Theydon Bois sewage works: storm overflow spills 2025

Theydon Bois sewage treatment works, run by Thames Water, has 1 monitored storm overflow discharging to Brookhouse Brook. In 2025 it recorded 85 spills totalling 635 hours, against 115 spills and 1,207 hours in 2024. By hours spilling, it ranks 50 of 233 Thames Water works, where 1 spilled longest.

Works size: 4,289 population equivalent (THEYDON BOIS 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, 03:45	Theydon Bois	2.8 days	1,961 – 12,361 m³
9 Feb 2025, 21:30	Theydon Bois	1.8 days	1,259 – 7,933 m³
24 Feb 2025, 03:45	Theydon Bois	1.5 days	1,054 – 6,642 m³
27 Jan 2025, 00:30	Theydon Bois	1.2 days	849 – 5,350 m³
18 Dec 2025, 13:00	Theydon Bois	1.1 days	746 – 4,705 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 estimate46,651 m³range 18,582 – 117,120 m³

In litres46.7 millionrange 18.6 million – 117.1 million L

Olympic swimming pools19at 2,500 m³ each

Organic load, as people's raw sewage69,977person-days of untreated BOD

Assumed spill rate while spilling: 8.1 – 51.2 L/s (central 20.4 L/s), from a dry-weather flow of 8.1 L/s.

Pollutant released	Low	Central	High	Concentration used (low / typical / high)
BOD₅ (organic load)	743 kg	4,199 kg	23,424 kg	40 / 90 / 200 mg/L Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi)
Suspended solids	1,951 kg	19,827 kg	84,443 kg	105 / 425 / 721 mg/L Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi)
Ammonium-N	35 kg	294 kg	1,089 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	121 kg	632 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	1.9 trillion organisms	46.7 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.

Theydon Bois

Inlet overflow at sewage works · discharges to Brookhouse Brook

Spills 2025: 85 (635 hours) · down 26% on 2024
Spills 2024: 115 (1,207 hours)
Long-term average: 96.7 spills a year (monitored since 2019)
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: Brookhouse Brook (GB106037028150)
Outlet location: TQ4769097720 (OS grid reference) · View on Apple Maps
EA ID / permit: TWL00413 · TEMP.2954 · company name: Theydon Bois 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.