An environmental group says municipalities across the province are allowing billions of litres of raw sewage to be dumped into the Great Lakes every year through outdated sewer systems - a problem they say could be prevented with green infrastructure.

In a report released Monday, Ecojustice Canada says that 89 Ontario municipalities are operating with antiquated infrastructure that allows sludge to overflow into the province's waterways.

The culprit is combined sewers, which carry untreated sewage and rainwater in a single pipe, said Liat Podolsky, a science researcher at Ecojustice and co-author of the report.

During wet weather, the volume of flow often exceeds the sewers' capacity, causing the untreated sewage and storm water to overflow into local waterways.

"The problem is increasing now because the populations in these cities are increasing with time, so there's more strain on the system itself just from greater amounts of sewage, and from greater amounts of storm water as a result of climate change," Podolsky said.

Most of these sewers were built in the early 20th century and aren't able to meet today's demands, Podlosky said.

The best way to prevent the problem is to implement green infrastructure - such as green roofs - that will absorb the rainwater rather than sending it through the sewage system.

"The conventional practices will treat the storm water and the sewage once it's already in the system, while green infrastructure treats it at the source," Podolsky said. "So it acts to minimize and limit the amount of storm water and sewage that's going into the system and therefore needs to be treated."

Though green roofs are the most popular and well-known solution, downspout disconnection - removing a roof drainage system from the sewer connection and directing rainwater towards gardens or lawns - can also reduce the amount of storm water collected in sewers. Even increasing the amount of trees in a city can help absorb the first half-inch of rainfall during a storm, the report reads.

Ecojustice also outlines which cities are adopting solutions to help ease the strain on combined sewers.

Toronto is the obvious leader, Podlosky said, due to its mandatory downspout disconnection program and green roof initiative.

Windsor, London, and St. Catharines, and Kingston are among the cities moving towards implementing green infrastructure.