**Abstract**

Due to the insufficiency of freshwater resources and to supply the drinking water populations, many desalination processes such as reverse osmosis, electrodialysis and distillation are widely used. However, these processes are of large-scale consumers of energy and confronted with various problems such as corrosion and scale formation. In most cases, scales are made of calcium carbonate CaCO3. In the present chapter, an overview on the effect of operating parameters such as temperature, pH and supersaturation on the precipitation kinetics, microstructure, and polymorphism of CaCO3 is given. Additionally, I put special emphasis on the effect of foreign ions such as magnesium, sulphate, and iron ions on CaCO3 precipitation since they are present at significant concentrations in natural waters. Also, the mechanisms by which these ions affect the crystal growth of CaCO3 were pointed out. Knowledge about these operating parameters as well as the effects of foreign ions allow elucidating the polymorphs growth during water treatment. The control of these operating parameters allows reducing scale formation during drinking water and wastewater treatment. The economic impact is of greatest importance since this favorably affects the treatment costs, increases the equipment life, and allows enhanced product water recovery.

**Keywords:** water treatment, scale, calcium carbonate, cement, growth, magnesium, sulphate, iron, CO2, supersaturation, pH, temperature
