Wastewater Treatment Terms Explained: Air Flotation Unit
An air flotation unit is a machine that uses small bubbles or microbubbles to bring impurities in the medium up to the water surface. It can be applied to fine particles in water whose specific gravity is close to that of water and which can hardly sink or float on their own weight.
Air flotation water purification is a solid-liquid separation technology with a long history, originating in mineral processing. Its application in the water treatment field dates back to the 1860s; shallow air flotation technology began to be applied in the late 1940s; and after pressurized dissolved air flotation was born in the 1950s, the technology entered a stage of rapid development. [10]
In water supply and drainage, except for raw water with a high sand content and wastewater carrying heavy mechanical impurities, most suspended matter in pretreated water consists of light particles with a specific gravity very close to that of water. Examples include algae in lakes, reservoirs and some rivers; plant residues and fine colloidal impurities; dye particles from the printing and dyeing industry; short fibers from the papermaking and chemical fiber industries; petroleum and organic solvent microdroplets from the refining and chemical industries; heavy metal ions in electroplating and pickling wastewater; electrophoretic paint wastewater; and so on. For such raw water, if traditional sedimentation is still used, the results will inevitably be poor, especially under low-temperature winter conditions, when deteriorated coagulation and hydraulic conditions make treatment performance even harder to guarantee. It is easy to imagine that forcing hard-to-settle flocs to sink yields half the result with twice the effort; it would be better to follow the trend and deliberately introduce bubbles into the water so that they adhere to the flocs, thereby greatly reducing the overall density of the flocs and, taking advantage of the rising speed of the bubbles, forcing them to float upward, thus achieving rapid solid-liquid separation. In this sense, the advent of air flotation technology was a revolution in gravity sedimentation, opening up a new field of solid-liquid separation technology.
An air flotation unit is water treatment equipment in which a dissolved air system produces a large number of fine bubbles in the water, so that air attaches to suspended particles in the form of highly dispersed tiny bubbles, creating a state with a density lower than that of water; the buoyancy principle then floats them to the surface, achieving solid-liquid separation [2]. The air flotation purification process includes the collision, under certain hydraulic conditions, between hydrophobic impurities in the water and hydrophobic microbubbles dispersed in the water, which adhere together through intermolecular van der Waals forces; in addition, it also includes the process in which larger hydrophilic flocs in the water capture tiny bubbles through net capture, bridging and sweeping actions, and are removed by floating to the surface under buoyancy [3].
Depending on how bubbles are generated, air flotation technology can be mainly divided into electrolytic coagulation flotation, dispersed air flotation and dissolved air flotation. Among them, partial pressurization dissolved air flotation is the most commonly used air flotation method both in China and abroad [3].
Pressurized dissolved air flotation equipment dissolves air into the water to be treated under a certain pressure, generally 0.2-0.6 MPa, then suddenly reduces the pressure to atmospheric; the air dissolved in the water escapes in the form of microbubbles (generally 20-100 um in diameter), adheres to the suspended solids in the water and floats with them to the surface to form scum, which is scraped by a skimmer into a scum tank for removal; the clarified water flows out from the lower part of the flotation tank, achieving solid-liquid separation [3].
Shallow air flotation equipment is high-efficiency flotation equipment successfully designed using the "shallow pool theory" and the "zero velocity principle" [2-3]. The "zero velocity principle" keeps the influent from disturbing the flow in the tank, providing a static environment for solid-liquid separation. Shallow air flotation equipment has a short hydraulic retention time of only 3-5 minutes [3].
The structure of air flotation units varies with their type, but mainstream equipment generally follows a certain division of functional modules. Taking the most widely used pressurized dissolved air flotation unit as an example, its structure mainly consists of three parts: the dissolved air system, the air release system and the separation system [3].
A typical dissolved air flotation unit specifically includes the tank body, flocculation zone, flotation zone, collection zone, mixing device and scum scraping drive assembly. The flocculation zone, flotation zone and collection zone are arranged in sequence along the working direction inside the tank; the mixing device is used to thoroughly mix chemicals with wastewater in the flocculation zone; and the drive assembly drives the scraper to push the scum formed on the surface of the flotation zone into the collection zone [4-5].
Depending on the type and nature of the suspended solids contained in the wastewater and the degree of treatment required, dissolved air flotation can be divided into three flow schemes: full pressurization dissolved air flotation, partial pressurization dissolved air flotation, and partial recycle pressurized dissolved air flotation [3].
Other types of air flotation units each have their own structural features: the shallow air flotation unit is designed with the "shallow pool theory" and the "zero velocity principle" [2-3], with a shallower tank depth, short hydraulic retention time [3] and compact structure [9]; the cavitation air flotation unit mainly generates microbubbles through high-speed rotation of an aerator [2], and its structure includes an aeration section, a flotation section and a scum scraping system [3].
The cavitation air flotation unit mainly relies on the high-speed rotating aeration impeller of the aerator to quickly disperse gas in the liquid, achieving the flotation effect. The aeration impeller rotates at 2,900 revolutions per minute, repeatedly shearing and dispersing the drawn-in gas until tiny bubbles are finally formed, producing the flotation effect.
