Storm overflow register · Yorkshire Water

South Elmsall sewage works: storm overflow spills 2025

South Elmsall sewage treatment works, run by Yorkshire Water, has 2 monitored storm overflows discharging to Hemsworth Dyke. In 2025 they recorded 134 spills totalling 818 hours, against 103 spills and 521 hours in 2024. By hours spilling, it ranks 52 of 283 Yorkshire Water works, where 1 spilled longest.

Works size: 35,138 population equivalent (SOUTH ELMSALL 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.

JanFebMarAprMayJunJulAugSepOctNovDecSouth Elmsall WwTW292 spills, 141.3 h, 1 Jan to 10 Jan18 spills, 4.5 h, 15 Jan to 17 Jan3 spills, 0.8 h, 18 Jan to 18 Jan4 spills, 1 h, 22 Jan to 23 Jan48 spills, 16.3 h, 23 Jan to 25 Jan76 spills, 47 h, 26 Jan to 29 Jan2 spills, 0.5 h, 30 Jan to 30 Jan71 spills, 25.3 h, 22 Mar to 24 Mar25 Mar 2025 23:00 GMT, 15 min8 spills, 3 h, 21 Apr to 21 Apr10 spills, 2.5 h, 24 May to 24 May26 spills, 7 h, 25 May to 27 May23 spills, 8 h, 28 May to 29 May16 spills, 7 h, 5 Jun to 6 Jun2 spills, 1 h, 6 Jul to 6 Jul20 spills, 9 h, 19 Jul to 20 Jul4 spills, 1.5 h, 21 Jul to 21 Jul13 spills, 3.5 h, 4 Sep to 4 Sep10 spills, 7 h, 17 Sep to 17 Sep26 spills, 15.8 h, 20 Sep to 21 Sep26 Sep 2025 14:45 GMT, 15 min9 spills, 6.5 h, 28 Sep to 28 Sep8 spills, 10.3 h, 3 Oct to 4 Oct8 spills, 4.5 h, 19 Oct to 20 Oct6 spills, 11.5 h, 22 Oct to 23 Oct31 Oct 2025 21:15 GMT, 90 min4 spills, 3 h, 10 Nov to 10 Nov12 Nov 2025 22:30 GMT, 15 min135 spills, 51.8 h, 14 Nov to 16 Nov3 spills, 0.8 h, 18 Nov to 18 Nov20 spills, 15.3 h, 23 Nov to 24 Nov86 spills, 56.5 h, 29 Nov to 2 Dec189 spills, 96.5 h, 4 Dec to 12 Dec60 spills, 32.5 h, 15 Dec to 17 Dec134 spills, 67 h, 17 Dec to 23 DecSouth Elmsall WwTW16 spills, 13.8 h, 1 Jan to 1 Jan37 spills, 34.3 h, 5 Jan to 7 Jan5 spills, 3.3 h, 27 Jan to 27 Jan5 spills, 3.3 h, 22 Mar to 23 Mar21 Apr 2025 20:45 GMT, 15 min24 May 2025 02:00 GMT, 15 min25 May 2025 22:15 GMT, 15 min3 spills, 1 h, 20 Jul to 20 Jul2 spills, 0.8 h, 21 Jul to 21 Jul3 Sep 2025 17:00 GMT, 30 min4 Sep 2025 15:00 GMT, 30 min6 spills, 2.5 h, 17 Sep to 17 Sep5 spills, 10.8 h, 20 Sep to 21 Sep4 spills, 1.3 h, 28 Sep to 28 Sep4 spills, 1.3 h, 3 Oct to 3 Oct2 spills, 0.5 h, 4 Oct to 4 Oct3 spills, 1 h, 19 Oct to 20 Oct5 spills, 1.3 h, 23 Oct to 23 Oct2 spills, 1.8 h, 31 Oct to 31 Oct3 spills, 0.8 h, 4 Nov to 4 Nov4 spills, 1.3 h, 10 Nov to 10 Nov33 spills, 26.5 h, 14 Nov to 16 Nov18 Nov 2025 16:30 GMT, 15 min5 spills, 2.8 h, 23 Nov to 23 Nov24 spills, 17.5 h, 28 Nov to 30 Nov3 spills, 1 h, 1 Dec to 1 Dec8 spills, 3.8 h, 4 Dec to 4 Dec13 spills, 4.8 h, 5 Dec to 6 Dec19 spills, 8.3 h, 9 Dec to 9 Dec13 spills, 4 h, 15 Dec to 16 Dec12 spills, 6.5 h, 17 Dec to 19 Dec8 spills, 2.5 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.

