Over the past 20 years some aspects of the water industry, particularly pumping and aeration, have seen a significant increase in the use of variable flow pumps and blowers as well...
Conventional mixing
Conventional single speed submersible mixers have traditionally been used for applications including activated sludge treatment, sludge holding and digestion. Mixer selection is typically based on accommodating a plant’s peak design load with a single speed mixer operating as if peak load is constant. However, peak load rarely occurs at most plants, therefore much more energy is being consumed than is needed. Interestingly, aeration control based on oxygen need has been widely practiced for quite some time. Mixing by aerators varies as aeration is adjusted according to oxygen demand. This holds for mechanical and fine bubble diffused aeration alike. Control algorithms are still under development to further maximize the nitrification output per kilowatt hour (kWh) of energy spent while maintaining sufficient capacity. In contrast, in mixing of unaerated (anoxic or anaerobic) zones or in oxidation ditches with high efficiency fine bubble diffused aeration and submersible mixers, for example, mixer capacity turndown is rarely practiced. This is surprising since, if correctly implemented, variable thrust mixer operation can lead to significantly lower energy consumption and increased mixing reliability.
Mixer selection considerations
Selecting adjustable thrust mixing technology offers many advantages during the design process. Although numerous parameters should be considered when selecting a mixer, specific mixing conditions and other considerations are not always known when a plant is in design stage, such as:
The tank’s internal dimensions, including eventual obstacles such as pipe work
- Actual rate of flow
- Dry matter content
- Consistency of the fluid to mix
- Actual level/results of pre-treatment (e.g. grit removal)
- Inlet and outlet locations
- Presence of aeration equipment and associated airflow
These design stage uncertainties do not create a challenge in variable thrust mixing, because it provides the engineer more flexibility right up to the time all facts are known (typically when the mixer is commissioned). Having the ability to change mixer thrust after installation to meet the facility’s specific conditions, can save energy cost significantly - sometimes by 50 percent or more, or thousands of dollars per mixer. This flexibility also allows a treatment plant to use only the energy the process requires today, while remaining well prepared for future increases in demand.
Demand-based mixing
Many wastewater plants use single speed mixers in ‘switch-on/switch-off’ mode, with plant operators manually turning one or more mixers off when thrust requirement is reduced, such as when an oxidation ditch is running in full anoxic mode. The logical next step is to reap the additional energy savings of a more efficient automated operation with all mixers running, but at the lowest produced thrust per unit required, as conditions vary. The following three examples illustrate opportunities for energy savings using ‘demand-based’ mixing:
- The flow rate into an anoxic or anaerobic tank is reduced, and the risk of bypassing decreases.
- One hundred percent activated sludge suspension is not required 100 percent of the time.
- The oxidation ditch diffused aeration is reduced or turned off – mixer speed can then be simply reduced to meet the lower thrust requirement.
The result in all three situations is reduced energy consumption as the lowest power consumption is employed while achieving sufficient mixing. Another important cost benefit is increased equipment life thanks to abrupt starts and stops being replaced with continuously varying speed. This kind of optimal mixing style also reduces maintenance required.
Some plants install tank-side Variable Frequency Drives (VFD) to add a degree of operational flexibility to a single speed mixer. This modification, however, introduces electromagnetic compatibility (EMC) and other installation and commissioning issues. Tank-side VFDs exposed to the environment are also inherently less reliable than an internal VFD. Xylem’s Flygt 4320 low speed submersible mixer enables frequent, demand-based thrust regulation, without the need for a separate, tank-side VFD. This adjustable thrust mixer allows for dynamic mixing resulting in an energy-minimizing operation. The company’s integrated drive technology allows continuous thrust variation to be easily established, and operates more reliably and efficiently than manual ‘switch-on/switch-off’ practices. The following case study illustrates how adjustable mixer speed reduces energy consumption while delivering optimal mixing.
Case study
Milwaukee, Wisconsin, United States - Only 0.28 W/m3 Energy Consumption.
A comprehensive mixer energy pilot study was conducted at the Milwaukee Metropolitan Sewerage District South Shore Water Reclamation Facility in Wisconsin, US, with the goal of minimizing energy consumption. Results show that energy consumption of just 0.28 watts per cubic meter (W/m3) is sufficient to fully homogenize activated sludge. This was achieved using a high-efficiency adjustable thrust submersible mixer, Xylem’s Flygt 4320, optimally positioned in the plant’s 48 feet (ft) long x 30ft wide x 15ft deep, biological nutrient removal (BNR) selector zone. Total Suspended Solids (TSS) profiles, phosphorous removal and energy measurements were studied as mixing energy in the selector basin was gradually reduced over time. The study concluded that one element key to this success was the use of a high efficiency submersible mixer with adjustable thrust fine-tuned to the basin shape. Other key elements were positioning the mixer to produce an efficient bulk flow loop that takes full advantage of basin shape and using a hydraulically optimized propeller.