Storm overflow register · Wessex Water

Wincanton sewage works: storm overflow spills 2025

Wincanton sewage treatment works, run by Wessex Water, has 1 monitored storm overflow discharging to River Cale. In 2025 it recorded 32 spills totalling 251 hours, against 62 spills and 487 hours in 2024. By hours spilling, it ranks 143 of 242 Wessex Water works, where 1 spilled longest.

Works size: 7,940 population equivalent (WINCANTON STW, UWWTD 2018). Used for the spill volume estimate.

Every spill in 2025

Each blue mark is one spill, placed by when it started and sized by how long it lasted. Hover over a mark for the date and duration.

JanFebMarAprMayJunJulAugSepOctNovDecWincanton Water…11 spills, 4.4 h, 1 Jan to 1 Jan49 spills, 46.9 h, 5 Jan to 7 Jan21 spills, 6.1 h, 23 Jan to 25 Jan14 spills, 61.6 h, 26 Jan to 29 Jan63 spills, 28.5 h, 23 Feb to 25 Feb55 spills, 11 h, 26 Feb to 26 Feb7 spills, 2.4 h, 21 May to 21 May4 spills, 0.5 h, 12 Jun to 12 Jun6 spills, 2.4 h, 31 Oct to 1 Nov19 spills, 2 h, 10 Nov to 10 Nov30 spills, 6 h, 12 Nov to 12 Nov33 spills, 19.9 h, 14 Nov to 15 Nov14 spills, 5.3 h, 1 Dec to 2 Dec43 spills, 7.3 h, 4 Dec to 4 Dec30 spills, 11.4 h, 5 Dec to 6 Dec28 spills, 5.8 h, 9 Dec to 9 Dec20 spills, 9.4 h, 16 Dec to 16 Dec36 spills, 20 h, 18 Dec to 19 Dec

Estimated spill volume by month, 2025

Hours spilling each month × the estimated spill rate for a works of this size. The bars are central estimates and the whiskers show the low-high range. Method.

020k40k60kJan 2025: 119 h spilling ≈ 16,190 m³ (range 6,449-40,645 m³)JanFeb 2025: 40 h spilling ≈ 5,374 m³ (range 2,141-13,491 m³)FebMar 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)MarApr 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)AprMay 2025: 2.4 h spilling ≈ 327 m³ (range 130-820 m³)MayJun 2025: 0.5 h spilling ≈ 68 m³ (range 27-171 m³)JunJul 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)JulAug 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)AugSep 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)SepOct 2025: 1.6 h spilling ≈ 218 m³ (range 87-546 m³)OctNov 2025: 29 h spilling ≈ 3,918 m³ (range 1,561-9,837 m³)NovDec 2025: 59 h spilling ≈ 8,068 m³ (range 3,213-20,254 m³)Dec
Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDurationEstimated volume
26 Jan 2025, 11:46Wincanton Water Recycling Centre2.4 days3,130 - 19,730 m³
5 Jan 2025, 07:36Wincanton Water Recycling Centre1.2 days1,577 - 9,939 m³
23 Feb 2025, 22:28Wincanton Water Recycling Centre14.6 h791 - 4,987 m³
18 Dec 2025, 11:24Wincanton Water Recycling Centre12.3 h667 - 4,201 m³
14 Nov 2025, 14:21Wincanton Water Recycling Centre10.5 h571 - 3,598 m³

Estimated 2025 spill volume and pollution

Estimate, not a measurement. Monitors record how long an overflow spills, not how much. This calculator back-calculates a spill rate from the size of the works, using the Environment Agency's permit formulae, and multiplies it by published storm-sewage concentrations. How it works.

