Toronto Water says the strange taste and smell some Toronto residents started noticing in their tap water last night is connected to treatment plant upgrades.

Staff investigated the issue overnight and found that the odour was related to system upgrades at the R.C. Harris Treatment Plant on Queen Street East in Scarborough, a statement from the city division said.

Toronto Water uses aluminum sulphate as part of the normal water treatment process, the statement continued, which can generate hydrogen sulphide gas that "resembles the smell of boiled eggs."

"This gas entered into some of the treated drinking water when the basin was put back into operation," the statement reads. "Toronto Water has taken the settling basin out of operation this morning and the taste and odour will dissipate by the end of today."

The odour had no effect on the city's water quality, according to the statement.

"The water remains safe to drink and meets all regulatory requirements."

In addition to the treatment plant work, Toronto Water has also been gradually increasing chlorine levels in the drinking water, as per regulatory requirements.

Residents describe 'chemical' taste

The strange smell worried some Toronto residents.

Ted Gorsline, who lives near Danforth and Pape avenues, said he first noticed a strange smell to the water on Thursday evening.

Is anyone else noticing their tap water tasting of chemicals? <a href="https://twitter.com/hashtag/SOTS?src=hash">#SOTS</a> <a href="https://twitter.com/hashtag/topoli?src=hash">#topoli</a> <a href="https://twitter.com/hashtag/chemicalwaterTO?src=hash">#chemicalwaterTO</a> <a href="https://twitter.com/hashtag/eaatyork?src=hash">#eaatyork</a> —@Tedgorsline

"We noticed our tap water tasting of strong chemicals last night at dinner," he told CBC Toronto. "Since then it has been undrinkable. We plan on using bottled water until this is returned to normal."

A neighbour one block away says she too noticed a strange smell.

"It kind of tasted like there was chemicals in it," said Siobhan Fahey, who said the smell and taste reminded her of fertilizer.

Her family has been drinking distilled water since Thursday evening, and they're boiling the tap water for their dogs.

Fahey's mother Moira Callahan says while the water has improved since Thursday evening, it is not yet back to normal.

"I'm still a bit concerned about drinking it," she said.

Gorsline also says neighbours have been posting warnings to local Facebook groups.

Other Toronto residents tweeted complaints to the @311Toronto account, with reports from various neighbourhoods, including Rosedale and Seaton Village.

<a href="https://twitter.com/CalebWellum">@CalebWellum</a> <a href="https://twitter.com/steven_heipel">@steven_heipel</a> TW investigated, found slight increase in chlorine. Level within regulation, safe to drink. Continue 2 monitor. —@311Toronto