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Ynys Môn

Polluted by sewage 3,829 times in 2025 lasting 36,236 hours - That's 4.19 years!

Monitoring was disabled in some locations, so could be more

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

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Live data for Ynys Môn in 2026

So far this year (as of 18 May 2026, 12:00) there has been 10,466 hours of sewage overflows, from 95 CSO Combined Sewage Overflow - a device that, when operating, pours combined rainwater & sewage into a waterway s - that's 1.21 years!

Totals for Ynys Môn in 2025

82 Sites polluted by Dwr Cymru Welsh Water
3,829 sewage dumps
36,236 hours duration
That's 4.19 years. On average 10.5 times a day

Change Over Time

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Pollution Summary 2025

Here are all the reported sewage overflow totals for Ynys Môn in 2025. Sometimes sewage can be being dumped for months at a time.

Reporting refers to how much of the time the monitoring was active. In this constituency 2 of the total 93 CSO Combined Sewage Overflow - a device that, when operating, pours combined rainwater & sewage into a waterway locations had reporting enabled for less than half the year. Of these 2 had zero reporting.

Company Waterway Site Sewage Dumps Duration (Hours) Reporting %
Dwr Cymru Welsh Water Menai Strait BRYNSCIENCYN STW 184 2,785 99.9
Dwr Cymru Welsh Water Unnamed Ditch LLANGAFFO CAE BERLLAN SPS 121 2,332 99.6
Dwr Cymru Welsh Water Coastal Waters - Aberffraw Bay ABERFFRAW WASTE WATER TREATMENT WKS 107 2,217 99.7
Dwr Cymru Welsh Water Menai Strait LLANDEGFAN (MENAI) PUMPING STATION 85 1,707 93.7
Dwr Cymru Welsh Water Braint Gaerwen Station SPS 74 1,483 99.5
Dwr Cymru Welsh Water Unnamed Draining To New Harbour Porth y Felin Sewage Pumping Station 87 1,451 99.8
Dwr Cymru Welsh Water Un-Named Watercourse VALLEY STORM PS (GRAVITY DISCHARGE) 82 1,340 99.9
Dwr Cymru Welsh Water Cadnant PONT LLANDEGFAN SEWAGE PUMPING STAT 72 1,252 94
Dwr Cymru Welsh Water River Goch Craig Y Don SPS 75 1,218 99.8
Dwr Cymru Welsh Water Bull Bay Bull Bay P.S. 82 1,142 99.7
Dwr Cymru Welsh Water Unnamed Ditch TALWRN SPS 63 1,133 99.9
Dwr Cymru Welsh Water Nant Nodwydd PENTRAETH STW 86 971 99.9
Dwr Cymru Welsh Water Cefni Reservoir Bypass Channel BODFFORDD SEWAGE TREATMENT WORKS 72 957 98.8
Dwr Cymru Welsh Water Estuarial Waters Of River Alaw Gorad PS Gorad NR Valley Anglesey 60 869 99.6
Dwr Cymru Welsh Water River Braint LLANGAFFO STW 104 798 99.9
Dwr Cymru Welsh Water Irish Sea RHOSNEIGR SPS RHOSNEIGR 44 696 99.9
Dwr Cymru Welsh Water Unnamed Watercourse To River Goch Rhosybol Gorslwyd PS 96 692 99.9
Dwr Cymru Welsh Water Coastal Waters 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 Braint LLANFAIR PG SPS 58 618 99.9
Dwr Cymru Welsh Water Ditch Hermon Malltraeth PS 49 609 93.4
Dwr Cymru Welsh Water Menai Strait LLANFAIR PG WWTW 107 590 99.9
Dwr Cymru Welsh Water Coastal Waters RHOSNEIGR STW (SETTLED STORM/EMERGE 39 587 99.9
Dwr Cymru Welsh Water Estuary Of River Braint NEWBOROUGH STW 82 559 99.9
Dwr Cymru Welsh Water Un-Named Watercourse Valley Storm PS (Pumped Discharge) 76 556 99.9
Dwr Cymru Welsh Water Llyn Maelog BRYN DU SEWAGE PUMPING STATION 51 546 99.7
Dwr Cymru Welsh Water River Briant LLANDDANIEL PONT Y CRUG SPS 57 528 99.9
Dwr Cymru Welsh Water River Cadnant Gorsllwyd Sewage Pumping Station 53 489 99.9
Dwr Cymru Welsh Water Esturial Waters Of Inland Sea TREARDDUR BAY PS (PUMPED DISCHARGE) 64 370 99.9
Dwr Cymru Welsh Water Menai Strait PS NO 1 (FAELOG CAUSEWAY) (EMERGENC 60 338 99.9
Dwr Cymru Welsh Water Menai Strait PS NO 1 (FAELOG CAUSEWAY) (EMERGENC 60 338 99.9
Dwr Cymru Welsh Water Menai Strait LLANFAES WWTW 53 332 99
Dwr Cymru Welsh Water Old Harbour Holyhead Wastewater Treatment Works SPS 87 330 99.9
Dwr Cymru Welsh Water New Harbour, Holyhead WATERSIDE PUMPING STATION 84 303 99.9
Dwr Cymru Welsh Water River Goch Storm Tank Army Cadet Centre CSO 50 296 99.9
Dwr Cymru Welsh Water River Cefni GAERWEN WWTW FINAL EFFLUENT 28 294 97.1
