Storm overflow register · Yorkshire Water

North Ferriby sewage works: storm overflow spills 2025

North Ferriby sewage treatment works, run by Yorkshire Water, has 1 monitored storm overflow discharging to River Humber. In 2025 it recorded 108 spills totalling 581 hours, against 126 spills and 670 hours in 2024. By hours spilling, it ranks 79 of 283 Yorkshire Water works, where 1 spilled longest.

Works size: 6,696 population equivalent (NORTH FERRIBY 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.

JanFebMarAprMayJunJulAugSepOctNovDecNorth Ferriby STW7 spills, 26.5 h, 1 Jan to 2 Jan17 spills, 63.5 h, 5 Jan to 8 Jan3 spills, 1.8 h, 24 Jan to 24 Jan10 spills, 27 h, 26 Jan to 28 Jan9 Feb 2025 00:15 GMT, 45 min6 spills, 7.8 h, 10 Feb to 11 Feb23 Feb 2025 22:45 GMT, 3.3 h26 Feb 2025 09:00 GMT, 2 h3 spills, 4 h, 12 Mar to 13 Mar14 Mar 2025 16:45 GMT, 30 min17 Mar 2025 11:00 GMT, 30 min4 spills, 2.3 h, 22 Mar to 23 Mar4 spills, 3.8 h, 15 Apr to 16 Apr21 Apr 2025 14:45 GMT, 105 min24 May 2025 03:00 GMT, 3.3 h27 May 2025 13:45 GMT, 30 min2 spills, 4 h, 29 May to 29 May3 Jun 2025 06:15 GMT, 45 min3 spills, 9.8 h, 5 Jun to 6 Jun3 spills, 2.3 h, 9 Jun to 10 Jun2 spills, 1.5 h, 22 Jun to 23 Jun24 Jun 2025 22:15 GMT, 45 min2 spills, 5.3 h, 2 Jul to 2 Jul2 spills, 4.8 h, 6 Jul to 6 Jul7 Jul 2025 22:00 GMT, 45 min2 spills, 5.5 h, 15 Jul to 15 Jul18 Jul 2025 07:00 GMT, 105 min19 Jul 2025 12:45 GMT, 5.3 h20 Jul 2025 13:30 GMT, 6.3 h22 Jul 2025 18:30 GMT, 45 min23 Jul 2025 16:45 GMT, 30 min3 spills, 3.5 h, 4 Aug to 4 Aug2 spills, 2.3 h, 28 Aug to 28 Aug30 Aug 2025 22:15 GMT, 75 min1 Sep 2025 14:30 GMT, 75 min2 spills, 2.8 h, 3 Sep to 3 Sep4 Sep 2025 12:45 GMT, 5 h3 spills, 4.5 h, 10 Sep to 11 Sep3 spills, 2.5 h, 14 Sep to 15 Sep2 spills, 7 h, 16 Sep to 17 Sep20 Sep 2025 11:00 GMT, 17.5 h28 Sep 2025 04:45 GMT, 2 h1 Oct 2025 10:15 GMT, 30 min5 spills, 13 h, 2 Oct to 4 Oct7 spills, 8.8 h, 19 Oct to 20 Oct23 Oct 2025 00:45 GMT, 7.5 h24 Oct 2025 23:30 GMT, 105 min31 Oct 2025 15:15 GMT, 75 min3 spills, 1.3 h, 1 Nov to 2 Nov3 Nov 2025 05:00 GMT, 45 min2 spills, 7.3 h, 4 Nov to 5 Nov2 spills, 1.5 h, 8 Nov to 8 Nov9 spills, 18.8 h, 10 Nov to 12 Nov12 Nov 2025 18:45 GMT, 6 h17 spills, 41 h, 14 Nov to 15 Nov14 spills, 22.5 h, 18 Nov to 21 Nov2 spills, 2 h, 22 Nov to 22 Nov5 spills, 8.5 h, 23 Nov to 24 Nov26 Nov 2025 20:15 GMT, 60 min28 spills, 64 h, 28 Nov to 2 Dec39 spills, 42.8 h, 4 Dec to 7 Dec12 spills, 23.3 h, 9 Dec to 10 Dec12 Dec 2025 13:00 GMT, 2.3 h32 spills, 55.5 h, 14 Dec to 20 Dec2 spills, 1.3 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: 119 h spilling ≈ 13,630 m³ (range 5,429-34,219 m³)JanFeb 2025: 14 h spilling ≈ 1,583 m³ (range 631-3,975 m³)FebMar 2025: 7.2 h spilling ≈ 826 m³ (range 329-2,074 m³)MarApr 2025: 5.5 h spilling ≈ 631 m³ (range 251-1,584 m³)AprMay 2025: 7.8 h spilling ≈ 895 m³ (range 356-2,247 m³)MayJun 2025: 15 h spilling ≈ 1,721 m³ (range 686-4,321 m³)JunJul 2025: 31 h spilling ≈ 3,534 m³ (range 1,408-8,872 m³)JulAug 2025: 7.0 h spilling ≈ 803 m³ (range 320-2,016 m³)AugSep 2025: 43 h spilling ≈ 4,876 m³ (range 1,942-12,242 m³)SepOct 2025: 33 h spilling ≈ 3,763 m³ (range 1,499-9,448 m³)OctNov 2025: 152 h spilling ≈ 17,382 m³ (range 6,924-43,638 m³)NovDec 2025: 148 h spilling ≈ 16,980 m³ (range 6,764-42,630 m³)Dec
Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDurationEstimated volume
5 Jan 2025, 05:00North Ferriby STW2.0 days2,159 - 13,610 m³
29 Nov 2025, 04:30North Ferriby STW1.4 days1,554 - 9,793 m³
14 Nov 2025, 04:00North Ferriby STW1.4 days1,554 - 9,793 m³
1 Jan 2025, 00:00North Ferriby STW21.8 h994 - 6,265 m³
20 Sep 2025, 11:00North Ferriby STW17.5 h800 - 5,041 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 estimate66,602 m³range 26,529 - 167,207 m³
In litres66.6 millionrange 26.5 million - 167.2 million L
Olympic swimming pools27at 2,500 m³ each
Organic load, as people's raw sewage99,903person-days of untreated BOD

Assumed spill rate while spilling: 12.7 - 80.0 L/s (central 31.9 L/s), from a dry-weather flow of 12.7 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 1,061 kg5,994 kg33,441 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 2,786 kg28,306 kg120,556 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 50 kg420 kg1,555 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 32 kg173 kg903 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 2.7 trillion organisms66.6 trillion organisms1.7 × 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.

North Ferriby STW

Inlet overflow at sewage works · discharges to River Humber

Spills 2025
108 (581 hours) · down 14% on 2024
Spills 2024
126 (670 hours)
Long-term average
135.8 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
HUMBER MIDDLE (GB530402609202)
Outlet location
SE9913025150 (OS grid reference) · View on Apple Maps
EA ID / permit
YWS00085 · 2137 · company name: NORTH FERRIBY/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.