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The Ultimate DAF System Buyer’s Guide: Part 1

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 pump

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.Nikuni regenerative turbine pump

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

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 pump

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. FRC DAF Controls App

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:

  1. Open all recirculation valves
  2. Energize whitewater pump
  3. Energize air compressor and feed pump x lpm of air at y psi
  4. Begin closing off whitewater discharge valve until discharge pressure gauge reads x-value
  5. Begin closing off whitewater suction valve until pressure switch registers y-value
  6. Open air bleed-off valve until air saturation tank reads z-value
  7. Keep tweaking all valves until whitewater looks good on visual inspection
  8. Run system for 10 minutes to saturate tank with whitewater
  9. Open wastewater influent valve and begin treatment

A well designed and automated DAF system should operate something like this:

  1. Push START button
  2. 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

The Ultimate DAF System Buyer’s Guide: Part 2

If you haven’t read it yet, make sure you to read Part 1 of the Ultimate DAF System Buyer’s Guide

3. Materials of Construction

When you buy a DAF system, you want something that is going to hold up in a harsh environment. You’re going to use the thing for the foreseeable future and the last thing you want is something you have to replace because it didn’t hold up.

There are several options for materials of construction with regards to the tank structure, each with their own advantages and disadvantages. Let’s look at what’s on the market:

Concrete
Steel-reinforced concrete basins are typically used in large municipal wastewater treatment plants. Concrete DAF basins are sturdy and leak proof, but can be very expensive because of the required civil work involved, i.e. excavation, steel reinforcement, concrete forming, coating, etc. Since they can only be built in the location where they’re going to operate, concrete DAF basins are not often selected for use in industrial facilities.

Polypropylene
Some manufacturers prefer polypropylene because of its lower material cost, good strength and stiffness, and generally broad chemical resistance.
Significant problems arise when polypropylene is exposed to extreme temperatures or UV radiation. If placed outdoors, UV light from the sun causes tertiary carbon bond breakdown and results in discoloration and cracks.

Polypropylene will not hold its shape at temperatures above 260°F and it will crack at temperatures below 32°F. Refurbishing polypropylene tanks for reuse is rarely pursued as the material degrades over time. DAF manufacturers that use polypropylene generally warranty the tank structures for 10 years.

Epoxy-Coated Carbon Steel
Coated carbon steel offers the strength of steel and the general corrosion-resistance of an epoxy coating. Structures of this sort are particularly useful in applications with high Total Dissolved Solids (TDS). But for applications in the food industry, this material isn’t recommended because free fatty acids in floating sludge will eat through the epoxy coatings and rust the steel, compromising the tank’s structural integrity. Epoxy-coated carbon steel is commonly thought of as less expensive than stainless steel, however when constructed to provide the same structural strength and corrosion resistance as stainless, the cost-savings of coated carbon steel is negligible.

Stainless Steel
Stainless steel is used in DAF tank construction for a variety of reasons. Stainless steel resists rust formation because of a passive film of chromium oxide on its surface that blocks oxygen diffusion and corrosion. Stainless steel retains its strength between temperatures of -320°F to 1500°F, so it holds up in indoor and outdoor applications. Welders can easily make modifications to the material without needing to re-apply paint or other coatings. Even after decades of use, stainless steel tanks can be retrofitted and work for decades longer because the steel structure remains sound.

Stainless steel, however, does come at a higher price than some other materials and it is not particularly well-suited for applications with significant concentrations of chlorides, which could potentially cause pitting or corrosion.

Stainless steel DAF tanks retain a high resale value and many manufacturers are willing to buy them back, even after prolonged use.

 

4. Sludge Thickening Mechanisms

The purpose of a DAF system in the wastewater treatment process is to remove solid and oily contaminants. The trick to increased efficiency and cost-reduction is to get the DAF generating fluffy, thick sludge – not a watery slurry. Sludge thickness is a function of hydraulic, mechanical, and chemical processes – when they come together, engineered in a thoughtful manner, the results speak for themselves.

Co-Current vs. Counter-Current Skimming

The internal hydraulic flow patterns in a DAF system vary from manufacturer to manufacturer and by DAF configurations. DAF systems that employ plate packs often direct wastewater in a down-flow or cross-flow pattern so the following discussion has less relevance to this type of DAF unit. Open style DAF tanks, however, generally employ a flow-through or end-to-end hydraulic process so this conversation is very important.

When considering an open tank DAF system, take a good look at which direction the skimmer assembly rotates. Does it rotate in the same direction (co-current) as the hydraulic flow of the wastewater through the system? Or does it rotate against (counter-current) that flow? Here’s why it matters.

