When the small team at the Chinook (Montana) Wastewater Treatment Plant selected new mixers for a plant upgrade in 2004, they could not have known that the same mixers would still be dutifully working 16 years later — and contributing to nutrient removal.

The mixers were purchased so that a 50 hp surface rotor aerator could be switched off during night hours of low BOD loading. The two 5 hp submersible mixers have saved $18,000 per year on electricity, achieving payback on the entire installation in slightly less than 4.5 years.

In addition, by trying the mixers on different on-off cycles, plant team members of Eric Miller, Cory Fox and Matthew Finley have significantly reduced effluent nitrogen and phosphorus in a plant never designed mainly for nutrient removal. The introductions of a SCADA system and automated control of ORP (oxidation-reduction potential) have further enhanced treatment performance and efficiency.

“The savings speak for themselves, but the benefits have been manifold,” says Miller, plant superintendent.

Targeting nutrients

Chinook, the largest city in Blaine County with 1,500 residents, lies in the far north of Montana and in the heart of the Milk River Valley. Farming and ranching still prevail there, but the valley is known as the home of the Sugarbeeters, from the days of the Utah-Idaho Sugarbeet Co., which operated in the city for 26 years.

Chinook’s treatment plant went online in 1984 as a single oxidation ditch equipped with dual aeration rotors; it was constructed to provide TSS and BOD removal. The original installation was designed for ammonia removal, but not for reduction of total nitrogen or total phosphorus.

Miller, who has served the plant for more than 20 years, observes, “Before the mixers were installed, we were over 4 mg/L for phosphorus and close to 30 mg/L for nitrogen. Now, along with the energy savings, we achieve effective, consistent nitrogen removal from the same 1984-vintage oxidation ditch plant that was modified in 2004 for energy efficiency.

“Today, with much tougher regulations, we have our ammonia at almost zero, nitrogen at the 2 mg/L required for permit reissuance since 2012, and phosphorus at just 1 mg/L, all from the same mixers installed 16 years ago.”

Seamless installation

In the original plant, both oxidation ditch rotors ran continuously, introducing a surplus of dissolved oxygen. To allow for the cycling of the fixed-speed aeration equipment, the mixers were installed to produce liquid velocity and mixing so that solids would remain suspended and continue to circle the ditch with the rotors turned off.

The low-speed mixers (Landia) are essentially gentle flowmakers that enable operators to adjust the angle of the stainless steel propeller blades so that energy consumption can be fine-tuned. Typically, at speeds up to 47 rpm for aeration applications, the mixers ensure that no process-damaging floc shear is caused.

With no other tank to go to, shutting the plant down for installation was not an option. Instead, the submersible mixers were installed in a full tank using Landia’s guide rail system, enabling the plant to operate nonstop. Landia also provided a set of custom-built service platforms. “Landia made it very simple for us,” says Miller. “On the very rare occasions when we’ve needed a spare part in 16 years, they have ever since. The mixers just keep on going.”

Knowledge at work

In 2012, the Chinook staff attended a two-day training class sponsored by the Montana Department of Environmental Quality. Using the knowledge they gained, they experimented with extended air-off cycle times.

By allowing dissolved oxygen in the ditch to cycle between anoxic and oxic conditions, the plant achieved an immediate 50% improvement in nitrogen removal. No equipment was purchased; no funds were expended. In fact, because of reduced rotor operating time, electricity costs were further reduced.

As a result of the lower tank dissolved oxygen, there also was some improvement in total phosphorus removal. Now, the ORP system, allowing control of the on-off cycles of the rotors and mixers, has been a welcome addition.

Even this very remote small city receives all the usual non-flushables that treatment plants have to contend with, though Miller admits that the wastewater it receives from its one industrial user — a local slaughterhouse — is a helper rather than a hinderer for BOD levels.

Economical operation

“The introduction of SCADA has made life a bit easier, but then the mixers have never given us any trouble,” says Miller. “Back in 2004 they were our biggest capital expense, but now they enable us to achieve full nutrient removal and process optimization.

“The old 50 hp rotors produced lots of air that we simply didn’t need all the time. We needed to save money and had looked at soft starts to reduce spikes, but running the mixers for about 13.5 hours per day has proved much more economical than what we had before.

“Where electricity costs are higher, the return on investment would be even better than the savings we’ve enjoyed here in Chinook. Over the years we’ve understandably had to see to the odd seal, but with their standard bearings, maintaining the mixers is a piece of cake — just a bit of grease and oil.”