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How To Balance Dissolved Oxygen Levels For Maximum Efficiency

• 22.06.2020
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May 26, 2020

Source: KLa Systems

While dissolved oxygen (DO) levels are critical considerations in aerobic wastewater treatment processes, not every application maintains a consistent level of demand. How industrial wastewater treatment plants (WWTPs) track and fulfill those needs make a big difference in the optimum performance and cost-efficiency of their operations — including the equipment used to manage their processes.

Dissolved oxygen levels can vary more widely in industrial WWTP applications such as food, beverage, or chemical processing due to production throughput changes or washdown periods. In addition, any breakthrough in headworks screening equipment can clog submerged aeration systems and elevate DO requirements quickly.

Whatever the requirement, matching the required DO level as close to demand as practical is key to efficient performance, but there are other factors at play as well.

The History Of DO In WWTPs

The measurement of DO and its use in activated sludge wastewater treatment are relatively recent concepts. The process of measuring DO accurately was developed in 1888 by Lajos Winker, and the activated sludge process based on aerating wastewater was developed shortly thereafter in 1913. Over the next few decades of the 20^th century, that process became widely adopted in the U.S.

Since then, maintaining a dissolved oxygen level of about 1 to 2 mg/L has been an important benchmark in wastewater treatment. Not all that long ago, however, achieving that benchmark was typically a manually controlled operation. A submerged oxygen sensor connected to a display mounted above the treatment basin gave WWTP operators the opportunity to observe the trend in DO within the basin and manually adjust blower output accordingly.

One problem with that arrangement was that sensors often became fouled with a variety of debris in the treatment basin. Another was that other plant responsibilities or distractions might keep an operator from monitoring the oxygen levels as frequently as desired.

Setting The Stage For DO Control

Fortunately, sensor technologies have improved, and screens and other pretreatment processes have helped to provide more consistent wastewater composition to reduce fouling opportunities. Also, as distributed control and SCADA systems became more commonplace, plant designers were able to connect DO sensor output directly to automatic blower control systems to keep DO levels at optimum levels more consistently. Today, with variable frequency drive (VFD) blowers and computerized control systems, WWTPs are able to maintain close to optimum DO levels with greater energy efficiency than ever before. 

But as refined as the relationship between DO sensors and the blower control systems outside the basins has become, how effectively that forced aeration performs beneath the surface of the basin depends on other factors.

Maximizing DO Efficiency In Practice

Blower output is only one aspect of operating efficiency in a wastewater treatment basin. How that oxygen is introduced into the process and how it relates to the process can enhance efficiency beyond just the volume of O₂ and the cost of blower operation.

• Bubble Size vs. Performance. Bubble size is important in that smaller bubbles offer a greater surface-area-to-volume ratio for oxygen transfer than fewer coarse bubbles carrying the same volume of air. But the real-world operating parameters of different technologies can vary with more than just the theoretical bubble size of their original design.

• Diffusers. The principle behind fine-bubble diffusers delivering 1- to 3-millimeter diameter bubbles is a desirable start. Getting maximum performance from such a system, however, requires full-floor coverage of the diffusers. That can make it more capital-intensive than other aeration alternatives. Those micron-size diffuser orifices can also be prone to clog quickly — sometimes in as little as six months — depending on the concentration of dissolved solids in the aeration basin. Such loss of efficiency caused by fouled diffusers can cause oxygen-transfer performance to drop off quickly.

• Jet Nozzles. Jet aeration nozzles that mix blower air with a pumped stream of mixed liquor in the basin can generate a strong current of midsize bubbles (+/-3 millimeters) in an arcing path to the surface of the basin, allowing more time for oxygen transfer at the air/liquid interface (Figure 1). Self-cleaning back-flush features in this aeration design also resist clogging and make it easier to maintain high efficiency without taking the basin out of service for cleaning. The stream of pumped liquid helps to mix the basin as well, regardless of the blower speed used to maintain the optimum 2 mg/L DO levels used in wastewater treatment.

Graphic courtesy of KLa Systems

Figure 1. Jet nozzles and slot injectors can provide angled discharge streams of liquid and entrained air bubbles to create a longer flow path and longer exposure time for oxygen exchange as that stream mixes and slowly ascends through the wastewater basin.

• Slot Injectors. Like jet aeration nozzles, slot injectors also create a mix of pumped air and mixed liquor to generate powerful streams of fine- to midsize bubbles. They also include the same type of backflush feature to combat clogging and minimize the need for maintenance. Unlike jet aerators, however, slot injectors can offer more cost-efficient control over total energy consumption in certain applications with their ability to control both the liquid pump and the aeration blower independently (Figure 2).

Chart courtesy of KLa Systems

Figure 2. Jet aerators can save energy by trimming airflow, but not liquid flow, down to meet lower oxygen demand. This chart demonstrates the superior performance of slot injectors as compared to conventional jet nozzles — thanks to their ability to optimize both air and liquid flows in a treatment basin across a wide range of air-to-liquid ratios without reductions in aeration efficiency.

• Mixing Efficiency. Unlike air bubbles that take the shortest vertical path from a diffuser to the surface of the basin, the bubble output from jet aerators and slot injectors can be angled in various directions to create a longer path, more contact time, and more efficient mixing before reaching the surface (Figure 1). Also, the fact that jet aerators and slot injectors pump liquid with or without air addition enables them to mix the contents of the basin thoroughly for well-distributed biological activity, whether in an aerobic, anaerobic, or anoxic phase of treatment.

Whatever the original size of the bubbles generated, how they flow, or their impact on oxygen transfer in the wastewater treatment process, one way to appreciate what is happening to air bubbles below the surface of a treatment basin is to observe the surface of the water being treated. If it shows large boils instead of a well-dispersed pattern of smaller bubbles rising to the surface, it could be an indication of clogged aeration devices and inconsistent blending of the mixed liquor.

Added Energy Efficiency Through Slot Injector Aeration

Although both jet aerators and slot injectors provide aeration and mixing by pumping air and liquid, slot injectors offer the added advantage of being able to cut back on both aeration and liquid pumping efforts as oxygen demand drops. Jet aerators discharge liquid at a lower pressure than slot injectors, so even if their blowers are turned down as DO demand decreases, the liquid pumps must still run at full power in order to maintain appropriate mixing of the basin, but from an aeration-efficiency perspective this is wasted energy. Because slot injectors start at a higher liquid pumping velocity and pressure, they can maintain good mixing characteristics even if the liquid pump is slowed down in proportion to the aeration blower. This can enable up to 60 percent pumping energy savings when running the system in low oxygen demand mode. Doing so enables the treatment plant to optimize DO levels and mixing performance at lower energy costs, even as biochemical oxygen demand levels fluctuate below peak demand or as colder water temperatures require less aeration. Most plants that adjust liquid pumping rates do so on a manual basis, although the future goal is for pump control to be automated just like DO/aeration control of the blowers.