Storm overflow register · Wessex Water

Yeovil Without sewage works: storm overflow spills 2025

Yeovil Without sewage treatment works, run by Wessex Water, has 2 monitored storm overflows discharging to Tributary of the Bearley Brook. In 2025 they recorded 89 spills totalling 752 hours, against 169 spills and 1,443 hours in 2024. By hours spilling, it ranks 47 of 242 Wessex Water works, where 1 spilled longest.

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.

JanFebMarAprMayJunJulAugSepOctNovDecYeovil Without STW4 spills, 3.8 h, 1 Jan to 1 Jan8 spills, 74.5 h, 5 Jan to 8 Jan3 spills, 21.8 h, 24 Jan to 25 Jan6 spills, 90.5 h, 26 Jan to 30 Jan5 spills, 7.5 h, 31 Jan to 31 Jan4 spills, 4.3 h, 21 Feb to 21 Feb42 spills, 65.9 h, 23 Feb to 27 Feb4 spills, 1.9 h, 20 Oct to 20 Oct12 spills, 10.1 h, 10 Nov to 10 Nov9 spills, 16.4 h, 11 Nov to 12 Nov18 spills, 15.4 h, 14 Nov to 15 Nov22 spills, 22 h, 1 Dec to 2 Dec66 spills, 51.8 h, 4 Dec to 7 Dec25 spills, 40.9 h, 8 Dec to 10 Dec147 spills, 145.5 h, 16 Dec to 23 DecYeovil Without STW1 Jan 2025 10:52 GMT, 15 min29 spills, 26.4 h, 4 Jan to 6 Jan12 spills, 7.1 h, 23 Jan to 24 Jan25 spills, 52.8 h, 26 Jan to 28 Jan2 spills, 0.3 h, 31 Jan to 31 Jan12 spills, 15.7 h, 23 Feb to 25 Feb6 spills, 2.9 h, 26 Feb to 26 Feb21 Apr 2025 15:02 GMT, 2 min21 May 2025 11:10 GMT, 17 min3 Jun 2025 11:35 GMT, 6 min7 Jun 2025 15:24 GMT, 12 min12 Jun 2025 09:45 GMT, 38 min4 spills, 0.5 h, 20 Jul to 20 Jul2 spills, 1.2 h, 29 Aug to 29 Aug1 Sep 2025 13:05 GMT, 42 min3 spills, 0.7 h, 3 Sep to 4 Sep3 spills, 0.2 h, 7 Sep to 7 Sep10 Sep 2025 09:38 GMT, 20 min11 Sep 2025 13:03 GMT, 16 min3 Oct 2025 23:21 GMT, 2 min20 Oct 2025 09:20 GMT, 38 min31 Oct 2025 23:15 GMT, 33 min4 Nov 2025 16:45 GMT, 4 min5 spills, 2.1 h, 10 Nov to 10 Nov10 spills, 3.1 h, 11 Nov to 12 Nov4 spills, 7.2 h, 14 Nov to 14 Nov29 Nov 2025 07:57 GMT, 15 min11 spills, 3.5 h, 1 Dec to 1 Dec3 spills, 3.5 h, 4 Dec to 4 Dec8 spills, 6.7 h, 5 Dec to 5 Dec11 spills, 18 h, 9 Dec to 9 Dec8 spills, 5.7 h, 15 Dec to 16 Dec20 spills, 17.5 h, 17 Dec to 19 Dec2 spills, 0.8 h, 21 Dec to 21 Dec

Hours spilling by month, 2025

0100200300400Jan 2025: 285 hours spillingJanFeb 2025: 89 hours spillingFebMar 2025: 0.0 hours spillingMarApr 2025: 0.0 hours spillingAprMay 2025: 0.3 hours spillingMayJun 2025: 0.9 hours spillingJunJul 2025: 0.5 hours spillingJulAug 2025: 1.2 hours spillingAugSep 2025: 2.2 hours spillingSepOct 2025: 3.1 hours spillingOctNov 2025: 55 hours spillingNovDec 2025: 316 hours spillingDec
Hours, measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDuration
26 Jan 2025, 03:00Yeovil Without STW3.5 days
17 Dec 2025, 20:57Yeovil Without STW2.2 days
5 Jan 2025, 00:15Yeovil Without STW2.1 days
26 Jan 2025, 09:42Yeovil Without STW1.9 days
23 Feb 2025, 21:30Yeovil Without STW1.7 days

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.

This works serves fewer than 2,000 people, or was not matched to a UWWTD report, so its size is not published. The population equivalent above is a placeholder of 1,000. Enter the population served if you know it.

Volume, central estimate12,883 m³range 5,132 - 32,344 m³
In litres12.9 millionrange 5.1 million - 32.3 million L
Olympic swimming pools5.2at 2,500 m³ each
Organic load, as people's raw sewage19,325person-days of untreated BOD

Assumed spill rate while spilling: 1.9 - 11.9 L/s (central 4.8 L/s), from a dry-weather flow of 1.9 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 205 kg1,160 kg6,469 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 539 kg5,475 kg23,320 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 9.8 kg81 kg301 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 6.2 kg33 kg175 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 513.2 billion organisms12.9 trillion organisms323.4 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.

Yeovil Without STW

Storm tank at sewage works · discharges to Tributary of the Bearley Brook

Spills 2025
44 (572 hours) · down 56% on 2024
Spills 2024
101 (1,142 hours)
Long-term average
73.0 spills a year (monitored since 2023)
Monitor uptime
100% of the year
High-spill reason (company)
Not Applicable - Ongoing investigation
Investigation
No investigation activity in reporting period
Improvement
No improvement action in reporting period
WFD catchment
Bearley Brook (GB108052015610)
Outlet location
ST5607018720 (OS grid reference) · View on Apple Maps
EA ID / permit
WXW01295 · 071670 · company name: YEOVIL WITHOUT OFF STONE LANE STORM TANK

Yeovil Without STW

Inlet overflow at sewage works · discharges to Tributary of the Bearley Brook

Spills 2025
45 (180 hours) · down 34% on 2024
Spills 2024
68 (301 hours)
Long-term average
62.7 spills a year (monitored since 2023)
Monitor uptime
100% of the year
High-spill reason (company)
Not Applicable - Ongoing investigation
Investigation
No investigation activity in reporting period
Improvement
No improvement action in reporting period
WFD catchment
Bearley Brook (GB108052015610)
Outlet location
ST5607018720 (OS grid reference) · View on Apple Maps
EA ID / permit
WXW01294 · 071670 · company name: YEOVIL WITHOUT OFF STONE LANE INLET

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 Wessex 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.