The City of Thunder Bay is increasing its rate of testing wastewater for COVID-19 as cases climb in the community in northwestern Ontario.

Thunder Bay is among a number of municipalities in the province that are submitting wastewater samples to labs for analysis.

Water samples taken at Thunder Bay's municipal pollution control plant go to the Great Lakes Institute for Environmental Research at the University of Windsor.

Thunder Bay chief chemist Ian Morgan said while the city did slow its sample rates in recent months due to a lack of COVID-19 cases in the city, that changed this week.

"With this current wave going on, we decided to ramp up for more data points, and are sampling the wastewater three times a week," Morgan said.

"At the beginning of the surveillance, we did see a little blip in February and March, which correlated nicely with the reported cases. Then when [case numbers] dropped down to essentially nothing, the data was negligible.

"However, now, when we're starting to see the cases come back up, the data is rising and correlating," he said.

Mike McKay, executive director of the Great Lakes Institute for Environmental Research, said samples are received by the lab the day after they're taken and analysis starts immediately.

"Samples are kept cold," McKay said. "The viral signal can persist probably for a week or longer when it's kept cold, so we're not worried about about the signal degrading.

"But it's important that we would be able to process samples as quickly as possible to get the information back."

That information is then provided to public health units to help them with the pandemic response, McKay said.

For example, some municipalities have used targeted wastewater testing to track cases in specific locations, like hospitals or long-term care facilities.

The University of Windsor also monitored wastewater at one of its residence halls, he said. In some cases, the samples showed COVID-19 in the water, which in turn led to broader testing and quarantining of asymptomatic individuals.

The process is especially important now, McKay said, due to a shortage of tests in Ontario.

"We're really flying blind. We've lost sight of the true size of the pandemic because of the lack of testing that's going on, the challenges for testing, whether it's supply chain or whether it's just staff shortages because of COVID.

"And so right now, there are limited metrics that health units can use to determine the true extent of the pandemic in their communities," McKay said. "Several of them are lagging, such as hospitalizations, and so there's renewed interest in wastewater surveillance right now to inform public health of trends of infection within the community."

Over about the last year, the Ontario wastewater surveillance program has grown to cover about 75 per cent of the province's population, McKay said.

"With Omicron, a lot of municipalities, a lot of public health units, are are looking at wastewater right now. We're seeing [it's] one of the true, unbiased indicators of what's really going on in the community.

"So I think there's going to be a future for wastewater monitoring even after we get past COVID," McKay said. "We're starting to look at this now as to whether or not we could use wastewater to detect other diseases that might be circulating in the community."

In an email to CBC News, a spokesperson for the Ministry of Environment, Conservation and Parks (MECP) said wastewater from all of Ontario's 34 public health unit service areas is being sampled and tested as part of the program.

Specifically, the samples are being taken at more than 170 locations, including wastewater treatment plants, long-term care facilities, universities, correctional facilities and First Nation communities.

The sampling and analysis program is being run by the MECP and Ministry of Health.