Storm overflow register · Yorkshire Water

Darley sewage works: storm overflow spills 2025

Darley sewage treatment works, run by Yorkshire Water, has 2 monitored storm overflows discharging to Trib of River Nidd. In 2025 they recorded 130 spills totalling 1,641 hours, against 232 spills and 3,186 hours in 2024. By hours spilling, it ranks 12 of 283 Yorkshire Water works, where 1 spilled longest.

Works size: 2,220 population equivalent (DARLEY 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.

JanFebMarAprMayJunJulAugSepOctNovDecDarley Sewage…51 spills, 155.8 h, 1 Jan to 9 Jan61 spills, 95.3 h, 13 Jan to 19 Jan20 spills, 21.5 h, 24 Jan to 25 Jan50 spills, 90.5 h, 26 Jan to 31 Jan6 spills, 5.8 h, 20 Feb to 20 Feb6 spills, 13.8 h, 22 Mar to 23 Mar26 May 2025 23:00 GMT, 2.8 h4 spills, 6 h, 29 May to 30 May2 spills, 8.8 h, 15 Jul to 16 Jul5 spills, 20 h, 19 Jul to 21 Jul2 spills, 15 h, 15 Sep to 16 Sep20 Sep 2025 08:45 GMT, 40.5 h7 spills, 4 h, 28 Sep to 28 Sep4 spills, 56.8 h, 3 Oct to 5 Oct2 spills, 23.8 h, 19 Oct to 20 Oct292 spills, 927 h, 11 Nov to 28 Dec30 Dec 2025 20:00 GMT, 15 min9 spills, 2.8 h, 31 Dec to 31 DecDarley Sewage…2 spills, 12.3 h, 1 Jan to 1 Jan4 spills, 17.3 h, 5 Jan to 6 Jan2 spills, 8 h, 14 Jan to 14 Jan24 Jan 2025 14:30 GMT, 15 min2 spills, 4 h, 27 Jan to 27 Jan2 spills, 0.5 h, 15 Jul to 15 Jul19 Jul 2025 14:30 GMT, 15 min20 Jul 2025 15:00 GMT, 60 min3 spills, 0.8 h, 15 Sep to 15 Sep4 spills, 7.5 h, 20 Sep to 21 Sep8 spills, 3.5 h, 3 Oct to 4 Oct19 Oct 2025 23:15 GMT, 75 min2 spills, 3.3 h, 11 Nov to 12 Nov3 spills, 4.3 h, 12 Nov to 13 Nov11 spills, 30.5 h, 14 Nov to 16 Nov2 spills, 1.3 h, 19 Nov to 19 Nov23 Nov 2025 12:15 GMT, 30 min24 Nov 2025 08:30 GMT, 30 min2 spills, 6.3 h, 1 Dec to 1 Dec8 spills, 27 h, 5 Dec to 7 Dec4 spills, 15.5 h, 9 Dec to 10 Dec3 spills, 5.5 h, 17 Dec to 18 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: 405 h spilling ≈ 15,398 m³ (range 6,133-38,657 m³)JanFeb 2025: 5.8 h spilling ≈ 221 m³ (range 88-554 m³)FebMar 2025: 14 h spilling ≈ 525 m³ (range 209-1,318 m³)MarApr 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)AprMay 2025: 8.8 h spilling ≈ 335 m³ (range 133-840 m³)MayJun 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)JunJul 2025: 31 h spilling ≈ 1,164 m³ (range 464-2,922 m³)JulAug 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)AugSep 2025: 68 h spilling ≈ 2,575 m³ (range 1,026-6,465 m³)SepOct 2025: 85 h spilling ≈ 3,245 m³ (range 1,292-8,146 m³)OctNov 2025: 437 h spilling ≈ 16,623 m³ (range 6,621-41,732 m³)NovDec 2025: 587 h spilling ≈ 22,344 m³ (range 8,900-56,095 m³)Dec
Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDurationEstimated volume
5 Dec 2025, 03:15Darley Sewage Treatment Works6.6 days2,398 - 15,112 m³
13 Nov 2025, 13:00Darley Sewage Treatment Works4.1 days1,485 - 9,359 m³
17 Nov 2025, 15:45Darley Sewage Treatment Works2.9 days1,072 - 6,756 m³
5 Jan 2025, 16:00Darley Sewage Treatment Works2.5 days894 - 5,634 m³
23 Nov 2025, 06:15Darley Sewage Treatment Works2.3 days852 - 5,372 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 estimate62,421 m³range 24,864 - 156,711 m³
In litres62.4 millionrange 24.9 million - 156.7 million L
Olympic swimming pools25at 2,500 m³ each
Organic load, as people's raw sewage93,632person-days of untreated BOD

Assumed spill rate while spilling: 4.2 - 26.5 L/s (central 10.6 L/s), from a dry-weather flow of 4.2 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 995 kg5,618 kg31,342 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 2,611 kg26,529 kg112,988 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 47 kg393 kg1,457 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 30 kg162 kg846 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 2.5 trillion organisms62.4 trillion organisms1.6 × 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.

Darley Sewage Treatment Works

Storm tank at sewage works · discharges to Trib of River Nidd

Spills 2025
97 (1,490 hours) · down 34% on 2024
Spills 2024
146 (2,260 hours)
Long-term average
109.7 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, Bathing Water and/or Shellfish Water investigation ongoing
Improvement
No improvement action in reporting period
WFD catchment
Nidd from Ashfoldside Beck to Birstwith (GB104027068295)
Outlet location
SE2118259656 (OS grid reference) · View on Apple Maps
EA ID / permit
YWS00249 · 27/21/0205 · company name: DARLEY/STW

Darley Sewage Treatment Works

Inlet overflow at sewage works · discharges to Trib of River Nidd

Spills 2025
33 (151 hours) · down 62% on 2024
Spills 2024
86 (926 hours)
Long-term average
58.7 spills a year (monitored since 2020)
Monitor uptime
100% of the year
High-spill reason (company)
Performance - SW Infiltration
Investigation
UWWTR investigation ongoing, Bathing Water and/or Shellfish Water investigation ongoing
Improvement
No improvement action in reporting period
WFD catchment
Nidd from Ashfoldside Beck to Birstwith (GB104027068295)
Outlet location
SE2118259656 (OS grid reference) · View on Apple Maps
EA ID / permit
YWS00250 · 27/21/0205 · company name: DARLEY/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.