Beside a sign saying “No swimming”, Pierre Fuzeau defiantly pulled on his swimming cap, slipped into the green water of the Ourcq canal on Paris’s northern edge, and set off with a strong front-crawl.

The 66-year-old company director regularly joins his open-water swimming group for well-organised illegal dips, including in the River Seine, where swimming has been banned since 1923 largely as a result of the health risk from unclean water and bacteria from human waste.

“I’ve never, ever been sick after swimming,” Fuzeau said. “There’s a wonderful feeling of freedom and boosting your immune system in cold water. There’s something great about being submerged in nature in an urban setting, it’s rare to have such a sense of aquatic freedom in the city, and the camaraderie with other swimmers is a joy.”

Cleaning up the murky River Seine to make it swimmable for athletes in this summer’s Paris Olympics has been one of the longest-running, most expensive and high-stakes endeavours of the Games.

The €1.4bn (£1.2bn) state-backed plan has entailed several years of work on wastewater management, treatment plants, filtering stations and storm basins to lower the river’s bacterial contamination from faecal waste.

It is a highly political undertaking that goes well beyond the Olympic Games. Paris’s summer temperatures are soaring amid the climate crisis. After the Games, authorities are planning to set up local beaches and swimming areas in the Seine and Marne rivers that will be open from 2025. Like Copenhagen, Munich or Zurich, Paris and its surrounding area wants residents to be able to use urban open water to cool off – something which was common practice in the 17th century when nude bathing in the city was the norm.

“When you’re passing a river, you just want to get in and swim,” said the Paris mayor, Anne Hidalgo, who, like Emmanuel Macron, plans to swim in the Seine next month.

For the Olympics, the open-water swimming events and triathlon will start at Pont Alexandre III, a marvel of 19th-century engineering near the foot of the Champs-Élysées, with the Eiffel Tower looming in the background. The river water will be tested regularly before each event and authorities are certain it will be clean. By the time of the Games, 75% of identified bacterial pollution will have been eliminated, they pledge – higher than the target set.

There is no plan B for the Olympic events, Paris region prefect Marc Guillaume has said, except to push the date back by a few days in case of prolonged, very heavy rain that could temporarily spill untreated water into the river.

The Seine in Paris is a relatively small and narrow river for a capital city, and has always been under pressure from the sanitation of the large communities that live along it. When residential buildings were hastily put up on the outskirts of Paris during the housing crisis after the second world war, many were not properly connected to wastewater networks. The problem in the decades after was largely human waste. When there was heavy rain in the Paris area, rainwater entering the sewage system caused an overspill, with untreated wastewater discharging straight into the Seine and increasing the presence of bacteria from faecal matter. Recent years have seen a slow improvement.

“Water quality in the River Seine has been improving gradually for decades, but the Games led us to speed up the process, especially over the last three years,” said the prefect, Pierre-Antoine Molina, secretary general of public policy for the Île-de-France region.

“The water quality has been improved through three different approaches. First, by improving the performance of our wastewater facilities. Then, by modernising the sewage system in order to separate rainwater and wastewater in a more systematic way. Finally, we have had to correct misconnections to the system, in which wastewater was discharged into rainwater systems and vice-versa.” Studies had shown, he said, that thousands of buildings were affected, having been connected to the wrong network.

Authorities have counted more than 30 species of fish in the Seine in Paris, compared with three in 1970.

Those who are happy about the changes include the people living on the roughly 250 houseboats in Paris, some of whom had no proper connections to the water system, meaning their waste went into the Seine. After a programme of state-subsidised improvements, almost all are now properly plumbed into the city’s wastewater system.

“It’s progress – the Olympic Games have been a great vector for that, it helped all the politicians agree and put the finances on the table,” said Jean-Philippe, 53, a lawyer who lived on a renovated 1900s tugboat moored on the left bank. He was happy that fish in the river were now flourishing.

The river’s strong current and the high traffic of industrial boats and barges had also made Parisians traditionally fear the Seine.

“In Paris, the Seine is still largely seen as a forbidden river,” said Baptiste Saint-Laurent, technical director of the Rosa Bonheur, a bar and dance venue on a boat moored near the spot where the Olympic swimming events will take place. With up to 2,000 customers a day, the Rosa Bonheur had always been fully connected to the wastewater network using a pump system.

“The pre-Olympics clean-up will change Parisians’ relationship to the water,” said Saint-Laurent, a former member of the French navy. “It won’t just be a stretch of water to cross on a bridge in a hurry. I grew up on the edge of the Seine in Normandy, I learnt to swim in the Seine when I was seven. So historically, this is an important moment because people in the city will reclaim the river.”

Meanwhile, special boats will travel the river to pick out litter, ranging from plastic bottles to branches during the Olympic Games.

Antoine, 28, a history teacher, lowering himself into the Ourcq canal water for his first open-swim, said he was looking forward to taking his new hobby to the Seine. “The pleasure of open-water swimming is something you usually associate with being on holiday. To have that in the city is amazing. I’ll swim in the Seine, with no worries for my health.”