(Bloomberg) -- Late at night on a quiet London street, workers in high-vis clothing put their ears to the ground. Lit up by the glare of headlights from passing cars, they face each other and shuffle in a steady sideways procession. One is wearing a large pair of headphones; his attention is fixed on a small LED screen. Another carries a device resembling an oversized plunger, which he lifts and replaces gently on the ground as they move. 

These are London’s leak hunters, constantly listening to the city’s underground world for wear, tear and breaks in its massive network of water pipes. This particular crew comes from Thames Water, which has more than 800 workers either pinpointing the locations of suspected leaks or fixing the ones that get discovered. The company’s 13,000 square-kilometer (5,000-square mile) remit includes central London and extends from Gloucestershire in the west of England through to parts of Kent and Essex in the east. Thames Water repairs more than 1,300 leaks per week, one every seven and a half minutes. 

Famous for its constant drizzle, the UK is now pressuring its water industry to reduce waste as lack of supply threatens water security. Many pipes are over a century old, and the system will be increasingly tested by extreme weather and a warmer climate that brings periods of drought followed by heavy rain. As it stands, a fifth of water supplied through the UK’s major water companies is lost to leaks, which makes it difficult to persuade the public to step up individual conservation. That’s where the leak hunters come in.

Read More:  Britain Is Racing to Fix Its Broken Water System

The most basic tool in the team’s arsenal is a listening stick, a steel cable with a wooden or plastic earpiece. The metal end is placed into a hatch in the pavement so workers can listen for flowing water amid the amplified sounds of the world below.

Putting your ear to the listening stick is like holding a stethoscope up to the chest of the city: The sound of every passing bus is exaggerated, and an untrained ear struggles to make out the soft noise that betrays running water — though it is unmistakable to the workers who spend their shifts searching for it. The Thames Water team works 8 p.m. through 6 a.m. to minimize the city sounds it must contend with.

To pinpoint a leak’s exact location, something higher-tech than a listening stick is sometimes needed. That’s when the crew hauls out the ground microphone, which is exactly what it sounds like. Donning a pair of headphones, a worker moves up and down the length of a pipe listening for a peak in the noise, like a metal detector zeroing in on underground gold. A third piece of equipment, known as a correlator, works via sensors at either end of the leaking pipe that triangulate the breach. Finding a leak is a little like a game of Clue — or Cluedo, as it’s known in the UK.

Once a leak’s location is identified, the water company must ask the local council for permission to dig up the pavement or road to fix it, a process that can be disruptive and sometimes takes weeks. Workers also need to rule out other possibilities, including leaks on private property, which are flagged to property owners for repair.

On a cool night in May, the Thames Water leak hunters are poking around behind some bins. They’re looking for a hatch in the ground housing a pipe that might be leaking. Curious pedestrians eye the team of three warily — they are neither the most popular nor the most colorful people walking London’s streets in the middle of the night. 

Leak-hunting is a physical job, and the engineers spend a lot of time in the cold, bending down and carrying heavy things, which is tough on the knees and back. But there’s also an almost theatrical flourish to it, especially the part where everyone falls silent, eyes on the worker with their ear pressed to the listening stick.

“It could be such a minute noise, and you just have to follow the lead,” says Matthew Morris, an engineer on the team. A leak finder’s listening stick is like a conductor’s baton, or a potter’s wheel: an ordinary object which, in the right hands, becomes a conduit for great skill. Every engineer has a slightly different approach, and some prefer to go back to basics. “You have all of this tech, but you could do it all with that,” Morris says, gesturing to the stick.

One recurring challenge for the crew is distinguishing between a leak and a tap in someone’s home: Sometimes suspected leaks turn out to just be people who use a lot of water. An active faucet can sound like a leak from the street, but it’s hard to be sure without temporarily switching off a household’s water supply. “A customer could be running a bath,” says Stewart Knowles, a manager on the Thames Water team.

Sometimes the leak hunters do briefly close off supply to a property, then measure the flow to see if it drops once that part of the network is cut out. It’s another reason the team works unsociable hours, to avoid disrupting anyone’s nightly wash. 

Leak-hunting teams are not new, but their ranks are growing in tandem with pressure on Britain’s water companies. Leaks that a decade ago might have been deemed too expensive to worry about are now searched for and mended.  

Thames Water is the UK’s worst-performing company on leakage, losing 630 million liters a day, a figure its regulator has demanded be cut by 20% between 2020 and 2025. In March, the company was told it would have to improve its leak-fixing operation before it could implement plans to take more water from rivers. Shane Gloster, Thames Water’s leakage performance manager, says 95% of leaks happen underground. The company has added more than 60 leak hunters to its ranks in the past three years.

England’s other major water companies employ similar teams, and are also layering in newer techniques — like permanent sensors that use AI to compare the noise and vibrations of a network against a library of leak sounds.

This arsenal of approaches will be increasingly necessary as climate change drives more extreme weather; last summer’s drought was a case in point. The UK suffered its driest July since 1935, and temperatures topped 40C (104F) for the first time ever. Water restrictions were implemented and the number of leaks shot up. “When the ground dries out, it moves, but then when it starts to rain again, it starts to swell,” Gloster says. That disturbs the pipes underground, making them more likely to break. 

Back on the streets of London, Knowles has struck leak-hunting gold. The correlator has traced a faint noise to a patch about halfway between two points, signified by a spike in the graph marking the spot behind the bins. Right where the listeners first heard it. 


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