Storm overflow register · South West Water

Honiton sewage works: storm overflow spills 2025

Honiton sewage treatment works, run by South West Water, has 2 monitored storm overflows discharging to River Otter. In 2025 they recorded 223 spills totalling 2,425 hours, against 369 spills and 4,899 hours in 2024. By hours spilling, it ranks 24 of 334 South West Water works, where 1 spilled longest.

Works size: 13,839 population equivalent (HONITON 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.

JanFebMarAprMayJunJulAugSepOctNovDecHoniton Wastewater…8 spills, 4.9 h, 1 Jan to 1 Jan81 spills, 80 h, 4 Jan to 9 Jan3 spills, 2.4 h, 20 Jan to 20 Jan12 spills, 1.4 h, 21 Jan to 22 Jan76 spills, 101.2 h, 23 Jan to 29 Jan15 spills, 3.5 h, 31 Jan to 31 Jan4 Feb 2025 14:52 GMT, 22 min14 Feb 2025 16:54 GMT, 41 min55 spills, 30.8 h, 19 Feb to 22 Feb32 spills, 60 h, 23 Feb to 27 Feb21 Mar 2025 16:20 GMT, 109 min2 spills, 1 h, 3 Apr to 3 Apr3 spills, 2.5 h, 15 Apr to 16 Apr20 spills, 35.2 h, 18 Apr to 20 Apr21 Apr 2025 05:30 GMT, 30 min7 spills, 9.5 h, 22 Apr to 23 Apr2 spills, 0.8 h, 21 May to 21 May3 Jun 2025 09:58 GMT, 28 min2 spills, 0.8 h, 5 Jun to 5 Jun6 spills, 5.1 h, 6 Jun to 7 Jun2 spills, 5.2 h, 12 Jun to 12 Jun22 Jun 2025 10:53 GMT, 22 min3 Jul 2025 10:28 GMT, 2 min2 spills, 0.6 h, 17 Jul to 17 Jul5 spills, 4.5 h, 19 Jul to 20 Jul21 Jul 2025 09:30 GMT, 15 min27 Jul 2025 13:52 GMT, 30 min4 Aug 2025 14:38 GMT, 10 min27 Aug 2025 13:10 GMT, 32 min9 spills, 9.6 h, 28 Aug to 29 Aug116 spills, 79.1 h, 30 Aug to 6 Sep69 spills, 53.9 h, 7 Sep to 13 Sep13 spills, 4.3 h, 14 Sep to 15 Sep12 spills, 2.4 h, 17 Sep to 17 Sep20 Sep 2025 13:16 GMT, 3 min22 spills, 7.7 h, 2 Oct to 4 Oct5 Oct 2025 09:14 GMT, 3 min42 spills, 37.6 h, 19 Oct to 21 Oct93 spills, 24.5 h, 22 Oct to 26 Oct5 spills, 0.5 h, 28 Oct to 28 Oct25 spills, 6.2 h, 30 Oct to 2 Nov2 spills, 0.2 h, 3 Nov to 3 Nov25 spills, 34.4 h, 4 Nov to 6 Nov26 spills, 10 h, 7 Nov to 8 Nov234 spills, 159 h, 9 Nov to 20 Nov74 spills, 64.2 h, 22 Nov to 26 Nov5 spills, 0.2 h, 27 Nov to 28 Nov469 spills, 478.6 h, 29 Nov to 25 Dec5 spills, 0.3 h, 26 Dec to 26 Dec4 spills, 0.2 h, 27 Dec to 27 DecHoniton Wastewater…3 spills, 5.3 h, 1 Jan to 1 Jan9 spills, 103 h, 4 Jan to 9 Jan6 spills, 38 h, 23 Jan to 25 Jan26 Jan 2025 10:53 GMT, 84.4 h3 spills, 3.1 h, 31 Jan to 31 Jan6 spills, 48.3 h, 20 Feb to 22 Feb6 spills, 89.8 h, 23 Feb to 27 Feb2 spills, 38.4 h, 18 Apr to 20 Apr2 spills, 10 h, 23 Apr to 23 Apr2 spills, 6.6 h, 6 Jun to 7 Jun3 spills, 4.2 h, 12 Jun to 12 Jun2 spills, 3.6 h, 29 Aug to 29 Aug31 Aug 2025 11:57 GMT, 2.4 h2 spills, 29.7 h, 19 Oct to 20 Oct8 spills, 15 h, 22 Oct to 23 Oct4 Nov 2025 16:11 GMT, 30 h7 Nov 2025 18:22 GMT, 5 h3 spills, 29.6 h, 9 Nov to 10 Nov9 spills, 101.5 h, 11 Nov to 16 Nov5 spills, 57.3 h, 22 Nov to 24 Nov3 spills, 2.9 h, 29 Nov to 29 Nov21 spills, 234 h, 1 Dec to 13 Dec5 spills, 102.7 h, 15 Dec to 19 Dec17 spills, 51.2 h, 20 Dec to 23 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.

