Oily substances usually exist in three states in wastewater.
■The oil floats, and the particle size of the oil droplets is greater than 100 μm, which is easy to separate from the wastewater.
■ Dispersed oil. The particle size of the oil droplets is between 10 and 100 μm, and they float in water.
■ Emulsified oil. The particle size of the oil droplets is less than 10μm, which is not easy to separate from the wastewater. Because the oil concentration in wastewater discharged from different industrial sectors is very different, such as wastewater generated in the oil refining process, the oil content is about 150-1000mg/L, the tar content in the coking wastewater is about 500-800mg/L, and the wastewater discharged from the gas generating station The tar content can reach 2000-3000mg/L.
Therefore, the treatment of oily wastewater should first use an oil trap to recover slick or heavy oil. The treatment efficiency is 60% to 80%, and the oil content in the effluent is about 100 to 200 mg/L. Emulsified oil and dispersed oil in wastewater are difficult Therefore, the emulsification phenomenon 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 second is to minimize the number of pumps to lift the wastewater during the treatment process to avoid increasing the degree of emulsification. The treatment method usually adopts air flotation method and demulsification method.