Storm overflow register · Severn Trent Water

Shrewsbury Monkmoor sewage works: storm overflow spills 2025

Shrewsbury Monkmoor sewage treatment works, run by Severn Trent Water, has 2 monitored storm overflows discharging to River Severn. In 2025 they recorded 53 spills totalling 254 hours, against 127 spills and 1,196 hours in 2024. By hours spilling, it ranks 143 of 435 Severn Trent Water works, where 1 spilled longest.

Works size: 105,643 population equivalent (SHREWSBURY 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.

JanFebMarAprMayJunJulAugSepOctNovDecShrewsbury Monkmoor…3 spills, 15.1 h, 1 Jan to 1 Jan5 Jan 2025 04:50 GMT, 28.5 h2 spills, 0.3 h, 9 Jan to 9 Jan11 spills, 9.4 h, 27 Jan to 27 Jan23 Feb 2025 22:35 GMT, 78 min6 spills, 0.8 h, 23 Apr to 23 Apr20 May 2025 17:16 GMT, 6.2 h2 spills, 1.2 h, 27 May to 27 May2 spills, 1.7 h, 13 Jun to 14 Jun4 spills, 0.6 h, 6 Jul to 6 Jul20 Jul 2025 17:30 GMT, 4.6 h4 Sep 2025 10:52 GMT, 6.5 h3 spills, 1.9 h, 7 Sep to 7 Sep3 spills, 2.6 h, 14 Sep to 14 Sep8 spills, 0.7 h, 17 Sep to 17 Sep2 spills, 15.5 h, 20 Sep to 21 Sep4 Nov 2025 19:02 GMT, 4.8 h8 spills, 0.8 h, 11 Nov to 11 Nov14 Nov 2025 07:29 GMT, 16.7 h3 spills, 0.2 h, 19 Nov to 19 Nov5 spills, 9.2 h, 29 Nov to 29 Nov10 spills, 20 h, 5 Dec to 6 Dec17 spills, 30.5 h, 15 Dec to 16 Dec11 spills, 13.8 h, 17 Dec to 19 DecShrewsbury Monkmoor…32 spills, 4.4 h, 5 Jan to 6 Jan8 spills, 0.6 h, 27 Jan to 27 Jan4 spills, 0.1 h, 23 Apr to 23 Apr18 spills, 1 h, 20 May to 20 May2 spills, 0 h, 27 May to 27 May5 spills, 0.3 h, 13 Jun to 14 Jun3 spills, 0.1 h, 6 Jul to 6 Jul14 spills, 0.8 h, 20 Jul to 20 Jul3 spills, 0.3 h, 4 Sep to 4 Sep3 spills, 2.5 h, 20 Sep to 20 Sep104 spills, 43.9 h, 14 Nov to 17 Nov2 spills, 0.2 h, 19 Nov to 19 Nov13 spills, 6.8 h, 29 Nov to 29 Nov5 spills, 0.3 h, 1 Dec to 1 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.

0100k200k300k400kJan 2025: 58 h spilling ≈ 105,531 m³ (range 42,035-264,939 m³)JanFeb 2025: 1.3 h spilling ≈ 2,353 m³ (range 937-5,908 m³)FebMar 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)MarApr 2025: 0.9 h spilling ≈ 1,629 m³ (range 649-4,090 m³)AprMay 2025: 8.4 h spilling ≈ 15,205 m³ (range 6,057-38,173 m³)MayJun 2025: 2.0 h spilling ≈ 3,620 m³ (range 1,442-9,089 m³)JunJul 2025: 5.9 h spilling ≈ 10,680 m³ (range 4,254-26,812 m³)JulAug 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)AugSep 2025: 30 h spilling ≈ 54,304 m³ (range 21,630-136,332 m³)SepOct 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)OctNov 2025: 83 h spilling ≈ 149,336 m³ (range 59,484-374,914 m³)NovDec 2025: 65 h spilling ≈ 116,935 m³ (range 46,577-293,569 m³)Dec
Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDurationEstimated volume
5 Jan 2025, 04:50Shrewsbury Monkmoor STW1.2 days20,573 - 129,667 m³
14 Nov 2025, 22:49Shrewsbury Monkmoor STW21.7 h15,670 - 98,765 m³
14 Nov 2025, 07:29Shrewsbury Monkmoor STW16.7 h12,041 - 75,892 m³
15 Dec 2025, 07:54Shrewsbury Monkmoor STW16.1 h11,596 - 73,089 m³
6 Dec 2025, 06:45Shrewsbury Monkmoor STW11.1 h8,003 - 50,443 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 estimate459,593 m³range 183,065 - 1.2 million m³
In litres459.6 millionrange 183.1 million - 1.2 billion L
Olympic swimming pools184at 2,500 m³ each
Organic load, as people's raw sewage689,389person-days of untreated BOD

Assumed spill rate while spilling: 200.3 - 1,262.3 L/s (central 502.8 L/s), from a dry-weather flow of 200.3 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 7,323 kg41,363 kg230,765 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 19,222 kg195,327 kg831,908 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 348 kg2,895 kg10,731 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 220 kg1,195 kg6,231 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 18.3 trillion organisms459.6 trillion organisms1.2 × 10¹⁶ 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.

Shrewsbury Monkmoor STW

Storm tank at sewage works · discharges to River Severn

Spills 2025
34 (193 hours) · down 64% on 2024
Spills 2024
94 (1,124 hours)
Long-term average
115.7 spills a year (monitored since 2020)
Monitor uptime
100% of the year
High-spill reason (company)
Not asset maintenance - Hydraulic capacity
Investigation
Env Act (SODRP) investigation ongoing, UWWTR investigation ongoing, Operational investigation completed
Improvement
Operational improvement completed
WFD catchment
Severn - Sundorne Bk to conf M Wenlock-Farley Bk (GB109054049141)
Outlet location
SJ5204513263 (OS grid reference) · View on Apple Maps
EA ID / permit
SVT02039 · S/02/56373/R · company name: MONKMOOR STW (SSTO)

Shrewsbury Monkmoor STW

Inlet overflow at sewage works · discharges to River Severn

Spills 2025
19 (61 hours) · down 42% on 2024
Spills 2024
33 (71 hours)
Long-term average
29.6 spills a year (monitored since 2021)
Monitor uptime
100% of the year
Investigation
Env Act (SODRP) investigation ongoing, UWWTR investigation ongoing, Operational investigation ongoing
Improvement
No improvement action in reporting period
WFD catchment
Severn - Sundorne Bk to conf M Wenlock-Farley Bk (GB109054049141)
Outlet location
SJ5204513263 (OS grid reference) · View on Apple Maps
EA ID / permit
SVT02038 · S/02/56373/R · company name: MONKMOOR STW (CSO)

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 Severn Trent 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.