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The nose shouldn’t know you live near South Treatment Plant. But if you smell something, we’re all ears

By April 19, 2018August 26th, 2020No Comments

If you work or live on the Eastside, or south of I-90, there’s a good chance you’re one of the 742,000 people who make a daily “contribution” to our South Treatment Plant in Renton.

While it doesn’t smell like roses inside a wastewater treatment plant, we put lot of investment into good operations and maintenance practices to keep nuisance odors from leaving the site and bothering our neighbors.

Still, fugitive odors can sometimes jump the fence line, and that’s when we need the public’s help to identify and catch them.

The best thing neighbors can do? Call our 24/7 odor control hotline right away at 206-263-1760 if you smell something so our operators can track down the source. If it’s related to our operation, we’ll take steps to address it, whether it’s coming from the plant or a pump station or maintenance hole farther away.

“We encourage our neighbors to call us when they smell something,” said Rick Butler, a process control supervisor at South Treatment Plant in Renton. “Odors can be aggravating, even if they only last a short time.”

Every odor complaint is logged and tracked, and plant operations makes it their goal to respond and identify the issue within two hours.


South Treatment Plant’s assistant manager Mike Wohlfert stresses a proactive approach to odor control.

It Starts with Prevention

Odor control is a feature at all of our treatment plants, with South Plant getting major upgrades in the late 1990’s and mid-2000’s. Aeration basins were covered, and ventilation systems were equipped with odor-reducing chemical units and carbon filters to trap and scrub foul odors.


A ventilation system cleans air pulled from the headworks, which is the place where wastewater enters the plant.

South Plant operations crews also play a big role in odor control, especially when tackling the jobs that can get smelly, like draining and cleaning tanks.

“We don’t generate odors on purpose, and we do everything in our power to minimize the potential for odors,” said Mike Wohlfert, assistant plant manager at South Plant. “Being proactive is far more effective than responding or reacting.


One of South Plant’s “undercover” aeration basins.

“We schedule shift crews to start draining tanks at night so the 6 a.m. crews can go out there and start hosing out most of the solids before the sun starts glaring in and drying things, because once it starts to dry, it gets ripe quickly,” Wohlfert said.

“We’re not going to drain a tank on a Friday and wait for somebody to come and clean it on Monday.”


A lot of coordination goes into cleaning the Daft Tanks (short for”Dissolved Air Flotation Thickener” Tanks) to avoid odors that could bother neighbors. This is an area where solids are processed, so operations is especially conscientious.

Because South Plant is in a commercial area, the operations crews can also schedule work outside business hours when the nearest neighbors could be affected.

Keeping the odor control units in good working order and checking to make sure they’re functioning properly is another standard practice. So is keeping doors closed in the dewatering buildings where the Loop® Biosolids are processed, and keeping the Biosolids trucks covered. Care is also taken to minimize the number of trucks on site at one time.


Biosolids trucks are kept cleaned and covered to avoid unnecessary odor.

“We also have other options,” said Butler. “There are chemicals to treat the air, and we can do certain things to the sludge and wastewater to reduce the production of odors. We can also add bleach, which helps control odors that can become fugitive.”

Butler added that short-term odors might be caused with good intent, for instance when workers need to hose off sludge that unexpectedly accumulates on a clarifier during the last stage of the secondary treatment process. The work might generate a brief odor issue, but it will head off a much bigger and stinkier problem.

What’s that Smell?

Different parts of the wastewater treatment process create different odors, which is why it’s so important for hotline callers to be ready with as much information as possible, including an address and a description of the smell.

There’s a slightly different chemistry to sewage odors, and that can help operators identify possible sources.

For example, the unpleasant “rotten egg” smell most people associate with sewage is hydrogen sulfide, a gas created inside sewer pipelines that’s most noticeable where the wastewater enters the treatment plant. Strong nitrogen and ammonia odors are more commonly associated with the Biosolids processing areas, which is where the solids are separated and dewatered. People with very sensitive noses might even pick up scents like cat urine or rotting cabbage. Wind and atmospheric conditions offer additional clues.


Not all wastewater odors are the same (Click Image to Enlarge)

“It helps if people can ask friends or neighbors if they’re experiencing a similar odor,” said Wohlfert. “All the complaints we get have credence, but it’s a pretty good indicator when three buildings and a complex say ‘something’s ripe.’”

But the business of identifying the odors and their source can sometimes be tricky, even for seasoned wastewater professionals.

“For the most part, everyone here at the plant is pretty astute at picking up odors,” said Butler. “They know the likely culprits. But sometimes, you could be standing right next to the tank that’s the cause of the smell, and you can’t smell it when it leaves the tank. The odor compounds may react with the air that carries them offsite where passersby can smell it.”



Odor control units use chemical solutions and carbon filters to target sulfides and other smelly compounds in wastewater.

Season of the Whiff

Butler said odor issues at South Plant typically occur on summer evenings when warm days coincide with prevailing wind patterns, so there’s a bit of a routine about how odors might travel.

“In the summertime when we have those northerly evening winds, when it’s really warm, that’s when we’re going to tend to get the odors that are tricky to for operators to notice, but can affect the community.

“We have people actually drive or walk around the plant to see what they can smell,” he said, adding that shift crew members will notify Main Control if they smell something out of the ordinary on their way to or from work.


Aerial image of South Plant

Potential for nuisance odors is more likely in the summer because wastewater arrives at the plant even smellier than other times of the year. That’s because less rain and more water conservation means wastewater sits longer inside the pipes, which creates more hydrogen sulfide, the stuff that smells like rotten eggs.

“Last summer, because it was so hot and dry, we saw record levels of hydrogen sulfide coming into the plant,” Wohlfert said. “Our operations crews really had to be on top of it. They had to change fans and chemical dosages, and we have had to completely change the odor control approach in a way we’ve never done.”

“This is going to be one of the challenges of people conserving water,” Butler said. “Everything is more concentrated, and it’s going to take a little longer to go through the sewers to get here. And so we probably are going to have higher and higher hydrogen sulfide levels, and that’s going to be quite a challenge to keep on top of.”

Being Outranked

Of course, there are many sources of odor beyond wastewater treatment plants. The decaying seaweed that washes up on beaches around Puget Sound every August is a big source of hydrogen sulfide, the same smelly compound in sewage, and winds can carry the odor far and wide.

Foul smells around South Plant might also be coming from nearby rending plants, transfer stations, decaying fish or vegetation, or even a neighbor’s fertilizer product.

We can’t do anything about those odors, but if there’s one thing our treatment plant operators want people to know it’s how dedicated they are to making sure our plants are good neighbors to people who live and work nearby.

“It’s not our interest to try to mask the odors,” said Wohlfert. “I worked at a plant where we did use a masking agent and it just smelled like orange poop. It was even worse. If people are educated on what we do and how we try to do things, and know we put more effort into odor control, way more than we did when the plant started up in the ‘60’s.”

“There’s an old proverb that basically says ‘100 sermons are wasted by one fart’,” Butler added. “It gave me a chuckle, but it capsulizes how vigilant we need to be in our approach to odor control management and responding to odor complaints.”