Storm overflow register · Yorkshire Water

Carleton sewage works: storm overflow spills 2025

Carleton sewage treatment works, run by Yorkshire Water, has 2 monitored storm overflows discharging to River Little Went. In 2025 they recorded 105 spills totalling 569 hours, against 115 spills and 646 hours in 2024. By hours spilling, it ranks 80 of 283 Yorkshire Water works, where 1 spilled longest.

Works size: 6,265 population equivalent (CARLETON 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.

JanFebMarAprMayJunJulAugSepOctNovDecCarleton No 2 STW11 spills, 23.5 h, 1 Jan to 2 Jan33 spills, 65 h, 5 Jan to 8 Jan11 spills, 11.5 h, 27 Jan to 28 Jan3 spills, 1 h, 23 Mar to 23 Mar2 spills, 2.8 h, 19 Jul to 19 Jul4 spills, 4 h, 20 Jul to 20 Jul5 spills, 1.3 h, 4 Sep to 4 Sep2 spills, 1.3 h, 17 Sep to 17 Sep4 spills, 12.3 h, 20 Sep to 21 Sep2 spills, 1 h, 3 Oct to 3 Oct3 spills, 3 h, 31 Oct to 31 Oct2 spills, 0.5 h, 12 Nov to 12 Nov4 spills, 1.3 h, 12 Nov to 12 Nov13 spills, 43.8 h, 14 Nov to 16 Nov7 spills, 6.5 h, 23 Nov to 24 Nov32 spills, 48 h, 29 Nov to 2 Dec48 spills, 35 h, 4 Dec to 7 Dec15 spills, 20.3 h, 9 Dec to 10 Dec15 spills, 22.3 h, 15 Dec to 16 Dec29 spills, 44 h, 17 Dec to 20 Dec9 spills, 6.8 h, 21 Dec to 22 DecCarleton No 2 STW3 spills, 15.8 h, 1 Jan to 1 Jan3 spills, 34 h, 5 Jan to 7 Jan24 Jan 2025 04:00 GMT, 75 min27 Jan 2025 02:15 GMT, 2.8 h2 spills, 0.8 h, 28 Jan to 28 Jan20 Feb 2025 15:30 GMT, 30 min26 Feb 2025 08:15 GMT, 60 min2 spills, 1.5 h, 7 Mar to 7 Mar2 spills, 2.8 h, 22 Mar to 23 Mar21 Apr 2025 20:15 GMT, 15 min24 May 2025 01:15 GMT, 45 min29 May 2025 02:15 GMT, 15 min4 spills, 2 h, 5 Jun to 6 Jun2 spills, 2 h, 6 Jul to 6 Jul7 Jul 2025 22:30 GMT, 15 min15 Jul 2025 21:15 GMT, 30 min3 spills, 3.5 h, 19 Jul to 19 Jul3 spills, 3.8 h, 20 Jul to 20 Jul21 Jul 2025 16:00 GMT, 45 min4 Aug 2025 12:45 GMT, 15 min27 Aug 2025 14:15 GMT, 30 min3 spills, 1.3 h, 4 Sep to 4 Sep2 spills, 1 h, 11 Sep to 11 Sep3 spills, 3.5 h, 17 Sep to 17 Sep20 Sep 2025 15:30 GMT, 10.3 h2 spills, 2.5 h, 28 Sep to 28 Sep3 spills, 3.8 h, 2 Oct to 4 Oct3 spills, 2.3 h, 19 Oct to 19 Oct2 spills, 1 h, 23 Oct to 23 Oct31 Oct 2025 16:30 GMT, 3.5 h2 spills, 1 h, 4 Nov to 4 Nov2 spills, 0.8 h, 10 Nov to 10 Nov4 spills, 3 h, 11 Nov to 12 Nov9 spills, 31.5 h, 14 Nov to 15 Nov18 Nov 2025 15:15 GMT, 30 min22 Nov 2025 05:45 GMT, 60 min4 spills, 2.8 h, 23 Nov to 23 Nov4 spills, 23 h, 28 Nov to 30 Nov1 Dec 2025 20:00 GMT, 60 min5 spills, 9.8 h, 4 Dec to 5 Dec6 Dec 2025 18:45 GMT, 15 min3 spills, 11.5 h, 9 Dec to 9 Dec4 spills, 10 h, 15 Dec to 15 Dec3 spills, 9.8 h, 17 Dec to 18 Dec4 spills, 4 h, 21 Dec to 22 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: 155 h spilling ≈ 16,585 m³ (range 6,606-41,638 m³)JanFeb 2025: 1.5 h spilling ≈ 161 m³ (range 64-404 m³)FebMar 2025: 5.2 h spilling ≈ 558 m³ (range 222-1,401 m³)MarApr 2025: 0.2 h spilling ≈ 21 m³ (range 8.6-54 m³)AprMay 2025: 1.0 h spilling ≈ 107 m³ (range 43-269 m³)MayJun 2025: 2.0 h spilling ≈ 215 m³ (range 86-539 m³)JunJul 2025: 18 h spilling ≈ 1,889 m³ (range 753-4,743 m³)JulAug 2025: 0.8 h spilling ≈ 86 m³ (range 34-216 m³)AugSep 2025: 33 h spilling ≈ 3,575 m³ (range 1,424-8,974 m³)SepOct 2025: 15 h spilling ≈ 1,557 m³ (range 620-3,908 m³)OctNov 2025: 152 h spilling ≈ 16,349 m³ (range 6,512-41,045 m³)NovDec 2025: 186 h spilling ≈ 19,934 m³ (range 7,940-50,046 m³)Dec
Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDurationEstimated volume
5 Jan 2025, 13:30Carleton No 2 STW2.1 days2,106 - 13,273 m³
14 Nov 2025, 07:00Carleton No 2 STW1.7 days1,742 - 10,982 m³
18 Dec 2025, 14:30Carleton No 2 STW1.4 days1,390 - 8,759 m³
29 Nov 2025, 09:00Carleton No 2 STW1.3 days1,315 - 8,287 m³
14 Nov 2025, 09:00Carleton No 2 STW1.2 days1,240 - 7,815 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 estimate61,027 m³range 24,308 - 153,210 m³
In litres61.0 millionrange 24.3 million - 153.2 million L
Olympic swimming pools24at 2,500 m³ each
Organic load, as people's raw sewage91,540person-days of untreated BOD

