Swanage Tunnel sewage works: storm overflow spills 2025

Swanage Tunnel sewage treatment works, run by Wessex Water, has 1 monitored storm overflow discharging to Swanage Bay. In 2025 it recorded 20 spills totalling 76 hours, against 31 spills and 134 hours in 2024. By hours spilling, it ranks 192 of 242 Wessex Water works, where 1 spilled longest.

Works size: 22,451 population equivalent (SWANAGE 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.

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.

Cubic metres (m³), estimate. Hours are measured by the event duration monitor.

Longest single spills

Started (GMT)	Overflow	Duration	Estimated volume
5 Jan 2025, 01:31	Swanage Tunnel	16.7 h	2,554 – 16,096 m³
18 Dec 2025, 10:23	Swanage Tunnel	11.3 h	1,729 – 10,897 m³
26 Jan 2025, 22:39	Swanage Tunnel	6.5 h	993 – 6,261 m³
31 Jan 2025, 06:37	Swanage Tunnel	3.7 h	572 – 3,606 m³
7 Dec 2025, 11:32	Swanage Tunnel	3.4 h	526 – 3,316 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 estimate29,198 m³range 11,630 – 73,302 m³

In litres29.2 millionrange 11.6 million – 73.3 million L

Olympic swimming pools12at 2,500 m³ each

Organic load, as people's raw sewage43,796person-days of untreated BOD

Assumed spill rate while spilling: 42.6 – 268.3 L/s (central 106.9 L/s), from a dry-weather flow of 42.6 L/s.

Pollutant released	Low	Central	High	Concentration used (low / typical / high)
BOD₅ (organic load)	465 kg	2,628 kg	14,660 kg	40 / 90 / 200 mg/L Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids	1,221 kg	12,409 kg	52,851 kg	105 / 425 / 721 mg/L Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N	22 kg	184 kg	682 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	14 kg	76 kg	396 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	1.2 trillion organisms	29.2 trillion organisms	733.0 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.

Swanage Tunnel

Storm tank at sewage works · discharges to Swanage Bay

Spills 2025: 20 (76 hours) · down 35% on 2024
Spills 2024: 31 (134 hours)
Long-term average: 24.7 spills a year (monitored since 2019)
Monitor uptime: 100% of the year
Investigation: Env Act (SODRP) investigation ongoing, UWWTR investigation ongoing
Improvement: Env Act (SODRP) improvement ongoing, Operational improvement completed
Bathing water: Swanage Central
WFD catchment: Dorset / Hampshire (GB620705550000)
Outlet location: SZ0424378665 (OS grid reference) · View on Apple Maps
EA ID / permit: WXW01097 · 400785 · company name: SWANAGE MARINE PARADE TUNNEL

How the estimate works

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].
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.
Low spill rate = DWF. This is the method used by Giakoumis & Voulvoulis (2026), which they note likely underestimates storm loads [G&V].
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.
Volume = spill rate × hours spilling, with hours taken from the event duration monitor.
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

← All Wessex 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.