When the Idaho city of Caldwell replaced two old digesters with a new and larger one, Salvador Arreola had to make a decision about mixing technology.

All the mixers he knew about seemed to have problems with scum lines forming at the top, sand and grit settling at the bottom, and dead spots where the solids wouldn’t break down fast enough.

Consulting engineer Larry Rupp of Keller and Associates in Boise suggested that a linear motion mixer might be a good fit. After some investigating, Arreola decided to go with it, and he hasn’t been sorry. “Every other kind of mixer I have experienced has had its flaws, but I haven’t found a flaw with this yet,” says Arreola, plant superintendent, who has worked in the wastewater treatment industry since 1989.

More gas production

Arreola describes the mixer as working like a soda fountain milkshake mixer, but instead of the container moving around the spinning disk, the disk moves up and down inside the container. At Caldwell, the container is a 986,000-gallon digester.

The mixer is working as well as or better than Arreola had hoped. It is also saving the city money by producing more methane, reducing the volume of solids sent to landfill and being more efficient to operate. Arreola expects it to go much longer between cleanings — another significant savings.

“Anytime you drop a digester for maintenance, it’s a big undertaking,” he says. “In wastewater plants, we count down how many digester cleanings we have before we retire.”

At the Caldwell Wastewater Treatment Plant (18.5 mgd design, 8.5 mgd average), the wastewater goes through screening and grit removal, to an aeration basin, into final clarifiers and then through sand filtration and UV disinfection before discharge to the Boise River.

The plant serves a population just over 50,000; it has a staff of six. The new mixer, an Ovivo LM, went online in 2016. After it operated long enough to demonstrate its effectiveness, the Caldwell plant won the 2019 Pacific Northwest Clean Water Association Sustainability Award.

Performance documented

In Arreola’s experience, routine maintenance on digesters usually reveals areas where mixing is incomplete or inefficient: “Usually when you take a digester down, every three to five years, you see where the sand and grit buildup is and you see where the dead spots are in the digesters. It’s long been assumed that that’s just the way it is.

“Well, I’m always looking for something better, something more efficient, something that will mix thoroughly and keep the scum layer broken and be more efficient, and this really blew away my expectations.”

His staff took the digester with the linear motion mixer offline for inspection after about eight months, and they did not find dead spots or buildups of sand and grit. Arreola says the mixer takes significant energy to get started, but the overall power use is way down.

“Can you imagine a million gallons being mixed at once?” he says. “It takes a spike of energy to get that total volume to move. Once it is moving, the power use goes way back down and it’s really efficient. Our power bills have been reduced dramatically.”

He finds the mixer is effective at keeping the temperature consistent throughout the digester and at keeping sand and grit suspended. More efficient mixing has resulted in 12% more biogas produced. The gas is scrubbed to reduce moisture and hydrogen sulfide and then burned in the boilers that heat the digesters.

“We capture the gas inside a gas sphere, which is just a big bubble with a bladder on the inside,” he says. “We use that methane to power up our boilers, and it gives us a very reduced cost for natural gas.” Previously, the biogas was flared. While some is still flared, so much is used in the boilers that the plant saves $2,200 to $3,000 a month on its gas bill.

Looking deeper

The more efficient mixing has also reduced the volume of and improved the quality of the biosolids that are dewatered and sent to landfill. “When it goes to our belt filter press, it uses a lot less polymer, and we get a good 18% solids cake,” Arreola says. “That’s very dry. Before, we were getting 12%-15% solids.”

The success of the mixer led Arreola to think about other ways of boosting sustainability. One is to retrofit a linear mixer in an existing digester that now uses a nozzle mixer. He expects that project to be done in the next two or three years. The digester tank will need extra support because of the movement of the different type of mixer.

Eventually Arreola would like to set up three or more digesters in series; the final product would then be Class A biosolids that would be valuable to local farmers as a soil supplement and could create a revenue stream. It would also eliminate the cost of landfilling, about $225,000 per year.

Caldwell, which is part of the Boise metropolitan area, has seen significant population growth in recent years. The city’s master plan expects the plant to be expanded to 30 mgd within 20 years. Arreola says the sustainability efforts are essential to expanding without raising rates.

“As Idaho grows, we’ll grow, but everybody’s worried about bills going up,” he says. “With new revenue streams, we can add to the infrastructure without raising rates.” He finds the public more understanding now that wastewater can be turned into valuable products such as biosolids, water for irrigation and clean energy.

“People are more aware about sustainability,” he says. “For years wastewater treatment plants have had this abundance of energy and were just burning it off. Now people are looking for ideas. We’re always looking for better, more innovative ways.”