Why 'unwanted water' in London, Ont. sewers is a pricey problem and a big flood factor

It's a problem that's costing the City of London about $1.4-million a year while adding to the risk of basement flooding. 

The engineering term is "inflow infiltration," but a report coming to the city's Civic Works committee on Tuesday uses a simpler one: Unwanted water.

Essentially, it's water that ends up in the city's sewer system that shouldn't be there. Instead of water that comes from sinks and toilets and requires treatment, this is water that enters sewer pipes through other sources, including:

• Perimeter drains. Also known as weeping tiles, these are underground water collection systems placed around foundation walls to collect water and prevent it from flooding buildings. Sump pumps are often used as part of these systems. This water isn't supposed to be directed in the sewer but it often is, especially in older homes. 
   
• Combined sewers: This is outdated city infrastructure that, when it was built, actually directed stormwater into the sewer system. The city is working to disconnect these and the $7-million project to rebuild York Street a few years ago was a big part of this.
   
• Damaged pipes. Older sewer lines often have cracks that allow water to infiltrate the system. Regular upgrades help address this. 

Add it all up and fully 44 per cent of the water sent to London's five sewage treatment plants is actually "unwanted water." Keeping it out of the sewer system is a big focus for engineers like Ashley Rammeloo.

To answer the "who cares?" question, she says unwanted water can cause big problems, including icky and expensive sewer backups and basement floods.  

Also, when the sewer system gets inundated during intense rainstorms, the treatment plants can't fully treat the incoming water and some of it has to be discharged into the Thames River.

"During wet weather events ... they just can't take all that water," she said. "That results in overflows to the river and bypasses at the treatment plant and possibly basement flooding." 

Rammeloo said the city was flooded with calls during September's storm when 100 mm of rain fell in the London area in 48 hours.

"We received hundreds of calls," she said. Another problem is that extra water in the sewer system also limits its capacity at a time when the city is growing and adding new connections.

The extra annual cost of unnecessarily treating this water is also absorbed by taxpayers. 

"That hits everybody who uses water," she said.

So what's the fix? 

The city is working to separate combined sewers — as it did on York Street — while also looking to help homeowners disconnect drainage systems that send stormwater into the sewer. The basement flooding grant provides a 90 per cent subsidy to separate weeping tiles from the sanitary sewer and to install backflow prevention valves. Through this process the water is directed to the stormwater system on the street, not the wastewater system.

It's a big job, the city report estimates there are more than 50,000 of these connections in London. It's also tricky, because the work would happen on private property.

"This really requires the co-operation of the homeowners," said Rammeloo.

The report says the city will continue to study the problem and come up with new solutions to aggressively cut off sources of unwanted water in sewers. 

Meanwhile, Rammeloo said climate change is giving new urgency to the effort.

"What we're seeing now is a real intensity in storms and that's what really overwhelms the sanitary sewer because you get so much water so quickly, it just doesn't have anywhere to go," she said. "And that's when we get more basement flooding."