A system and method are provided for producing from a first fluid having particles of a substance suspended or dissolved therein with a first concentration of the substance, a second fluid having a second concentration of the substance lower than the first concentration and a separated-fluid product, in which concentration of the substance is greater than that in the first fluid. The system comprises a pre-treatment module for processing the first fluid to produce aggregates from the substance and at least one separation duct in fluid communication with an outlet of the pre-treatment module, the duct having at least one bay portion having such design as to cause aggregates to accumulate along a pre-determined wall of the bay portion, thereby facilitating separation between the second fluid and the separated-fluid product.