Going for the Green

The AquaDAF high-rate clarifier has proven itself in effective removal of algae and other low-density solids from surface water sources.
Going for the Green
A schematic shows the basic structure and operation of the AquaDAF high-rate clarifier.

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Surface waters that contain algae can be challenging to treat in conventional settling and filtration processes. Being light and buoyant, algae and certain other organic materials found in lakes and reservoirs can resist settling and inhibit downstream filtration.

For removing such substances, Infilco Degremont (SUEZ ENVIRONNEMENT) offers the AquaDAF high-rate clarifier, a dissolved air flotation (DAF) process. Essentially a sedimentation process in reverse, it uses tiny air bubbles to float less-dense, flocculated impurities to the surface, where they are skimmed off.

The technology originated in Finland in the 1960s; Infilco Degremont licensed it and brought it to North America in 2000. It is designed to treat large volumes of low-turbidity, algae-laden water — 10 to 20 gpm per square foot of collection zone — in a small footprint. It can be installed for new treatment plants as well as retrofitted in existing facilities, even where space is constrained.

Patrick Daniel, application engineer with Infilco Degremont, talked about the technology and its applications for drinking water treatment in an interview with Water System Operator.

wso: When and where would a drinking water utility apply this technology?

Daniel: The real key is that you have something in the water that wants to float out. For heavy particles, like silt in river water, it’s not the best application. We’re trying to float the particles out of the water, and there are only so many air bubbles you can attach to a small piece of floc to float it out.

The ideal application is for water that contains algae but otherwise is pretty clean with low turbidity. Lakes and reservoirs are typically low in turbidity because everything heavy has already settled out by the time the raw water gets to the intake.

wso: Where does this technology fit in the treatment scheme?

Daniel: We see it in brand new plants, and in older failing plants. An old plant might have a conventional system with a really long basin where they try to settle out the particles. But if the water is from a lake that has algae problems, conventional treatment is not the best, because algae is very lightweight. It ends up going through the clarifier and then gets clogged up in the filter.

We see this technology applied where someone wants to get rid of an aging system and put in a system that is more suited for the type of water they have. We have a couple of installations where a utility did exactly that. Their system was failing. They were getting really short filter run times, and the water tasted bad, too, because algae made it through the system.

wso: What kind of chemistry is used in this process?

Daniel: It’s basically standard jar testing chemistry. When we get a project, we do jar tests and look at how the floc flows. It’s the standard chemistry you see in a conventional treatment plant, but instead of sinking the floc, we’re floating it.

wso: In summary, how does the process work?

Daniel: Raw water entering the unit goes through a rapid mixer or static mixer where we introduce the coagulant. The coagulant destabilizes the particles in the water so they start to clump together. The water then proceeds into a flocculation basin where we form a floc and get it the right size.

The water then enters the base of the flotation zone, where a saturated air-water recycle stream is added. This stream is produced by recycling some of the already clarified water to a pressurized saturator vessel. The recycle stream is then depressurized through a series of release nozzles. This creates thousands of micro-bubbles, which disperse into the flotation zone.

The air bubbles are really small, on the order of 40 to 70 micrometers. The water takes the appearance of milk — people call it whitewater. The bubbles attach to the floc particles formed in the previous step and float them to the surface. The clarified water exits the bottom of the unit through a perforated collector floor and moves on to filtration, which can be a media filter or a membrane system.

wso: How is the floated material removed from the unit?

Daniel: There are two ways to remove the sludge — hydraulically or mechanically. In hydraulic removal, we slowly close off the bafflement at intervals so that the water can’t leave the system. Water then lifts up a couple of inches and then spills over a weir like a waterfall, into a trough on the side. You get a more dilute liquid sludge that way.

The other method is to use a mechanical scraper. A motor- and gear-driven scraper blade comes down at intervals and pushes the sludge into the same trough. It’s thicker sludge when you remove the material this way, and the utility needs to have the proper equipment at the plant to handle that.

wso: What accounts for the high volume this system handles in a small footprint?

Daniel: It goes to the way we collect the water underneath the unit. We introduce a headloss through the floor and force the water to take a particular path. Rather than letting the water take the least-resistant path, we force it to take a more indirect route. That allows us to push more flow through the system.

wso: What effluent quality does the clarifier produce?

Daniel: We typically see 90 percent algae removal in our pilot studies and our full-scale treatments. Some algae species are more difficult to remove, but on the other hand many of our systems get as high as 93 to 94 percent removal. Because we start with low-turbidity water, we typically see 1 NTU coming off the clarifier.

wso: How does this unit’s speed compare with conventional settling?

Daniel: A conventional clarifier might take a couple of hours from when the water enters until it leaves. It typically takes less than 30 minutes, and more likely 20 minutes, to get through this system. It’s quick to turn on and lets you produce water a lot faster.

wso: How easy are these systems to install?

Daniel: There are nine package units in capacities from 100 to 1,750 gpm. They show up as a tank and skid bearing the pump and saturation vessel. A contractor simply has to wire it, plumb it and it’s ready. It’s much quicker than doing a new concrete installation.


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