In October 2023, Storm Babet hit the River Wey basin. Bentley STW discharged sewage twice in five days. The following month, two more: 49 hours, then 40 hours of discharge, straight into the river upstream of Farnham and Guildford. By January 2025, the site had recorded 187 spill events in two years.
A storm tank was promised. A 252-cubic-metre tank — enough to hold roughly two hours of peak flow — was installed in September 2024, six months ahead of schedule. The upgrade was marked complete in March 2025.
WaterWatch exists to check whether upgrades like this actually deliver what they claim. We have no relationships with any water company and we’re not paid to publish these reports. What follows is what the data shows — nothing more, nothing less.
Key events at Bentley STW. Dashed vertical line marks the WaterWatch upgrade marker (Mar 2025).
What the numbers mean
Since the upgrade: zero spill events across fourteen months, including periods of heavy rainfall and elevated river levels that would previously have triggered discharges. Before the upgrade: 1,984 hours of discharge across 187 events.
A raw before/after spill count can mislead — a drier post-upgrade period will always look better regardless of the engineering. WaterWatch normalises by environmental pressure: a daily score combining rainfall, ground saturation, river levels, and groundwater data.
Before the upgrade the spill rate was 2.28 per 100 pressure units. Since the upgrade it’s 0.00 — including on the heaviest-pressure days recorded in the post-upgrade window.
Why the confidence rating is LOW — and why it’s counterintuitive
The analysis carries a LOW confidence rating. It doesn’t mean the result looks suspicious. It means there aren’t yet enough post-upgrade observations to compute a statistically meaningful rate.
To build a reliable comparison, WaterWatch needs to observe at least ten spills after the upgrade. With zero recorded, there’s no “after” rate to compare against the pre-upgrade figure of 2.28 per 100 pressure units — only a confirmation that the rate hasn’t exceeded what’s been observed so far.
A few spills this winter would actually increaseconfidence. If Bentley records five or six events but at a rate substantially below 2.28 per 100 units, that’s a real before/after comparison — far more meaningful than continued zero counts. Right now the signal is positive, but we need some data from the “after” side to have something to measure.
What was actually built
The upgrade itself was straightforward: an 18 by 6-metre storm tank, 252 cubic metres of capacity, installed in September 2024 — six months ahead of schedule. A phosphorus reduction scheme followed in 2025, bringing treated effluent to below 0.9 mg/l.
The site had been on the upgrade list for years. Bentley sits above a chalk aquifer that feeds the Wey through groundwater, which means high-pressure events hit the site harder than simple rainfall figures suggest. The Storm Babet episodes in late 2023 made the case hard to ignore.
Whether £6–8 million represents genuine long-term commitment or the bare minimum to satisfy regulators is a separate argument. What the data shows is that for now, at least, the tank is doing its job.
What happens next
WaterWatch will update this analysis automatically as new data comes in. If a wet winter arrives and the site holds, the confidence rating goes up. If events start appearing at a rate comparable to before, that tells a different story.
The River Wey runs through Alton, Farnham, and Guildford. The people who live alongside it deserve to know, with honesty about what the data can and can’t yet confirm, which story they’re in.
187 spills. An upgrade. Then zero. Independent analysis, no spin — just the data from Bentley STW on the River Wey.