Storm overflow register · South West Water

South Molton sewage works: storm overflow spills 2025

South Molton sewage treatment works, run by South West Water, has 2 monitored storm overflows discharging to River Mole. In 2025 they recorded 261 spills totalling 2,554 hours, against 278 spills and 3,235 hours in 2024. By hours spilling, it ranks 19 of 334 South West Water works, where 1 spilled longest.

Works size: 5,794 population equivalent (SOUTH MOLTON 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.

JanFebMarAprMayJunJulAugSepOctNovDecSouth Molton WwTW14 spills, 14.6 h, 1 Jan to 2 Jan13 spills, 47.8 h, 5 Jan to 7 Jan9 spills, 2.7 h, 9 Jan to 9 Jan16 spills, 2.1 h, 10 Jan to 10 Jan6 spills, 0.7 h, 11 Jan to 12 Jan20 spills, 13.6 h, 23 Jan to 24 Jan61 spills, 117.4 h, 26 Jan to 1 Feb2 Feb 2025 20:12 GMT, 15 min16 spills, 15.3 h, 14 Feb to 15 Feb86 spills, 111.5 h, 20 Feb to 28 Feb95 spills, 85.3 h, 15 Apr to 21 Apr17 spills, 32.9 h, 22 Apr to 24 Apr9 spills, 13.2 h, 26 Apr to 26 Apr3 spills, 8.1 h, 27 May to 27 May5 spills, 2.6 h, 6 Jun to 6 Jun4 spills, 4.3 h, 7 Jun to 7 Jun31 spills, 66.8 h, 12 Jun to 16 Jun7 spills, 2.6 h, 5 Jul to 5 Jul32 spills, 20.8 h, 20 Jul to 21 Jul1 Aug 2025 12:57 GMT, 2.3 h4 Aug 2025 13:24 GMT, 2 h5 spills, 3.6 h, 29 Aug to 29 Aug2 spills, 1.3 h, 30 Aug to 30 Aug5 spills, 5.9 h, 1 Sep to 1 Sep14 spills, 30.9 h, 2 Sep to 4 Sep62 spills, 198.6 h, 9 Sep to 21 Sep12 spills, 20.6 h, 3 Oct to 4 Oct13 spills, 45.6 h, 19 Oct to 21 Oct56 spills, 290.9 h, 22 Oct to 7 Nov25 spills, 225.3 h, 9 Nov to 19 Nov73 spills, 426.9 h, 22 Nov to 13 Dec19 spills, 197.4 h, 15 Dec to 23 DecSouth Molton WwTW2 spills, 5.4 h, 1 Jan to 1 Jan2 spills, 50.2 h, 4 Jan to 7 Jan4 spills, 4.5 h, 23 Jan to 24 Jan7 spills, 42.4 h, 26 Jan to 29 Jan31 Jan 2025 03:19 GMT, 2.3 h14 Feb 2025 11:28 GMT, 39 min15 Feb 2025 07:27 GMT, 19 min2 spills, 0.5 h, 19 Feb to 20 Feb4 spills, 3.8 h, 21 Feb to 21 Feb2 spills, 9.9 h, 23 Feb to 24 Feb2 spills, 3.7 h, 26 Feb to 26 Feb28 Mar 2025 03:17 GMT, 14 min6 spills, 15.7 h, 15 Apr to 16 Apr94 spills, 79.2 h, 17 Apr to 21 Apr3 spills, 5.9 h, 22 Apr to 23 Apr11 May 2025 14:44 GMT, 10 min12 May 2025 13:44 GMT, 12 min2 spills, 1 h, 21 May to 21 May2 spills, 2.3 h, 27 May to 27 May3 Jun 2025 08:24 GMT, 56 min3 spills, 2.3 h, 6 Jun to 6 Jun2 spills, 2.7 h, 7 Jun to 7 Jun2 spills, 0.5 h, 10 Jun to 10 Jun31 spills, 37.5 h, 12 Jun to 14 Jun22 Jun 2025 22:13 GMT, 19 min2 spills, 0.5 h, 24 Jun to 24 Jun26 Jun 2025 06:59 GMT, 18 min3 spills, 3.6 h, 21 Jul to 21 Jul29 Jul 2025 08:12 GMT, 4 min2 spills, 0.5 h, 31 Jul to 31 Jul4 Aug 2025 12:01 GMT, 2.8 h28 Aug 2025 22:43 GMT, 112 min2 spills, 1.3 h, 30 Aug to 30 Aug3 spills, 2.2 h, 1 Sep to 1 Sep8 spills, 6.5 h, 2 Sep to 4 Sep2 spills, 1.1 h, 6 Sep to 7 Sep8 Sep 2025 18:02 GMT, 25 min12 spills, 6.7 h, 9 Sep to 13 Sep5 spills, 12.1 h, 14 Sep to 15 Sep4 spills, 2.2 h, 17 Sep to 18 Sep3 spills, 3.1 h, 20 Sep to 20 Sep7 spills, 6 h, 3 Oct to 4 Oct3 spills, 1.3 h, 18 Oct to 18 Oct9 spills, 9.7 h, 19 Oct to 21 Oct22 Oct 2025 16:51 GMT, 2.4 h7 spills, 6 h, 23 Oct to 24 Oct25 Oct 2025 21:36 GMT, 31 min2 spills, 4.7 h, 26 Oct to 26 Oct28 Oct 2025 04:34 GMT, 25 min2 spills, 1.1 h, 29 Oct to 29 Oct30 Oct 2025 20:51 GMT, 88 min5 spills, 3.6 h, 1 Nov to 2 Nov2 spills, 2 h, 4 Nov to 5 Nov2 spills, 0.1 h, 7 Nov to 8 Nov4 spills, 8.5 h, 9 Nov to 10 Nov5 spills, 11.5 h, 11 Nov to 12 Nov7 spills, 34.6 h, 13 Nov to 15 Nov3 spills, 1.5 h, 18 Nov to 18 Nov7 spills, 13.6 h, 22 Nov to 24 Nov27 Nov 2025 18:01 GMT, 8 min2 spills, 2.5 h, 29 Nov to 29 Nov11 spills, 10.3 h, 1 Dec to 2 Dec12 spills, 31.7 h, 4 Dec to 7 Dec2 spills, 20.5 h, 8 Dec to 10 Dec12 Dec 2025 04:13 GMT, 70 min5 spills, 27.1 h, 15 Dec to 16 Dec5 spills, 17.7 h, 17 Dec to 18 Dec3 spills, 1.3 h, 20 Dec to 20 Dec2 spills, 1 h, 22 Dec to 22 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.

