Event Duration Monitoring · annual returns
The headline figures
Mogden is the dominant sewage works on this reach. When rain overwhelms it, a storm tank discharges screened but untreated storm sewage straight to the tidal Thames, and the Environment Agency's Event Duration Monitoring (EDM) records each discharge. These are the numbers it reports for permit CNTD.0085.
Storm-tank discharge by year, 2020–2025
Mogden storm-tank discharge duration by year
Hours of discharge per year, with EA counted spills labelled above each bar · permit CNTD.0085
19f6064d…; the underlying XLSX returns were parsed by this
programme. Bars show discharge duration in hours; the figure above each bar
is the EA "counted spills" for that year. There is no volume axis because no volume is
published. Provenance: RAW-PULL.
Discharge duration peaked in 2024 at 471.7 hours across 50 counted spills — the most recorded at this asset — then fell to 202.5 hours in 2025. The fall is real but must be read with care: spill frequency tracks rainfall, and 2025 was a drier year. There is no monotonic "worse every year" trend in these six points, and — most importantly — none of them measure how much sewage entered the river, only how long the storm tank was discharging.
| Year | Duration (hours) | Counted spills | EDM operational | Long-term avg (spills/yr) | Source return |
|---|---|---|---|---|---|
| 2020 | 412.8 | 42 | 99.9% | — | Thames Water 2020 return |
| 2021 | 438.3 | 43 | 98.1% | — | All-WaSC 2021 |
| 2022 | 164.3 | 20 | 100% | 33.3 | All-WaSC 2022 |
| 2023 | 221.1 | 28 | 99.7% | 32.2 | All-WaSC 2023 |
| 2024 | 471.7 | 50 | 100% | 35.2 | All-WaSC 2024 |
| 2025 | 202.5 | 17 | 100% | 32.6 | All-WaSC 2025 |
Within the Teddington–Richmond focus box, the active spiller in 2024 was the Petersham Road CSO (18 h / 8 spills to the River Thames); Kingston Main and Amyand Park Road (Twickenham) recorded 0 / 0. Mogden, at Isleworth just downstream, dwarfs them all. RAW-PULL, same dataset.
How to read this
Caveats a regulator would press — stated up front
- Duration, never volume. This is the single most important caveat. EDM records how long and how often the storm tank discharged, never how much. No spill volume or pollutant load is published for Mogden — or anywhere in the EDM dataset — and Mogden's own receiving-water discharge points return no observations in the public water-quality archive. There is no open measure of how much sewage enters the tidal Thames here.
- "Counted spills" is a construct. The spill counts use the EA's 12–24 hour block-count method — a regulatory convention, not a raw tally of discrete events.
- Volatile, not monotonic. Spills track rainfall. 2025's fall to 202.5 h most likely reflects a drier year, not investment — so avoid reading a steady "worse every year" trend into six volatile points.
- Geography. Mogden sits at Isleworth, downstream of Richmond and just outside the Teddington–Richmond focus box. Its outfall is WFD-coded
GB530603911403— adjacent to, but not the same body as, the "Thames Middle"GB530603911402cited on the WFD page. It is nonetheless the dominant local sewage input. - Reliability is good here. EDM was 98–100% operational every year, so these durations are dependable — unlike sites with poor monitor coverage.
- Same asset throughout. Recorded as MOGDEN STW in 2020 and MOGDEN WWTW from 2021; permit CNTD.0085 is unchanged.
Sources & provenance
- RAW-PULL EA Event Duration Monitoring (EDM) Storm Overflow Annual Returns — data.gov.uk dataset
19f6064d-7356-466f-844e-d20ea10ae9fd. Underlying per-year XLSX returns downloaded and parsed by this programme. - Asset: MOGDEN WWTW, permit CNTD.0085, "Storm tank at WwTW", receiving water River Thames (tidal), WFD body
GB530603911403. - Working dataset:
/tmp/thames_datastore/sewage_edm/mogden_edm_2020_2025.csv(and the six source ZIPs alongside it).