In nanofiltration and reverse osmosis membrane systems 75-90% of the feed water is conversed to product, and as a consequence dissolved salts are concentrated in the membrane concentrate. Subsequently, sparingly soluble inorganic compounds such as calcium carbonate and barium sulphate become supersaturated and may precipitate in a membrane element (scaling). Scaling leads to an increase in energy consumption and chemical cleaning frequency. From an economic and environmental point of view this is not desired and early detection of scaling is therefore necessary. With the aim to detect scaling in an early stage a scaling monitor has been developed and tested. The scaling monitor, named ScaleGuard(R), measures the normalised flux of a small membrane element fed by a part of the concentrate of the full-scale plant. This scaling monitor was tested at a pilot plant treating anaerobic groundwater for the production of drinking water. The monitor demonstrated to be able to detect scaling before it took place in the last stage of the pilot plant.