0100k200k300k400kJan 2025: 262 h spilling ≈ 157,983 m³ (range 62,928-396,623 m³)JanFeb 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)FebMar 2025: 29 h spilling ≈ 17,279 m³ (range 6,883-43,381 m³)MarApr 2025: 3.2 h spilling ≈ 1,927 m³ (range 767-4,837 m³)AprMay 2025: 18 h spilling ≈ 10,837 m³ (range 4,317-27,207 m³)MayJun 2025: 7.0 h spilling ≈ 4,214 m³ (range 1,679-10,581 m³)JunJul 2025: 13 h spilling ≈ 8,008 m³ (range 3,190-20,103 m³)JulAug 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)AugSep 2025: 49 h spilling ≈ 29,200 m³ (range 11,631-73,309 m³)SepOct 2025: 34 h spilling ≈ 20,230 m³ (range 8,058-50,787 m³)OctNov 2025: 157 h spilling ≈ 94,224 m³ (range 37,531-236,553 m³)NovDec 2025: 247 h spilling ≈ 148,591 m³ (range 59,187-373,043 m³)Dec
Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDurationEstimated volume
5 Jan 2025, 18:15South Elmsall WwTW23.5 h5,636 - 35,521 m³
30 Nov 2025, 14:00South Elmsall WwTW10.0 h2,398 - 15,115 m³
14 Nov 2025, 16:30South Elmsall WwTW9.8 h2,338 - 14,737 m³
20 Sep 2025, 17:30South Elmsall WwTW8.3 h1,978 - 12,470 m³
23 Oct 2025, 01:45South Elmsall WwTW8.0 h1,919 - 12,092 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 estimate492,493 m³range 196,170 - 1.2 million m³
In litres492.5 millionrange 196.2 million - 1.2 billion L
Olympic swimming pools197at 2,500 m³ each
Organic load, as people's raw sewage738,739person-days of untreated BOD

Assumed spill rate while spilling: 66.6 - 419.9 L/s (central 167.2 L/s), from a dry-weather flow of 66.6 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 7,847 kg44,324 kg247,285 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 20,598 kg209,310 kg891,461 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 373 kg3,103 kg11,499 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 235 kg1,280 kg6,677 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 19.6 trillion organisms492.5 trillion organisms1.2 × 10¹⁶ 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.

South Elmsall WwTW

Storm tank at sewage works · discharges to Hemsworth Dyke

Spills 2025
88 (660 hours) · up 96% on 2024
Spills 2024
45 (302 hours)
Long-term average
64.8 spills a year (monitored since 2020)
Monitor uptime
100% 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
Ea Beck from Frickley Beck to the Skell (GB104027063200)
Outlet location
SE4830010650 (OS grid reference) · View on Apple Maps
EA ID / permit
YWS01417 · WRA7507 · company name: SOUTH ELMSALL/STW/3XDWF OVERFLOW

South Elmsall WwTW

Inlet overflow at sewage works · discharges to Hemsworth Dyke

Spills 2025
46 (158 hours) · down 21% on 2024
Spills 2024
58 (220 hours)
Long-term average
48.0 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
Improvement
No improvement action in reporting period
WFD catchment
Ea Beck from Frickley Beck to the Skell (GB104027063200)
Outlet location
SE4821010640 (OS grid reference) · View on Apple Maps
EA ID / permit
YWS01418 · WRA7507 · company name: SOUTH ELMSALL/STW/6XDWF OVERFLOW

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.