London's £4.1bn 'super sewer' is kicking up a stink as campaigners label it a 'monster'

The Thames Tideway Tunnel will be the deepest structure under the capital – if it ever gets built

The executive in charge of the £4.1bn tunnel project to transform London's Victorian-era sewer network is quick to point out he's an engineer by profession, not a politician or a PR man.

Thames Water's Phil Stride does have a special sewer-use-only suit though, and he's become quite adept at taking tours of researchers, campaigners and politicians down to explore London's ageing Victorian sewer network.

"The Thames Tideway Tunnel is absolutely urgently needed," he told The Independent on Sunday before a trip down Thames Water's Victorian sewer at Blackfriars in central London. "London's main sewers were built a 150 years ago at a time when there were just over two million people living in the city. Now there are eight million."

Proposals for the 15-mile tunnel, known as the "super sewer" and running from Acton in west London to Abbey Mills in the east, are with the Planning Inspectorate but face string of determined opposition groups; Stride will need all his persuasive skills to finish the project by 2023.

Campaigners have labelled it a "monster" and accused Thames Water of "overcharging" for the project, but Mr Stride's argument in its favour is incredibly simple; that roughly once a week London's sewer system overflows and raw sewage is pumped directly into the river Thames. The consequences of not acting would be "catastrophic", according to Stride, when we are already "dumping tens of millions of tons of sewage into the Thames every year".

London's sewage problems aren't new. In the summer of 1858 the "Great Stink" descended on the Houses of Parliament. The smell of sewage in the River Thames became unbearable and MPs allocated £3m for engineer Sir Joseph Bazalgette to build a system of tunnels to catch London's sewage and divert it away from the capital.

Today there are about 100 miles of Bazalgette's tunnels. They are still in good condition but as little as 2mm of rainfall at the wrong time of day can send untreated sewage down one of the 57 overflow points directly into the river Thames at the rate of up to 50 tons a second.

According to Thames Water, 55 million tons of untreated sewage was flushed into the river last year and the Thames Tideway Tunnel is the only viable way to remove reliance on Bazalgette's system. It will go as deep as 61 metres, which is "deeper than all of London's existing underground infrastructure", and will run under 1,300 homes.

If Mr Stride and his team are successful, it will be the deepest, longest and widest sewer the UK has ever seen; without it, they say, London will dump as much as 70 million tons of raw sewage a year into the Thames by 2020.

But engineering prowess hasn't stopped the project causing a stink. With five construction shafts, 17 smaller interception shafts and long-term disruption, it's spawned half a dozen protest groups and become a metaphor for modern Nimbyism.

Campaigner Ana Williams, from Save Your Riverside at the Chambers Wharf shaft site on the south bank, just east of Tower Bridge, said the sewer "will ruin the lives of thousands of local residents and school children in an area that just isn't suitable for a project of this scale". When the tunnel is complete, she says, Thames Water will still dump "around two million tons" of sewage into the river each year.

Further east, on the north bank, at Shadwell, the chairman of the Save King Edward Memorial Park campaign, Carl Dunsire, said Thames Water sees construction and disruption at the park as an "acceptable cost" of the super sewer and that the firm had "rejected" alternative brownfield sites.

Mr Stride is quick to point to ComRes poll data from 2012 showing 85 per cent of Londoners back the tunnel: "People who are against the tunnel are generally against it because of local impact … you are never going to build a tunnel across London without causing some disturbance."

According to Thames Water, local protests don't take into account the 100 years of benefit the system will provide. "Bazalgette's era ...was a time when we led the world in terms of infrastructure," Mr Stride said. "Today we have a problem separating the long-term benefits for the nation and how we manage those against local disruption."

Stephen Halliday, author of The Great Stink of London, a history of Bazalgette's sewers, agrees: "In the 1800s, most people accepted there was a need for a new sewer system. The main debate was about who would pay for it, whereas today most objections seem to come from people living in the areas where the shafts are going to be sunk."

He added: "Personally I can't really see an alternative, but people don't want these sites at the end of their street and I can't really blame them."

It's a dirty job, but...

As you climb down into the Fleet Sewer at Blackfriars in central London the most unpleasant thing isn't the discomfort from your clunky safety gear, the bilious sludge below, or even the musty smell of half a million Londoners' bowel movements; it's the brown mulch and shreds of used toilet roll clinging to the ladder just inches from your face.

The experience is a sensory overload that couldn't be further from Thames Water's glossy websites and slick corporate videos in support of the "super sewer". It's made worse when I realise some of the fibres clinging to my gloves are strands of sanitary products and nappies.

My guide, Thames Water's chief flusher, Rob Smith, notices I'm holding my face as far away from the slippery ladder as possible. "It's fresh sewage here," he says when I reach the bottom. "Try visiting a treatment plant. That's a different story."

Mr Smith leads us down a narrow brick-lined tunnel while explaining that the 318 million bricks that make up London's Victorian sewer network are in still remarkably good condition.

Fascinating, I'm sure, but I'm too busy concentrating hard on not tripping over to think much about it. I want to avoid a panicked scrubbing in the decontamination truck parked six metres above us.

Eventually we reach a point where the long-covered river Fleet meets an interception sewer. Thick build-ups of fibres and "human deposits" hang from the ceiling like stalactites. My protective suit is already pebble-dashed with faeces; this is the most unpleasant place I've ever been.

Pushing deeper, we come to giant gates separating us from the Thames. When it rains, Mr Smith tells me, 50 tons of sewage a second can flow out here. I decide to take his word for it rather than hang around to find out.

Jamie Merrill