Storm overflow register · Yorkshire Water

Skidby sewage works: storm overflow spills 2025

Skidby sewage treatment works, run by Yorkshire Water, has 1 monitored storm overflow discharging to Tributary of Mill Beck. In 2025 it recorded 14 spills totalling 89 hours, against 18 spills and 119 hours in 2024. By hours spilling, it ranks 225 of 283 Yorkshire Water works, where 1 spilled longest.

Works size: 2,180 population equivalent (SKIDBY 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 Jan 2025, 13:00	Skidby STW	1.0 days	357 – 2,251 m³
14 Nov 2025, 17:15	Skidby STW	9.5 h	141 – 891 m³
29 Nov 2025, 16:30	Skidby STW	8.5 h	126 – 797 m³
20 Sep 2025, 19:30	Skidby STW	7.0 h	104 – 656 m³
10 Nov 2025, 16:15	Skidby STW	5.5 h	82 – 516 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 estimate3,317 m³range 1,321 – 8,327 m³

In litres3.3 millionrange 1.3 million – 8.3 million L

Olympic swimming pools1.3at 2,500 m³ each

Organic load, as people's raw sewage4,975person-days of untreated BOD

Assumed spill rate while spilling: 4.1 – 26.0 L/s (central 10.4 L/s), from a dry-weather flow of 4.1 L/s.

Pollutant released	Low	Central	High	Concentration used (low / typical / high)
BOD₅ (organic load)	53 kg	299 kg	1,665 kg	40 / 90 / 200 mg/L Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids	139 kg	1,410 kg	6,004 kg	105 / 425 / 721 mg/L Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N	2.5 kg	21 kg	77 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	1.6 kg	8.6 kg	45 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	132.1 billion organisms	3.3 trillion organisms	83.3 trillion 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.

Skidby STW

Storm tank at sewage works · discharges to Tributary of Mill Beck

Spills 2025: 14 (89 hours) · down 22% on 2024
Spills 2024: 18 (119 hours)
Long-term average: 31.8 spills a year (monitored since 2020)
Monitor uptime: 100% of the year
Investigation: UWWTR investigation ongoing, Operational investigation ongoing
Improvement: No improvement action in reporting period
WFD catchment: Beverley and Barmston Drain (GB104026067211)
Outlet location: TA0179033970 (OS grid reference) · View on Apple Maps
EA ID / permit: YWS01119 · WA6412 · company name: SKIDBY/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.