| In a recent project, these multi-stage centrifugal blowers were replaced with a new 60 HP Kaeser positive displacement blower with variable frequency drives. |
Before discussing simple payback, it's important to first understand how an aeration system functions and the different types of aeration systems available. WWTPs typically use either surface or submerged aeration systems to facilitate the activated sludge process. Submerged aeration systems typically use blowers to compress air and transfer oxygen into the wastewater through diffusers mounted on the basin floor. The oxygen transfer supports the aeration process within the basin.
In contrast, surface aeration systems are installed on the water's surface and use rotating brushes or disks to transfer air into the water. Surface aerators cost less to install when compared to submerged systems. However, surface systems have a higher lifetime operating cost due to lower oxygen transfer efficiencies.
WWTPs using submerged systems also typically employ either positive displacement blowers or multi-stage centrifugal blowers to transfer air from the surface to beneath the water. These technologies were state of the art for many years. However, recent advancements have superseded the efficiency of traditional blowers.
The next generation of aeration blowers is 10 to 20 percent more energy efficient than previous systems. These new blowers are called high-speed centrifugal blowers or turbo blowers. They operate at 10,000 to 60,000 rotations per minute while traditional blowers operate within the range of 900 to 3,600 rpm. The higher speeds are achieved by using gear increases, air bearings or magnetic bearings. Air and magnetic bearing are a recent technology breakthrough.
A significant advantage of turbo blowers is the ability to maintain efficiency when wastewater flows are lower. For example, a plant may require 90 percent of the maximum rated flow of a blower system during peak flows and may only require 45 percent under normal operating conditions. Most turbo blowers have integrated variable frequency drives to scale down the blower and improve energy efficiency during low-flow times. These blowers can also modulate blower output in response to signals from dissolved oxygen sensors, allowing the air flow to meet the changing conditions of a WWTP automatically throughout the day.
The improved efficiency of these blowers, even without a dissolved oxygen (DO) sensor, helps reduce energy costs, enabling WWTPs to pay for the blower upgrades within 3 to 10 years on average, depending on the size of the WWTP and application.
Calculating Simple Payback
Plant managers should weigh their options and decide whether investing in newer blower technology would benefit the plant. Upgrading to a new and more energy-efficient aeration system can be expensive, but the subsequent savings can pay for the improvements. The key is to calculate simple payback to help better understand the potential financial benefits and then use this information to decide if upgrading makes sense.
