Air flotation has been used for many years in the beneficiation of ores. Its first application in the wastewater-treatment field was in the flotation of suspended solids (SS), fibers, and other low-density solids. Flotation also is used for the thickening of activated sludge and flocculated chemical sludges. More recently, air flotation has been applied to the removal of oils and greases from wastewater because it is a practical, reliable, and efficient treatment
process.
Air flotation is widely used to treat wastes from a wide variety of sources: paper making, refineries, ship’s bilge and ballast waste, deinking operations, metal plating, meat processing, laundries, iron and steel plants, soap manufacturing, chemical processing and manufacturing plants, barrel and drum cleaning, washrack and equipment maintenance, glass plants, soybean processing, mill waste, and aluminum forming.

The process of flotation consists of four basic steps:
1. Bubble generation in the wastewater
2. Contact between the gas bubble and the particle or oil droplet suspended in the water
3. Attachment of the particle or oil droplet to the gas bubble
4. Rise of the air/solids combination to the surface where the floated material are skimmed off

The overall objectives of the biological treatment of wastewater are to:

1. Transform dissolved and particulate biodegradable constituents into acceptable end products,
2. Capture and incorporate suspended and nonsettleable colloidal solids into a biological floc or biofilm,
3. Transform or remove nutrients, such as nitrogen and phosphorus,
4. In some cases, remove specific trace organic constituents and compounds. For industrial wastewater, the objective is to remove or reduce the concentration of organic and inorganic compounds. Because some of the constituents and compounds found in industrial wastewater are toxic to microorganisms, pretreatment may be required before the industrial wastewater can be discharged to a municipal collection system. For agricultural irrigation return wastewater, the objective is to remove nutrients, specifically nitrogen and phosphorus, that are capable of stimulating the growth of aquatic plants.

Chemical processes, in conjunction with various physical operations, have been developed for the complete secondary treatment of untreated wastewater, including the removal of either nitrogen or phosphorus or both. Chemical processes have also been developed to remove phosphorus by chemical precipitation, and are designed t be used in conjunction with biological treatment. Other chemical processes have been developed for the removal of heavy metals and for specific organic compounds and for the advanced treatment of wastewater. Currently the most important applications of chemical unit processes in wastewater treatment are for:

1. The disinfection of wastewater.
2. The precipitation of phosphorus.
3. The coagulation of particulate matter found in wastewater at various stages in the treatment process.

Gravity separation of solids from liquid, producing a clarified overflow and a thickened solids underflow, has long been used in the wastewater treatment industry. Often, the terms clarification and thickening or sedimentation are used to describe gravity separation unit operations, depending on if the process focus, or objective, is on the clarified liquid or the thickened solids, respectively.

The process objective of primary clarifiers is to remove settleable TSS, whether these solids already exist in the raw wastewater or if they are precipitated solids generated as a result of chemical addition for enhanced suspended solids, phosphorus, or heavy metal removal.

The function of a final clarifier is clarification, which is a solids-separation process that results in the removal of biological floc from the liquid stream. During the subsequent thickening process, sludge particles are conveyed to the bottom of the tank, resulting in a concentrated underflow (RAS).

Filtration involves the separation of particulate and colloidal matter from a liquid. In membrane filtration the range of particle sizes is extended to include dissolved constituents. The role of the membrane is to serve as a selective barrier that will allow the passage of certain constituents and will retain other constituents found in the liquid.

Membrane processes include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), dialysis, and electrodialysis (ED).

Granular filtration involves the removal of particulate material suspended in a liquid by passing the liquid through a filter bed comprised of a granular or compressible filter medium. Although granular filtration is one of the principal unit operation used in the treatment of potable water, the filtration of effluents from wastewater treatment processes is becoming more common. Granular filtration is now used to achieve supplemental removals of suspended solids from wastewater effluents of biological and chemical treatment processes to reduce the mass discharge of solids and, perhaps more importantly, as a conditioning step that will allow for the effective disinfection of the filtered effluent. Granular filtration is also used as a pretreatment step for membrane filtration.