Grendon Underwood STW
Storm overflow operated by Thames Water. The source names Oxon Ray as the receiving water.
- 31
- Spills in 2025
- 306
- Recorded hours
- 99.7%
- Monitor operation
- 3
- Years listed
Checking the latest monitor report…
How it compares in 2025
Grendon Underwood STW recorded 31 spills in 2025. That is more than 75% of the 14,239 overflows in England with a reported figure, and ranks 3,344th nationally. Among Thames Water overflows it ranks 102nd of 579; the median Thames Water overflow recorded 6 spills.
| Group | Rank by spills | Median spills | Median hours |
|---|---|---|---|
| This outlet | – | 31 | 306 |
| England | 3,344th of 14,239 | 11 | 22.3 |
| Thames Water | 102nd of 579 | 6 | 20.9 |
Rank 1 has the most spills. Equal counts share a rank. Only overflows with a reported count are ranked. Counts do not measure volume, so a higher rank does not mean more sewage.
Annual spill history
| Year | Spills | Duration | Monitor operation | Source |
|---|---|---|---|---|
| 2025 | 31 | 306 hours | 99.7% | Official data |
| 2024 | 68 | 793.8 hours | 87.2%Low coverage | Official data |
| 2023 | 76 | 840.8 hours | 60.8%Low coverage | Official data |
A missing figure means the source did not provide a usable value. A zero is shown as zero. Monitor operation below 90% can make comparison less reliable.
Outlet details
- Water company
- Thames Water
- Outlet type
- Storm tank at a wastewater treatment works
- Permit
- TEMP.2626
- Activity reference
- 1 activity on permit
- Grid reference
- SP6799021150
- Waterbody ID
- GB106039030100Official waterbody record ↗
- Location source
- Official annual grid reference
- Location precision
- About 1 metres
- Location note
- Unknown
About the name and type
The annual return lists this asset as “Storm tank at WwTW”: storm tank at a wastewater treatment works. The overflow comes from storm storage at a treatment works.
The waterbody link opens the Environment Agency record for the exact ID in the return. It is a source lookup; we do not import its classification.
Where it is
The recorded position is about 0.7 km north-west of the Grendon Underwood reference point in the ONS built-up areas data.
Area and water pages
Check the official record
Public access to this outlet is unknown. Do not enter restricted land or use this page as a route guide.
Open official source ↗Other outlet records nearby
- Ludgershall STWCurrent status unknown
Thames Water · Ludgershall Brook
- Marsh Gibbon STWCurrent status unknown
Thames Water · Summerstown Ditch
- Poundon STWCurrent status unknown
Anglian Water · Poundon Brook NT
- Twyford-School Lane TpsCurrent status unknown
Anglian Water · Tributary of Padbury Brook
Questions about Grendon Underwood STW
- Who operates Grendon Underwood STW?
- Thames Water is the operator named in the annual record. The permit reference is TEMP.2626.
- Where does Grendon Underwood STW discharge?
- The source names Oxon Ray as the receiving water. This does not show how far a discharge travelled.
- How many times did Grendon Underwood STW spill in 2025?
- The 2025 return records 31 counted spill events and 306 hours of recorded discharge. Spills are counted with the 12/24-hour method, so this is not a count of every start signal.
- Can I trust the 2025 figures for this outlet?
- The monitor worked for 99.7% of the reporting period, so the annual figures cover most of the year. They still do not measure discharge volume.
- Is Grendon Underwood STW discharging right now?
- This record is linked to a company live feed. The panel above shows the latest report we hold, with its date. That report can be old, and the monitor can be offline, so check the date before relying on it. A report that no discharge is taking place is not a water-quality result.
- How accurate is the map marker?
- The marker comes from the grid reference in the official annual return, which is accurate to about 1 metres. It shows the recorded outlet position, not the extent of any discharge.
- What kind of outlet is Grendon Underwood STW?
- The annual return lists the asset type as “Storm tank at WwTW”, which means storm tank at a wastewater treatment works. The overflow comes from storm storage at a treatment works.
- How does Grendon Underwood STW compare with other overflows?
- Grendon Underwood STW recorded 31 spills in 2025. That is more than 75% of the 14,239 overflows in England with a reported figure, and ranks 3,344th nationally. Among Thames Water overflows it ranks 102nd of 579; the median Thames Water overflow recorded 6 spills. Ranks compare counted spill events only. They do not compare volume or harm.
- Does zero spills mean the water was safe?
- No. A zero means the annual return recorded zero counted spill events for this outlet and year. It is not a water-quality test, and pollution can come from other sources.