*3.3. Blank C Experiment Results*

This experiment has to demonstrate non-inhibited RO membrane gypsum scaling as a reference to the inhibited one. Figure <sup>6</sup> reveals a linear increase of Ca2<sup>+</sup> content from 0.015 up to 0.025 mol·dm<sup>−</sup>3. When CaSO4·2H2O saturation is achieved (K = 2), the Ca2<sup>+</sup> concentration reaches the maximum. Then [Ca2<sup>+</sup>] decreases due to the gypsum crystals deposition, and at the final moment (K = 5) [Ca2<sup>+</sup>] corresponds almost to its initial level. At the same time 80% of calcium gets deposited as gypsum scale on membrane surface.

**Figure 6.** Variation of Ca2<sup>+</sup> concentration in a Blank C (a,b,c,d) and in GSI experiment (c'): (a) total calcium concentration in retentate + membrane; (b) Ca2<sup>+</sup> concentration in permeate; (c,c') "free" calcium in retentate; and (d) calcium content on membrane surface as gypsum, expressed in units of calcium concentration.

The corresponding scale on membrane surface fits well typical stick-shaped gypsum crystals morphology [6], Figure 7A,B. These images leave an impression that the scale is formed by crystal precipitation from the bulk, rather than by their initial formation on the membrane surface: several crystals lying on top of each other are clearly visible.

**Figure 7.** SEM images of gypsum deposit on membrane surface at the end of the Blank C (**A**,**B**) and GSI (**C**) experiment. Scale marker corresponds to 20 (**A**,**C**) and 5 (**B**) μm.
