Dissolved air flotation (DAF) systems are a critical part of many wastewater treatment plants. They work by dissolving air into pressurized water to remove suspended solids, biochemical oxygen demand (BOD) and other contaminants from wastewater streams. DAFs are especially effective in treating wastewater with fats, oils and greases (FOG), fibers and other low-density materials, making them the preferred treatment option in industries like pulp and paper, food and beverage and industrial laundry, among others. For municipal wastewater plants, DAF upgrades can be enhanced by the World Water Works Dissolved Air Generator (DAG).

Yet despite their importance in modern wastewater applications, not all DAFs are created equal. Factors, such as the components that make up these systems, will influence how well they work and how long they last. In our next few blog posts, we'll take a deep dive into what to look for in your DAF system to take its performance and efficiency to the next level.

Why the Bubbles Matter

The first DAF component to consider is your air dissolving pump. DAFs rely on the introduction of whitewater — a highly saturated and pressurized stream of air and DAF effluent — into the wastewater to create microbubbles. These microbubbles attach to the particles, lowering their density and causing them to rise to the surface. From there, the contaminants can be skimmed off and removed.

With these operating principles in mind, efficient flotation depends on the quality of the dissolved air, making your choice of pump an important consideration in your DAF setup.

Evaluating the Right DAF Pump

The keys to a high-performing air dissolving system for your DAF include saturation efficiency, bubble size and maintenance requirements.

Saturation. High air/water saturation efficiencies require less energy and water to recirculate in the system — which is desirable in your DAF setup. Water recirculation reduces retention time, thus limiting the DAF system’s capacity and performance. In addition to high saturation, it’s also important to have consistent air flow. Inconsistent air flow leads to inconsistent saturation and, by extension, inconsistent DAF performance.

Bubble size. The size of the microbubbles will be an indicator of DAF performance. The smaller the bubble, the smaller the particle that can be removed from the wastewater. Smaller particles also require less chemistry to coagulate the products into larger particles. Besides the obvious operational cost savings, less chemical consumption unlocks dewatering and sustainability benefits. 

Maintenance. Air dissolving techniques shouldn’t require a lot of maintenance. This is a critical part of the DAF clarification process. Unfortunately, most pump systems are unreliable in terms of maintenance needs. 

The Nikuni Pump: Operating Principles

An example of a pump system that checks all the right technical boxes is the Nikuni pump — a regenerative turbine pump that achieves over 95% saturation efficiency for 5- to 15-micron bubbles. This system performs three functions in one unit, simultaneously drawing in air and water, mixing the air and then dissolving the air with its turbine impeller.

Thanks to its unique design, the Nikuni pump is highly efficient at dissolving the air into water, creating highly pressurized and air-enriched whitewater. To do so, it takes advantage of three forces: First, friction pushes the liquid in the direction of rotation. Second, centrifugal forces swing the liquid out into the crosswise direction of rotation. And third, axial forces push the liquid to both sides of the impeller. Together, these forces achieve the air dissolving action, creating a constant, stable and high-pressure flow with high saturation efficiency and fine microbubbles.

The Benefits of the Nikuni Pump

The Nikuni system overcomes many of the challenges of traditional DAF pumps. First, it achieves high saturation efficiency. Whereas most pump systems have 20 to 50% saturation efficiencies, the Nikuni pump system is 95% efficient, sometimes higher. It also achieves 10 to 20% recycle rates — compared to the 30 to 50% average range — leading to more water retention, more separation time and better DAF performance. Because it uses atmospheric air to operate, the Nikuni pump also achieves a consistent air flow and air-to-water ratio.

A second challenge the Nikuni pump overcomes relates to bubble size. While many pump systems create bubbles that are over 100 microns, the Nikuni pump creates 20-micron bubbles or less (see Figure 1).

Figure 1: Testing shows the majority of microbubbles produced by the Nikuni DAF pumps are below 10 microns.

A third challenge the Nikuni pump overcomes involves maintenance. Unlike other DAF pumps and conventional DAF systems, the Nikuni pump doesn’t require compressed air. This feature, along with its multifunctional design, means the Nikuni pump eliminates the need for large, costly ancillary equipment like air compressors, ejectors, static mixers and saturation vessels, simplifying DAF operation, lowering maintenance needs and reducing costs.

Other benefits of the Nikuni pump include:

• Better DAF performance and increased capacity.
• The elimination of costly compressed air systems.
• A 10-40 percent reduction in chemical requirements.
• Energy savings and a reduced carbon footprint.
• A compact size and small installation footprint.
• An ability to fit new and existing DAF systems.

Get Started With the DAF-Nikuni Pump Combination

At World Water Works, we’ve incorporated the Nikuni pumps into both new and existing DAF systems. To upgrade your system, our wastewater treatment experts can calculate the most suitable Nikuni model. To get started, we’ll need certain DAF system operational data including influent flow rate, influent total suspended solids (TSS), influent temperature and influent pH. 

Once we have this information, we can provide you with a quote for the most suitable model, eliminating the need for compressed air, reducing your energy needs, cutting maintenance costs and improving the efficiency of your DAF system.

To learn more about our Ideal DAF™ systems, check out our technology page. You can also contact us to begin the process of upgrading your DAF with the Nikuni pump.