Storm overflow register · South West Water

Gunnislake sewage works: storm overflow spills 2025

Gunnislake sewage treatment works, run by South West Water, has 2 monitored storm overflows discharging to River Tamar (Estuarine). In 2025 they recorded 166 spills totalling 1,396 hours, against 244 spills and 3,285 hours in 2024. By hours spilling, it ranks 75 of 334 South West Water works, where 1 spilled longest.

Works size: 2,121 population equivalent (GUNNISLAKE 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.

JanFebMarAprMayJunJulAugSepOctNovDecGunnislake WwTW9 spills, 13.7 h, 1 Jan to 2 Jan24 spills, 42 h, 4 Jan to 7 Jan9 spills, 45.2 h, 8 Jan to 10 Jan8 spills, 21.3 h, 13 Jan to 15 Jan2 spills, 20.3 h, 24 Jan to 25 Jan2 spills, 26.4 h, 26 Jan to 27 Jan7 spills, 3.3 h, 28 Jan to 29 Jan22 spills, 8.9 h, 31 Jan to 1 Feb13 spills, 25.7 h, 2 Feb to 5 Feb19 Feb 2025 20:39 GMT, 2.7 h21 spills, 35.2 h, 20 Feb to 24 Feb2 spills, 2.9 h, 3 Mar to 3 Mar5 spills, 16.5 h, 4 Mar to 5 Mar17 spills, 17.6 h, 15 Apr to 16 Apr12 spills, 15.2 h, 18 Apr to 19 Apr2 spills, 2.4 h, 22 Apr to 23 Apr2 spills, 0.5 h, 11 May to 11 May3 Jun 2025 08:44 GMT, 96 min2 spills, 30.1 h, 5 Jun to 7 Jun5 spills, 8.4 h, 12 Jun to 12 Jun5 spills, 7.2 h, 13 Jun to 13 Jun2 spills, 3.1 h, 28 Aug to 29 Aug30 Aug 2025 17:24 GMT, 20 min6 spills, 10.4 h, 3 Sep to 3 Sep4 spills, 8 h, 8 Sep to 8 Sep6 spills, 8.5 h, 10 Sep to 10 Sep2 spills, 2.5 h, 27 Sep to 28 Sep3 Oct 2025 20:27 GMT, 6.1 h5 spills, 144.9 h, 19 Oct to 26 Oct3 spills, 142.1 h, 4 Nov to 10 Nov5 spills, 2.4 h, 11 Nov to 12 Nov17 Nov 2025 12:46 GMT, 111 min4 spills, 87 h, 24 Nov to 28 Nov14 spills, 25.2 h, 30 Nov to 4 Dec5 Dec 2025 23:19 GMT, 15.9 h2 spills, 106.3 h, 9 Dec to 13 Dec5 spills, 0.3 h, 15 Dec to 16 Dec24 Dec 2025 10:38 GMT, 8 h29 Dec 2025 13:34 GMT, 2 h3 spills, 34 h, 30 Dec to 1 JanGunnislake WwTW1 Jan 2025 10:10 GMT, 10 min11 spills, 34.2 h, 4 Jan to 7 Jan4 spills, 4.5 h, 23 Jan to 24 Jan7 spills, 23.5 h, 26 Jan to 28 Jan19 Feb 2025 19:40 GMT, 100 min2 spills, 0.5 h, 21 Feb to 21 Feb3 spills, 25.7 h, 23 Feb to 24 Feb26 Feb 2025 06:22 GMT, 70 min21 Mar 2025 19:27 GMT, 8 min2 spills, 1.2 h, 15 Apr to 15 Apr3 spills, 0.4 h, 18 Apr to 18 Apr2 spills, 1.7 h, 22 Apr to 22 Apr11 May 2025 09:49 GMT, 12 min27 May 2025 13:17 GMT, 8 min5 Jun 2025 08:19 GMT, 4 min2 spills, 1.3 h, 7 Jun to 7 Jun3 spills, 1.9 h, 12 Jun to 12 Jun2 spills, 0.5 h, 13 Jun to 13 Jun20 Jul 2025 00:24 GMT, 23 min27 Aug 2025 11:13 GMT, 10 min2 spills, 0.4 h, 28 Aug to 28 Aug4 spills, 1.2 h, 2 Sep to 4 Sep2 spills, 2.4 h, 8 Sep to 8 Sep9 Sep 2025 14:53 GMT, 2 min14 Sep 2025 11:41 GMT, 12 min15 Sep 2025 06:50 GMT, 9 min2 spills, 0.4 h, 20 Sep to 20 Sep27 Sep 2025 20:36 GMT, 37 min3 Oct 2025 20:27 GMT, 5 min5 spills, 3.7 h, 19 Oct to 20 Oct30 Oct 2025 21:02 GMT, 20 min8 spills, 10.2 h, 4 Nov to 5 Nov7 Nov 2025 13:11 GMT, 19 min9 spills, 6 h, 9 Nov to 10 Nov44 spills, 49.9 h, 11 Nov to 14 Nov7 spills, 4.5 h, 22 Nov to 23 Nov2 spills, 0.4 h, 24 Nov to 24 Nov3 spills, 1.1 h, 28 Nov to 29 Nov16 spills, 28.5 h, 30 Nov to 2 Dec4 spills, 1.1 h, 4 Dec to 4 Dec98 spills, 121 h, 5 Dec to 12 Dec10 spills, 107.3 h, 15 Dec to 19 Dec2 spills, 1.2 h, 20 Dec to 20 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: 235 h spilling ≈ 8,555 m³ (range 3,408-21,478 m³)JanFeb 2025: 101 h spilling ≈ 3,660 m³ (range 1,458-9,188 m³)FebMar 2025: 20 h spilling ≈ 709 m³ (range 282-1,779 m³)MarApr 2025: 39 h spilling ≈ 1,399 m³ (range 557-3,513 m³)AprMay 2025: 0.8 h spilling ≈ 29 m³ (range 12-73 m³)MayJun 2025: 51 h spilling ≈ 1,853 m³ (range 738-4,653 m³)JunJul 2025: 0.4 h spilling ≈ 15 m³ (range 5.8-36 m³)JulAug 2025: 3.9 h spilling ≈ 142 m³ (range 56-356 m³)AugSep 2025: 34 h spilling ≈ 1,243 m³ (range 495-3,120 m³)SepOct 2025: 155 h spilling ≈ 5,637 m³ (range 2,245-14,151 m³)OctNov 2025: 314 h spilling ≈ 11,422 m³ (range 4,550-28,676 m³)NovDec 2025: 442 h spilling ≈ 16,067 m³ (range 6,400-40,336 m³)Dec
Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDurationEstimated volume
4 Nov 2025, 11:19Gunnislake WwTW5.2 days1,807 - 11,387 m³
9 Dec 2025, 01:58Gunnislake WwTW4.4 days1,531 - 9,651 m³
22 Oct 2025, 16:11Gunnislake WwTW3.4 days1,180 - 7,439 m³
24 Nov 2025, 15:44Gunnislake WwTW3.2 days1,126 - 7,098 m³
8 Dec 2025, 20:52Gunnislake WwTW2.6 days920 - 5,797 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 estimate50,734 m³range 20,208 - 127,369 m³
In litres50.7 millionrange 20.2 million - 127.4 million L
Olympic swimming pools20at 2,500 m³ each
Organic load, as people's raw sewage76,100person-days of untreated BOD

