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.

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.

Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)	Overflow	Duration	Estimated volume
5 Dec 2025, 03:15	Darley Sewage Treatment Works	6.6 days	2,398 – 15,112 m³
13 Nov 2025, 13:00	Darley Sewage Treatment Works	4.1 days	1,485 – 9,359 m³
17 Nov 2025, 15:45	Darley Sewage Treatment Works	2.9 days	1,072 – 6,756 m³
5 Jan 2025, 16:00	Darley Sewage Treatment Works	2.5 days	894 – 5,634 m³
23 Nov 2025, 06:15	Darley Sewage Treatment Works	2.3 days	852 – 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 released	Low	Central	High	Concentration used (low / typical / high)
BOD₅ (organic load)	995 kg	5,618 kg	31,342 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 kg	26,529 kg	112,988 kg	105 / 425 / 721 mg/L Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N	47 kg	393 kg	1,457 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 kg	162 kg	846 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 organisms	62.4 trillion organisms	1.6 × 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

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].
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.
Low spill rate = DWF. This is the method used by Giakoumis & Voulvoulis (2026), which they note likely underestimates storm loads [G&V].
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.
Volume = spill rate × hours spilling, with hours taken from the event duration monitor.
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

← 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.