Storm overflow register · Wessex Water
Shepton Mallet sewage works: storm overflow spills 2025
Shepton Mallet sewage treatment works, run by Wessex Water, has 1 monitored storm overflow discharging to River Sheppey. In 2025 it recorded 36 spills totalling 460 hours, against 98 spills and 1,976 hours in 2024. By hours spilling, it ranks 89 of 242 Wessex Water works, where 1 spilled longest.
Works size: 42,378 population equivalent (SHEPTON MALLET 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.
Longest single spills
| Started (GMT) | Overflow | Duration | Estimated volume |
|---|---|---|---|
| 26 Jan 2025, 15:15 | Shepton Mallet WwTW | 3.4 days | 23,934 – 150,850 m³ |
| 5 Jan 2025, 04:15 | Shepton Mallet WwTW | 2.8 days | 19,740 – 124,417 m³ |
| 14 Nov 2025, 05:53 | Shepton Mallet WwTW | 1.7 days | 12,099 – 76,261 m³ |
| 18 Dec 2025, 11:33 | Shepton Mallet WwTW | 1.3 days | 9,323 – 58,760 m³ |
| 24 Feb 2025, 01:32 | Shepton Mallet WwTW | 1.2 days | 8,253 – 52,015 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.
Assumed spill rate while spilling: 80.3 – 506.4 L/s (central 201.7 L/s), from a dry-weather flow of 80.3 L/s.
| Pollutant released | Low | Central | High | Concentration used (low / typical / high) |
|---|---|---|---|---|
| BOD₅ (organic load) | 5,324 kg | 30,075 kg | 167,785 kg | 40 / 90 / 200 mg/L Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi) |
| Suspended solids | 13,976 kg | 142,019 kg | 604,866 kg | 105 / 425 / 721 mg/L Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi) |
| Ammonium-N | 253 kg | 2,105 kg | 7,802 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 | 160 kg | 869 kg | 4,530 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 | 13.3 trillion organisms | 334.2 trillion organisms | 8.4 × 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.
Shepton Mallet WwTW
Storm tank at sewage works · discharges to River Sheppey
- Spills 2025
- 36 (460 hours) · down 63% on 2024
- Spills 2024
- 98 (1,976 hours)
- Long-term average
- 53.9 spills a year (monitored since 2018)
- Monitor uptime
- 100% of the year
- High-spill reason (company)
- Not Applicable - Ongoing investigation
- Investigation
- Operational investigation completed
- Improvement
- Operational improvement completed
- WFD catchment
- Sheppey (GB108052021221)
- Outlet location
- ST6030043970 (OS grid reference) · View on Apple Maps
- EA ID / permit
- WXW00983 · 101543 · company name: SHEPTON MALLET DARSHILL STORM TANK
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
- EA: Water companies – environmental permits for storm overflows and emergency overflows
- EA: Water resources 2024 to 2025 – per capita consumption
- HBF: Foul sewer design – historic conventions (citing Escritt 1984 on infiltration)
- Giakoumis & Voulvoulis (2026), Environ. Sci.: Water Res. Technol., doi:10.1039/D5EW00860C
- Botturi et al. (2021), Crit. Rev. Environ. Sci. Technol. 51:1585 – CSO quality review, Table 2
- US EPA (2004) Report to Congress on CSOs and SSOs, chapter 4
- Urban Waste Water Treatment Directive, Article 2(6): 1 p.e. = 60 g BOD5/day
- Olympic-size swimming pool: 2,500 m³ at the nominal 2 m depth
← 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.