Say you’re looking at the side of an open style DAF tank that’s 40 feet long and 8 feet wide. Wastewater is fed into the DAF unit on the left side and exits out the right. As a layer of sludge begins to form at the water’s surface, the skimmer flights are going to start pushing that sludge in one of two directions. The co-current skimmer is going to push the sludge from left to right, the same direction as the wastewater flow. The counter-current skimmer folds the sludge back over the flow from right to left. Still with me?Co-Current vs. Counter-Current DAF Skimmer

Now, consider where the sludge first begins to reach the surface of the water – is it near the inlet, middle, or outlet side of the tank? Most DAF systems hit the inlet stream of wastewater with whitewater immediately upon entry into the vessel, so sludge forms near the inlet side of the DAF tank. If sludge forms near the inlet side of the DAF tank, what good reason is there to co-currently push that sludge across the entire 40 foot length of the unit? It  makes much more sense to use a counter-current skimmer to push the sludge a shorter distance.

Proponents of the co-current configuration would argue that while sludge is being pushed across the long length of a DAF system, it’s being thickened. However, field data and reviews from operators would argue the opposite. Skimming co-currently more frequently results in watery sludge, extra build-up of material near the sludge ramp, sludge splashing into the effluent chamber, and faster wear and tear on the skimmer motor and flights.

With open-style DAF systems, the counter-current skimmer consistently wins the performance battle, especially when an extra measure is taken to lock sludge in place when it reaches the surface of the water – more on that to follow.

Sludge Dewatering Grid

Another key mechanical feature of DAF systems that are engineered to generate the driest sludge possible is a static grid that sits at the surface of the water, commonly referred to as a Dewatering Grid.

A dewatering grid is a rectangular framework of angled steel plates that help lock sludge in place as it reaches the surface of the water. As sludge accumulates in the dewatering grid, it is held in place until it reaches the point where the skimmer blades slice it off the top and start pushing it toward the sludge ramp. By allowing for a little retention time in the grid, float materials self-dewater before they’re skimmed away. This results in higher dry solids content, or less-watery sludge.

Without the grid the skimmer blades start pushing sludge along before it has had any time to dewater in place. Often times what happens is that the float begins to accumulate right near the sludge ramp and can be forced back down into the water, completely undoing any dewatering that may have occurred before removal. The diagrams below illustrate this point.

Dewatering Grid Diagram

So what’s the big deal with watery sludge? As long as the discharge water meets the requirements we’re good right? Well, that depends on how important saving money is to your operation. More water in sludge means more required storage capacity, bigger dewatering equipment, and more chemical expenses on re-processing filtrate water (the water that separates out of sludge after dewatering) through the DAF system. You have everything to gain by going with a DAF system that’s engineered to produce drier sludge. Ask the DAF vendor how their design helps generate drier, thicker sludge. What mechanisms do they provide to allow operators to adjust sludge thickness?

 

Keep reading: Part 3 of the Ultimate DAF System Buyer’s Guide

 

 

The Ultimate DAF System Buyer’s Guide Part: 3

Make sure you’ve read the Ultimate DAF System Buyer’s Guide Parts 1 & 2 before starting here.

Part 1 of the Ultimate DAF System Buyer’s Guide

Part 2 of the Ultimate DAF System Buyer’s Guide

5. Dissolved Air Distribution Methodology

The method used to distribute whitewater into the incoming wastewater stream makes all the difference in the way sludge forms and floats inside the DAF vessel. Let’s examine two different approaches.

Some DAF manufacturers employ a specialty whitewater pump, vertical saturation pipe and diaphragm valve assembly to generate and inject whitewater into the incoming wastewater. Others use ANSI recycle pumps, an angled air saturation tube, and a whitewater distribution manifold with multiple ball valve and tubing assemblies. A comparison of these two configurations might sound like splitting hairs, but it’s definitely worth some consideration. air bleed off diagram

