A method for treating a purge stream derived from a sodium carbonate, sesquicarbonate, wegsheiderite, or bicarbonate crystallizer, said purge stream comprising sodium carbonate and/or sodium bicarbonate and at least 1 % by weight of sodium chloride and/or sodium sulfate, the method comprising : - causticizing at least 50 mol. % of the sodium from sodium carbonate and/or sodium bicarbonate into a caustic solution and into a calcium carbonate mud with lime and water, - separating the calcium carbonate mud from the caustic solution; - concentrating the caustic solution by removing part of the water to obtain a concentrated caustic solution comprising at least 25 % NaOH, and a crystallized solid comprising sodium carbonate and sodium chloride and/or sulfate, - separating the crystallized solid from the concentrated caustic solution, said crystallized solid to be disposed of or to be further valorized, - recycling part of the concentrated caustic solution to the sodium carbonate, sesquicarbonate, wegsheiderite, or bicarbonate crystallizer.