The treatment of refinery wastes is carried out in three steps:

1)Physical separation of free oil.

2)Chemical coagulation of emulsified oil.

3)Biological treatment for the removal of BOD, phenol and other toxic materials.

• The physical separation of oil is accomplished in gravity separators.
• These are essentially rectangular tanks with the arrangements for oil skimming and sludge scraping.
• The design of gravity separator is based on the principles of sedimentation.
• Dissolved air flotation with or without flocculation is also tried to remove the oil content of refinery wastes.
• Various coagulating agents like ferrous sulphate, ferric sulphate, chlorinated copperas, calcium chloride have been tried to de-emulsify an oil separator effluent.
• The chemical treatment not only produces effluent of low oil content but reduces the sulphides content considerably.
• Biological treatment includes waste stabilization pond, aerated lagoon, trickling filter and activated sludge process.
• As biological flocs formed in the above process are adsorbed with some quantity of oil, the final settling tank following the aeration tank must be designed with a low surface settling rate and larger surface area.