050k100k150kJan 2025: 297 h spilling ≈ 29,455 m³ (range 11,733-73,949 m³)JanFeb 2025: 153 h spilling ≈ 15,160 m³ (range 6,038-38,059 m³)FebMar 2025: 0.2 h spilling ≈ 20 m³ (range 7.9-50 m³)MarApr 2025: 232 h spilling ≈ 23,042 m³ (range 9,178-57,848 m³)AprMay 2025: 12 h spilling ≈ 1,162 m³ (range 463-2,916 m³)MayJun 2025: 119 h spilling ≈ 11,784 m³ (range 4,694-29,585 m³)JunJul 2025: 28 h spilling ≈ 2,740 m³ (range 1,091-6,879 m³)JulAug 2025: 15 h spilling ≈ 1,509 m³ (range 601-3,788 m³)AugSep 2025: 270 h spilling ≈ 26,765 m³ (range 10,661-67,195 m³)SepOct 2025: 285 h spilling ≈ 28,304 m³ (range 11,274-71,058 m³)OctNov 2025: 555 h spilling ≈ 55,069 m³ (range 21,935-138,253 m³)NovDec 2025: 589 h spilling ≈ 58,454 m³ (range 23,284-146,752 m³)Dec
Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDurationEstimated volume
5 Dec 2025, 05:23South Molton WwTW5.9 days5,552 - 34,993 m³
11 Nov 2025, 06:13South Molton WwTW5.8 days5,466 - 34,453 m³
15 Dec 2025, 03:26South Molton WwTW4.9 days4,693 - 29,576 m³
30 Nov 2025, 16:06South Molton WwTW2.5 days2,366 - 14,913 m³
23 Oct 2025, 16:28South Molton WwTW2.3 days2,218 - 13,982 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 estimate253,533 m³range 100,988 - 636,505 m³
In litres253.5 millionrange 101.0 million - 636.5 million L
Olympic swimming pools101at 2,500 m³ each
Organic load, as people's raw sewage380,300person-days of untreated BOD

Assumed spill rate while spilling: 11.0 - 69.2 L/s (central 27.6 L/s), from a dry-weather flow of 11.0 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 4,040 kg22,818 kg127,301 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 10,604 kg107,752 kg458,920 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 192 kg1,597 kg5,920 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 121 kg659 kg3,437 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 10.1 trillion organisms253.5 trillion organisms6.4 × 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.

South Molton WwTW

Storm tank at sewage works · discharges to River Mole

Spills 2025
143 (2,014 hours) · down 16% on 2024
Spills 2024
170 (2,729 hours)
Long-term average
102.3 spills a year (monitored since 2017)
Monitor uptime
100% of the year
High-spill reason (company)
Not asset maintenance - hydraulic capacity
Investigation
Env Act (SODRP) investigation ongoing, Operational investigation completed
Improvement
Env Act (SODRP) improvement ongoing
WFD catchment
Mole (Yeo to Burcombe Stream) (GB108050014130)
Outlet location
SS72252572 (OS grid reference) · View on Apple Maps
EA ID / permit
SBB01101 · NRA-SW-5654 · company name: SOUTH MOLTON STW_SSO_SOUTH MOLTON

South Molton WwTW

Inlet overflow at sewage works · discharges to River Mole

Spills 2025
118 (540 hours) · up 9% on 2024
Spills 2024
108 (506 hours)
Long-term average
117.6 spills a year (monitored since 2017)
Monitor uptime
98% of the year
High-spill reason (company)
Not asset maintenance - hydraulic capacity
Investigation
Env Act (SODRP) investigation ongoing, Operational investigation completed
Improvement
Env Act (SODRP) improvement ongoing
WFD catchment
Mole (Yeo to Burcombe Stream) (GB108050014130)
Outlet location
SS72252572 (OS grid reference) · View on Apple Maps
EA ID / permit
SBB01100 · NRA-SW-5654 · company name: SOUTH MOLTON STW_SO_SOUTH MOLTON

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.