*2.2. Reverse Osmosis Membrane System*

Gypsum scaling experiments were carried out using an automatically controlled laboratory-scale cross-flow RO spiral wound module RE 1812 tailored with thin film composite BLN membrane (Figure 2). The test unit was operated in circulation mode whereby concentrate after membrane module was returned back to feed water tank. The feed water was added to feed water tank 1. The volume of tank 1 was 5 L. Feed water from tank 1 was supplied by small gear pump 2 to membrane module 3. In all experiments the commercially available spiral-wound filter elements (CSM RE1812-80 GPD) made of polyamide and manufactured by CSM (Seoul, South Korea) were used. These operated at constant feed flow rate of 72.0 <sup>±</sup> 0.2 dm3/h, permeate flow rate of 6.0–6.3 dm3/hour, constant temperature of 25.0 ± 2 ◦C; constant pressure 7.0± 0.2 bar in concentration mode.

**Figure 2.** Schematic diagram of laboratory RO unit for membrane scaling tests: 1-feed water tank; 2-pump; 3-spiral wound membrane module; 4-permeate tank; 5-heat exchanger; 6-pressure gauge; 7-feed water flow meter; 8-permeate flow meter; 9-concentrate flow meter; 10-by-pass adjusting valve; 11-feed water adjusting valve; 12-concentrate adjusting valve; 13-cooling water adjusting valve; 14-sampler.

Stabilized salt rejection, at a constant pressure of 7 bar, a solution temperature of 25 ◦C, and a pH value of 6.5–7.0 is reported to be 96.5–97.0% for a 200 mg·dm−<sup>3</sup> NaCl solution (manufacturer's data). Each run was performed with a virgin membrane sample.
