Conserving energy and saving costs are always on the minds of wastewater professionals. Aeration accounts for more than 50 percent of electrical usage at most treatment plants. Improved aeration efficiency will always work toward the goals of saving energy and reducing operating costs.
Using jet aeration for certain unit processes can reduce electrical usage. However, to take maximum advantage of jet aeration system efficiencies, engineers must concentrate on many factors during design. Details are important.
Optimizing Efficiency With Jet Aeration
The beauty of jet aeration is that you’re not only using a blower to provide mixing and aeration, but engaging a pump as well. Jet aerators transfer oxygen by pumping large volumes of mixed liquor suspended solids (MLSS) and compressed air through a series of jet nozzles. These systems provide intense mixing in an outward, then vertical, pattern. The fine-bubble cloud results in high alpha factors and very efficient oxygen transfer, even for industrial wastes and biosolids.
The pump accounts for 25 to 40 percent of the power used in a jet aeration system. So, additional consideration is required when designing these systems compared with typical diffused air designs.
Optimizing energy efficiency is a combination of the aerator configuration, device selection, and system engineering.
Aerator Configuration
As with all aeration systems, aerator configuration is a major factor for energy efficiency. It’s important to work closely with the jet aeration system supplier to determine the best spacing of the headers and location of individual nozzles.
Basin geometry determines the most efficient aeration design. For instance, with long, rectangular basins, a bi-directional jet configuration is typically more efficient than having jets along one side of the basin inducing a spiral-roll surface pattern. However, a long, narrow basin may be more economical with spiral-roll design. For circular basins, directional flow jets have a lot of advantages over bi-directional jets from both a mixing and oxygen transfer perspective. Again, the supplier can work with the engineer and provide a recommended layout for the system that meets manufacturers’ recommendations.
Device Selection
The rotating equipment chosen, whether blowers or pumps, has a great effect on system efficiency as they are designed to run 24/7. Pumps have become much more efficient hydraulically over the years. Pumps exceeding 80 percent efficiency are readily available. You may be able to meet the same duty point with a less expensive pump but lose 10 to 15 percent of your pumping electrical efficiency. So, lifecycle costing should be part of the design.
Blower choice has been expanded as well. Hybrid screw type blowers are now available, and in small and medium size plants are often more efficient than multistage centrifugal blowers. These machines are 5 to 10 points more efficient than positive displacement blowers and have a greater range of turn down when operated on variable frequency drives.
System Engineering
Most conventional jet pumps run at very low pressure — 14 to 20’ of total dynamic head (TDH) pressure drop at the nozzle, dependent on design liquid flow rate. They’re high-volume, low-pressure devices. If the pumps are located outside the basin, the engineering must account for enough piping system head loss to ensure the pumps run on the curve at the specified duty point. Unaccounted-for pressure drop across the outside piping system leads to a reduction in jet flow and results in lower aeration efficiency because of incorrect jet velocity and bubble size.
More often, though, the real issue with energy occurs because of too much pipe and too many bends and/or valves between the pump and the basin. For example, it’s typically recommended that the pumping system be located right next to the basin, with the pipe diameters chosen such that the liquid velocity is at a very efficient range of 4 to6’per second (fps) to avoid a lot of pressure drop. Tank isolation valves are needed on the suction and discharge side of the system. Full-port valves are recommended to reduce pressure drop. If configured this way, the pumping pressure should only increase an additional 2’ TDH.
Engineers often overthink the system. In addition, their clients may request that pumps and blowers be installed at specific locations some distance from the basins and/or in a designated building. They may want automatic restart with a standby pump(s), with additional check and isolation valves. As a result, pumps are located farther from the basin, with additional bends and valves and can lead to additional TDH requirements of 6 to 10’. Convenience may be enhanced at the cost of a significant increase in power consumption. In this type of configuration, it is not unusual for the pumping energy to increase by up to 50 percent and the overall energy (pump and blower) usage by 15 to 20 percent. Engineers should educate their clients so they understand the resulting loss of energy efficiency when adding distance and friction between the pump and tank.
Use Your Supplier As A Technical Advisor
Jet aeration system suppliers have the knowledge and experience to help engineers design energy-efficient systems.
Jet aeration systems require a sophisticated design effort. Use all of the resources available to create the most energy-saving system possible.
