ROCKLAND — It’s hard to imagine what tourists thought as they wandered from art gallery to gift shop on Main Street this week.

Some local people were worried that visitors might presume the city always smells as it did for much of this week. But if Steve MacDonald, public works superintendent, is right, the oppressive smell of sewage that hung over the downtown was the last gasp of an old era.

The city’s waste water treatment plant is in the middle of a $7.1 million upgrade that began in March. About $2.2 million of the work is dedicated to improvements that will remove the odor that often wafts from the plant, lifted by breezes off the harbor onto the adjacent Main Street and business district.

What caused this week’s stink was unrelated to the upgrade.

MacDonald explained that an arm that sweeps around one of two clarifying or settling tanks snapped, effectively putting it out of commission for four to five days.

To further complicate matters, a valve embedded 20 feet from the top of the tank that is supposed to close it off also snapped, and repeated efforts to empty the tank failed, as effluent continued to leak by the gates.

“We’d pump it down, but it’d fill up,” MacDonald said Friday.

With time, and a little help from hot and humid weather, things got ripe.

“It basically went septic,” he said.

Workers have used a hose to spray a gas over several of the tanks in the system to try to neutralize some of the odor. MacDonald blocked off parts of the open-air sections of the plant with blue tarps so the gas — which actually strips the smell out of the air — could work more effectively.

“If we hadn’t done that, it would have been twice as bad,” he said.

Chlorinated water is also being introduced into effluent near the end of the process to help kill some of the bacteria.

MacDonald said it is obvious that the upgrade is coming none too soon.

The plant is 21 years old and had a life expectancy of 15 years, he said. The valve that broke is slated for replacement with an improved setup that might prevent this week’s mishap.

When the work is done by the end of the year, the plant will more efficiently treat an average of 3.3 million gallons of effluent per day, up from the current average capacity of 2.2 million gallons. The new plant will also be able to handle 12 million to 33 million gallons of storm water runoff.

Much of the plant will be sealed when the work is completed. Covers will be added to most of the areas that are now open to the air. And a “negative pressure” system will be employed throughout the new system, so that when a door opens, air from the outside will be sucked in, rather than odor escaping out.

MacDonald also has plans for landscaping the site, including planting pines to help block any breezes from carrying wayward odors up to Main Street.

MacDonald and his crew will work throughout the weekend and early Monday and Tuesday to get the plant caught up on its processing so it can shut down some activities in time for the opening of the city’s annual Maine Lobster Festival.