City sewer overflow hits high mark in 2017

Equipment upgrades reduce the number of times diluted sewage empties into river

A massive volume of diluted sewage emptied into the Ottawa River from City of Ottawa sewers during a seven-month reporting period in 2017, but the number of times there was an overflow was at its lowest in recent years.

The city recorded 1.6 billion litres of liquid muck spilling into the river between April 15 and Nov. 15, a volume not recorded in the 12 years of data published by city hall. The closest the city has come to that volume was in 2006 when nearly 1.1 billion litres of diluted sewage poured into the river.

Combined sewers capture sanitary sewage and stormwater, and divert the liquid to the sewage treatment plant. Most of the city has separated sewers, but the older areas have combined sewers. When the combined sewers fill up during heavy rainfall, the overflow sewage drains into the river. It’s an accepted practice in Ontario.

While the volume of combined sewer overflow was huge in 2017, the city only recorded 79 times when an overflow happened, one better than the previous low of 80 in 2015. The city has done better cutting down on the occurrences, thanks to upgrades to sewer monitoring equipment. Instead of hundreds of occurrences each reporting period, there are now dozens.

If Ottawa wants to blame the volume of overflow on one thing, it can blame it on the rain. The city recorded 923 millimetres of precipitation during the provincial reporting period in 2017, the most over the 12-year period. Springtime can also see an increase in volumes when snowmelt is mixed in.

The city is in the middle of a $232-million construction project downtown that aims to reduce the number of combined sewer overflows. A tunnel network, measuring six kilometres in total, will capture the overflow liquid and send it to the sewage treatment plant starting in 2020. It’s the cornerstone project of the city’s Ottawa River action plan.

While the city draws its drinking water from the Ottawa River through purification plants, it says combined sewer overflows don’t affect the quality of the municipal water supply. The province gives the city’s drinking water top ratings.

The City of Gatineau also has sewage overflows into the river, but a spokesperson for that city says it’s able to discharge small amounts of overflow at a time. The province of Quebec doesn’t take into account the volume of overflows, only the number of events, the city says, but it didn’t provide the number of events for 2017.

Because Gatineau’s overflow points are spread across the city, it would be difficult to build a sewage-collection tunnel network like what’s being constructed in Ottawa, according to the Gatineau spokesperson, but the city is reducing the number of overflow points. The city says it continues working on separating the stormwater sewers from the sanitary sewers during infrastructure rehabilitation projects.

The City of Gatineau says it’s preparing an overflow management plan for consideration by council this year.