Dwr Cymru Welsh Water Un-Named Watercourse HOLYHEAD WWTW FINAL EFFLUENT PS 80 289 99.9
Dwr Cymru Welsh Water Un-Named Watercourse TREARDDUR BAY PS (GRAVITY) 42 254 99.2
Dwr Cymru Welsh Water Outer Harbour, Holyhead Morawelon SPS 41 230 99.8
Dwr Cymru Welsh Water River Alaw Bodedern Sewage Pumping Station 25 225 99.9
Dwr Cymru Welsh Water Coastal Waters Off Benllech, A BENLLECH WWTW (STW) LSO BENLLECH 44 210 99.1
Dwr Cymru Welsh Water Cefni - Cefni Reservoir West Lon Sardis CSO (BODFFORDD OPP CAPEL SARDIS) 75 199 98.9
Dwr Cymru Welsh Water Coastal Waters Of Irish Sea CEMAES PS/BALANCING TANK CEMAES BAY 43 198 99.9
Dwr Cymru Welsh Water New Harbour Coastguard CSO 83 193 99.9
Dwr Cymru Welsh Water Menai Strait PS NO 3 (SUSPENSION BRIDGE) (EMERGE 60 185 99.8
Dwr Cymru Welsh Water Menai Strait PS NO 3 (SUSPENSION BRIDGE) (EMERGE 60 185 99.8
Dwr Cymru Welsh Water River Meddanen LLANFECHELL PUMPING STATION 19 179 99.3
Dwr Cymru Welsh Water River Meddanen Llanfechell Wastewater Treatment Works 22 175 79.9
Dwr Cymru Welsh Water Menai Strait MOUNT FIELD P.S. BEAUMARIS , 34 163 99.9
Dwr Cymru Welsh Water Menai Strait LLEINIOG PS 52 145 99.9
Dwr Cymru Welsh Water River Cefni Llangefni Main Pumping Station 22 138 99.9
Dwr Cymru Welsh Water Menai Strait BRYNSCIENCYN PS , , 13 103 99.9
Dwr Cymru Welsh Water Cefni Estuary MALLTRAETH WWTW TERMINAL 38 97 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 Alan Catchment RHOSYBOL PLAS Y FFRWD SPS 29 88 100
Dwr Cymru Welsh Water Adjacent Ditch Llangefni Rhostrehwfa CSO 18 69 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 Nant Cwyrtai GWALCHMAI STW 33 54 99.5
Dwr Cymru Welsh Water Coastal Waters - Amlwch Harbour Amlwch Port CSO 22 51 99.5
Dwr Cymru Welsh Water Cefni MALLTRAETH VILLAGE CSO 22 47 92.2
Dwr Cymru Welsh Water Eastuary - River Cefni MALLTRAETH VILLAGE PUMPING STATION 22 47 92.2
Dwr Cymru Welsh Water Cefni CSO Llangefni MH Front Ynys Mon Of 46 45 98.3
Dwr Cymru Welsh Water Y Brenin PONT Y BRENIN PS 13 43 99.9
Dwr Cymru Welsh Water New Harbour Hibernia Row Pumping Station 11 40 99.9
Dwr Cymru Welsh Water Via A Drainage To River Clai CSO at Llangefni MH Rear of Burgess 19 29 99.8
Dwr Cymru Welsh Water Unnamed Stream VALLEY DYFFRYN AND PENDYFFRYN 7 29 99.9
Dwr Cymru Welsh Water River Cefni LLANGEFNI STW LLANGEFNI ANGLESEY 38 24 99.9
Dwr Cymru Welsh Water Unnamed Ditch NEWBOROUGH MILLBANK SPS 10 10 99.9
Dwr Cymru Welsh Water Coastal Waters WENDON CAR PARK PS BENLLECH , , , 3 6 85.8
Dwr Cymru Welsh Water Menai Strait GLYN GARTH PS LLANDEGFAN 7 4 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 PENTRAETH TAN Y GRAIG SPS 2 1 99.9
Dwr Cymru Welsh Water River Ffraw ABERFFRAW SPS GLAN Y MOR ABERFFRAW 2 0 99.9
Dwr Cymru Welsh Water Braint DWYRAN BY SCHOOL - SSO 2 0 99.6
Dwr Cymru Welsh Water Beddmanarch Bay PENRHOS BEACH PUMPING STATION 1 0 100
Dwr Cymru Welsh Water Coastal Waters CYNLAI PS LLANGOED ANGLESEY 1 0 99.9
Dwr Cymru Welsh Water River Cadnant LLANSADWRN PS 1 0 99.9
Dwr Cymru Welsh Water Un-Named Of Alaw BODEDERN STW 0 0 0
Dwr Cymru Welsh Water Unnamed Stream GWALCHMAI ISAF PS 0 0 99.9
Dwr Cymru Welsh Water River Goch LLANERCHYMEDD STW 0 0 99.9
Dwr Cymru Welsh Water Cadnant LLANSADWRN PS 0 0 0
Dwr Cymru Welsh Water Menai Strait LLYN Y FELIN PS (EMERGENCY) 0 0 95.5
Dwr Cymru Welsh Water Menai Strait LLYN Y FELIN PS (EMERGENCY) , , 0 0 95.5
Dwr Cymru Welsh Water River Alaw Newlands Beach PS Valley Angle 0 0 99.9
Dwr Cymru Welsh Water Menai Strait PS NO 2 (PORTH WRACH) (EMERGENCY) 0 0 99.8
Dwr Cymru Welsh Water Menai Strait PS NO 2 (PORTH WRACH) (EMERGENCY) 0 0 99.8
Dwr Cymru Welsh Water River Goch Settled Storm Tank at Llanerchymedd STW 0 0 100
Dwr Cymru Welsh Water Adj Ditch TREARDDUR BAY HAUL A MOR SPS 0 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

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.

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Analysing sewage pollution by the Water Industry

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