Storm overflow register · South West Water

St Dennis Wastewater Treatment Work sewage works: storm overflow spills 2025

St Dennis Wastewater Treatment Work sewage treatment works, run by South West Water, has 2 monitored storm overflows discharging to River Fal. In 2025 they recorded 162 spills totalling 1,435 hours, against 253 spills and 3,182 hours in 2024. By hours spilling, it ranks 72 of 334 South West Water works, where 1 spilled longest.

Works size: 2,544 population equivalent (ST DENNIS 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.

JanFebMarAprMayJunJulAugSepOctNovDecSt Dennis Wastewater…1 Jan 2025 11:57 GMT, 5.1 h3 spills, 39.3 h, 4 Jan to 6 Jan8 Jan 2025 12:42 GMT, 11 h10 Jan 2025 05:12 GMT, 41.7 h24 Jan 2025 00:53 GMT, 5.9 h2 spills, 11.1 h, 26 Jan to 26 Jan19 Feb 2025 19:50 GMT, 2.2 h21 Feb 2025 08:03 GMT, 4.7 h3 spills, 15.9 h, 23 Feb to 24 Feb2 spills, 4.6 h, 26 Feb to 26 Feb21 Mar 2025 19:46 GMT, 33 min2 spills, 17.4 h, 14 Apr to 16 Apr4 spills, 17.2 h, 18 Apr to 19 Apr3 spills, 3.3 h, 22 Apr to 23 Apr2 spills, 0.2 h, 7 Jun to 7 Jun12 Jun 2025 04:43 GMT, 3 min14 Jun 2025 03:04 GMT, 29 min28 Aug 2025 22:06 GMT, 5 min2 spills, 0.4 h, 2 Sep to 2 Sep17 Sep 2025 02:49 GMT, 4 min2 spills, 0.3 h, 20 Sep to 20 Sep19 Oct 2025 06:42 GMT, 58 min2 spills, 2.1 h, 23 Oct to 23 Oct5 spills, 6.9 h, 4 Nov to 5 Nov7 Nov 2025 12:38 GMT, 28 min9 Nov 2025 12:05 GMT, 76 min2 spills, 0.6 h, 12 Nov to 12 Nov2 spills, 4.3 h, 13 Nov to 14 Nov20 Nov 2025 12:47 GMT, 6.3 h4 spills, 6.2 h, 22 Nov to 23 Nov2 spills, 6.2 h, 24 Nov to 24 Nov27 Nov 2025 16:41 GMT, 2.3 h4 spills, 5.8 h, 29 Nov to 29 Nov11 spills, 17.2 h, 1 Dec to 2 Dec2 spills, 1.2 h, 5 Dec to 5 Dec2 spills, 3.5 h, 8 Dec to 9 Dec12 Dec 2025 00:13 GMT, 51 min3 spills, 65.1 h, 15 Dec to 18 Dec12 spills, 153.3 h, 20 Dec to 28 DecSt Dennis Wastewater…2 spills, 1 h, 1 Jan to 1 Jan72 spills, 201.2 h, 4 Jan to 13 Jan24 Jan 2025 01:01 GMT, 6.9 h487 spills, 99.7 h, 26 Jan to 6 Feb2 spills, 2.3 h, 19 Feb to 19 Feb16 spills, 8.3 h, 21 Feb to 21 Feb380 spills, 85.2 h, 23 Feb to 3 Mar8 spills, 1.5 h, 4 Mar to 4 Mar24 spills, 25.8 h, 14 Apr to 16 Apr35 spills, 33.2 h, 18 Apr to 20 Apr3 spills, 24.8 h, 22 Apr to 23 Apr26 Apr 2025 00:37 GMT, 5.9 h43 spills, 5.7 h, 6 Jun to 9 Jun44 spills, 14.3 h, 11 Jun to 14 Jun19 Jul 2025 22:25 GMT, 13 min2 spills, 0.6 h, 21 Jul to 21 Jul2 spills, 0.6 h, 29 Jul to 29 Jul4 spills, 0.4 h, 31 Jul to 31 Jul4 Aug 2025 11:39 GMT, 2.5 h28 Aug 2025 21:37 GMT, 79 min2 spills, 0.6 h, 2 Sep to 3 Sep2 spills, 0.7 h, 7 Sep to 7 Sep2 spills, 0.3 h, 8 Sep to 8 Sep17 Sep 2025 02:22 GMT, 21 min2 spills, 1 h, 20 Sep to 20 Sep4 spills, 1.3 h, 19 Oct to 19 Oct20 Oct 2025 05:27 GMT, 51 min5 spills, 1.9 h, 23 Oct to 23 Oct30 Oct 2025 20:26 GMT, 10 min12 spills, 4.1 h, 4 Nov to 5 Nov4 spills, 0.5 h, 7 Nov to 7 Nov3 spills, 0.8 h, 9 Nov to 9 Nov3 spills, 1.1 h, 12 Nov to 12 Nov5 spills, 3.4 h, 13 Nov to 14 Nov4 spills, 3.1 h, 20 Nov to 20 Nov9 spills, 3.5 h, 22 Nov to 23 Nov6 spills, 2.1 h, 24 Nov to 24 Nov2 spills, 0.3 h, 27 Nov to 27 Nov7 spills, 2.4 h, 29 Nov to 29 Nov13 spills, 3.3 h, 1 Dec to 1 Dec5 spills, 1.7 h, 3 Dec to 4 Dec43 spills, 32.7 h, 5 Dec to 9 Dec7 spills, 4.7 h, 11 Dec to 12 Dec3 spills, 10.8 h, 15 Dec to 15 Dec9 spills, 9.2 h, 17 Dec to 18 Dec2 spills, 4 h, 20 Dec to 20 Dec22 Dec 2025 02:53 GMT, 59 min

