Three months after the city of Montreal dumped billions of litres of untreated sewage into the St. Lawrence river, studies show the impact on the environment was minimal.

Mayor Denis Coderre's administration said that hindsight shows its largest problem was in communicating the exact nature of the work that had to take place, why it was happening, and its impact.

In November crews had to stop sewage from flowing into the main collector leading to city's wastewater treatment facility in order to repair support beams that were falling apart.

Despite the plan being discussed for several months, it was only a few days before work began that city officials explained the rotting support beams were could cause so much damage that Montreal's wastewater treatment plant risked being shut down indefinitely.

In the end, the beams were replaced within 89 hours, or about half the time it was expected the job would take.

About 4.9 billion litres of untreated sewage was dumped into the river from about two dozen locations.

Many of those sewer pipes were surrounded by nets and floating booms in order for treatment crews to fish out large objects.

Crews then finished work on a snowmelt collector, but were able to time that so as to reduce the amount of untreated sewage going into the river.

Richard Fontaine, the director of Montreal's wastewater treatment facility, said the city did the best it could.

"The impact is there. It's never a good idea to overflow without treatment. What we're saying is that the impact was limited in time, it was limited in distance, and the situation back to normal, was acquired at the earliest four days after we stopped the overflow. In one sector, the McGill sector, ten days," said Fontaine.

Water tests showed the effect of the dump was limited to an area about 250 metres wide and stretching 10 km downstream.

Fecal coliform tests in nearby Repentigny showed no change during and after the dump.

Opposition councillors in Montreal took credit for sounding the alarm about the sewage dump, and said without the work of Projet Montreal, citizens would have been in the dark about the work taking place and its possible environmental effects.

Prior to the 2015 sewage dump, Montreal had let untreated sewage flow into the river on two occasions in the past 15 years, one of those times dumping 18 billion litres of sewage into the St. Lawrence.

In 2013 there were 45,000 occasions when cities and towns in Quebec dumped raw sewage into the St. Lawrence river.

Montreal's wastewater treatment facility was only completed in 1998, and until then untreated sewage flowed directly into the waterways.