ScaleGuard® is a continuous on-line monitor that detects scaling at an early stage when fed with the concentrate of a pilot- or full-scale plant. Scaling is observed in the ScaleGuard before it occurs in the full-scale installation. The ScaleGuard is used to optimize the recovery of two full-scale plants: one reverse osmosis plant treating pretreated surface water and one nanofiltration plant treating anaerobic groundwater. Based on the results of the ScaleGuard the conversion of both the reverse osmosis plant and the nanofiltration plant has been increased from 80% to 82%. The interpretation of the results of the ScaleGuard which was connected with the reverse osmosis plant was not unambiguous. This was due to the mass transfer coefficient decline in the ScaleGuard which could not be attributed to scaling. Results in the nanofiltration plant did not give room for any doubt. In addition, safe supersaturation ratios were derived from experience in pilot- and full-scale plants using anti-scalants for seven sparingly soluble inorganic compounds.