Assumed spill rate while spilling: 11.9 - 74.9 L/s (central 29.8 L/s), from a dry-weather flow of 11.9 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 972 kg5,492 kg30,642 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 2,552 kg25,936 kg110,465 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 46 kg384 kg1,425 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 29 kg159 kg827 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 2.4 trillion organisms61.0 trillion organisms1.5 × 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.

Carleton No 2 STW

Storm tank at sewage works · discharges to River Little Went

Spills 2025
45 (355 hours) · down 4% on 2024
Spills 2024
47 (392 hours)
Long-term average
63.3 spills a year (monitored since 2020)
Monitor uptime
100% of the year
High-spill reason (company)
Not asset maintenance - hydraulic capacity
Investigation
UWWTR investigation ongoing, Operational investigation completed
Improvement
No improvement action in reporting period
WFD catchment
Went from Hoyle Mill Stream to Blowell Drain (GB104027063360)
Outlet location
SE4632019380 (OS grid reference) · View on Apple Maps
EA ID / permit
YWS00769 · 381 · company name: CARLETON/NO 2 STW

Carleton No 2 STW

Inlet overflow at sewage works · discharges to River Little Went

Spills 2025
60 (214 hours) · down 12% on 2024
Spills 2024
68 (254 hours)
Long-term average
71.7 spills a year (monitored since 2020)
Monitor uptime
99% of the year
High-spill reason (company)
Not asset maintenance - hydraulic capacity
Investigation
UWWTR investigation ongoing, Operational investigation completed
Improvement
No improvement action in reporting period
WFD catchment
Went from Hoyle Mill Stream to Blowell Drain (GB104027063360)
Outlet location
SE4632019380 (OS grid reference) · View on Apple Maps
EA ID / permit
YWS00770 · 381 · company name: CARLETON/NO 2 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.