Storm overflow register · South West Water

Altarnun sewage works: storm overflow spills 2025

Altarnun sewage treatment works, run by South West Water, has 1 monitored storm overflow discharging to Penpont Water. In 2025 it recorded 165 spills totalling 3,295 hours, against 206 spills and 3,918 hours in 2024. By hours spilling, it ranks 7 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.

JanFebMarAprMayJunJulAugSepOctNovDecAltarnun STW (Storm…70 spills, 982 h, 1 Jan to 13 Feb15 spills, 5.8 h, 13 Feb to 14 Feb20 Feb 2025 11:28 GMT, 34 min3 spills, 384.9 h, 21 Feb to 9 Mar4 spills, 22.9 h, 15 Apr to 16 Apr6 spills, 101 h, 18 Apr to 23 Apr12 May 2025 04:19 GMT, 6.2 h21 spills, 10.8 h, 27 May to 27 May6 spills, 2.3 h, 3 Jun to 3 Jun9 spills, 7.2 h, 7 Jun to 7 Jun12 spills, 23 h, 12 Jun to 14 Jun2 spills, 2 h, 27 Aug to 27 Aug2 spills, 1.9 h, 28 Aug to 29 Aug2 spills, 1.4 h, 1 Sep to 1 Sep2 spills, 4.5 h, 7 Sep to 7 Sep9 Sep 2025 19:07 GMT, 103 min14 spills, 9.5 h, 14 Sep to 15 Sep4 spills, 2.6 h, 17 Sep to 17 Sep9 spills, 5.5 h, 20 Sep to 20 Sep3 spills, 1.7 h, 27 Sep to 27 Sep2 spills, 5.5 h, 3 Oct to 4 Oct11 Oct 2025 10:23 GMT, 4 h13 spills, 28.3 h, 19 Oct to 20 Oct11 spills, 1679.7 h, 22 Oct to 31 Dec

Hours spilling by month, 2025

0200400600800Jan 2025: 711 hours spillingJanFeb 2025: 461 hours spillingFebMar 2025: 202 hours spillingMarApr 2025: 124 hours spillingAprMay 2025: 17 hours spillingMayJun 2025: 32 hours spillingJunJul 2025: 0.0 hours spillingJulAug 2025: 3.9 hours spillingAugSep 2025: 27 hours spillingSepOct 2025: 254 hours spillingOctNov 2025: 720 hours spillingNovDec 2025: 744 hours spillingDec
Hours, measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDuration
19 Nov 2025, 00:39Altarnun STW (Storm Tank)26.5 days
28 Oct 2025, 15:27Altarnun STW (Storm Tank)20.8 days
15 Dec 2025, 13:15Altarnun STW (Storm Tank)16.4 days
6 Jan 2025, 11:32Altarnun STW (Storm Tank)15.7 days
25 Jan 2025, 01:27Altarnun STW (Storm Tank)12.4 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 estimate56,449 m³range 22,485 - 141,718 m³
In litres56.4 millionrange 22.5 million - 141.7 million L
Olympic swimming pools23at 2,500 m³ each
Organic load, as people's raw sewage84,674person-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) 899 kg5,080 kg28,344 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 2,361 kg23,991 kg102,179 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 43 kg356 kg1,318 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 27 kg147 kg765 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 2.2 trillion organisms56.4 trillion organisms1.4 × 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.

Altarnun STW (Storm Tank)

Storm tank at sewage works · discharges to Penpont Water

Spills 2025
165 (3,295 hours) · down 20% on 2024
Spills 2024
206 (3,918 hours)
Long-term average
166.2 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
Penpont Water (GB108047007700)
Outlet location
SX22408140 (OS grid reference) · View on Apple Maps
EA ID / permit
SBB00043 · SWWA 154 · company name: ALTARNUN STW_SSO_ALTARNUN

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.