Storm overflow register · South West Water

St Dennis Wastewater Treatment Work sewage works: storm overflow spills 2025

St Dennis Wastewater Treatment Work sewage treatment works, run by South West Water, has 2 monitored storm overflows discharging to River Fal. In 2025 they recorded 162 spills totalling 1,435 hours, against 253 spills and 3,182 hours in 2024. By hours spilling, it ranks 72 of 334 South West Water works, where 1 spilled longest.

Works size: 2,544 population equivalent (ST DENNIS 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
20 Dec 2025, 19:36	St Dennis Wastewater Treatment Work	5.3 days	2,191 – 13,811 m³
8 Jan 2025, 06:29	St Dennis Wastewater Treatment Work	5.2 days	2,161 – 13,617 m³
15 Dec 2025, 04:03	St Dennis Wastewater Treatment Work	2.6 days	1,072 – 6,758 m³
4 Jan 2025, 17:39	St Dennis Wastewater Treatment Work	2.3 days	943 – 5,942 m³
10 Jan 2025, 05:12	St Dennis Wastewater Treatment Work	1.7 days	724 – 4,563 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 estimate62,530 m³range 24,907 – 156,984 m³

In litres62.5 millionrange 24.9 million – 157.0 million L

Olympic swimming pools25at 2,500 m³ each

Organic load, as people's raw sewage93,795person-days of untreated BOD

Assumed spill rate while spilling: 4.8 – 30.4 L/s (central 12.1 L/s), from a dry-weather flow of 4.8 L/s.

Pollutant released	Low	Central	High	Concentration used (low / typical / high)
BOD₅ (organic load)	996 kg	5,628 kg	31,397 kg	40 / 90 / 200 mg/L Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids	2,615 kg	26,575 kg	113,185 kg	105 / 425 / 721 mg/L Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N	47 kg	394 kg	1,460 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	30 kg	163 kg	848 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	2.5 trillion organisms	62.5 trillion organisms	1.6 × 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.

St Dennis Wastewater Treatment Work

Storm tank at sewage works · discharges to River Fal

Spills 2025: 65 (817 hours) · down 48% on 2024
Spills 2024: 124 (1,929 hours)
Long-term average: 170.3 spills a year (monitored since 2019)
Monitor uptime: 100% of the year
High-spill reason (company): Not asset maintenance - hydraulic capacity
Investigation: Env Act (SODRP) investigation ongoing, UWWTR investigation completed, Operational investigation completed
Improvement: Env Act (SODRP) improvement ongoing
WFD catchment: Upper River Fal (GB108048001390)
Outlet location: SW94115764 (OS grid reference) · View on Apple Maps
EA ID / permit: SBB01129 · NRA-SW-3885 · company name: ST DENNIS STW_SSO_ST DENNIS

St Dennis Wastewater Treatment Work

Inlet overflow at sewage works · discharges to River Fal

Spills 2025: 97 (617 hours) · down 25% on 2024
Spills 2024: 129 (1,253 hours)
Long-term average: 92.5 spills a year (monitored since 2019)
Monitor uptime: 100% of the year
High-spill reason (company): Performance - SW Infiltration
Investigation: Env Act (SODRP) investigation ongoing, UWWTR investigation completed, Operational investigation completed
Improvement: Operational improvement completed
WFD catchment: Upper River Fal (GB108048001390)
Outlet location: SW94115764 (OS grid reference) · View on Apple Maps
EA ID / permit: SBB01128 · NRA-SW-3885 · company name: ST DENNIS STW_SO_ST DENNIS

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.