0200k400k600kJan 2025: 427 h spilling ≈ 101,275 m³ (range 40,340-254,256 m³)JanFeb 2025: 230 h spilling ≈ 54,538 m³ (range 21,724-136,921 m³)FebMar 2025: 1.8 h spilling ≈ 427 m³ (range 170-1,072 m³)MarApr 2025: 97 h spilling ≈ 23,025 m³ (range 9,171-57,804 m³)AprMay 2025: 0.8 h spilling ≈ 190 m³ (range 76-476 m³)MayJun 2025: 23 h spilling ≈ 5,406 m³ (range 2,153-13,573 m³)JunJul 2025: 5.9 h spilling ≈ 1,399 m³ (range 557-3,512 m³)JulAug 2025: 34 h spilling ≈ 8,086 m³ (range 3,221-20,300 m³)AugSep 2025: 122 h spilling ≈ 28,905 m³ (range 11,514-72,568 m³)SepOct 2025: 119 h spilling ≈ 28,170 m³ (range 11,221-70,723 m³)OctNov 2025: 507 h spilling ≈ 120,198 m³ (range 47,877-301,761 m³)NovDec 2025: 856 h spilling ≈ 203,049 m³ (range 80,879-509,762 m³)Dec
Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDurationEstimated volume
5 Dec 2025, 08:21Honiton Wastewater Treatment Works5.7 days12,891 - 81,250 m³
26 Jan 2025, 10:53Honiton Wastewater Treatment Works3.5 days7,967 - 50,214 m³
17 Dec 2025, 06:18Honiton Wastewater Treatment Works3.3 days7,378 - 46,503 m³
4 Jan 2025, 22:03Honiton Wastewater Treatment Works3.0 days6,897 - 43,467 m³
26 Jan 2025, 08:43Honiton Wastewater Treatment Works2.7 days6,119 - 38,566 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 estimate574,953 m³range 229,016 - 1.4 million m³
In litres575.0 millionrange 229.0 million - 1.4 billion L
Olympic swimming pools230at 2,500 m³ each
Organic load, as people's raw sewage862,430person-days of untreated BOD

Assumed spill rate while spilling: 26.2 - 165.4 L/s (central 65.9 L/s), from a dry-weather flow of 26.2 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 9,161 kg51,746 kg288,688 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 24,047 kg244,355 kg1.0 million kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 435 kg3,622 kg13,424 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 275 kg1,495 kg7,795 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 22.9 trillion organisms575.0 trillion organisms1.4 × 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.

Honiton Wastewater Treatment Works

Inlet overflow at sewage works · discharges to River Otter

Spills 2025
148 (1,329 hours) · down 1% on 2024
Spills 2024
150 (1,091 hours)
Long-term average
117.2 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
No improvement action in reporting period
WFD catchment
Middle River Otter (GB108045009180)
Outlet location
ST15220094 (OS grid reference) · View on Apple Maps
EA ID / permit
SBB00591 · NRA-SW-3869 · company name: HONITON STW_SO_HONITON

Honiton Wastewater Treatment Works

Storm tank at sewage works · discharges to River Otter

Spills 2025
75 (1,096 hours) · down 66% on 2024
Spills 2024
219 (3,808 hours)
Long-term average
140.3 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
No improvement action in reporting period
WFD catchment
Middle River Otter (GB108045009180)
Outlet location
ST15220094 (OS grid reference) · View on Apple Maps
EA ID / permit
SBB00592 · NRA-SW-3869 · company name: HONITON STW_SSO_HONITON

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.