Storm overflow register · South West Water

St Neot sewage works: storm overflow spills 2025

St Neot sewage treatment works, run by South West Water, has 1 monitored storm overflow discharging to St Neot River. In 2025 it recorded 167 spills totalling 2,031 hours, against 245 spills and 2,522 hours in 2024. By hours spilling, it ranks 36 of 334 South West Water works, where 1 spilled longest.

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 Neot STW29 spills, 5.1 h, 1 Jan to 2 Jan313 spills, 249.5 h, 4 Jan to 21 Jan403 spills, 143.7 h, 23 Jan to 4 Feb395 spills, 196.5 h, 19 Feb to 3 Mar4 spills, 0.5 h, 3 Apr to 3 Apr34 spills, 5.4 h, 14 Apr to 16 Apr200 spills, 23.6 h, 17 Apr to 21 Apr11 spills, 4.5 h, 22 Apr to 23 Apr24 Apr 2025 05:56 GMT, 2 min5 spills, 0.2 h, 25 Apr to 26 Apr8 spills, 1.5 h, 11 May to 12 May2 spills, 0.1 h, 24 May to 24 May16 spills, 2.6 h, 27 May to 27 May2 Jun 2025 07:02 GMT, 2 min4 spills, 0.3 h, 3 Jun to 3 Jun5 Jun 2025 08:06 GMT, 2 min12 spills, 3 h, 6 Jun to 7 Jun9 spills, 1.7 h, 11 Jun to 12 Jun14 spills, 3.5 h, 13 Jun to 14 Jun2 spills, 0.1 h, 22 Jun to 22 Jun2 spills, 0.2 h, 6 Jul to 6 Jul11 spills, 1.2 h, 19 Jul to 19 Jul21 Jul 2025 10:51 GMT, 1 min5 spills, 0.7 h, 4 Aug to 4 Aug3 spills, 0.2 h, 27 Aug to 27 Aug9 spills, 2.8 h, 28 Aug to 29 Aug14 spills, 1 h, 30 Aug to 30 Aug3 spills, 0.4 h, 31 Aug to 31 Aug7 spills, 1.5 h, 1 Sep to 1 Sep12 spills, 3.8 h, 2 Sep to 4 Sep5 spills, 1.1 h, 7 Sep to 7 Sep28 spills, 3.5 h, 9 Sep to 12 Sep14 spills, 1.5 h, 14 Sep to 15 Sep4 spills, 1.1 h, 17 Sep to 17 Sep2 spills, 2.4 h, 20 Sep to 20 Sep9 spills, 1.3 h, 27 Sep to 27 Sep12 spills, 1.7 h, 2 Oct to 4 Oct528 spills, 189.8 h, 19 Oct to 2 Nov354 spills, 1176.3 h, 4 Nov to 28 Dec31 Dec 2025 15:24 GMT, 5 min

Hours spilling by month, 2025

0200400600800Jan 2025: 343 hours spillingJanFeb 2025: 198 hours spillingFebMar 2025: 54 hours spillingMarApr 2025: 34 hours spillingAprMay 2025: 4.1 hours spillingMayJun 2025: 8.6 hours spillingJunJul 2025: 1.4 hours spillingJulAug 2025: 5.1 hours spillingAugSep 2025: 16 hours spillingSepOct 2025: 190 hours spillingOctNov 2025: 559 hours spillingNovDec 2025: 619 hours spillingDec
Hours, measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDuration
4 Nov 2025, 08:56St Neot STW15.2 days
4 Dec 2025, 17:18St Neot STW10.7 days
15 Dec 2025, 09:19St Neot STW9.5 days
5 Jan 2025, 17:05St Neot STW6.9 days
23 Nov 2025, 01:18St Neot STW3.5 days

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.

This works serves fewer than 2,000 people, or was not matched to a UWWTD report, so its size is not published. The population equivalent above is a placeholder of 1,000. Enter the population served if you know it.

Volume, central estimate34,803 m³range 13,863 - 87,375 m³
In litres34.8 millionrange 13.9 million - 87.4 million L
Olympic swimming pools14at 2,500 m³ each
Organic load, as people's raw sewage52,205person-days of untreated BOD

Assumed spill rate while spilling: 1.9 - 11.9 L/s (central 4.8 L/s), from a dry-weather flow of 1.9 L/s.

Pollutant releasedLowCentralHighConcentration used (low / typical / high)
BOD₅ (organic load) 555 kg3,132 kg17,475 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 1,456 kg14,791 kg62,998 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 26 kg219 kg813 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 17 kg90 kg472 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 1.4 trillion organisms34.8 trillion organisms873.8 trillion 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 Neot STW

Inlet overflow at sewage works · discharges to St Neot River

Spills 2025
167 (2,031 hours) · down 32% on 2024
Spills 2024
245 (2,522 hours)
Long-term average
228.3 spills a year (monitored since 2021)
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
St Neot River (GB108048007640)
Outlet location
SX18706737 (OS grid reference) · View on Apple Maps
EA ID / permit
SBB01155 · NRA-SW-3674 · company name: ST NEOT STW_SO_ST NEOT

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.