Proper mixing is a key to high-quality wastewater treatment and consistent permit compliance.

Mixers of different designs deployed in aeration basins, sludge storage tanks and other treatment processes have varying degrees of effectiveness. They can also be significant consumers of energy, especially if operated in an “always on” mode.

Now Xylem offers a compact submersible wastewater mixer with easily adaptable output and the potential to reduce mixing energy usage by up to 50 percent. The company says its Flygt 4220 adaptive mixer lets operators manage variable mixing conditions by simply adjusting the mixer output. Since the mixer then delivers no more than the power actually required, energy is saved.

Operators can modify the output by way of a tank-side control panel, a programmable controller or plantwide SCADA system to match mixing needs. The unit also delivers comprehensive information about the mixer’s performance and status: speed, power, alarms, runtime and energy consumption.

The device is designed to accommodate a variety of common mixer applications and is well-suited for retrofits. Matthew Szurgot, global product manager with Xylem, talked about the offering in an interview with Treatment Plant Operator.

TPO: What was the rationale for bringing this product to market?

Szurgot: The ethos of the Flygt brand is low total cost of ownership, and that was our starting point for this product. Having worked closely with customers, we saw a great opportunity for energy efficiency. Mixers, unlike pumps, tend to operate 100 percent of the time, 8,760 hours per year. So energy savings even at fractional kilowatt levels can add up quite quickly.

TPO: Was there also potential to improve process performance through optimized mixing?

Szurgot: Yes. Mixing is a bit of a black art compared to pumping. In determining how much mixing is required, there is a lot of uncertainty, and safety factors tend to get built up as well. In visiting plants, we often observe that at a given time, the amount of mixing being done in a tank is too much, and there is an opportunity to turn that down and to adjust for temporary or long-term changes in demand for mixing performance.

TPO: Where would mixers like the Flygt 4220 be applied in a wastewater treatment plant?

Szurgot: The No. 1 application would be within the biological treatment process. For example, in a conventional activated sludge plant doing biological nitrogen or phosphorus removal, there would be mixers in the nonaerated tanks. Another application would be in any of a variety of sludge holding tanks. Less common applications would be around retention basins, stormwater basins and pump stations.

TPO: From a design perspective, how is this mixer different from others on the market?

Szurgot: From the outside, the Flygt 4220 looks similar to a traditional single-speed mixer, but inside, things are quite different. Single-speed mixers have a single duty point. Once a mixer is configured at the factory, it will always deliver the same output. With our product, we’re moving from fixed output to one that is variable — from a duty point to an interval.

TPO: In general terms, how is this accomplished?

Szurgot: Thrust is industry standard for measuring mixer output. What makes the new mixer so powerful is the Flygt Dirigo technology platform, which marries a super-premium, high-efficiency motor and an integrated motor control system. So between the hardware components and the software, we provide a complete platform to help clients take control of mixing.

TPO: Beyond electronics, is anything different about the physical attributes of the mixer?

Szurgot: The propellers are optimized for geometry and blade angle. We’ve also increased the size of the propeller, which leads to greater efficiency. So we have optimized the hydraulics to be the absolute best they can be. Another advantage is in overload protection. A conventional mixer is very binary: If the motor is overheating, it shuts down. You don’t get any warning, and the mixer doesn’t react in a smart way. Our mixer, with Dirigo technology, monitors itself. When it senses a near overload condition, instead of shutting off, it slows itself down, so the process is still up and running. That’s a great benefit in terms of process resiliency and compliance.

TPO: How does this mixer respond to different mixing conditions?

Szurgot: There are three levels. The first is no regulation of thrust. Like a conventional mixer, it has a fixed speed, but it has its built-in efficiency because of the premium motor and optimized hydraulics. The second level is occasional regulation of thrust, which is handled manually by the operator at tank-side through a control panel. The operator can dial in a new duty point, a new thrust. A great place for that to happen is at startup of a tank after the design phase, to adapt and fine-tune for the real-world conditions. The third level is continuous regulation of thrust, which is handled by connecting the mixer into a plantwide SCADA system. This enables the mixer to respond to changing process parameters by adjusting the thrust in real time.

TPO: How do you account for the claimed 50 percent potential energy savings?

Szurgot: The bulk of that comes from the out-of-the-box efficiency. A second aspect is the super-premium efficiency motor. The remaining fraction is around the fine-tuning at startup. Often in the industry, there is a propensity to be conservative with design, and understandably so, because meeting effluent discharge permit levels is essential. But there is often an opportunity to turn that down and optimize the mixer output at startup.

TPO: Are there any other advantages from the flexibility of this mixer?

Szurgot: Yes. The mixer’s ability to deliver a wide range of duty points enables a single unit to replace multiple conventional mixers. That reduces the amount of spare mixers and replacement parts a plant needs to stock. Moving from an operating point to an operating interval also allows flexibility to relocate mixers from one tank to another.