Rachel Cole-Wilkin hates paying to use the toilet. In the United States, where she’s from, public toilets are free. It was a bit of a shock for her when she moved to London, where that’s not the case.

Cole-Wilkin, 29, moved to London from San Francisco seven years ago to study theatre but decided to stay after she became obsessed with finding the city’s public toilets and turned it into a business.

Since 2012, Cole-Wilkin has been offering Loo Tours around the restrooms of London. At the start of each tour, she identifies herself by holding a plunger in the air. She then uses it to guide the participants around various facilities in the British capital.

The tour begins at Waterloo Station before winding its way across the Thames to Covent Garden. One of the highlights is the Jubiloo, possibly the city’s most patriotic public toilet, near the London Eye.

Cole-Wilkin leads tours of toilets in London. — Cornelia Neumeyer/dpa

Inside, the design screams “best of British”. Pictures of the Queen stare down at you from the walls, and Union Jack patterns decorate the toilet seats, bins and mirrors. It’s eco-friendly, too, collecting rainwater to help with flushing.

The Jubiloo is Cole-Wilkin’s personal favourite – even though it’s not free. But you get what you pay for: The patriotic toilet seats get a wipe after every customer.

From the Jubiloo, the tour heads north towards the next stop on the Golden Jubilee Bridge, above what is essentially London’s biggest public toilet: the River Thames.

Every year, 40 million cubic metres of sewage flow into the river – mainly as a result of London’s ancient sewage system, which dates to 1865. Its two main channels run underground alongside the river and were designed more than 150 years ago.

One of the highlights of Loo Tours is the Jubiloo, possibly the city’s most patriotic public toilet, near the London Eye. Inside, the design screams ‘best of British’. — Ilya Fisher/Loo Tours

In the meantime, the city’s population has grown to almost nine million.

Everyone knows that more people produce more waste – but no one in London quite knows what to do with it. The best solution anyone has come up with so far is to let whatever doesn’t fit in the two channels overflow into the Thames.

But in the early 2000s, it started to become clear that this was less than ideal. This is why a kind of overflow basin is being built in the Thames opposite the London Eye to catch the waste.

It’s due to be finished in six years. Until then, some 300 Olympic swimming pools of untreated waste will continue to roar down the river each year. – dpa/Cornelia Neumeyer