No Pressure Vessel, No Compressor: What Cavitation Air Flotation Changes on Site

Most flotation discussions start with the dissolved-air vessel, because for decades that vessel was unavoidable. Cavitation air flotation (CAF) removes it. The CAF system is a patented device, listed as an export-recommended technology by the U.S. Department of Commerce and the Environmental Protection Agency, and it exists to pull grease, colloids and suspended solids (SS) out of industrial and municipal wastewater without the pressurised side of the process. Five sections make up the whole installation: the aeration zone, the flotation tank, the sludge scraper and screw, the overflow arrangement and the recirculation pipe.

How the bubbles get in without a saturator

Pre-treated wastewater enters a small aeration section fitted with a cavitation aerator. The aerator is deceptively simple: a bottom disperser impeller spins fast enough to create a vacuum zone in the water, and an air-intake duct feeds air from above the surface down into that vacuum to fill it. Microbubbles form, spiral upward, and dissolve oxygen as they rise. Nothing is pressurised, nothing is pre-saturated. Because the impeller never stops making bubbles, there is nothing to clog.

What happens once the bubbles are in the tank

The gas-water mixture is lighter than the surrounding liquid, so a vertical buoyancy force develops and carries SS to the surface. Bubbles attach to the particles on the way up and then hold them there. A chain scraper travels the full liquid surface on an intermittent cycle, pushing the floating solids from the inlet end toward a sludge-discharge pipe at the outlet end, where a horizontal screw conveyor moves them into a collection pit. Scraper and conveyor run off the same motor, rated at 0.5 hp. Clarified water passes under an inclined plate into the overflow trough, which sets the tank water level and keeps liquid out of the sludge pipe, then flows by gravity to biological treatment.

The open recirculation pipe runs from the aeration section along the tank bottom. While the aerator is making bubbles it also creates a negative-pressure zone at the bottom where that pipe sits, drawing wastewater back from the tank floor into the aeration zone and then into the flotation section again. Roughly 40% of the flow recirculates this way, which is why the flotation section keeps working even when influent stops.

The numbers that decide the argument

The tank itself is shallow by flotation standards, and that is deliberate. Working water depth no greater than 2.0 m keeps the bubble rise path short, so a droplet does not have to travel far before it reaches the surface layer, and the length-to-width ratio of no less than 4 keeps the flow from short-circuiting toward the outlet. Shallow tanks also mean lower civil cost, which combined with the missing compressor room is where most of the claimed investment saving actually comes from.

Two comparisons sell the technology. A CAF unit with Q=200 m3/h occupies 36.15 m2 and draws 5.435 kW; a dissolved-air flotation (DAF) system of the same capacity draws 65 kW, because it has to run a pressure vessel, an air compressor and a circulation pump. Energy use lands somewhere around one-eighth to one-tenth of conventional flotation. Against DAF and IAF on a 2000 t/d basis with no chemical feed, the picture holds: retention time 15-20 min, surface loading 5-10 m3/(m2 h), working water depth in the tank no greater than 2.0 m, and a tank length-to-width ratio no less than 4.

Why the absence of a saturator matters operationally

Removing the pressure vessel, the air compressor and the circulation pump does more than cut the capital line. Start-up stops being a procedure: there is no saturator to vent, no compressor to load and unload, and no recycle pump to prime. Turndown is forgiving, because bubble generation follows impeller speed rather than a saturation pressure that has to be re-established every time the flow changes. Operators who have spent nights chasing a lost air supply on a DAF will recognise the value immediately. The trade-off is that bubble size distribution is a function of impeller condition and submergence, so wear on the disperser shows up as performance drift rather than as an alarm, and it belongs on the preventive-maintenance list.

Performance and where it fits

Oil and suspended solids removal runs above 80%. BOD and COD removal reaches over 60%, where a DAF system typically manages about 35%. Sulphides get oxidised, so the sulphur content of the wastewater drops, and colour improves visibly. The unit handles both floatable solids, with specific gravity below water, and settleable ones, with specific gravity above water, and it removes most of the non-dissolvable COD. Mechanically there are only two moving parts, which is why labour and maintenance workloads stay small and why slag discharge can be automated and quiet.

Application sits mainly in papermaking, chemicals, dyeing and printing, food, breweries, pharmaceuticals and slaughtering. Because CAF pairs mechanical aeration with chemical flotation, it works as a physico-chemical stage ahead of biology; on lightly loaded streams the clarified water can be reused or discharged to standard once biological treatment is added. The honest caveat is that recirculation is driven by the aerator rather than metered, so carrier distribution and dead-zone behaviour still need watching on site, the same issue that later produced the Moving Bed Sequencing Batch Biofilm Reactor (MBSBBR) thinking in biofilm work.