against 105%.

So much of any drop is weather, not fixed pipes. The live page shows rainfall and sewage side by side, day by day.

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Pollution Summary 2025

Here are all the reported sewage overflow totals for Ynys Mon / Anglesey in 2025.

How we count, and how accurate it is

A CSO Combined Sewage Overflow - a device that, when operating, pours combined rainwater & sewage into a waterway counts towards Ynys Mon / Anglesey when it sits inside the designated area, or within 200 metres of its boundary. An overflow close to two AONBs is counted in both.

We use data from the water companies, but we are seeing many anomalies that make us believe that location information is quite poor. The situation is likely far worse than reported here.

Other CSO Combined Sewage Overflow - a device that, when operating, pours combined rainwater & sewage into a waterway s just outside the 200 metre boundary are not counted, though they may still affect water quality inside the area.

Company Waterway Site Sewage Dumps Duration (Hours) Reporting %
Dwr Cymru Welsh Water Menai Strait Brynsiencyn Wastewater Treatment Works 184 2,785 99.9
Dwr Cymru Welsh Water Coastal Waters - Aberffraw Bay Aberffraw Wastewater Treatment Works 107 2,217 99.7
Dwr Cymru Welsh Water Menai Strait LLANDEGFAN (MENAI) PUMPING STATION 85 1,707 93.7
Dwr Cymru Welsh Water River Goch Craig Y Don SPS 75 1,218 99.8
Dwr Cymru Welsh Water Menai Strait Treborth Wastewater Treatment Works 88 1,168 99.9
Dwr Cymru Welsh Water Bull Bay Bull Bay P.S. 82 1,142 99.7
Dwr Cymru Welsh Water Nant Nodwydd Pentraeth Wastewater Treatment Works 86 971 99.9
Dwr Cymru Welsh Water Estuarial Waters Of River Alaw Gorad PS Gorad NR Valley Anglesey 60 869 99.6
Dwr Cymru Welsh Water Coastal Waters Settled Storm and Emergency Overflow at Llaneilian SPS 54 688 99.9
Dwr Cymru Welsh Water Estuarial Waters Of Holy Island Strait South Four Mile Bridge SPS Holyhead 52 672 99.9
Dwr Cymru Welsh Water Ditch Hermon Malltraeth PS 49 609 93.4
Dwr Cymru Welsh Water Menai Strait Llanfair PG Wastewater Treatment Works 107 590 99.9
Dwr Cymru Welsh Water Estuary Of River Braint Newborough Wastewater Treatment Works 82 559 99.9
Dwr Cymru Welsh Water Llyn Maelog Storm and Emergency Overflow at Llanfaelog (Bryn Du) SPS 51 546 99.7
Dwr Cymru Welsh Water River Cadnant Gorsllwyd Sewage Pumping Station 53 489 99.9
Dwr Cymru Welsh Water Esturial Waters Of Inland Sea Settled Storm and Emergency Overflow at Trearddur Bay SPS 64 370 99.9
Dwr Cymru Welsh Water Menai Strait PS NO 1 (FAELOG CAUSEWAY) (EMERGENCY) 60 338 99.9
Dwr Cymru Welsh Water Menai Strait PS NO 1 (FAELOG CAUSEWAY) (EMERGENC 60 338 99.9
Dwr Cymru Welsh Water Un-Named Watercourse HOLYHEAD WWTW FINAL EFFLUENT PS 80 289 99.9
Dwr Cymru Welsh Water Un-Named Watercourse CSO and EO at Trearddur Bay SPS (Gravity Overflow) 42 254 99.2
Dwr Cymru Welsh Water Menai Strait PS NO 3 (SUSPENSION BRIDGE) 60 185 99.8
Dwr Cymru Welsh Water Menai Strait PS NO 3 (SUSPENSION BRIDGE) (EMERGENCY) 60 185 99.8
Dwr Cymru Welsh Water Menai Strait LLEINIOG PS 52 145 99.9
Dwr Cymru Welsh Water Menai Strait BRYNSCIENCYN PS , , 13 103 99.9
Dwr Cymru Welsh Water Cefni Estuary Malltraeth Wastewater Treatment Works 36 95 99.9
Dwr Cymru Welsh Water Menai Strait BRYNSCIENCYN PS 12 90 99.9
Dwr Cymru Welsh Water Unnamed Ditch FRYARS BAY P.S- SCREENED EMERG 22 88 99.9
Dwr Cymru Welsh Water Menai Strait MOUNT FIELD P.S. BEAUMARIS , 17 81 99.9
Dwr Cymru Welsh Water Menai Strait CSO at Mount Field SPS 17 81 99.9
Dwr Cymru Welsh Water Coastal Waters MOELFRE PS MOELFRE 20 69 99.9
Dwr Cymru Welsh Water Menai Strait RHIANFA PS LLANDEGFAN 22 61 99.9
Dwr Cymru Welsh Water Menai Strait WEST END PS BEAUMARIS ANGLESEY 16 55 99.9
Dwr Cymru Welsh Water Eastuary - River Cefni MALLTRAETH VILLAGE PUMPING STATION 22 47 92.2
Dwr Cymru Welsh Water Cefni MALLTRAETH VILLAGE CSO 22 47 92.2
Dwr Cymru Welsh Water Y Brenin PONT Y BRENIN PS 13 43 99.9
Dwr Cymru Welsh Water Menai Strait TREBORTH STW (FINAL) BANGOR 18 19 99.8
Dwr Cymru Welsh Water Unnamed Ditch NEWBOROUGH MILLBANK SPS 10 10 99.9
Dwr Cymru Welsh Water Coastal Waters Benllech Old Tidal Tank CSO 3 6 85.8
Dwr Cymru Welsh Water Menai Strait GLYN GARTH PS LLANDEGFAN 7 4 99.9
Dwr Cymru Welsh Water Cefni Estuary Malltraeth WwTW Terminal SPS 2 2 99.9
Dwr Cymru Welsh Water Braint DWYRAN RHYDWYN - SSO 9 2 99.9
Dwr Cymru Welsh Water Unnamed Watercourse LLANFAES PS (EMERGENCY) , , 1 2 99.7
Dwr Cymru Welsh Water Of Braint LLANFAIRPWLL SSO NO. 2 7 2 96.1
Dwr Cymru Welsh Water Nodwydd CSO and EO at Tan Y Graig SPS 2 1 99.9
Dwr Cymru Welsh Water Beddmanarch Bay PENRHOS BEACH PUMPING STATION 1 0 100
Dwr Cymru Welsh Water River Ffraw Aberffraw SPS 2 0 99.9
Dwr Cymru Welsh Water Braint DWYRAN BY SCHOOL - SSO 2 0 99.6
Dwr Cymru Welsh Water Coastal Waters CYNLAI PS LLANGOED ANGLESEY 1 0 99.9

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. Live sewage data from Stream Water API

Annual figures up to and including 2025 come from official water company returns. The 2026 figure is our own estimate, derived continuously from the companies' live data feeds.

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.