Storm overflow register · Yorkshire Water

Thorne sewage works: storm overflow spills 2025

Thorne sewage treatment works, run by Yorkshire Water, has 1 monitored storm overflow discharging to River Don (Tidal). In 2025 it recorded 31 spills totalling 204 hours, against 31 spills and 192 hours in 2024. By hours spilling, it ranks 173 of 283 Yorkshire Water works, where 1 spilled longest.

Works size: 40,160 population equivalent (THORNE 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.

JanFebMarAprMayJunJulAugSepOctNovDecThorne STW15 spills, 15 h, 1 Jan to 1 Jan28 spills, 44 h, 5 Jan to 7 Jan20 spills, 12.5 h, 27 Jan to 28 Jan2 spills, 2.8 h, 19 Jul to 19 Jul5 spills, 3.3 h, 20 Jul to 20 Jul3 spills, 3.5 h, 3 Sep to 3 Sep17 Sep 2025 10:45 GMT, 15 min5 spills, 12.3 h, 20 Sep to 21 Sep2 spills, 2 h, 19 Oct to 20 Oct4 spills, 3.5 h, 10 Nov to 11 Nov3 spills, 2.5 h, 12 Nov to 13 Nov15 spills, 29.5 h, 14 Nov to 15 Nov4 spills, 1.5 h, 19 Nov to 19 Nov4 spills, 4.5 h, 23 Nov to 24 Nov2 spills, 15.3 h, 29 Nov to 29 Nov11 spills, 18.8 h, 4 Dec to 6 Dec12 spills, 12.5 h, 9 Dec to 9 Dec15 spills, 11.8 h, 15 Dec to 16 Dec5 spills, 9 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.

050k100k150kJan 2025: 72 h spilling ≈ 49,200 m³ (range 19,598-123,520 m³)JanFeb 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 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: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)MayJun 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)JunJul 2025: 6.0 h spilling ≈ 4,129 m³ (range 1,645-10,365 m³)JulAug 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)AugSep 2025: 16 h spilling ≈ 11,010 m³ (range 4,385-27,641 m³)SepOct 2025: 2.0 h spilling ≈ 1,376 m³ (range 548-3,455 m³)OctNov 2025: 57 h spilling ≈ 39,085 m³ (range 15,568-98,125 m³)NovDec 2025: 52 h spilling ≈ 35,782 m³ (range 14,253-89,833 m³)Dec
Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDurationEstimated volume
5 Jan 2025, 17:30Thorne STW1.3 days8,428 - 53,122 m³
14 Nov 2025, 10:00Thorne STW18.3 h5,002 - 31,528 m³
29 Nov 2025, 07:45Thorne STW15.0 h4,111 - 25,913 m³
20 Sep 2025, 20:00Thorne STW11.0 h3,015 - 19,003 m³
1 Jan 2025, 03:15Thorne STW8.5 h2,330 - 14,684 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 estimate140,514 m³range 55,970 - 352,766 m³
In litres140.5 millionrange 56.0 million - 352.8 million L
Olympic swimming pools56at 2,500 m³ each
Organic load, as people's raw sewage210,771person-days of untreated BOD

Assumed spill rate while spilling: 76.1 - 479.9 L/s (central 191.1 L/s), from a dry-weather flow of 76.1 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 2,239 kg12,646 kg70,553 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 5,877 kg59,718 kg254,344 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 106 kg885 kg3,281 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 67 kg365 kg1,905 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 5.6 trillion organisms140.5 trillion organisms3.5 × 10¹⁵ 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.

Thorne STW

Storm tank at sewage works · discharges to River Don (Tidal)

Spills 2025
31 (204 hours) · no change on 2024
Spills 2024
31 (192 hours)
Long-term average
38.5 spills a year (monitored since 2020)
Monitor uptime
92% of the year
High-spill reason (company)
Performance - SW Infiltration
Investigation
UWWTR investigation ongoing, Operational investigation completed
Improvement
No improvement action in reporting period
WFD catchment
Don from Mill Dyke to River Ouse (GB104027064243)
Outlet location
SE6746014680 (OS grid reference) · View on Apple Maps
EA ID / permit
YWS00701 · 3332 · company name: THORNE/STW

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 Yorkshire 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.