This problem causes the City of London to lose approximately US$1.4 million a year, and at the same time increases the risk of basement flooding. 

The engineering term is "inflow infiltration," but a report submitted to the city’s municipal engineering committee on Tuesday used a simpler term: unwanted water.

In essence, the water that eventually enters the urban sewer system should not exist. This water does not come from sinks and toilets and needs to be treated, but water that enters the sewer through other sources, including:

Adding all of this together, 44% of the water sent to the five sewage treatment plants in London is actually "unwanted water." Keeping it away from the sewer system is the focus of engineers such as Ashley Rammeloo.

Responding to the "who cares?" question, she said that unwanted water can cause big problems, including annoying and expensive sewer backup and basement flooding.  

In addition, when the sewer system is submerged during heavy rains, the treatment plant cannot completely treat the inflowing water, and some of it must be discharged into the Thames.

"During wet weather events... they just can't take all the water away," she said. "This causes overflow into the river and bypasses the treatment plant, and may cause flooding in the basement." 

Rammeloo said that during the September storm, the London area reached 100 mm of rainfall in 48 hours, and the entire city was flooded by telephones.

"We received hundreds of calls," she said. Another problem is that as cities develop and add new connections, the excess water in the sewer system will also limit its capacity.

The additional annual costs of unnecessary treatment of this water are also borne by taxpayers. 

"This affects everyone who uses water," she said.

The city is working to separate confluence sewers—as it did on York Street—and also hopes to help homeowners disconnect the drainage system that drains rainwater into the sewers. The basement flood protection subsidy provides a 90% subsidy to separate leaking tiles from sanitary sewers and install backflow prevention valves. Through this process, the water is directed to the rainwater system on the street instead of the wastewater system.

This is a daunting task, and the city report estimates that there are more than 50,000 such connections in London. This is also tricky, because the work will be done on private property.

"This really requires the cooperation of the homeowner," Lamelu said.

The report stated that the city will continue to study this problem and propose new solutions to actively cut off unwanted water sources in sewers. 

At the same time, Lamelu said that climate change has brought new urgency to this work.

"What we are seeing now is the true intensity of the storm, which is what really overwhelmed the sewer, because you get so much water so quickly, it has nowhere to go," she said. "That was when our basement was flooded." 

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