Stewkley sewage works: storm overflow spills 2025

Stewkley sewage treatment works, run by Thames Water, has 1 monitored storm overflow discharging to Hardwick Brook. In 2025 it recorded 143 spills totalling 3,069 hours, against 217 spills and 4,482 hours in 2024. By hours spilling, it ranks 2 of 233 Thames 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
21 Jan 2025, 21:00	Stewkley Wastewater Treatment Works	35.6 days
26 Feb 2025, 15:30	Stewkley Wastewater Treatment Works	26.9 days
6 Jan 2025, 11:15	Stewkley Wastewater Treatment Works	14.8 days
18 Dec 2025, 21:45	Stewkley Wastewater Treatment Works	5.2 days
7 Dec 2025, 09:15	Stewkley Wastewater Treatment Works	4.9 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 estimate52,589 m³range 20,947 – 132,027 m³

In litres52.6 millionrange 20.9 million – 132.0 million L

Olympic swimming pools21at 2,500 m³ each

Organic load, as people's raw sewage78,883person-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)	838 kg	4,733 kg	26,405 kg	40 / 90 / 200 mg/L Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids	2,199 kg	22,350 kg	95,191 kg	105 / 425 / 721 mg/L Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N	40 kg	331 kg	1,228 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	25 kg	137 kg	713 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	2.1 trillion organisms	52.6 trillion organisms	1.3 × 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.

Stewkley Wastewater Treatment Works

Storm tank at sewage works · discharges to Hardwick Brook

Spills 2025: 143 (3,069 hours) · down 34% on 2024
Spills 2024: 217 (4,482 hours)
Long-term average: 138.8 spills a year (monitored since 2020)
Monitor uptime: 100% of the year
High-spill reason (company): Performance - GW inundation
Investigation: Env Act (SODRP) investigation ongoing
Improvement: No improvement action in reporting period
WFD catchment: Hardwick Brook (Source to Thame) (GB106039030430)
Outlet location: SP8498024860 (OS grid reference) · View on Apple Maps
EA ID / permit: TWL00391 · CSSC.2368 · company name: Stewkley STW

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 Thames 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.