At a certain point in the project development process you will have qualified a few dissolved air flotation (DAF) system manufacturers from a field of many. Now you have to decide which one will best suit your needs. How do you choose?
Whether you’re an engineer specifying equipment on a client’s behalf, a plant owner working to solve your own wastewater treatment challenge, or someone who is just looking to deepen their understanding of wastewater process equipment, this guide will help you gain clear insight into an expert DAF system manufacturer’s design choices.
Dive into mechanical and process design elements within this series of posts and gain valuable insight as to how you should evaluate DAF systems. With this knowledge you’ll be able to determine design superiority and most importantly, be prepared to make the right purchase decision. You only buy a DAF system once, get it right the first time.
1. DAF Recycle Pump
The recycle pump is the heart and soul of a DAF System – it’s only smart to start here. On it rides the greatest capital and maintenance expenses associated with a dissolved air flotation system. So, what do you look for in a DAF pump?
First off, what kind of pump is it? If the DAF manufacturer is calling it a specialty “whitewater pump,” they’re likely referring to a multistage impeller pump.
Multistage Impeller Pumps
These pumps draw atmospheric air (or receive an inlet feed of compressed air) into the pump chamber where impellers whip the air around with water to form micron-sized bubbles that dissolve into solution. While these specialty pumps do generate quality whitewater, there is cause for concern in a wastewater environment.
Multistage impeller pumps have low solids tolerance and will fail when oily, stringy, or gritty materials enter the pump chamber. On DAF systems carry over solids in the effluent can end up in the pump, and that’s cause for concern.
When used in DAF applications, these pumps operate at discharge pressures from 90-120 psi. Again, any solids in the recirculation piping can cause pressure drops and trip pressure alarms resulting in system shut-down.
Finally, multistage DAF pumps use components that are specially machined for that specific pump design. Should any part of the pump require repair, the end-user is limited to the DAF manufacturer as their sole-source for replacement pump parts. This is particularly troublesome for manufacturers that use pumps made overseas, as lead times can extend to several weeks.
There are other applications where multistage impeller pumps perform very well, particularly with higher pressure services like boiler feed water, condensate, pipelines, reverse osmosis, and descaling applications. But in wastewater environments and as an integral component of a DAF system whose sole purpose is to remove solids and oily materials, the multistage pump is just a little out of its element.
If the DAF system manufacturer employs a whitewater pump, but it’s not a multistage impeller pump, they’re likely using a regenerative turbine pump.
Regenerative Turbine Pumps
These are a type of centrifugal pump that use a rotating impeller to increase velocity, however the impeller looks quite different from what you would see in a centrifugal pump. Instead of vanes, the turbine impeller has radially oriented buckets or blades, which make it look like a turbine rotor.
As water enters the pump chamber, it moves in a circular path through the turbine buckets. In a DAF application compressed air is fed into the pump and dissolved into solution as the turbine spins and blends the air/water mixture.
Unique to the regenerative turbine pump is its ability to generate high pressures in a compact machine. Clearances in a turbine pump are much tighter than in a traditional centrifugal pump so the pumped liquid must be very clean. The tighter clearances also make this pump type noiser than a standard centrifugal pump.
Similar to multistage impeller pumps, regenerative turbine pumps operate at discharge pressures between 90-120 psi in a DAF application. High operating pressure and very tight internal clearances require liquids to be entirely devoid of solids or oily materials.
The most popular regenerative turbine pump used by DAF system designers is built by Japanese manufacturer, Nikuni Co. Ltd. Any parts that are required for repair have to be acquired through Nikuni’s representative network, which is limited to two companies for all of North America. A spare pump should be kept on-site at all times because lead time on a replacement pump often extends as long as 10-12 weeks.
DAF manufacturers relatively new to the wastewater market have adopted the multistage impeller and regenerative turbine pumps because they are marketed as “whitewater pumps” and under perfect operating conditions, they work very well. There is, however, another DAF pump option that is better suited for the wastewater environment – the ANSI end-suction centrifugal pump.
ANSI pumps are the only dimensionally standard pump type in the pump industry. All components are interchangeable – motor, coupling, impeller, volute, bedplate, etc. End-users who buy DAF systems that employ ANSI pumps don’t have to go to go the DAF manufacturer to source spare or replacement parts because they are readily available from any reputable pump vendor.
ANSI pumps are designed specifically to function in food processing, oil refinery, general manufacturing, pulp and paper, and chemical applications. They can pump liquids with or without solids and can be fitted with various alloys for operation in corrosive environments. When used in DAF applications, ANSI pumps operate at discharge pressures between 70-90 psi.
So how do ANSI pumps generate whitewater?
The trick is, they don’t. The ANSI pump is used to transfer water and that’s it. Whitewater is generated in a static “air dissolving tube” downstream of the pump as water comes in contact with small volumes of compressed air.
By placing the responsibility for generating whitewater on a static tube made of stainless steel, the ANSI pump can focus on what it does best – move water.
DAF pump selection says a lot about the design approach of the manufacturer. Be sure to ask why they use the pump they do. Ask for the reasons why they don’t use the other options. Remember, the DAF pump is the heart and soul of the entire system – it’s worth discussing.
2. Controls & Automation
Think here for a second about something completely unrelated to DAF systems. Grab your smartphone. Notice how smooth and easy it is to operate? The interface flows logically and simply through the tasks that you want to perform. That’s how a DAF system should operate.
Many manufacturers build systems with arduous operational procedures in the name of cost reduction, when in fact the headaches caused by such poor design end up costing infinitely more in time, labor, and frustration.
Some DAF systems we’ve gone out and serviced have ridiculous start-up routines that go something like this:
- Open all recirculation valves
- Energize whitewater pump
- Energize air compressor and feed pump x lpm of air at y psi
- Begin closing off whitewater discharge valve until discharge pressure gauge reads x-value
- Begin closing off whitewater suction valve until pressure switch registers y-value
- Open air bleed-off valve until air saturation tank reads z-value
- Keep tweaking all valves until whitewater looks good on visual inspection
- Run system for 10 minutes to saturate tank with whitewater
- Open wastewater influent valve and begin treatment
A well designed and automated DAF system should operate something like this:
- Push START button
- Watch controls do all the work while you eat a muffin
It’s somewhat comical how difficult certain DAF designs are to use. These systems don’t have to be that way. When you’re considering a buying a DAF system, take into account what it will actually be like running the thing. Ask for a write up of the operating procedure.
Continue to Part 2 of the Ultimate DAF System Buyer’s Guide