Reducing energy use in wastewater mixing

Mixers in aeration and sludge tanks can account for up to 10% of a wastewater treatment plant's overall energy use. By selecting energy-efficient mixers, operators can reduce energy consumption attributed to mixing by as much as 50%. Discover how three wastewater operators have achieved significant energy and cost savings with Xylem's Flygt mixers.

When looking for energy-efficient mixers, operators should consider several factors. While energy-efficient motors can reduce energy use and emissions, the two most important factors related to energy consumption are the mixer's hydraulics and whether the mixer can adapt its speed to actual process conditions.

A major contributor to a mixer's energy efficiency is its hydraulics, meaning how well the propeller's design and speed affect the flow created in wastewater. 40 years ago, Flygt set the standard for hydraulic efficiency with its "banana-blade" mixer, which has yellow, backswept propeller blades that maximize thrust (mixing force), lower energy use and prevent clogging.

After 40 years of research and the installation of over 300,000 Flygt mixers worldwide, Xylem continues to evolve its mixing technology to meet customer challenges. Learn how three forward-thinking wastewater operators are lowering energy use, reducing their carbon footprint, and saving costs with the help of Flygt's innovative mixers.

1. Italian wastewater treatment plant reduces mixer energy use in sludge tanks by up to 50%

The Publiacqua wastewater treatment plant handles wastewater for more than 600,000 people, including residents of Florence, Italy. The plant treats more than 65 million cubic meters of wastewater every year, processed through three lots of 24 sludge tanks. Each tank requires a submersible mixer to keep activated sludge in suspension.

When the plant identified that one lot of sludge tanks required new mixers, it compared and tested different brands of mixers. Compact, high-speed mixers are often selected for smaller tanks of this size, which can also lead to higher energy consumption.

The plant instead selected 24 Flygt 4530 mid-sized mixers from Xylem, which can operate at lower speeds due to their optimal propeller design and overall higher thrust-to-power ratio.

By installing Flygt low-speed mixers instead of a compact high-speed mixer, Publiacqua can reduce energy use at the plant by up to 50%. Additionally, the optimized shape of the Flygt mixers helps ensure uninterrupted, clog-free mixing, while reducing operational and lifecycle costs. Read more.

2. Saving $1 million over a decade with energy-efficient aeration mixers

The Ypsilanti Community Utilities Authority in Michigan, US, provides water to 112,000 people and operates a wastewater treatment plant with a capacity of 51 million gallons (193,000 cubic meters) per day.

At the wastewater treatment plant, the authority uses 30 mixers for biological treatment, as part of an anoxic nutrient removal process. The mixers were old, inefficient, and starting to fail.

The authority conducted an extensive comparison of the energy efficiency of several aeration mixers, including an older Flygt mixer, mixers from two other vendors, and the newer Flygt 4320 mixer with integrated intelligent speed control.

The authority selected the Flygt 4320 mixers, which generate the needed thrust while consuming as little as 50% of the power of a compact submersible mixer. In addition, the mixers require maintenance only once every two years, or every 16,000 hours (most mixers operate 24/7). Over a decade, the energy savings from the Flygt 4320 mixers could easily amount to $1 million. Read more.

3. Avoiding costly shutdowns and supporting industrial water reuse in Arizona

The Ocotillo Brine Reduction Facility, in Chandler, Arizona, US, is a water reuse facility that supports a nearby microchip manufacturing plant. Every day, the reuse facility processes 1.5 million gallons (5,600 cubic meters) of used water from the manufacturing plant, which is treated to drinking standards. The water is then reused by the manufacturer, minimizing its need for potable water from the city.

To treat the manufacturing plant's used water, the reuse facility uses a cold lime softening process. Lime softener sludge was constantly building up and settling in the equalization basin, which resulted in having to shut down operations for at least two days every several months. Each shutdown cost $15,000 and forced the manufacturing plant to rely on municipal potable water.

Because the level of the equalization basin fluctuated several times per day by as much as 15 feet (4.5 meters), Xylem representative JCH, Inc. recommended a Flygt adaptive mixer. These mixers can adapt the thrust they produce to actual conditions, which would help keep the lime sludge in suspension.

By matching the thrust of the mixer to the water level in the tank, the reuse facility has eliminated the need for tank cleanings, reduced costs associated with downtime, and minimized use of municipal potable water by local industry. Read more.

Learn how Xylem's Flygt mixers can help you optimize mixing processes.

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