Wastewater treatment processes have come a long way in the past century, but demand continues to grow for more reliable and efficient treatment technologies. As a result, industry professionals are searching for an all-encompassing solution to enhance their treatment plants and processes. One of the purest environmental aids on the market is oxygen.
Typically, secondary wastewater treatment is done using blowers and associated air-based equipment, including diffusers. Secondary treatment is used to remove dissolved and suspended biological and organic matter, and this works very well in many cases. However, there are instances where the use of pure oxygen to supplement the air system can be particularly effective.
When to Use Oxygen to Supplement, or Enrich Compressed Air
When should a wastewater treatment facility evaluate its equipment needs and look for a source of pure oxygen to supplement, or enrich compressed air? There are five situations to consider:
- If the facility is experiencing problems, or doesn’t have sufficient airflow at a secondary or biological treatment facility.
- If the treatment plant operator is trying to get more out of an existing system, or has limitations, power concerns and space issues.
- If a plant is looking to avoid issues with older equipment such as breakage, motor failure and clogging.
- With summer or heat-related issues that are not present in cooler months or climates.
- Peak conditions (seasonal changes in flow/loading, etc.).
Of course, if none of these, or other related issues exist, there is probably not a need for pure oxygen within a given facility. If they do exist, there is a good chance oxygen can help.
There are several energy-saving benefits to this supplemental oxygen process that could potentially make daily tasks easier. Traditionally, oxygen systems are pressure-based, which means they often can operate with significantly less power consumption than equivalent air systems and, in some cases, can operate with no power at all.
In addition, the fact that the oxygen-based equipment can almost always be incorporated into the treatment process without interruption provides much more flexibility with little impact on the plant operations.
An oxygen system is installed to supplement wastewater treatment processes.
What Happens When Oxygen is Used?
Sufficient transferred oxygen is the key to all secondary aerobic treatment operations. There are several ways that the use of pure oxygen can help to ensure the system operates correctly and has this needed oxygen level with fewer detrimental impacts.
Often as changes occur in production rates, flows, product specifications, and more, the loading oxygen demand (BOD/COD) at the treatment plant changes as well. Wastewater systems are often not sized sufficiently to handle these changes. This is yet another area where pure oxygen can provide some relief.
Oxygen can help to alleviate multiple other problems prevalent in secondary treatment systems. For example, dead zones - areas with no- or low-dissolved oxygen due to poor mixing or non-functioning equipment - can be addressed with targeted injection of oxygen.
In most cases, the oxygen is added via separate means in the treatment area, which means it can be used on an as-needed basis and operated independently. This provides flexibility to meet plant needs while minimizing chemical usage and operating costs.
When air is added to wastewater, it can strip hydrogen sulfide and/or VOCs (volatile organic compounds such as benzene, toluene or methanol) that might be present during the aeration process. Compared to the traditional compressed air approach, pure oxygen often achieves a higher level of dissolved oxygen. Pure oxygen injection can result in improved mass transfer since it is typically transferred at a high rate (70-90%) and there is no nitrogen to interfere. These factors help to minimize the potential stripping that is typical of air-based systems where VOCs are present.
In some cases, even low doses of oxygen utilized prior to secondary treatment can help maintain the system health by ensuring sufficient dissolved O2, which prevents filamentous and other undesired bacteria from developing.
Appropriate levels of oxygen lead to healthy bacteria, which lead to improved treatment capabilities. Pure oxygen can play an important role.
What Is an Oxygen System?
An oxygen system consists of three main parts:
- The oxygen storage and supply tank, usually called a bulk tank. It is a vessel designed to store cryogenic liquids, including oxygen. Note: Although the oxygen is used as a gas, it is stored in a concentrated form as an extremely cold liquid, approximately -250°F, depending on tank pressure.
- The oxygen is vaporized in an ambient air vaporizer, which looks like a giant radiator, and then flows to the valve train/control panel. The oxygen flow and pressure are adjusted appropriately depending on the particular system design.
- Finally, the oxygen is injected. There are several methods of injection. Simple devices such as spargers, diffusers or porous hoses can be used. More complex devices can provide additional mixing energy and typically improve transfer efficiency. These devices can be floating, sit on the bottom of the basin, or even be side stream and will vary based on the applicable criteria (e.g. basin depth and size, quantity of O2 needed, need for mixing, availability of power, and many other factors).
Configuring the Right Oxygen System for your Facility
There are multiple ways to add oxygen in water treatment based on the need and the existing system design. Regardless of configuration, there is almost certainly an option that would be a fit.
Typically, oxygen can be injected directly into the aerobic basin, or in a side-stream and then pumped back into the basin.
A well-informed oxygen supplier specialist will look at each individual site situation to design a solution that meets the unique needs of a specific plant. This will take into account oxygen demand, as well as criteria such as the water depth and existing aeration equipment.
How Does Pure Oxygen Improve Wastewater Treatment?
Using pure oxygen achieves the highest level of dissolved oxygen as compared to the traditional approach, strictly using air.
There are several potential benefits when utilizing pure oxygen to enhance the air system in secondary treatment:
- Reduction in odors
- Improved sludge settling
- Increased loading (BOD/oxygen demand)
- Reduced VOCs (volatile chemicals)
- Potential to improve ammonia treatment
- Increased flexibility in system operation (blower turndown, etc.)
- Ability to meet peak demands/high production
- Help meet permit limits (e.g., NPDES)
- Aid in emergency cases (i.e., aerator failure)
- Maintain system health (sufficient dissolved oxygen in pipelines, final dissolved oxygen)
- Permit limits can be met without substantial upgrades to the entire plant
This can be the most cost-effective process to attain goals - meeting permit limits, allowing higher production, lowering costs, and avoiding capital expenditure - year after year.
When assessing the need for pure oxygen within a wastewater treatment plant, a perfect place to start is by having a conversation with a reputable supplier who can guarantee a reliable supply of high-purity oxygen. With a steady supply and a top-notch specialist, wastewater treatment processes will be streamlined to ensure quality requirements are met, equipment is operating smoothly, and costs are in check. In the wastewater treatment field, it’s important to explore and know all options, as technology can lend a hand in making a plant more efficient for years to come.
About the Author
Vince Hartley is a senior applications specialist in Airgas’ Engineering Solutions Group. With more than 20 years of experience, Vince supports gas applications in industrial settings with a strong focus on water treatment. For more information, please contact Vince Hartley at firstname.lastname@example.org, tel:713-896-2382 or visit www.airgas.com.
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