Method for treating alkaline waste water

A method of treating alkaline waste water originating from washing spent oil with water, said alkaline waste water also containing organic material, the method comprising the following sequence of steps:

• a) bringing the alkaline waste water into contact, at a temperature in the range about 60° C. to 105° C. and for a period in the range about 5 to 60 minutes, with a mineral acid in a reaction wherein said mineral acid is present in a quantity 1% to 20% greater than a stoichiometric amount, transforming any alkali atoms present in the alkaline waste water to be treated into a mineral salt:
• b) allowing organic and aqueous phases obtained to settle at a temperature in the range about 60° C. to 105° C., to recover an acidic aqueous phase;
• c) forming a reaction medium by bringing the acidic aqueous phase into contact, at a temperature in the range about 40° C. to 95° C., with an alkaline solution in a quantity sufficient to obtain a pH in the range 7.5 to 9 in the reaction medium, then allowing the reaction medium to stand at a temperature in the range about 40° C. to about 95° C. to allow a flocculate to form and then settle out, and filtering to recover an aqueous phase.