Halifax Water says human health isn't at risk over the next two months as it shuts down its bacteria disinfection system for a pilot project, but a Dalhousie University study suggests humans aren't the ones we should be worried about.

Powerful ultra-violet lights at three wastewater treatment plants were shut off Monday and won't be turned back on until April 30.

The project is designed to see if rising bacteria levels in Halifax Harbour are worth the energy and money saved.

"There will be no change in terms of the clarity of the water, odours in the harbour — none of that will occur," said spokesman James Campbell.

The utility will monitor bacteria levels at eight sites in Halifax Harbour and report back to the province's environment department. If approved, seasonal disinfection would see the UV lights turned off between Nov. 1 and March 31 every year.

Halifax Water says just over 1.5 million kilowatt hours and $148,000 would be saved in a year. When the lights are turned back on, the utility says it will take two to three days for bacteria levels to return to normal.

But between April and October, humans aren't the only ones who use the harbour.

'We must be very careful'

Dalhousie University assistant professor Dr. Haibo Niu co-authored Prediction of Fecal Coliform Concentrations from Wastewater Discharges in Halifax Harbour. He agrees that swimmers aren't likely to be affected by seasonal disinfection.

But after creating model simulations with tidal, current, and water column data, he says there is a high probability that bi-valve shellfish — such as mussels and clams — will be affected in areas where recreational harvesting is done, such as Eastern Passage.

James Campbell, a spokesman with Halifax Water, said turning the UV treatment system off between November and April is not unheard of. (CBC)

"I personally think the money savings is one thing," Niu said. "I think we must be very careful before we take this approach because of the potential risks here."

The Canadian Shellfish Sanitation Program uses fecal coliform levels to measure shellfish safety. Standards for the outer harbour around McNabs Island are 14 units per 100 millilitres.

Ten per cent of those samples can't exceed 43 units per 100 millilitres or the shellfish is considered unsafe.

In Niu's six-month simulated study, fecal coliform levels between McNabs Island, Portuguese Cove and Eastern Passage range anywhere from 14 to 200 units per 100 millilitres when disinfection isn't taking place.

"We have quite a large area that have concentrations above the shellfish limit," he said.

Won't know until you try

The Canadian Food Inspection Agency was not able to provide an exact figure for bacteria levels in shellfish that would be toxic to human health.

Campbell says the utility spent the last five months testing water samples for baseline data. Of the eight sites from which samples have been drawn, he says the site farthest in the outer harbour is Herring Cove.

"It's completely up to our regulators. So we'll comply with whatever they tell us to do," Campbell said.

Niu agrees that data collected after the lights are shut off is important, and can only improve his simulation models.

But because this is Halifax's first crack at seasonal disinfection, he urges care. There aren't many examples of seasonal disinfection near coastal areas, he says, since most case studies take place near rivers.

"This is all kind of unique because when you're trying to look at this seasonal disinfection in the literature, there's not a whole lot," he said.

"I hope they [Nova Scotia Environment] will consider this, because this gives an indication of what might happen there."