Everything was running fine at the Lake Stevens Sewer District. Then the Washington Department of Ecology imposed a new permit limiting total inorganic nitrogen. The district had less than a year to comply.

A physical plant upgrade to meet the permit limit could have cost millions. Instead the clean-water plant team achieved compliance by optimizing the aeration process without any capital investment — and with significant energy cost savings in the bargain.

IMMEDIATE CHALLENGE

The Lake Stevens district serves a scenic community of 40,000 about 30 miles northeast of Seattle. Its Sunnyside Wastewater Treatment Plant was commissioned in 2012, replacing a lagoon system that lay in the Snohomish River flood plain. “We needed to get out of the flood plain and take on additional flow for our growing community,” notes Mariah Low, district general manager.

The new plant uses an activated sludge process with a membrane bioreactor (MBR). It discharges to Puget Sound. Influent passes through a headworks with two 6 mm band screens and two rag washer/compactors (JWC Environmental).

After passing through the headworks, the flow proceeds to two primary clarifiers (only one in service at any given time). Supernatant flows to the primary building with 2 mm fine band screens (also JWC). Primary sludge is sent from the clarifier to a WEMCO Hydrogritter (Trillium Flow Technologies) for grit removal. Flow then proceeds to a gravity thickener. From there, solids are sent to the anaerobic digesters and floatables return to the primary clarifiers.

Sludge wasted from the downstream end of the basins is sent to a thickening centrifuge (also Andritz D5LL). Thickened waste activated sludge goes to the digesters and supernatant to the return channel.

Screened wastewater goes through a flash mixer where it combines with return activated sludge and centrate, then to the three aeration basins (two online at any given time) and next to the MBR consisting of six trains, each with six membrane modules. The MBR effluent is UV disinfected (Ozonia) and discharged. Digested Class B biosolids are produced using Andritz D5LL decanter centrifuges and hauled out by a contractor for land application.

PERMIT SURPRISE

The challenge came in January 2021 when the Department of Ecology issued a new general nutrient permit on top of the plant’s existing permit, limiting total inorganic nitrogen releases to 127,000 pounds per year and an annual average of 10 mg/L in effluent. “TIN was not something we previously had to worry about,” says Low, although the previous permit did include an effluent ammonia limit that remains in effect.

“Before we began working to optimize our system, Ecology looked at the previous five years of operation and estimated the amount of TIN we had been putting out,” Low says. “In three of those five years we had exceeded the amount they were imposing as our new regulatory limit.”

Before the optimization project, effluent TIN was averaging about 18 mg/L.

The new permit took effect on Jan. 1, 2022. In the preceding year, the Lake Stevens team worked with Grant Weaver of Grant Tech, who specializes in helping clean-water plants reduce nutrients affordably and had assisted several plants nearby. His work was funded by the Snohomish County Public Utility District No. 1 as an energy-saving initiative.

SEEKING SOLUTIONS

Working with Weaver and the utility district staff, the Lake Stevens team went through a cost-benefit analysis of several nutrient reduction strategies, focusing on those that could be deployed with little or no capital improvement.

The team settled on a strategy involving adjustments in aeration. Previously, each aeration basin functioned with two anoxic zones and three oxic zones, according to the original plant design.

Initially they shut off the air to the first oxic zone in the two operating basins on an alternating schedule, according to Travis Rosencrans, plant supervisor.

“We would turn one basin off for 45 minutes, while the other basin remained on,” says Rosencrans. “Then we would go switch it and turn the one that was off back on. We did that for quite a few months and noticed a difference in our TIN going down.

“But it got to the point where that manual process became a hassle. We almost had to make it somebody’s job to manage it throughout the day. If it didn’t get done, we noticed the difference in our TIN going up.”

FINE-TUNING

As a different way to accomplish the same objective, the team changed to shutting off the air in the first zone of one basin 24 hours a day, except for a three- to four-minute “bump” of aeration at the start and at the end of the one daily eight-hour shift during which the plant is staffed.

With that approach, “We have been able to meet our yearly limit of TIN, and as of now we are a good bit under the limit every month,” says Low. Estimated baseline TIN discharges were 136,507 pounds in 2020 and 132,253 pounds in 2021.

For all of 2022, under the new operating scheme, TIN discharges totaled 104,577 pounds. “From January through September of 2023, we were at just 60% of our annual limit, or 76,465 pounds,” Low says. Annual average effluent TIN has been 11 to 12 mg/L.

In the future, the team hopes to fine-tune the process further by automating the aeration sequence; that will require some capital cost still to be determined. “That is a project on our list to get done in the short term,” says Low.

Rosencrans notes that the plant team enjoyed working with Weaver: “He was extremely knowledgeable, very helpful and offered quite a bit of support.”

Lower energy consumption was a substantial added benefit of reducing aeration to one basin. The utility district estimates savings at 243,198 kWh per year, translating to $21,888 in avoided energy costs annually. The utility district also provided a $55,610 Durable Operations and Maintenance Energy Savings incentive.

“The partnership with Grant Weaver and the Lake Stevens Sewer District was a win-win for everyone,” says Allison Grinczel, the utility district’s senior program manager. “It was very gratifying to work on a project that saved residents of Lake Stevens from having to pay for expensive infrastructure upgrades, while reducing the wastewater treatment plant’s impact on the grid and thus saving all our customers in the long run.”