Behind the scenes of Mount Vernon’s $38.5M sewage treatment plant

MOUNT VERNON — It may not be dinner table talk, but recent discussion of Mount Vernon’s $38.5 million wastewater treatment plant kept coming back to food.

The microbes that eat away at the yuck are like a “sourdough starter” that must be fed, Assistant Public Works Director Mike Love said recently at the plant.

The anaerobic digester, where some of the microbes devour the sludgy stuff, works like a “stomach” breaking down food, he added.

And the brown and bubbling substance floating on top of a huge basin looks like a “chocolate malt.”

New systems and upgrades that were part of the two-year project are now largely online at the plant, with some of the switches flipped just days ago.

Some changes are not much more technologically advanced than what was already there — new clarifiers, which are essentially huge tanks in which the floaters go to the top, the sludge sinks to the bottom and the stuff in between gets clearer, are meant to increase capacity.

Extra capacity will free up old clarifiers for use as massive storage tanks. That will come in handy in meeting the city’s goal, which is also a mandate from the state Department of Ecology, of reducing the number of times stormwater mixed with untreated sewage overflows into the Skagit River each year.

That happens about eight times a year now, Love said.

Before a 6-foot diameter pipe was installed downtown to hold additional sewage and stormwater in 1998, it was more like 100 overflows into the river each year.

By Jan. 1, 2015, the city has agreed to get that down to one “combined sewer overflow” per year.

The upgrade also switches over to safer treatment technology. Out is the old chlorine treatment method, and in are new banks of UV lights.

The chlorine posed potential pollution issues, said Department of Ecology Environmental Engineer Shawn McKone, who oversees the Mount Vernon plant.

Chlorine was also a greater hazard to the plant’s 12 employees, said Bill Fullner, manager of the plant.

The problem with the UV method, though, Mount Vernon Public Works Director Esco Bell said, is that the chlorine was what used to take the stench away.

It is, after all, raw sewage when it comes in, so the wastewater’s first stop, the influent building, does smell horrible, according to city officials.

During a recent tour, much of the plant proved virtually odorless, though — except for the occasional whiff of construction dust or a slightly unpleasant sniff near an open basin.

Odors are contained thanks to nearly 5-foot diameter pipe leading out of the influent building. It sends the smelly air to a fan, which blows it over a bed of bark, neutralizing the funk.

Besides the air duct, a sewage pipe sends the waste to the pretreatment building, where the wastewater is “mechanically” cleaned.

Inside the new pretreatment building, the cover is removed from the “finescreen,” revealing a perforated metal plate littered with green, white, amber and other-colored gobs strained from the wastewater.

The gunk ends up in a landfill, looking a little like beach glass plucked from ocean. No one volunteered to touch the finescreen’s collection, though.

From there, wastewater moves on to the primary classifier.

There “solids” sink, “scum” floats and both are pumped to an anaerobic digester.

The liquids in between the solids and scum are sent to the aeration basin. Largely unchanged by the upgrade, that basin is where biological processes take over.
The bacteria-stocked basin is brown and slightly foamy. But the basin topped with Love’s “chocolate malt” looks that way because of the hungry microorganisms in it, not the sewage.

In the basin, the bacteria feasts on waste.

Next, the now clearer waters move on to a secondary clarifier where the settling process repeats. Again sludge goes to the digester, where different bacteria dine.

Eventually the sludge inside the digester will become biosolids fit to fertilize some farm fields.

The liquids from the secondary clarifier, which did not go to the digester, go to the “effluent building.” UV lights there treat the wastewater to a stage where it looks like briefly steeped green tea and is ready to splash into the Skagit River.

The water is not drinkable, Bell said, but it meets Ecology’s standards with some cushion. Neither Fullner nor Bell knew quite how long it takes for a toilet flush at the plant to make it into the river, but Bell guessed it was a 10-hour trip from the influent building to the effluent building.

Nearly all of the wastewater’s stops — everywhere but the aeration basin and digester — are new, Bell said.

The plant’s last major upgrade was in 1989, Fullner said. That project replaced an old treatment system that “dribbled water over redwood slats” that were covered “with a slime layer.”

Mount Vernon grew out of that technique, Fullner said. And it will grow out of the new upgrades eventually, Love said.

“We have upgraded over time, and as we have upgraded, the permits have changed,” Love said, referring to Ecology’s ever-tightening treatment standards.

Bell said the nearly completed upgrade, which began in 2007 and was funded with city money and low-interest state loans, will help meet Mount Vernon’s goal of reducing overflows into the Skagit River and allow for some city growth.

Already though, he said sites have been picked for more clarifiers and other treatment tools.

• Elliott Wilson can be reached at 360-416-2147 or at .