Oily wastewater mainly comes from petroleum, petrochemical, steel, coking, gas generation station, mechanical processing and other industrial sectors. The relative density of oil pollutants in wastewater is less than 1, except that of heavy tar is above 1.1. Oil substances usually exist in three states in wastewater.

      (1) Floating on the oil, the particle size of the oil drop is more than 100 μ m, which is easy to be separated from the wastewater.

      (2) Dispersed oil, oil droplet size between 10-100 μ m, floating in water.

      (3) Emulsified oil, the particle size of oil drop is less than 10 μ m, which is not easy to separate from wastewater.

       Due to the great difference of oil concentration in the waste water discharged by different industrial departments, for example, the oil content in the waste water produced in the refining process is about 150-1000mg / L, the tar content in the coking waste water is about 500-800mg / L, and the tar content in the waste water discharged from the gas generation station is up to 2000-3000mg / L. Therefore, the treatment of oily wastewater should first use the oil separator to recover the floating oil or heavy oil. The treatment efficiency is 60% - 80%, and the oil content in the effluent is about 100-200mg / L. the emulsified oil and dispersed oil in the wastewater are difficult to treat, so the emulsification should be prevented or reduced. One of the methods is to reduce the emulsification of the oil in the wastewater during the production process; the other is to reduce the number of times of lifting the wastewater with the pump as much as possible during the treatment process so as not to increase the emulsification degree. Air flotation and demulsification are usually used for treatment