A method for purifying a target substance from a fluid to be treated comprising at least one impurity, in an installation comprising: - a first separation unit comprising at least a chromatography column, a fluid inlet and a fluid outlet; - a second separation unit comprising at least a chromatography column, a fluid inlet, a fluid outlet and a bypass line linked to at least a fluid inlet of the second separation unit at one of its ends and to at least a fluid outlet of the second separation unit at the other of its ends and bypassing the at least one chromatography column of the second separation unit; - a first reservoir in fluid connection with a fluid outlet of the first separation unit and a fluid inlet of the second separation unit; and - a second reservoir in fluid connection with at least a fluid inlet and a fluid outlet of the second separation unit; the method comprising the following steps: - providing a flow of fluid to be treated; - treating the flow of fluid to be treated by means of a chromatography step in the first separation unit; - collecting a fraction enriched in the target substance in the first reservoir; - the viral inactivation of the fraction enriched in the target substance, said viral inactivation comprising: ▪ passing the fraction enriched in the target substance collected in the first reservoir through the second separation unit, via the bypass line; ▪ passing a viral inactivation solution through the second separation unit, via the bypass line; ▪ mixing the fraction enriched in the target substance and the viral inactivation solution; and ▪ collecting the mixture of the fraction enriched in the target substance and the viral inactivation solution in the second reservoir; so as to obtain a fraction depleted of active virus; - treating the fraction depleted of active virus by means of a chromatography step in the second separation unit; and - collecting a fraction more enriched in the target substance at the outlet of the second separation unit. - a first separation unit comprising at least a chromatography column, a fluid inlet and a fluid outlet; - a second separation unit comprising at least a chromatography column, a fluid inlet, a fluid outlet and a bypass line linked to at least a fluid inlet of the second separation unit at one of its ends and to at least a fluid outlet of the second separation unit at the other of its ends and bypassing the at least one chromatography column of the second separation unit; - a first reservoir in fluid connection with a fluid outlet of the first separation unit and a fluid inlet of the second separation unit; and - a second reservoir in fluid connection with at least a fluid inlet and a fluid outlet of the second separation unit; the method comprising the following steps: - providing a flow of fluid to be treated; - treating the flow of fluid to be treated by means of a chromatography step in the first separation unit; - collecting a fraction enriched in the target substance in the first reservoir; - the viral inactivation of the fraction enriched in the target substance, said viral inactivation comprising: ▪ passing the fraction enriched in the target substance collected in the first reservoir through the second separation unit, via the bypass line; ▪ passing a viral inactivation solution through the second separation unit, via the bypass line; ▪ mixing the fraction enriched in the target substance and the viral inactivation solution; and ▪ collecting the mixture of the fraction enriched in the target substance and the viral inactivation solution in the second reservoir; - providing a flow of fluid to be treated; - treating the flow of fluid to be treated by means of a chromatography step in the first separation unit; - collecting a fraction enriched in the target substance in the first reservoir; - the viral inactivation of the fraction enriched in the target substance, said viral inactivation comprising: ▪ passing the fraction enriched in the target substance collected in the first reservoir through the second separation unit, via the bypass line; ▪ passing a viral inactivation solution through the second separation unit, via the bypass line; ▪ mixing the fraction enriched in the target substance and the viral inactivation solution; and ▪ collecting the mixture of the fraction enriched in the target substance and the viral inactivation solution in the second reservoir; ▪ passing the fraction enriched in the target substance collected in the first reservoir through the second separation unit, via the bypass line; ▪ passing a viral inactivation solution through the second separation unit, via the bypass line; ▪ mixing the fraction enriched in the target substance and the viral inactivation solution; and ▪ collecting the mixture of the fraction enriched in the target substance and the viral inactivation solution in the second reservoir; so as to obtain a fraction depleted of active virus; - treating the fraction depleted of active virus by means of a chromatography step in the second separation unit; and - collecting a fraction more enriched in the target substance at the outlet of the second separation unit. - treating the fraction depleted of active virus by means of a chromatography step in the second separation unit; and - collecting a fraction more enriched in the target substance at the outlet of the second separation unit.