Upgraded Blowers Can Help Meet Expansion Needs

By Calvin R. Wallace

Municipalities across the country are using rotary lobe blowers to increase oxygen supply to aerobic digesters.

Infrastructure spending in the United States has not kept pace with the population growth and many wastewater treatment plants are at peak capacity. The nation's wastewater systems and their ratepayers face a major challenge in funding the expansion, replacement, and maintenance of aging infrastructure over the next several decades. For this reason, plant engineers and operators have significantly increased the focus on lifecycle costs including initial price, installation, maintenance and energy costs. Additionally, new regulations require the reducing nutrients in the effluent discharged from existing plants.

One method to meet these challenges is simply to increase the size of the wastewater treatment plant. Another way is integrating processes such as supplemental biological contactors by incorporating subsurface aeration equipment into existing plant operations. This second means presents the added benefit of increasing plant capacity without requiring plant expansion.

Common to both methods is the need for an increased oxygen supply to support the process. Blowers have become the most common and most efficient method to introduce oxygen to aerobic digesters. Blowers can replace expensive liquid oxygen injection systems and inefficient mechanical surface splashers. Among blowers, there are two common options: centrifugal blowers and positive displacement rotary-lobe blowers.

Centrifugal blowers are dynamic machines designed to produce variable flows at a constant pressure. In other words, the volume delivery of the blower depends on the system total dynamic head. Rotary-lobe blowers, in contrast, are positive-displacement machines and deliver constant volumes at variable pressure. This influences the complexity of controls needed in wastewater applications.

Closely controlling the dissolved oxygen level in the digester is critical to microbial respiration and effective treatment. The wastewater's ability to hold oxygen depends on many variables. Plus, the amount of air needed to maintain the desired oxygen level changes proportionately with the volume of wastewater being processed. These variables mean that the demand for air in the process changes.

A blower's design and its energy consumption profile are extremely important in these variable flow conditions. Most manufacturers should provide an efficiency curve comparing brake horsepower and airflow in cubic feet per minute (cfm). A quick comparison between blower type and manufacturer could reveal efficiency differences of up to 20%.

Centrifugal blowers are highly effective for large flows at moderate to low pressures. Positive-displacement blowers are variable over a wide speed range and the power requirements are almost directly proportional to the changes in motor speed. This is a significant per unit energy advantage that can quickly add up to thousands of dollars in power costs.

HB 950 rotary lobe blower packages from Kaeser Compressors provide up to 3365 cfm each. Their sound dampening enclosures significantly reduce noise levels and noise pollution in the surrounding community.

Blower type also impacts the bill of materials and installation costs. Positive displacement blowers generally employ straightforward and economical controls. For example, a variable capacity control panel for a positive-displacement blower consists only of the variable frequency drive and a process follower such as a dissolved oxygen sensor.

Complete blower packages built and backed by the manufacturer offer excellent value with motors, silencers, and valves already included. Pre-engineered sound enclosures that reduce noise levels 18-20 dB(A) are available to reduce noise pollution and improve not only the working environment, but the surrounding community as well. Some blower packages are even specifically designed for easy "side-by-side" installation. These space-saving units not only reduce the amount of plant area dedicated to blowers, but they save on installation costs with simplified utility connections.

From a maintenance and repair perspective, evaluate a unit's routine maintenance requirements prior to purchase. Easy maintenance features such as oil sight glasses on bare blowers and piped oil drains and automatic belt tensioning devices on packaged units.

Positive displacement blowers are usually field-repairable, without the need for specialized equipment or specially trained technicians. Plant maintenance personnel and standard tools can usually repair them.

As our wastewater treatment infrastructure continues to age, the costs associated with new plants, plant expansions and plant modernization will foster ingenuity toward more effective aeration designs. The growing use of both rotary lobe and centrifugal blowers for aeration applications reflects the important cost and performance advantages they present over traditional methods. Water treatment plant designers, owners and operators now have even greater opportunities to optimize aeration systems by properly selecting blower technology.

About the Author:
Calvin R. Wallace is National Sales Manager for Kaeser Compressor's Omega Blowers and has nearly 20 years experience with blowers and rotating equipment.