Storm overflow register · Southern Water

Sandown sewage works: storm overflow spills 2025

Sandown sewage treatment works, run by Southern Water, has 2 monitored storm overflows discharging to The English Channel. In 2025 they recorded 140 spills totalling 1,621 hours, against 283 spills and 4,081 hours in 2024. By hours spilling, it ranks 5 of 215 Southern 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.

JanFebMarAprMayJunJulAugSepOctNovDecSandown WwTW11 spills, 29.5 h, 1 Jan to 2 Jan19 spills, 78.7 h, 4 Jan to 9 Jan3 spills, 20.4 h, 23 Jan to 25 Jan26 Jan 2025 12:50 GMT, 157 h8 spills, 21.8 h, 21 Feb to 22 Feb19 Apr 2025 09:34 GMT, 73 min2 spills, 6.4 h, 23 Apr to 23 Apr5 spills, 7.7 h, 27 May to 27 May2 spills, 3.1 h, 12 Jun to 12 Jun5 spills, 1.9 h, 19 Jul to 20 Jul2 spills, 4.2 h, 29 Aug to 29 Aug4 spills, 3.6 h, 30 Aug to 30 Aug10 spills, 24.7 h, 3 Sep to 4 Sep13 spills, 29.9 h, 10 Sep to 11 Sep12 spills, 19 h, 13 Sep to 14 Sep5 spills, 10.4 h, 3 Oct to 4 Oct11 spills, 31.1 h, 19 Oct to 21 Oct7 spills, 26.2 h, 22 Oct to 23 Oct6 spills, 9 h, 29 Oct to 29 Oct19 spills, 20.9 h, 31 Oct to 2 Nov13 spills, 8.8 h, 10 Nov to 10 Nov5 spills, 5.5 h, 14 Nov to 14 Nov10 spills, 26.4 h, 22 Nov to 23 Nov3 spills, 2.8 h, 29 Nov to 29 Nov54 spills, 235.6 h, 1 Dec to 12 Dec18 spills, 103.6 h, 18 Dec to 23 DecSandown WwTW4 spills, 25.7 h, 1 Jan to 2 Jan24 spills, 98.1 h, 4 Jan to 10 Jan23 Jan 2025 14:30 GMT, 2 min24 Jan 2025 09:24 GMT, 6.5 h17 spills, 137.9 h, 26 Jan to 1 Feb11 spills, 7.9 h, 21 Feb to 22 Feb35 spills, 45.5 h, 23 Feb to 27 Feb2 spills, 1.6 h, 23 Apr to 23 Apr31 Jul 2025 20:29 GMT, 70 min16 spills, 5.3 h, 29 Aug to 29 Aug4 spills, 1.5 h, 30 Aug to 30 Aug1 Sep 2025 10:13 GMT, 2 min13 spills, 15.8 h, 3 Sep to 4 Sep12 spills, 13.2 h, 10 Sep to 11 Sep6 spills, 6.1 h, 13 Sep to 13 Sep4 spills, 1.3 h, 14 Sep to 14 Sep11 spills, 2.6 h, 3 Oct to 4 Oct20 Oct 2025 06:23 GMT, 19.2 h4 spills, 23.5 h, 22 Oct to 23 Oct2 spills, 5.9 h, 29 Oct to 29 Oct31 Oct 2025 20:12 GMT, 3 h3 spills, 3.3 h, 1 Nov to 2 Nov8 spills, 23.9 h, 22 Nov to 23 Nov6 spills, 2.5 h, 29 Nov to 29 Nov100 spills, 189.8 h, 1 Dec to 11 Dec33 spills, 90.4 h, 18 Dec to 22 Dec

Hours spilling by month, 2025

0200400600800Jan 2025: 507 hours spillingJanFeb 2025: 122 hours spillingFebMar 2025: 0.0 hours spillingMarApr 2025: 9.2 hours spillingAprMay 2025: 7.7 hours spillingMayJun 2025: 3.1 hours spillingJunJul 2025: 3.1 hours spillingJulAug 2025: 15 hours spillingAugSep 2025: 110 hours spillingSepOct 2025: 135 hours spillingOctNov 2025: 91 hours spillingNovDec 2025: 619 hours spillingDec
Hours, measured by the event duration monitor.

Longest single spills

Started (GMT)OverflowDuration
26 Jan 2025, 12:50Sandown WwTW6.5 days
5 Dec 2025, 09:34Sandown WwTW5.6 days
18 Dec 2025, 02:08Sandown WwTW2.8 days
4 Jan 2025, 22:15Sandown WwTW2.7 days
26 Jan 2025, 12:50Sandown WwTW2.5 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 estimate27,773 m³range 11,063 - 69,726 m³
In litres27.8 millionrange 11.1 million - 69.7 million L
Olympic swimming pools11at 2,500 m³ each
Organic load, as people's raw sewage41,660person-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) 443 kg2,500 kg13,945 kg 40 / 90 / 200 mg/L
Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids 1,162 kg11,804 kg50,272 kg 105 / 425 / 721 mg/L
Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N 21 kg175 kg648 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 13 kg72 kg377 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 1.1 trillion organisms27.8 trillion organisms697.3 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.

Sandown WwTW

Storm tank at sewage works · discharges to The English Channel

Spills 2025
71 (889 hours) · down 54% on 2024
Spills 2024
155 (2,328 hours)
Long-term average
105.7 spills a year (monitored since 2016)
Monitor uptime
100% of the year
High-spill reason (company)
Performance - GW inundation
Investigation
UWWTR investigation ongoing
Improvement
No improvement action in reporting period
Bathing water
Sandown; Shanklin; Yaverland; Whitecliff Bay
WFD catchment
Isle of Wight East (GB650705530000)
Outlet location
SZ6127084690 (OS grid reference) · View on Apple Maps
EA ID / permit
SWS00741 · A00726 · company name: SANDOWN NEW NO.1 SSO - 115453

Sandown WwTW

Storm tank at sewage works · discharges to The English Channel

Spills 2025
69 (732 hours) · down 46% on 2024
Spills 2024
128 (1,753 hours)
Long-term average
77.6 spills a year (monitored since 2016)
Monitor uptime
100% of the year
High-spill reason (company)
Performance - GW inundation
Investigation
UWWTR investigation ongoing
Improvement
No improvement action in reporting period
Bathing water
Sandown; Shanklin; Yaverland; Whitecliff Bay
WFD catchment
Isle of Wight East (GB650705530000)
Outlet location
SZ6127084690 (OS grid reference) · View on Apple Maps
EA ID / permit
SWS00742 · A00726 · company name: SANDOWN NEW NO.2 SSO - 115454

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 Southern 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.