Thames Water customers will pay a £25 levy on their bills for about another two decades to repay the cost of the £4.5bn “super sewer”.

The 15-mile pipeline, which was officially declared in use on Monday, more than eight years after construction began, is being funded through a surcharge, currently three per cent, on domestic water bills.

A decade ago, when the project was first envisaged, Tideway, the firm that has built the super sewer, and Thames Water committed to charging “no more than £25 a year” at 2014/15 prices.

Andy Mitchell, chief executive of Tideway, said the project – which will drastically reduce the amount of raw sewage that ends up in the Thames – was “like a mortgage arrangement that quietly will be paid off over decades”.

Speaking to The Standard on Monday, Mr Mitchell said: “We believe this year that it will be at its peak. It will fall away thereafter.

“Quite at what pace it falls away really will be a matter between Tideway and Thames Water and the regulator every five years, to decide what should happen over the coming five years.

“It’s variable and decisions will be made in future decades as to how long that carries on.”

Thames Water has been approached for comment.

The project has created seven new public “piazzas” along the riverbank, including at Putney, Chelsea, Vauxhall, on the Victoria Embankment and beside Blackfriars bridge.

The space at Blackfriars – the largest of the seven – will be named Bazalgette Embankment, in honour of Joseph Bazalgette, the Victorian engineer who created the capital’s first sewage system.

“We don’t think he got enough credit for what he did, so we have been able to get that named after him,” Mr Mitchell said.

“This will be a continuation of what Bazalgette did. We hope it’s a place that people will enjoy for many, many decades to come.

“We have the Victoria site opposite site opposite the London Eye, which is going to be a fantastic site in future years to see the fireworks on New Year’s Eve.”

The capital’s flagship cycle route, the CS3 cycle superhighway, now known as cycleway 3, along the Victoria Embankment, is expected to be fully reinstated next summer, when Tideway vacate its riverside work site between Embankment Tube station and Blackfriars bridge.

Construction works have required the narrowing of the cycleway near the Tube station and the re-routing of the westbound and eastbound ramps that allow cyclists and vehicles to move between Blackfriars bridge and the Embankment.

Mr Mitchell said: “We are talking to TfL and have agreed that they will do that reinstatement. We are paying for it, but they will do it. Because we are demobilising it makes more sense for them to do it.

“As we finish that site next summer, we will vacate the space. That will allow TfL to put the superhighway back where it was before we started.”

The super sewer’s flood defences are operating in four out of 21 sites, all in west London – Acton, Barn Elms and two portals at King George’s Park.

Next week the central London portals at Victoria Embankment, Blackfriars Bridge and Albert Embankment are expected to come on stream.

All 21 sites should be in use by the end of the year. “We are hoping another six to eight weeks and we will be there,” Mr Mitchell said.

During heavy rainfall, they will direct overflows into the super sewer, which has been dug under the river between Hammersmith bridge and Limehouse, and takes the discharges direct to Beckton sewage treatment works in east London.

Prior to the super sewer opening, untreated sewage was discharged into the Thames about 60 days a year.

Mr Mitchell said: “This is going to make a fundamental difference into the health of the river.”

Asked about London mayor Sadiq Khan’s plan to make the Thames swimmable within a decade, Mr Mitchell said the super sewer would make it much cleaner – but he wouldn’t personally want to swim in the river.

He said: “If you are out there swimming in the tidal Thames, with all the traffic and a 7m tide range and the fast currents, arguably the quality of the water is the least of your problems. It’s not an advisable place to swim.

“But on the point of: ‘Will the water be an awful lot cleaner?’ Yes, it will.

“Technically, this water would be of a condition that – if that was the only consideration – you could [go swimming]. I wouldn’t.”