Storm overflow register · Anglian Water

Silverstone sewage works: storm overflow spills 2025

Silverstone sewage treatment works, run by Anglian Water, has 1 monitored storm overflow discharging to Tributary River Tove River Gre. In 2025 it recorded 69 spills totalling 660 hours, against 152 spills and 2,071 hours in 2024. By hours spilling, it ranks 25 of 419 Anglian Water works, where 1 spilled longest.

Works size: 2,984 population equivalent (SILVERSTONE 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.

JanFebMarAprMayJunJulAugSepOctNovDecSilverstone STW16 spills, 7 h, 1 Jan to 3 Jan81 spills, 75.4 h, 5 Jan to 11 Jan65 spills, 72.1 h, 25 Jan to 30 Jan33 spills, 17.4 h, 31 Jan to 1 Feb5 spills, 3.3 h, 3 Feb to 4 Feb12 Feb 2025 10:44 GMT, 8 min33 spills, 7.5 h, 15 Feb to 17 Feb45 spills, 28.4 h, 24 Feb to 28 Feb18 spills, 5.3 h, 23 Apr to 23 Apr8 spills, 3 h, 4 Sep to 5 Sep7 spills, 3.3 h, 14 Sep to 15 Sep7 spills, 1.6 h, 3 Oct to 4 Oct3 spills, 0.8 h, 20 Oct to 20 Oct14 spills, 18.4 h, 10 Nov to 11 Nov15 spills, 10 h, 11 Nov to 12 Nov18 spills, 73.5 h, 13 Nov to 18 Nov16 spills, 6.3 h, 19 Nov to 20 Nov50 spills, 33.4 h, 22 Nov to 25 Nov17 spills, 2.9 h, 29 Nov to 30 Nov24 spills, 30.9 h, 1 Dec to 3 Dec79 spills, 140.6 h, 4 Dec to 12 Dec68 spills, 118.6 h, 16 Dec to 23 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.

010k20k30k40kJan 2025: 169 h spilling ≈ 8,661 m³ (range 3,450-21,744 m³)JanFeb 2025: 42 h spilling ≈ 2,137 m³ (range 851-5,366 m³)FebMar 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)MarApr 2025: 5.3 h spilling ≈ 271 m³ (range 108-680 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: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)JulAug 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)AugSep 2025: 6.3 h spilling ≈ 322 m³ (range 128-809 m³)SepOct 2025: 2.4 h spilling ≈ 123 m³ (range 49-308 m³)OctNov 2025: 145 h spilling ≈ 7,388 m³ (range 2,943-18,548 m³)NovDec 2025: 290 h spilling ≈ 14,833 m³ (range 5,908-37,238 m³)Dec
Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDurationEstimated volume
19 Dec 2025, 00:00Silverstone STW1.0 days489 - 3,081 m³
6 Dec 2025, 00:00Silverstone STW1.0 days489 - 3,081 m³
15 Nov 2025, 00:00Silverstone STW1.0 days489 - 3,081 m³
14 Nov 2025, 00:00Silverstone STW1.0 days489 - 3,081 m³
27 Jan 2025, 00:00Silverstone STW1.0 days489 - 3,081 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 estimate33,730 m³range 13,435 - 84,680 m³
In litres33.7 millionrange 13.4 million - 84.7 million L
Olympic swimming pools13at 2,500 m³ each
Organic load, as people's raw sewage50,595person-days of untreated BOD

Assumed spill rate while spilling: 5.7 - 35.7 L/s (central 14.2 L/s), from a dry-weather flow of 5.7 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 537 kg3,036 kg16,936 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 1,411 kg14,335 kg61,054 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 26 kg212 kg788 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 16 kg88 kg457 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 1.3 trillion organisms33.7 trillion organisms846.8 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.

Silverstone STW

Storm tank at sewage works · discharges to Tributary River Tove River Gre

Spills 2025
69 (660 hours) · down 55% on 2024
Spills 2024
152 (2,071 hours)
Long-term average
107.2 spills a year (monitored since 2020)
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 completed
Improvement
Operational improvement completed
WFD catchment
Silverstone Brook (GB105033038250)
Outlet location
SP6708045490 (OS grid reference) · View on Apple Maps
EA ID / permit
AWS01136 · AWCNF1191

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 Anglian 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.