A method for treating a purge stream derived from an anhydrous sodium carbonate crystallizer, or a sodium carbonate monohydrate crystallizer, or a sodium carbonate decahydrate crystallizer, or a sodium sesquicarbonate crystallizer, or a wegsheiderite crystallizer, or a sodium bicarbonate crystallizer, said purge stream comprising sodium carbonate (Na 2 CO 3) and/or sodium bicarbonate (NaHCO 3) in a quantity of at least 7 % total alkalinity expressed as equivalent weight Na 2 CO 3, and at least 1 % by weight of a sodium salt selected among sodium chloride, sodium sulfate and mixtures thereof, the method comprising the following steps: - f) causticizing at least 50 mol. % of the sodium from sodium carbonate and/or sodium bicarbonate, into an aqueous sodium hydroxide solution and into a calcium carbonate mud by reaction of the purge stream by adding lime, in presence of water; - g) separating the calcium carbonate mud from the aqueous sodium hydroxide solution; - h) concentrating the aqueous sodium hydroxide solution by removing part of the water in order to obtain: a concentrated aqueous sodium hydroxide solution comprising at least 25 % NaOH, and a crystallized solid comprising sodium carbonate and comprising sodium chloride and/or sulfate, wherein lime added at step f) and water removed at step h) are controlled so that the weight ratio of sodium carbonate to the sum of the sodium chloride and/or sodium sulfate in crystallized solid is at most 2, - i) separating the crystallized solid comprising sodium carbonate and sodium chloride and/or sulfate from the concentrated aqueous sodium hydroxide solution, said crystallized solid to be disposed of or to be further valorized, - j) preferably recycling at least one part of the concentrated aqueous sodium hydroxide solution to the anhydrous sodium carbonate crystallizer, or to the sodium carbonate monohydrate crystallizer, or to the sodium carbonate decahydrate crystallizer, or to the sodium sesquicarbonate crystallizer, or to the bicarbonate crystallizer, or to processes upstream of the anhydrous sodium carbonate crystallizer, or of the sodium carbonate monohydrate crystallizer, or of the sodium carbonate decahydrate crystallizer, or of the sodium sesquicarbonate crystallizer, or of the bicarbonate crystallizer. - f) causticizing at least 50 mol. % of the sodium from sodium carbonate and/or sodium bicarbonate, into an aqueous sodium hydroxide solution and into a calcium carbonate mud by reaction of the purge stream by adding lime, in presence of water; - g) separating the calcium carbonate mud from the aqueous sodium hydroxide solution; - h) concentrating the aqueous sodium hydroxide solution by removing part of the water in order to obtain: a concentrated aqueous sodium hydroxide solution comprising at least 25 % NaOH, and a crystallized solid comprising sodium carbonate and comprising sodium chloride and/or sulfate, wherein lime added at step f) and water removed at step h) are controlled so that the weight ratio of sodium carbonate to the sum of the sodium chloride and/or sodium sulfate in crystallized solid is at most 2, a concentrated aqueous sodium hydroxide solution comprising at least 25 % NaOH, and a crystallized solid comprising sodium carbonate and comprising sodium chloride and/or sulfate, wherein lime added at step f) and water removed at step h) are controlled so that the weight ratio of sodium carbonate to the sum of the sodium chloride and/or sodium sulfate in crystallized solid is at most 2, - i) separating the crystallized solid comprising sodium carbonate and sodium chloride and/or sulfate from the concentrated aqueous sodium hydroxide solution, said crystallized solid to be disposed of or to be further valorized, - j) preferably recycling at least one part of the concentrated aqueous sodium hydroxide solution to the anhydrous sodium carbonate crystallizer, or to the sodium carbonate monohydrate crystallizer, or to the sodium carbonate decahydrate crystallizer, or to the sodium sesquicarbonate crystallizer, or to the bicarbonate crystallizer, or to processes upstream of the anhydrous sodium carbonate crystallizer, or of the sodium carbonate monohydrate crystallizer, or of the sodium carbonate decahydrate crystallizer, or of the sodium sesquicarbonate crystallizer, or of the bicarbonate crystallizer.