Its construction nearly a half century ago helped transform La Conner from what former Town Councilman Michael Hood once termed a “stubborn stagnation” into a bustling, popular destination whose residents are envied by the many visitors who descend here from around the globe.

Now, as it approaches late middle age, the La Conner wastewater treatment plant is in line for a major makeover within the next decade.

“This is nothing new,” Mayor Ramon Hayes said of mandated and age-necessitated plant upgrades for which Town officials have been bracing. “We’ve known this was coming for q...