Storm overflow register · South West Water

Delabole sewage works: storm overflow spills 2025

Delabole sewage treatment works, run by South West Water, has 2 monitored storm overflows discharging to River Allen. In 2025 they recorded 159 spills totalling 2,911 hours, against 236 spills and 4,059 hours in 2024. By hours spilling, it ranks 11 of 334 South West Water works, where 1 spilled longest.

Works size: 2,170 population equivalent (DELABOLE 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.

JanFebMarAprMayJunJulAugSepOctNovDecDelabole Wastewater…165 spills, 324 h, 1 Jan to 15 Jan24 Jan 2025 01:56 GMT, 4.3 h42 spills, 217.2 h, 26 Jan to 4 Feb78 spills, 428.4 h, 21 Feb to 12 Mar152 spills, 249 h, 15 Apr to 27 Apr5 spills, 4.5 h, 27 May to 27 May2 spills, 9.5 h, 7 Jun to 7 Jun38 spills, 14.8 h, 13 Jun to 14 Jun11 spills, 2.1 h, 20 Jul to 20 Jul9 spills, 3.7 h, 28 Aug to 29 Aug6 spills, 3.1 h, 3 Sep to 3 Sep196 spills, 249.3 h, 12 Sep to 24 Sep3 spills, 5.5 h, 19 Oct to 19 Oct20 Oct 2025 07:31 GMT, 6.7 h3 spills, 7.3 h, 23 Oct to 23 Oct8 spills, 30.2 h, 24 Oct to 27 Oct589 spills, 1101 h, 29 Oct to 17 Dec218 spills, 192.7 h, 19 Dec to 28 Dec37 spills, 50.7 h, 29 Dec to 31 DecDelabole Wastewater…5 Jan 2025 19:33 GMT, 11 min2 spills, 0.2 h, 26 Jan to 26 Jan2 spills, 0.2 h, 28 Jan to 28 Jan19 spills, 0.9 h, 23 Feb to 23 Feb4 spills, 0.2 h, 26 Feb to 26 Feb27 spills, 3.8 h, 15 Apr to 16 Apr4 spills, 0.1 h, 22 Apr to 22 Apr14 Sep 2025 11:58 GMT, 14 min6 spills, 1 h, 20 Sep to 20 Sep2 Nov 2025 04:06 GMT, 6 min7 spills, 0.9 h, 15 Dec to 15 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.

020k40k60k80kJan 2025: 462 h spilling ≈ 17,178 m³ (range 6,842-43,126 m³)JanFeb 2025: 238 h spilling ≈ 8,838 m³ (range 3,520-22,188 m³)FebMar 2025: 276 h spilling ≈ 10,255 m³ (range 4,085-25,745 m³)MarApr 2025: 253 h spilling ≈ 9,407 m³ (range 3,747-23,617 m³)AprMay 2025: 4.5 h spilling ≈ 167 m³ (range 67-420 m³)MayJun 2025: 24 h spilling ≈ 904 m³ (range 360-2,268 m³)JunJul 2025: 2.1 h spilling ≈ 78 m³ (range 31-196 m³)JulAug 2025: 3.6 h spilling ≈ 134 m³ (range 53-336 m³)AugSep 2025: 254 h spilling ≈ 9,429 m³ (range 3,756-23,673 m³)SepOct 2025: 105 h spilling ≈ 3,904 m³ (range 1,555-9,801 m³)OctNov 2025: 689 h spilling ≈ 25,618 m³ (range 10,204-64,316 m³)NovDec 2025: 601 h spilling ≈ 22,346 m³ (range 8,901-56,101 m³)Dec
Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDurationEstimated volume
4 Mar 2025, 17:48Delabole Wastewater Treatment Works2.3 days807 - 5,084 m³
25 Nov 2025, 11:42Delabole Wastewater Treatment Works2.2 days797 - 5,024 m³
20 Nov 2025, 06:00Delabole Wastewater Treatment Works2.1 days750 - 4,725 m³
27 Feb 2025, 19:05Delabole Wastewater Treatment Works1.9 days659 - 4,152 m³
1 Feb 2025, 14:41Delabole Wastewater Treatment Works1.7 days614 - 3,871 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 estimate108,232 m³range 43,111 - 271,721 m³
In litres108.2 millionrange 43.1 million - 271.7 million L
Olympic swimming pools43at 2,500 m³ each
Organic load, as people's raw sewage162,348person-days of untreated BOD

Assumed spill rate while spilling: 4.1 - 25.9 L/s (central 10.3 L/s), from a dry-weather flow of 4.1 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 1,724 kg9,741 kg54,344 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 4,527 kg45,999 kg195,911 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 82 kg682 kg2,527 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 52 kg281 kg1,467 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 4.3 trillion organisms108.2 trillion organisms2.7 × 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.

Delabole Wastewater Treatment Works

Storm tank at sewage works · discharges to River Allen

Spills 2025
148 (2,903 hours) · down 9% on 2024
Spills 2024
162 (3,264 hours)
Long-term average
140.2 spills a year (monitored since 2016)
Monitor uptime
99% 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, UWWTR improvement ongoing
WFD catchment
Allen (GB108049007050)
Outlet location
SX07298294 (OS grid reference) · View on Apple Maps
EA ID / permit
SBB00357 · NRA-SW-4934 · company name: DELABOLE STW_SSO_DELABOLE

Delabole Wastewater Treatment Works

Inlet overflow at sewage works · discharges to River Allen

Spills 2025
11 (7.7 hours) · down 85% on 2024
Spills 2024
74 (794 hours)
Long-term average
158.4 spills a year (monitored since 2016)
Monitor uptime
99% of the year
Investigation
Env Act (SODRP) investigation ongoing
Improvement
Env Act (SODRP) improvement ongoing, UWWTR improvement ongoing
WFD catchment
Allen (GB108049007050)
Outlet location
SX07298294 (OS grid reference) · View on Apple Maps
EA ID / permit
SBB00356 · NRA-SW-4934 · company name: DELABOLE STW_SO_DELABOLE

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.