The other part of the question concerns what happened and the city's choices. The City of Montreal had some work to do because the pipes where all the sewers converged were damaged. A lot of debris had built up and the city had to discharge water in order to clear the pipes and remove the debris.
The city had its works planned at the time, had all the necessary labour and equipment and was able, for example, to control the number of days to be allocated to the work. Failure to perform that work would have been risky because any breakage would have resulted in a spill in any case, and the city wouldn't know how much time was needed and wouldn't necessarily have the required equipment.
As regards the third part of the question, the final order, which the government issued under section 31 of the Canadian Environmental Assessment Act, included conditions that the City of Montreal had to meet if it decided to proceed with the discharge. It wasn't an authorization to proceed with the discharge.