The town of Ajax, one of the many bedroom communities that sprawl out from Toronto's border, has a waterfront unlike many of its neighbours. With the exception of one private property, it's seven uninterrupted kilometres of publicly owned, publicly accessible land, the longest stretch in the Greater Toronto Area. In the warm months, it's a runner's or cyclist's paradise: a seemingly endless stretch of green grass and tangles of trees lining the water of Lake Ontario.

But Ajax, a town of 110,000, is in a fight to protect its shores from the burgeoning population of York, the region on its western flank, and the proposed doubling of the sewage to be treated in Ajax's lakeside plant. For Mayor Steve Parish, more sewage means more algae, the pungent green nuisance that already imperils the town's beaches, rendering them unusable for much of the summer.

This particular species of algae, called cladophora, is common throughout the Great Lakes except Lake Superior. It proliferates when it has access to lots of phosphorus and light – both of which are in abundant supply along Ajax's shore. It grows on rock surfaces in the spring and then detaches and floats to the shore from the middle of the summer until mid-September. The mayor believes it threatens the future of his town.

Story continues below advertisement

"Developers in York Region, in developing and selling 150,000 household units, will get very significant profits from this on the backs of the people downstream – the people in Ajax," Mr. Parish said.

The local sewage treatment plant, jointly owned by Durham Region (which governs Ajax) and the more populous York Region, plans to increase the amount of sewage it treats from its current 340 million litres per day to 620 million litres per day in the coming years, in step with the anticipated population increase of 400,000 in York by 2031. The Duffin Creek Water Pollution Control Plant expels treated sewage (known as effluent) one kilometre off the shore of Lake Ontario, and the Town of Ajax believes the phosphorus in the effluent is causing the algae to take over its shoreline.

But the science is inconclusive: Some studies suggest stormwater and zebra mussels may be even more important factors in algae growth.

"It's not just fouling the beaches in terms of making them not swimmable or the rotting of the algae causing smells," Mr. Parish said. "We are a Lake Ontario waterfront community, and the waterfront has major economic impact in terms of the growth, tourism, etc. of our community." He's called for a study to look at its effect on E. coli in the area, as well, as he believes the algae traps and re-emits E. coli near Ajax's shore, which may be responsible for the frequent beach closings in the town in the summer months.

While the Town of Ajax believes the water quality along its shoreline will be most affected by this change at the sewage treatment plant, the two-tier government structure prevents it from having equal say in the decision. How this extra demand on the system will be managed is ultimately up to Durham and York regions (as well as the province's Ministry of Environment), which have called for a relatively minor adjustment to the equipment used to release the effluent into Lake Ontario rather than more drastic and expensive measures proposed by Ajax. The town favours adding an extra stage of treatment for the sewage to separate out more phosphorus and to release the treated sewage further offshore.

Durham and York regions are nearing the end of an environmental assessment on modifications to the plant that began in 2010. As part of that process, they hosted public consultations last week in Pickering, where the plant is located, and neighbouring Ajax. The Ajax meeting drew a crowd of about 80 locals, who pressed the two regions, as well as the consultants running the project, to "be leaders" in tackling the water quality problem in Lake Ontario.

"We fully recognize that cladophora is an issue for the Ajax-Pickering area. We don't dispute that," said Barry Laverick, a project engineer with Durham Region, at the meeting. "We've spent a lot of time and a lot of effort ranking the alternatives."

Story continues below advertisement

At the heart of the dispute is the cause of the algae growth.

The Town of Ajax cites a University of Michigan study that points to the phosphorus from the sewage plant's effluent as the culprit behind the algae, but the two regions have turned to other research from other sources, including the University of Waterloo, that suggests stormwater and the infestation of zebra mussels in Lake Ontario have more to do with the algae growth.

"There's no one gigantic source. If there was, it would be easy to deal with," said Peter Dillon, a professor at Trent University's Water Quality Centre and a Canadian expert on the biogeochemistry of lakes. "It's the cumulative effect of a lot of smaller sources, and you have to deal with them all best you can."

While he hasn't completed studies of Ajax's near-shore area, Dr. Dillon said the regions of York and Durham may very well be right that storm water is the biggest contributor of phosphorus to that part of Lake Ontario. But he said it's always easier to manage the phosphorus levels coming from a "point source," such as a sewage treatment plant, versus a broad range such as a lawn-fertilizer nutrients, dust on roads and agricultural runoff that could all end up in stormwater.

On a walk along the water in Rotary Park near the mouth of Duffins Creek (at this time of year, it's naked trees and calf-high slush), Mr. Parish said that since there's no consensus on the cause of the algae, the regions should put the environmental assessment on pause and commission an extensive study of the water quality along Ajax's shoreline – or even wait until a provincial study currently underway on the health of Lake Ontario is completed.

In the meantime, the Town of Ajax has submitted two alternative ways to handle the added pressure on the sewage-treatment system coming from York Region. One is constructing a longer outfall (the pipe that carries the effluent into the lake) from its current length of one kilometre to three kilometres. Both the town and the two regions agree the deeper water three kilometres offshore would allow for better dilution of the effluent, which could mean less algae on the waterfront. But that option comes with a $185-million to $240-million pricetag.

Story continues below advertisement

The town has also proposed tertiary treatment: an extra few stages of sewage treatment that would remove more phosphorus from the effluent prior to its release. The regions say not only would it be expensive (between $175-million and $230-million) but it's the option with the biggest carbon footprint, since it would involve constructing two new buildings on the plant's site. John Presta, Durham Region's director of environment services, says it won't do much to combat the algae in the lake, either.

"Tertiary treatment is like adding a clean drop of water in a bathtub. It's not going to provide much impact," he said.

But regardless of which research or modifications to the plant residents prefer, most who attended the public consultation seemed most riled up about the regional politics behind the plant's expansion.

Walter Donaldson, 66, who has lived in Ajax for almost three decades, said he thinks York and Durham regions have put cost-effectivness before all else in deciding how to handle the increased demands for sewage treatment from residents of York Region.

"There's a big pipe coming down from York Region – they were saying in [the meeting] it's 80 per cent from them," Mr. Donaldson said. "The town [of Ajax] isn't getting the responses they want. It seems to me like they're trying to rush this through."

York Region officials say they've listened and responded to the Town of Ajax's concerns, but say the option they've selected makes the most sense for the region when all factors – including social and financial – are considered. While some sewage from the region is sent to a plant along Lake Simcoe for treatment, it's not feasible to direct sewage from all future development there.

Story continues below advertisement

"The regions have expended a significant amount of money to go above and beyond the minimum requirements for an environmental assessment," said Daniel Kostopoulos, York Region's director of capital planning and delivery. "We're spending millions of dollars on it to make sure we get things right."