City pays the price for holding it in

Damage to Chilliwack's waste water treatment plant attributed to hydrogen sulphide


By Emma Gregory

Some smells you can’t just put a lid on.

The Chilliwack waste water treatment plant found this out the hard way, when its odor control system generated enough sulphuric acid to corrode the steel and concrete of one of its main system components.

“We were already engaged in a huge project, doing work on the inlet, when it came to our attention there was something wrong with one of the clarifiers,” said Jenny Tough, the City of Chilliwack’s utility manager.

The inlet is the point of entry for sewage into the facility. From there, it moves to the clarifier tanks, where solid waste is separated from water. Essentially a large concrete tank, a clarifier allows for solid waste to settle to the bottom of the tank and its water intake is controlled by a weir. One of three of the clarifier’s weirs fell off.

“One small part of concrete gave way, and now we have to do the whole repair.”

According to a staff report presented to city council on March 29. “Extensive corrosion of the structural concrete, structural steel and effluent weirs has led to a failure in one of the three tanks and put the remaining two tanks at risk of imminent failure.”

On Sept. 14, 2020 staff observed a clarifier tank was critically damaged, and they shut it down. It was drained and temporarily repaired.

Social responsibility led to the damage

According to the staff report, “In 2001 an odour control system was installed including stainless steele covering plates, ducting and mechanical ventilation. The odour control system is effective in minimizing the impact of the WWTF on neighbouring communities but does not provide sufficient ventilation to the clarifier tanks.”

“Back in the day the standard was big open tanks, but as the city grew up around the treatment plant, modern urban concerns included a desire to reduce the odor.” Tough said.

The problem is that while the odor control system worked to contain the odor, it also worked to contain the large amounts of hydrogen sulphide gas that is a product of a city’s sewage.

In an open air tank, hydrogen sulphide would release into the air and cause a smell. In an odor controlled tank, fitted with a steel lid, the hydrogen sulphide mixes with water and becomes sulphuric acid.

The lids were outfitted with fans at the top and were installed along with ventilation ducts, but this did not suffice, leading to the corroded weir and concrete fixture.

When asked which engineering company installed the odor control system, Tough said she did not know offhand as it was 20 years ago and. “I wouldn’t want to imply that the company involved had anything to do with the subsequent issues in the tanks.”

City council approved a change order to the contract with Stantec Consulting Ltd. and Tritech Group Ltd. to repair the clarifier, as they are already onsite doing other repairs.

Stantec did not respond to comment, and Tritech declined to comment on the situation.

While the staff report refers to this an emergency, the repairs will be held off until the dry season, because only when during a period of reduced water intake would the repairs be feasible.

Not all large volumes of gas are a pain though. The WWTP is also expanding its capacity to generate its own power using methane gas, which it collects from a different segment of the waste water treatment process.

The WWTP is in the direct vicinity of Shxwhá:y Village and Townsend Park.

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