Dissolved oxygen is central to the efficient treatment of waste in water processes, with a typical wastewater treatment plant leveraging four main stages of treatment:
- Primary
- Secondary
- Tertiary
- Sludge
The secondary treatment stage involves organic waste being oxidized to form carbon dioxide, water, and nitrogen compounds. Most modern plants use an activated sludge system to achieve this, with a culture of bacteria and other organisms feeding on organic materials present in the sewage.
At the right temperature, these bacteria and organisms leverage dissolved oxygen to burn or break down organic carbons into carbon dioxide, water, and energy, removing harmful substances from the water.
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The Importance of Accurate Dissolved Oxygen Control
Oxygen is one of the most important parameters in water quality monitoring because it is a key requirement for most types of life. Water operators are required to monitor oxygen levels closely throughout the water treatment process, from the waste treatment at the aeration stage to the point of final discharge.
This whitepaper from ABB examines the role of dissolved oxygen in these processes, outlining the benefits of ABB’s Variable Frequency Drives in these settings.
Optimizing Aeration Efficiency
Aeration provides the oxygen that bacteria need to effectively treat and stabilize wastewater. Aerobic bacteria in the wastewater use oxygen to break down carbon-containing organic matter into water and carbon dioxide.
Aeration accounts for approximately 66% of energy use in wastewater treatment processes. For instance, 30% to 40% of energy use can be attributed to wastewater treatment operations in a typical mid-sized city, with 25% of this related to aeration.
The efficiency of the aeration process is dependent on the careful control of dissolved oxygen levels. Dissolved oxygen levels should be maintained at between 1.5 and 2 ppm under ideal conditions.
Should there be insufficient dissolved oxygen available, aeration basins will be deprived of the oxygen required for effective bacterial growth, adversely affecting the rate of sewage breakdown and hindering the efficiency of treatment processes.
The presence of too much dissolved oxygen can also negatively impact operations. Because aeration processes account for more than half of a plant’s energy costs, it is imperative that their efficiency be sufficiently optimized. Failure to maintain control of dissolved oxygen levels within the 1.5 to 2 ppm range significantly increases the risk of operators incurring unnecessary energy costs.
A range of factors are involved in maximizing the effectiveness of dissolved oxygen in enabling the breakdown of organic matter, including the size of the basin itself, the characteristics of the sewage influent, and the sewage flow rate.
Benefits of Using ABB Variable Frequency Drives as Part of a Dissolved Oxygen Control Strategy
Fitting drives to aeration blowers affords operators greater oxygen transfer flexibility, allowing them to better meet demand. Avoiding ‘over aeration’ is also key to achieving significant annual energy savings.
Variable frequency drives enable precise motor acceleration and deceleration, helping to reduce mechanical stresses on the drive chain and reducing power consumption.
Utilizing the combined output of ammonia and DO sensors, PLC and variable frequency drives considerably reduces energy use via improved pump control.
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Acknowledgments
Produced from materials originally authored by ABB Measurement & Analytics.
This information has been sourced, reviewed and adapted from materials provided by ABB.
For more information on this source, please visit ABB.