Assumed spill rate while spilling: 4.0 - 25.3 L/s (central 10.1 L/s), from a dry-weather flow of 4.0 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 808 kg4,566 kg25,474 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 2,122 kg21,562 kg91,833 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 38 kg320 kg1,185 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 24 kg132 kg688 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 2.0 trillion organisms50.7 trillion organisms1.3 × 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.

Gunnislake WwTW

Storm tank at sewage works · discharges to River Tamar (Estuarine)

Spills 2025
94 (956 hours) · down 42% on 2024
Spills 2024
161 (3,051 hours)
Long-term average
119.5 spills a year (monitored since 2019)
Monitor uptime
100% of the year
High-spill reason (company)
Not asset maintenance - hydraulic capacity
Investigation
UWWTR investigation ongoing, Operational investigation completed
Improvement
Operational improvement ongoing
WFD catchment
Not part of a WFD catchment
Outlet location
SX43437067 (OS grid reference) · View on Apple Maps
EA ID / permit
SBB00523 · SWWA 2413 · company name: GUNNISLAKE STW_SSO_GUNNISLAKE

Gunnislake WwTW

Inlet overflow at sewage works · discharges to River Tamar (Estuarine)

Spills 2025
72 (440 hours) · down 13% on 2024
Spills 2024
83 (233 hours)
Long-term average
68.0 spills a year (monitored since 2019)
Monitor uptime
100% of the year
High-spill reason (company)
Not asset maintenance - hydraulic capacity
Investigation
UWWTR investigation ongoing, Operational investigation completed
Improvement
Operational improvement ongoing
WFD catchment
Not part of a WFD catchment
Outlet location
SX43437067 (OS grid reference) · View on Apple Maps
EA ID / permit
SBB00522 · SWWA 2413 · company name: GUNNISLAKE STW_SO_GUNNISLAKE

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 South West 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.