Storm overflow register · South West Water

Gwennap sewage works: storm overflow spills 2025

Gwennap sewage treatment works, run by South West Water, has 2 monitored storm overflows discharging to Hicks Mill Stream. In 2025 they recorded 154 spills totalling 2,701 hours, against 151 spills and 2,588 hours in 2024. By hours spilling, it ranks 15 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.

Hours spilling by month, 2025

Hours, measured by the event duration monitor.

Longest single spills

Started (GMT)	Overflow	Duration
12 Nov 2025, 10:54	Gwennap Wastewater Treatment Works	49.2 days
19 Feb 2025, 19:30	Gwennap Wastewater Treatment Works	15.9 days
4 Nov 2025, 08:54	Gwennap Wastewater Treatment Works	8.1 days
4 Jan 2025, 19:54	Gwennap Wastewater Treatment Works	4.9 days
17 Apr 2025, 21:47	Gwennap Wastewater Treatment Works	2.2 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 estimate46,285 m³range 18,436 – 116,201 m³

In litres46.3 millionrange 18.4 million – 116.2 million L

Olympic swimming pools19at 2,500 m³ each

Organic load, as people's raw sewage69,428person-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 released	Low	Central	High	Concentration used (low / typical / high)
BOD₅ (organic load)	737 kg	4,166 kg	23,240 kg	40 / 90 / 200 mg/L Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids	1,936 kg	19,671 kg	83,781 kg	105 / 425 / 721 mg/L Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N	35 kg	292 kg	1,081 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	22 kg	120 kg	627 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	1.8 trillion organisms	46.3 trillion organisms	1.2 × 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.

Gwennap Wastewater Treatment Works

Storm tank at sewage works · discharges to Hicks Mill Stream

Spills 2025: 141 (2,673 hours) · up 12% on 2024
Spills 2024: 126 (2,520 hours)
Long-term average: 128.8 spills a year (monitored since 2020)
Monitor uptime: 98% of the year
High-spill reason (company): Performance - SW Infiltration
Investigation: Env Act (SODRP) investigation ongoing, UWWTR investigation completed, Operational investigation completed
Improvement: No improvement action in reporting period
WFD catchment: Hicks Mill Stream (GB108048001150)
Outlet location: SW74864044 (OS grid reference) · View on Apple Maps
EA ID / permit: SBB00526 · NRA-SW-5475 · company name: GWENNAP STW_SSO_LANNER ST DAY

Gwennap Wastewater Treatment Works

Inlet overflow at sewage works · discharges to Hicks Mill Stream

Spills 2025: 13 (28 hours) · down 48% on 2024
Spills 2024: 25 (69 hours)
Long-term average: 26.2 spills a year (monitored since 2020)
Monitor uptime: 98% of the year
Investigation: Env Act (SODRP) investigation ongoing, UWWTR investigation completed
Improvement: UWWTR improvement completed
WFD catchment: Hicks Mill Stream (GB108048001150)
Outlet location: SW74864044 (OS grid reference) · View on Apple Maps
EA ID / permit: SBB00525 · NRA-SW-5475 · company name: GWENNAP STW_SO_LANNER ST DAY

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 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.