DAF units are everywhere in industrial wastewater treatment. You'll see one ahead of oily wastewater, dyeing wastewater, paper mill white water, and food processing wastewater. Its job is simple: use bubbles to lift suspended solids and oil droplets to the surface, then scrape them off. Sounds easy enough. But in real operation, a lot of DAF units don't do what they're supposed to. It's not the equipment. It's the bubbles. The key isn't whether you have bubbles. It's whether the bubbles are small enough, numerous enough, and sticky enough to grab onto what they're supposed to. If any one of those three is off, the DAF unit is just a tank.
A Real Case: The Dissolved Air Water Looked Fine, but the Effluent Never Hit Spec
A auto parts plant in Zhejiang generates about 300 tons per day of oily wastewater from its machining shop. The wastewater comes mainly from cleaning operations and contains emulsified oil and metal fines. The plant installed a DAF unit with a design flow of 15 m³/h, equipped with a dissolved air pump and a release unit, and PAC and PAM for chemical dosing.
The first month, effluent quality was acceptable. Oil was around 10 mg/L, and suspended solids were below 50 mg/L. Starting the second month, effluent began to deteriorate. Oil rose above 20 mg/L, suspended solids exceeded 100 mg/L, and the float layer on the DAF surface got thinner and thinner. Sometimes there was almost no float at all.
Operators checked the dissolved air pump. Pressure was normal, around 0.4 MPa. They checked the release unit. No clogging. They checked chemical dosing. PAC and PAM concentrations and flow rates were within set ranges. The dissolved air water looked normal too. When they sampled from the dissolved air line, they saw plenty of white bubbles.
The problem was the bubbles themselves.
Under a microscope, the bubble size in the dissolved air water was mostly above 100 microns, with some exceeding 200 microns. For a properly operating DAF, bubble size should be between 30 and 50 microns. Bubbles that big have a small specific surface area. The collision probability with oil droplets and flocs is low, and so is the attachment efficiency. A few bubbles lifted a little floc, but most of the floc settled to the bottom or flowed out with the effluent.
The root cause was in the dissolved air system. The air intake on the dissolved air pump hadn't been adjusted properly. The air-to-water ratio was too high. The supersaturated air in the dissolved air water didn't have time to form fine bubbles during release, so it coalesced into large ones. At the same time, the internal structure of the release unit had slight wear. It wasn't completely clogged, but release performance had already dropped, and bubble uniformity was worse.
They adjusted the air intake, brought the air-to-water ratio down to the right range, and replaced the release unit. After a week of operation, the float layer was noticeably thicker. Effluent oil dropped below 8 mg/L, and suspended solids fell to around 40 mg/L.
In this case, the dissolved air pump was running, the release unit wasn't clogged, and chemicals were being dosed. Everything looked normal. But bubble size—the single most important indicator—wasn't being watched. A DAF unit solves the problem of how to separate suspended solids and oil droplets from water. It doesn't solve the problem of bubbles too large to attach to anything.
How DAF Actually Works
The principle behind DAF isn't complicated. A dissolved air pump forces air into water under pressure. The air dissolves in the water. The dissolved air water passes through a release unit into the DAF tank. Pressure drops suddenly, and the dissolved air comes out as microbubbles. As these microbubbles rise, they collide with suspended solids, oil droplets, or flocs, attach to them, and carry them to the surface. A float layer forms, and a skimmer scrapes it off.
The key is "microbubbles." The smaller the bubble, the larger the specific surface area, and the higher the attachment efficiency. A 30–50 micron bubble has three to five times the specific surface area of a 100 micron bubble. Large bubbles have enough buoyancy, but they don't stick to anything. What floats up is just the bubble itself.
Several factors affect bubble size. Dissolved air pressure usually runs between 0.35 and 0.5 MPa. Too low, and not enough air dissolves, so fewer bubbles are released. Too high, and energy consumption goes up, and the release unit wears faster. The air-to-water ratio is generally 5% to 10%. Too much air, and bubbles tend to coalesce into larger ones during release. The release unit's internal structure determines how uniformly bubbles form. Wear or clogging will increase bubble size.
The Numbers You Need to Get Right When Selecting a DAF
The first number is recycle ratio. Dissolved air flotation typically uses partial recycle, with a recycle ratio between 20% and 40%. Too low, and there isn't enough dissolved air water, so bubble quantity is insufficient. Too high, and the hydraulic load on the DAF tank increases, cutting retention time. For oily wastewater with a high degree of emulsification, go with a higher recycle ratio. For suspended solids, a lower one works.
The second number is surface loading rate. DAF surface loading generally runs between 4 and 8 m³/(m²·h). Too high, and bubbles and flocs don't have time to rise before being carried out with the effluent. Too low, and the tank is oversized, wasting capital. For oily wastewater, use the lower end. For suspended solids, the higher end is fine.
The third number is chemical dosing. The dose and injection point for PAC and PAM are critical. PAC goes in first to compress the electrical double layer and destabilize fine particles. PAM goes in after to bridge flocs into larger agglomerates. Too much PAM, and the floc surfaces get coated with polymer, so bubbles can't attach. Too little, and the flocs are too small for bubbles to carry. If the injection point is too close to the DAF tank, flocculation isn't complete. Too far, and the flocs get sheared in the piping.
The fourth number is skimmer speed. Too fast, and the float layer gets disturbed, and some float sinks back into the water. Too slow, and the float stays on the surface too long, becoming compacted and hard to scrape. Typical speed is 0.5 to 1.0 m/min.
Where DAF Works and Where It Doesn't
Oily wastewater is the classic DAF application. Machining wastewater, steel cold-rolling wastewater, food processing wastewater—these contain oil in free and emulsified forms. DAF works well on free oil. Emulsified oil needs demulsification first.
Wastewater with high suspended solids also fits. Paper mill white water, dyeing wastewater, coal washing wastewater—DAF can quickly remove a large amount of suspended solids and reduce the load on downstream biological treatment.
For water with algae, DAF beats settling. Algae have low density and don't settle well. DAF actually floats them up more easily.
But there are cases where DAF has limited effect. Wastewater dominated by dissolved organics—DAF is basically useless. Those aren't things bubbles can carry. Wastewater that's heavily emulsified and hasn't been demulsified—oil droplets are wrapped in surfactants, and bubbles can't attach. You need a demulsifier first. And when flow rates swing widely, DAF is sensitive. A sudden increase in flow raises the surface loading rate, and effluent quality can easily