Volume, central estimate34,161 m³range 13,607 - 85,764 m³
In litres34.2 millionrange 13.6 million - 85.8 million L
Olympic swimming pools14at 2,500 m³ each
Organic load, as people's raw sewage51,242person-days of untreated BOD

Assumed spill rate while spilling: 15.1 - 94.9 L/s (central 37.8 L/s), from a dry-weather flow of 15.1 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 544 kg3,075 kg17,153 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 1,429 kg14,519 kg61,836 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 26 kg215 kg798 kg 1.9 / 6.3 / 9.3 mg/L
Low: Germany, Brombach 2005; typical/high: Paris range 3.3-9.3 midpoint and top, Gasperi 2012 (via botturi); no UK value found
Total phosphorus 16 kg89 kg463 kg 1.2 / 2.6 / 5.4 mg/L
Low/high: Paris, Gasperi 2012; typical: Slovakia (via botturi). UK figure of 10 mg/L treated as an outlier
E. coli 1.4 trillion organisms34.2 trillion organisms857.6 trillion organisms 10,000 / 100,000 / 1.0 million per 100 mL
Median range at CSO and retention-tank outlets, Stott et al. 2018 (via botturi); typical = log midpoint

Low combines the low spill rate with low concentrations, and high combines high with high, so the true figure is very likely inside the range. Long spills become more diluted as they go on, so the central figure tends to overstate the load of long events. The calculator does not estimate the effect on the receiving water, which depends on river flow or tide at the time of the spill.

Wincanton Water Recycling Centre

Storm tank at sewage works · discharges to River Cale

Spills 2025
32 (251 hours) · down 48% on 2024
Spills 2024
62 (487 hours)
Long-term average
50.8 spills a year (monitored since 2022)
Monitor uptime
100% of the year
High-spill reason (company)
Not Applicable - Ongoing investigation
Investigation
No investigation activity in reporting period
Improvement
Operational improvement completed
WFD catchment
Cale (GB108043015850)
Outlet location
ST7106027380 (OS grid reference) · View on Apple Maps
EA ID / permit
WXW01265 · 401609 · company name: WINCANTON STORM TANK

How the estimate works

  1. Dry-weather flow (DWF) = population × water use per person × (1 + infiltration). Water use is 136.5 L per person per day, England's per-capita consumption for 2024-25 [EA]. Infiltration of groundwater into sewers is taken as 0-40% of that flow [Escritt, via HBF]. The Environment Agency defines DWF as PG + I + E [EA].
  2. Population. The size of each works comes from the load entering it as reported under the Urban Waste Water Treatment Directive, in population equivalents (1 p.e. = 60 g of BOD a day) [UWWTD]. We treat population equivalent as population. That overstates flow where trade effluent adds load, and no official source links the two. Works serving fewer than 2,000 p.e. are not reported, so their size has to be entered by hand.
  3. Low spill rate = DWF. This is the method used by Giakoumis & Voulvoulis (2026), which they note likely underestimates storm loads [G&V].
  4. High spill rate = Formula A − flow to full treatment = (DWF + 1360P) − 3DWF. This is the largest flow the permit expects the storm tanks to handle before it passes on to treatment or overflows [EA]. The central rate is the geometric mean of the low and high rates.
  5. Volume = spill rate × hours spilling, with hours taken from the event duration monitor.
  6. Pollution = volume × published storm-sewage concentrations [Botturi et al.] [US EPA]. Organic load is also shown as person-days of untreated sewage at 60 g of BOD per person per day [UWWTD].

What this cannot tell you. It gives no real flow for any single spill, and no effect on the river or sea, which depends on the flow, tide and the other pressures on the water at the time. It gives no figure for spills from overflows on the sewer network, which serve catchments of unknown size. It is a way to put hours into rough physical terms, not a substitute for flow monitoring.

Sources

  1. EA: Water companies - environmental permits for storm overflows and emergency overflows
  2. EA: Water resources 2024 to 2025 - per capita consumption
  3. HBF: Foul sewer design - historic conventions (citing Escritt 1984 on infiltration)
  4. Giakoumis & Voulvoulis (2026), Environ. Sci.: Water Res. Technol., doi:10.1039/D5EW00860C
  5. Botturi et al. (2021), Crit. Rev. Environ. Sci. Technol. 51:1585 - CSO quality review, Table 2
  6. US EPA (2004) Report to Congress on CSOs and SSOs, chapter 4
  7. Urban Waste Water Treatment Directive, Article 2(6): 1 p.e. = 60 g BOD5/day
  8. Olympic-size swimming pool: 2,500 m³ at the nominal 2 m depth

← All Wessex Water works · Contains Environment Agency data licensed under the Open Government Licence v3.0. Spill start and stop times: © Environment Agency copyright and/or database right 2026. All rights reserved. They are water-company data and have not been verified by the Environment Agency. Works size: EEA Waterbase UWWTD. How to read the data.