Oil substances usually exist in three states in wastewater.

(1) Floating oil with droplet size greater than 100 μ m, easy to separate from wastewater. The dispersed particles of oil in wastewater are relatively large, with a particle size greater than 100 microns, making them easy to separate from the wastewater. In petroleum wastewater, this oil accounts for 60-80% of the total oil content in the water.

(2) Dispersed oil. The particle size of oil droplets ranges from 10 to 100 μ m and is suspended in water.

(3) Emulsified oil, with droplet size less than 10 μ m, has a very small dispersed particle size in wastewater and is in an emulsified state, making it difficult to separate from the wastewater.

(4) Dissolved oil, the state in which oil dissolves in water.

The oily substances contained in oily wastewater include natural petroleum, petroleum products, tar and its fractions, as well as edible animal and vegetable oils and fats.

The oily wastewater, after being treated with chemicals, enters the mixing zone of air flotation and comes into contact with the released dissolved air water, causing the flocs to adhere to fine bubbles before entering the air flotation zone. Under the action of buoyancy, flocs float towards the water surface to form floating scum. The clear water in the lower layer flows through the collector to the clear water tank, and a part of it is refluxed for use as dissolved water. The remaining clear water flows out through the overflow port. After the floating slag on the surface of the air flotation tank accumulates to a certain thickness, it is scraped into the sludge tank of the air flotation machine by a scraper and discharged.