The dissolved air flotation unit mainly works by pressurizing and dissolving gas in water to form a supersaturated solution, then suddenly decompressing it through a release device so that the dissolved gas is released in the form of microbubbles in the flotation tank, achieving the flotation effect.
The dispersed air flotation unit mainly achieves the flotation effect by shredding bubbles through a disperser.
The super-efficient shallow air flotation device is an advanced flotation system designed using the "shallow pool theory" and the "zero velocity principle". Integrating coagulation, flotation, scum skimming, sedimentation and sludge scraping in one unit, it is a highly efficient and energy-saving water purification device. Its retention time is only 3-5 minutes, with forced water distribution and static inlet and outlet. The adhesion between microbubbles and flocs occurs throughout the entire flotation separation process including the contact zone. Scum is discharged instantly, disturbance to the water body is small, effluent suspended solids are low and the scum solids content is high; the suspended solids removal rate can reach over 90%-99.5%, the COD removal rate can reach 65%-90%, and the color removal rate can reach 70%-95%.
Air flotation is specifically applied to oil-water separation of oily wastewater in the petroleum, chemical and machinery manufacturing industries; industrial wastewater treatment and the recovery of useful materials such as papermaking fibers; it is especially suitable as a replacement for secondary sedimentation tanks in processes prone to activated sludge bulking, for thickening waste activated sludge, and for removing algae and micro-flocs from water. [3] Shallow air flotation units or oil-removal-dedicated flotation units are particularly suitable for treating oily or high-suspended-solids wastewater such as food processing, machining cutting fluids, the downstream stage of restaurant grease interception, and petrochemical pretreatment. [9]
As a solid-liquid separation device, the air flotation unit features low investment, small footprint, high automation, and convenient operation and management [2] [6].
Key performance indicators of air flotation units include: suspended solids removal up to over 90%-99.5%, with SS removal stable above 95% [1-2] [6]; COD removal can reach 65%-90%; color removal can reach 70%-95% [2]; and the removal rate of animal and vegetable oils from oily wastewater is generally between 85% and 95% [9].
In terms of typical operating parameters, shallow air flotation units achieve a surface loading of 8-20 m3/(m2*h), a dissolved air pressure of 0.3-0.6 MPa, a hydraulic retention time of only 3-5 minutes, and a treatment capacity usually of 5-500 m3/h [2-3] [9].
High-quality air flotation units can last 8-10 years, and stainless steel units more than 15 years, with low daily maintenance costs [1] [9]. Choosing an efficient dissolved air system and energy-saving pumps can reduce energy consumption; through optimization and retrofit, dissolved air efficiency can be significantly improved and the recycle pump power can be reduced by about 50% [1] [7].
In terms of technical progress, magnetic loading air flotation technology can increase the settling velocity by 30% and save 20% of the equipment footprint [6].
Air flotation units are widely used in water supply, industrial wastewater and municipal sewage treatment. [2] Their application fields cover municipal sewage and industrial wastewater such as oily wastewater, printing and dyeing wastewater, papermaking white water, and wastewater treatment in the chemical, machinery manufacturing, tanning, fiberglass, dairy, automotive painting, papermaking and refining industries, as well as aquaculture wastewater and landfill leachate treatment. [1] [3] [6] [9]
In air flotation theory, besides the bubble adhesion, bubble buoyant support and floc adhesion mechanisms, there is also a "bubble entrainment" effect: part of the fine suspended matter not adsorbed by bubbles or flocs may, during the rise of dense bubble swarms, be carried in the gaps between bubbles and entrained to the water surface for separation, because the particle size of fine suspended matter is still far larger than that of water molecules. The denser the bubble swarm, the stronger this effect, and the finer the suspended matter that can be entrained.
With increasingly strict environmental policies and rising environmental awareness among enterprises, market demand for air flotation equipment continues to grow, and industry competition is becoming increasingly fierce [1].
According to the "2025-2030 China Air Flotation Equipment Industry Market Analysis and Development Prospect Forecast Report" released by the China Industry Research Institute, the market size of China's air flotation equipment reached RMB 38.2 billion in 2024 and is expected to exceed RMB 120 billion by 2030 [11]. The "2025 China Water Treatment Equipment Industry White Paper" points out that the market size of air flotation equipment maintains steady growth with a compound annual growth rate of about 18.7%, with subdivisions such as leachate flotation and waste transfer station flotation showing outstanding growth rates [6] [12].
The industry currently shows three major trends: precision, intelligence and integrated services, with customers' requirements for equipment purification efficiency, intelligence level and full-life-cycle services continuously rising [6]. Industry technology keeps advancing; for example, a research team improved dissolved air efficiency by more than 2 times and cut recycle pump power by about 50% by optimizing air flotation tank components [7]; and a number of enterprises have obtained utility model patents involving improved skimming efficiency and multi-equipment linkage [4-5]. The air flotation unit industry will develop in the direction of greater intelligence, energy efficiency and integration [6].
