A desupersaturation unit (DU), placed between the stages of either RO or NF systems is aimed at reducing the degree of supersaturation in the concentrate stream thus preventing membrane scaling in the last stage of a membrane filtration system while allowing conversion to be maximized up to design limits (>90%). Barium sulfate desupersaturation in an up-flow fixed bed reactor was demonstrated for ca. 200 h (Concentrate A). However, barium removal in the DU was shown to be source dependent and early break-through of the DU occurred with Concentrate B after ca. 30 h. Calculations showed that barium sulfate deposition in the DU was affected by natural organic matter (NOM). Blocking of seed crystal surface by rapid adsorption of certain NOM fractions hindered crystal growth and was believed to be the main reason for break-through in the DU. Recycling of these naturally occurring organic molecules as 'natural antiscalants' to maximize the conversion of RO (NF) systems may be possible if they can be isolated and characterized.