Partially treated wastewater was sent to Georgian Bay on Thursday as sewage flows to Collingwood's wastewater treatment plant spiked during a heavy rainstorm.
The town reported a partial bypass event on Thursday, Aug. 25. About 930 cubic metres of wastewater bypassed the final stage of treatment (UV disinfection) and went back to the bay. The municipality confirmed the water was treated with chlorine before being sent to the bay.
It was the heavy rain Thursday evening that caused the plant to be overwhelmed and skip a step to get water processed faster.
On Friday, the town's wastewater treatment plant is getting a flow of about 32,000 cubic metres per day. Comparatively, the flow rate during yesterday's storm was over 60,000 cubic metres per day.
According to Environment and Climate Change Canada, about 38.5 millimetres of rain fell in Collingwood on Aug. 25 between 4-7 p.m.
The town has notified the Ministry of the Environment, Conservation and Parks' spills action centre and the regional medical officer of health of the partial bypass and has collected samples of the water discharged during the bypass.
According to the town, public works staff are monitoring the sanitary system today, particularly in the area of the former shipyards.
On Sept. 23, 2021, another rainstorm in Collingwood overwhelmed the town's wastewater treatment plant, this time causing a full bypass for 14.5 hours and a partial bypass for 21.5 hours.
In the event of a full bypass, excess wastewater will overflow from the plant, bypassing the entire treatment process. It will mix with treated and partially treated water before discharging into the bay.
During the 2021 rain event, about 118.3 milimetres of rain fell in 35 hours. The Pretty River rose to record levels not seen since 2016, and the second-highest record since 2006.
A partial bypass of the town's wastewater treatment plant in June 2020 occurred when 26.5 millimetres of rain fell in about one hour.
The town's stormwater drains do not flow through the wastewater treatment plant. However, heavy rainfalls will increase the amount of water going to the wastewater treatment plant because of infiltration into the wastewater system.
In 2021, staff explained the extra water can come from various sources including downspouts and sump pumps connected to the sanitary sewer (though they are not supposed to be). Other sources include groundwater and rainwater getting into cracks in the sewer pipes and/or maintenance holes, and from overland flooding draining into sanitary maintenance holes.
The town's wastewater treatment plant is designed to have wastewater bypass (fully or partially) when the plant is overwhelmed in order to prevent sewer backups from occurring in local homes. Without bypassing, the wastewater would fill the sanitary sewer pipes and start backing up into residences and through maintenance holes on the street, according to town staff.
