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Halifax Water agreed test out the effectiveness of ozone generators using a 20-year-old machine that once was deployed in a hospital smoking room. ((CBC))

The Halifax water commission has ordered two $5,000 ozone generators in an effort to solve the persistent sewage gas odour problem in parts of the city's downtown.

The smell of raw sewage has permeated the waterfront area around the Halifax casino since a nearby sewage treatment plant failed and flooded in mid-January.

The main goal is to mitigate the stench, said James Campbell, spokesman for Halifax Water.

"For the businesses and the residents, that's what we want to do," he told CBC News on Monday.

The decision to buy the ozone generators comes after a 20-year-old model was lowered down a manhole on Cogswell Street 10 days ago and hydrogen sulfide gas readings dropped "significantly."

"The results were quite successful," Campbell said. "And that was with a smaller unit. We're hoping with a larger unit that we're going to get in the next week and half we'll have even better results."

Campbell said the machine produces ozone, which essentially oxidizes the hydrogen sulfide molecules and turns it into a water-based compound, thereby eliminating the odour and the gas.

Ozone chases odour away

The machine that convinced the municipality that ozone could solve the sewer gas problem belongs to Greg Murray, owner of Clean Air & Water Centre in Halifax. Murray said he was able to convince Ted Tam, the general manager of Halifax Harbour Solutions, to give his product a try.

Before the ozone generator was lowered into a catch basin, Murray said, hydrogen sulfide gas readings were around 150 parts per million.

"One minute after that machine was in place they were down to 60, and they would have kept going down if they left it longer. But one minute was all it took," Murray said.

The small, gold-coloured aluminum machine is more than 20 years old. It was once used as a smoke eater in a cigarette-smoking room at the Nova Scotia Rehabilitation Centre, part of the QEII Health Sciences Centre.

Simpson Environmental of Burlington, Ont., is supplying the two Model SF 5000 ozone generators.

"I would think it's going to make a huge difference. The ones we're bringing down are very potent machines, and they'll maintain ozone levels at quite a distance. If we're at the right place at the right time, we're going to get pretty close to zero residual odour down there," Murray said.

"Where the ozone is, the odour is going to be pretty much gone. We've been doing this for 25 years now and we've had a lot of good experience."

Deodorant blocks, carbon filters

Municipal officials have tried a number of methods to eliminate the stench of sewage.

Earlier this month, they installed carbon filters and large deodorant blocks at two locations in the catch basin by the Halifax casino. The filter traps the gas, while the block masks the smell.

Officials expect the failed sewage treatment plant will be back in operation by next spring.