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: 391 h spilling ≈ 17,044 m³ (range 6,789-42,789 m³)JanFeb 2025: 136 h spilling ≈ 5,924 m³ (range 2,360-14,872 m³)FebMar 2025: 21 h spilling ≈ 928 m³ (range 370-2,331 m³)MarApr 2025: 128 h spilling ≈ 5,562 m³ (range 2,215-13,964 m³)AprMay 2025: 0.0 h spilling ≈ 0.0 m³ (range 0.0-0.0 m³)MayJun 2025: 21 h spilling ≈ 902 m³ (range 359-2,265 m³)JunJul 2025: 1.7 h spilling ≈ 74 m³ (range 30-186 m³)JulAug 2025: 3.9 h spilling ≈ 170 m³ (range 68-427 m³)AugSep 2025: 3.6 h spilling ≈ 157 m³ (range 63-394 m³)SepOct 2025: 7.2 h spilling ≈ 314 m³ (range 125-788 m³)OctNov 2025: 61 h spilling ≈ 2,676 m³ (range 1,066-6,719 m³)NovDec 2025: 308 h spilling ≈ 13,443 m³ (range 5,355-33,750 m³)Dec
Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDurationEstimated volume
20 Dec 2025, 19:36St Dennis Wastewater Treatment Work5.3 days2,191 - 13,811 m³
8 Jan 2025, 06:29St Dennis Wastewater Treatment Work5.2 days2,161 - 13,617 m³
15 Dec 2025, 04:03St Dennis Wastewater Treatment Work2.6 days1,072 - 6,758 m³
4 Jan 2025, 17:39St Dennis Wastewater Treatment Work2.3 days943 - 5,942 m³
10 Jan 2025, 05:12St Dennis Wastewater Treatment Work1.7 days724 - 4,563 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 estimate62,530 m³range 24,907 - 156,984 m³
In litres62.5 millionrange 24.9 million - 157.0 million L
Olympic swimming pools25at 2,500 m³ each
Organic load, as people's raw sewage93,795person-days of untreated BOD

Assumed spill rate while spilling: 4.8 - 30.4 L/s (central 12.1 L/s), from a dry-weather flow of 4.8 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 996 kg5,628 kg31,397 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 2,615 kg26,575 kg113,185 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 47 kg394 kg1,460 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 30 kg163 kg848 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 2.5 trillion organisms62.5 trillion organisms1.6 × 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.

St Dennis Wastewater Treatment Work

Storm tank at sewage works · discharges to River Fal

Spills 2025
65 (817 hours) · down 48% on 2024
Spills 2024
124 (1,929 hours)
Long-term average
170.3 spills a year (monitored since 2019)
Monitor uptime
100% of the year
High-spill reason (company)
Not asset maintenance - hydraulic capacity
Investigation
Env Act (SODRP) investigation ongoing, UWWTR investigation completed, Operational investigation completed
Improvement
Env Act (SODRP) improvement ongoing
WFD catchment
Upper River Fal (GB108048001390)
Outlet location
SW94115764 (OS grid reference) · View on Apple Maps
EA ID / permit
SBB01129 · NRA-SW-3885 · company name: ST DENNIS STW_SSO_ST DENNIS

St Dennis Wastewater Treatment Work

Inlet overflow at sewage works · discharges to River Fal

Spills 2025
97 (617 hours) · down 25% on 2024
Spills 2024
129 (1,253 hours)
Long-term average
92.5 spills a year (monitored since 2019)
Monitor uptime
100% of the year
High-spill reason (company)
Performance - SW Infiltration
Investigation
Env Act (SODRP) investigation ongoing, UWWTR investigation completed, Operational investigation completed
Improvement
Operational improvement completed
WFD catchment
Upper River Fal (GB108048001390)
Outlet location
SW94115764 (OS grid reference) · View on Apple Maps
EA ID / permit
SBB01128 · NRA-SW-3885 · company name: ST DENNIS STW_SO_ST DENNIS

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.