Without an appropriate control strategy, even the best wastewater treatment plant (WWTP) equipment can struggle to deliver maximum efficiency. Because energy costs represent up to 60 percent of WWTP plant operating expense, air distribution designs and changing loading (biochemical/biological oxygen demand [BOD]/chemical oxygen demand [COD], ammonia) demands require control strategies designed to adapt to all eventualities efficiently. The following checklists, plus important Do’s and Don’ts, can help in identifying an underperforming control strategy and developing a better one.
Know The Warning Signs Of Inefficient Control
Improperly controlled or poorly tuned blower systems in WWTP plants manifest themselves in multiple ways:
1. Recognize The Two-Tiered Nature Of Control
Where possible, choose blowers with control systems that also provide the benefit of a fully engineered design for dynamic protection across multiple operating scenarios — including surge, overheating, and changing ambient conditions. If upset conditions do occur, deal with them as quickly as possible. Do not attempt to bypass alarms. Operating beyond design parameters can compromise system longevity. Instead, work with the supplier to reestablish operating parameters within machine limits.
2. Match Technologies To Processes
Knowing equipment and control system “sweet spots” and limitations makes it easier to plan ahead to accommodate wide swings in plant throughput volumes and operating temperatures. This makes it easier to adapt to a wider range of control scenarios.
Different blower designs — positive displacement (PD), hybrid (screw compressor), and eventually turbo blowers — have evolved to offer distinct and unique advantages in terms of purchase cost, operating cost, and ability to respond to changing air demands. Selecting the appropriate blower type to meet the demands of an application can yield significant operational advantages in robustness and energy usage.
Machines with limited turndown, or centrifugal technologies with a surge line, introduce additional control concerns. Using independent blower controls and valve controls that don’t communicate can lead to a surge condition if the supply-side (blower) is already running at a minimum and the demand side (digester) requires even less air. This is why it is important to work closely with a system manufacturer or a knowledgeable integrator to sequence valves and blowers to adapt blower performance to changes in process demands.
3. Take A Total System Approach
Controlling the system’s blowers separately or in parallel with the system’s airflow distribution valves in a variable BOD situation is a recipe for disaster (Figure 1). These systems can fight each other if not properly integrated, often leading to instability and premature component failure. Instead, design air distribution system blowers and valves to complement each other with a streamlined aeration system controller. When these systems are properly integrated to work together, it is possible to maximize the energy efficiency of the system without compromising oxygen setpoint tracking performance.
An airflow-based MOV control strategy directly correlates the demand of the process to the supply and enables achieving airflow distribution at the lowest possible system pressure (Figure 2). This is done by matching blower output with the system loading requirements to minimize wasted energy costs. It allows the system to operate at peak efficiency with the minimal amount of pressure necessary to achieve optimal flow distribution.
4. Practice These Do’s and Don’ts Of Efficient Process Control
Image credit: "aeration piping," Jason Turgeon © 2014, used under an Attribution 2.0 Generic license: https://creativecommons.org/licenses/by/2.0/