A calibration error was the cause of a November sewage spill that sent hundreds of cubic metres of wastewater flowing into the Ottawa River, the city says.
The malfunction involved the $232-million pair of tunnels that opened in late 2020, part of the city's plan to keep surface runoff and wastewater from flowing untreated into the Ottawa River during very wet weather.
On Nov. 16, the instrument that measures sewage levels in the Stanley Park shaft malfunctioned and the tunnels were filled past capacity.
City staff were monitoring the system and were able to stop the overflow quickly, but wastewater flowed into three outlets to the Ottawa River.
A total of 306 cubic metres spilled, a volume which represents about 0.1 per cent of the daily amount treated at the sewage treatment facility on a typical day.
On Friday, the city said a calibration error in the Stanley Park shaft instrument led to inaccurate measurements being reported to operations staff.
While a second, identical instrument exists in the shaft, it had been recently relocated, resulting in "inconsistent data that delayed an immediate response," said the memo from Marilyn Journaux, the city's director of water services.
Several recommendations have been put forward to prevent something like that from happening again, Journaux said.