  1. The vertical saturation pipe configuration is designed to bleed off air that doesn’t dissolve into solution. While most excess air will escape from the top of the tube, the side-mounted whitewater discharge line will draw some portion of it into the tank. As excess air enters the flotation cell, large bubbles form and burp at the water’s surface. These bubbles disturb already floating sludge and immerse solids back into the water.With an angled configuration, excess air escapes out the elevated portion of the tube as whitewater flows out from the lowest point. No undissolved air can enter the flotation tank because the water flows from top to bottom, not the other way around.
  2. DAF systems that use vertical tubes typically only have one dissolved air injection port. When flowing 100 gpm (1.67 gallons per second) of wastewater into the flotation cell, a single dissolved air injection point is unable to effectively and evenly spread whitewater over the entire incoming solution. Some portion of the flow gets a huge shot of whitewater, while other parts go entirely without. It’s up to the hydraulic patterns inside the flotation cell to distribute the bubbles. Often times this works, but it’s unpredictable and uncontrollable. A habitual problem with this design is that solids cannot float to the water’s surface quickly enough to keep up with the incoming flow. By all calculations, a system designed to process 100 gpm of wastewater is unable to achieve that flow rate because solids are not separating quickly enough.air distribution manifold DAF systems with whitewater distribution manifolds do not have this issue. Numerous whitewater injection ports are placed across the width of the flotation cell at different heights to evenly distribute micro-bubbles all throughout the DAF tank in a controllable manner.
  3. The start-up routines for these two configurations also varies by nature of the whitewater injection system. A single air injection port saturates the tank with whitewater from one origin. As tanks are sized longer and wider, it takes more and more time for the bubbles to disperse throughout the whole tank. Some manufacturers’s DAF operation manuals suggest allowing 5-10 minutes for tank saturation before opening the wastewater influent valve and proceeding with treatment. DAF systems with an air distribution manifold are fully saturated within 60 seconds or less. Shorter start-up time means less waiting around and more getting work done.

The lesson is is to get in closer and examine the design differences. The details matter. Ask the manufacturer about their design justifications.

6. Application Specific Design

Think of a hammer, a tool meant to deliver impact to an object. Now, picture one in your mind. What do you see?

If you’ve ever worked in carpentry maybe you visualized a claw hammer.  Metalworking? A ball-peen hammer with a hemispherical head. Maybe you’re a stonemason and thought of a brick hammer with it’s long chisel-shaped blade. Everyone thinks of the hammer they use in their trade. But, no one should know better than the hammer-maker which one is the most appropriate for a specific task.

A DAF system manufacturer should have that same level of expertise and knowledge about which DAF system design is most suited to a specific application. If a designer is using the same DAF system for a dairy acidulation process as they use in a cattle abattoir plant, that’s like a rail worker using a judge’s gavel to pound in a railroad spike – similar tool, wrong application.Railroad worker with gavel

The application specific design largely comes down to materials selection and process engineering. Take for example a DAF system used in a cattle kill plant. Without understanding that these types of facilities generate wastewater laden with sandy, gritty materials, a manufacturer may opt to use their design standard, say a cast iron pump. Unfortunately for the person who buys this DAF system, their gritty wastewater will wear pin holes in the pump casing and eventually cause a pump failure. This results in down-time and replacement of the most expensive component on the system. When something like this happens, not only does it reflect poorly on the manufacturer, but also on the engineer/consultant that presented it to the end client as the right solution. The master craftsman would understand the gritty nature of cattle wastewater and select a pump casing with high Brinell Hardness, like CD4MCu.

On the process side, the DAF designer should understand how to properly calculate and interpret hydraulic surface loading rate, solids loading rate, and air-to-solids ratio based on the application at hand. Solids in poultry kill plant wastewater separate infinitely easier than those from the effluent of an activated sludge system. Using the same values for each of these processes would likely result in an oversized poultry DAF and an undersized bio-mass separator.

You’re the one buying the system, don’t be afraid to question why a specific configuration was selected over another and be sure to ask for an explanation of the calculations behind your system sizing.

 

Continue to the Conclusion of the Ultimate DAF System Buyer’s Guide

The Ultimate DAF System Buyer’s Guide: Conclusion

At face value DAF systems are pretty much like any other piece of industrial equipment – they’re an investment that you only want to make once. And in the same vane, some designs are simply better than others. With this guide, hopefully you’ve learned which design elements separate one DAF unit from another.

In summary, let’s recap the things you should discuss with a DAF manufacturer before setting a purchase in motion:

  1. What type of DAF pump is used? Why?
  2. What materials are used in DAF tank construction?
  3. What mechanical measures are taken to thicken sludge?
  4. How is dissolved air distributed into the DAF tank?
  5. What is the operating procedure for the system?
  6. What specific design considerations were weighed for your application?

The DAF system manufacturer should be able to address each of these questions with great detail and explain why they design their systems the way they do. If they can’t articulate why they do things one way and not another, then there’s a lack of attention in their engineering approach. Conversely, the manufacturer that can discuss each of these topics is better equipped to become a useful resource and partner in your project as it moves forward.

If you’re satisfied with the reasons behind the manufacturer’s design decisions, then the next step is to take a look at their project history. If they’ve only got a few projects under their belt, their understanding of DAF design may still be largely theoretical or academic. When it comes to large capital expenditures, you don’t want theoretical solutions, you want something that’s guaranteed to work.

So, whether you’re an engineer or an end-user, remember that you only want to select a DAF system once. Make sure you get it right.

If you’d like to discuss a wastewater application where you’re thinking about employing a DAF system, contact us – we’re glad to help.