Storm overflow register · Northumbrian Water

Knitsley sewage works: storm overflow spills 2025

Knitsley sewage treatment works, run by Northumbrian Water, has 1 monitored storm overflow discharging to Smallhope Burn. In 2025 it recorded 41 spills totalling 280 hours, against 54 spills and 484 hours in 2024. By hours spilling, it ranks 72 of 183 Northumbrian Water works, where 1 spilled longest.

Works size: 5,239 population equivalent (KNITSLEY 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.

JanFebMarAprMayJunJulAugSepOctNovDecKnitsley Sewage…11 spills, 10 h, 1 Jan to 2 Jan2 spills, 23.5 h, 6 Jan to 7 Jan3 spills, 24.8 h, 13 Jan to 14 Jan3 spills, 4 h, 24 Jan to 24 Jan11 spills, 25.3 h, 26 Jan to 28 Jan10 Feb 2025 16:44 GMT, 19.8 h3 spills, 3.3 h, 12 Feb to 12 Feb4 spills, 3 h, 22 Mar to 23 Mar3 spills, 8.2 h, 19 Jul to 20 Jul2 spills, 1.2 h, 10 Sep to 11 Sep3 spills, 0.9 h, 12 Sep to 12 Sep3 spills, 1.8 h, 14 Sep to 15 Sep7 spills, 18.9 h, 20 Sep to 21 Sep1 Oct 2025 09:08 GMT, 3.2 h4 spills, 5.5 h, 3 Oct to 3 Oct4 spills, 2.1 h, 19 Oct to 20 Oct26 spills, 50.4 h, 12 Nov to 15 Nov29 spills, 18.8 h, 24 Nov to 25 Nov11 spills, 3.5 h, 28 Nov to 28 Nov5 spills, 7.4 h, 29 Nov to 29 Nov8 spills, 19.8 h, 1 Dec to 2 Dec5 spills, 5.3 h, 5 Dec to 6 Dec5 spills, 6 h, 7 Dec to 7 Dec29 spills, 11.9 h, 9 Dec to 9 Dec5 spills, 1.3 h, 15 Dec to 15 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.

05,00010k15k20kJan 2025: 88 h spilling ≈ 7,855 m³ (range 3,129-19,719 m³)JanFeb 2025: 23 h spilling ≈ 2,065 m³ (range 822-5,183 m³)FebMar 2025: 3.0 h spilling ≈ 269 m³ (range 107-676 m³)MarApr 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)AprMay 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)MayJun 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)JunJul 2025: 8.2 h spilling ≈ 736 m³ (range 293-1,848 m³)JulAug 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)AugSep 2025: 23 h spilling ≈ 2,047 m³ (range 815-5,138 m³)SepOct 2025: 11 h spilling ≈ 961 m³ (range 383-2,411 m³)OctNov 2025: 80 h spilling ≈ 7,199 m³ (range 2,868-18,074 m³)NovDec 2025: 44 h spilling ≈ 3,977 m³ (range 1,584-9,984 m³)Dec
Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDurationEstimated volume
12 Nov 2025, 23:38Knitsley Sewage Treatment Works1.6 days1,399 - 8,819 m³
6 Jan 2025, 00:59Knitsley Sewage Treatment Works22.5 h805 - 5,071 m³
13 Jan 2025, 09:29Knitsley Sewage Treatment Works21.3 h760 - 4,789 m³
10 Feb 2025, 16:44Knitsley Sewage Treatment Works19.8 h706 - 4,451 m³
1 Dec 2025, 05:10Knitsley Sewage Treatment Works18.4 h659 - 4,154 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 estimate25,108 m³range 10,001 - 63,034 m³
In litres25.1 millionrange 10.0 million - 63.0 million L
Olympic swimming pools10at 2,500 m³ each
Organic load, as people's raw sewage37,662person-days of untreated BOD

Assumed spill rate while spilling: 9.9 - 62.6 L/s (central 24.9 L/s), from a dry-weather flow of 9.9 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 400 kg2,260 kg12,607 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 1,050 kg10,671 kg45,448 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 19 kg158 kg586 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 12 kg65 kg340 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 1.0 trillion organisms25.1 trillion organisms630.3 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.

Knitsley Sewage Treatment Works

Storm tank at sewage works · discharges to Smallhope Burn

Spills 2025
41 (280 hours) · down 24% on 2024
Spills 2024
54 (484 hours)
Long-term average
45.0 spills a year (monitored since 2021)
Monitor uptime
100% of the year
High-spill reason (company)
Performance - Asset configuration (e.g. PS/rising main/storm tanks)
Investigation
Env Act (SODRP) investigation ongoing, Operational investigation ongoing
Improvement
Operational improvement ongoing
WFD catchment
Smallhope Burn from Source to Browney, Stocke (GB103024077330)
Outlet location
NZ1244748118 (OS grid reference) · View on Apple Maps
EA ID / permit
NWL00766 · 244/1028

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 Northumbrian 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.