In the process of water purification, the finest particles dissolved in the water do not settle while the water is at rest, as they are negatively electrically charged and repel each other. In order to achieve a better effect of their removal, chemical treatment is used. In this method, positively charged substances are introduced into the water, which enter into a chemical reaction with those dissolved in water and form aggregated neutrally charged compounds. The agglomerations thus obtained in the form of large flocs are rapidly deposited, and in the presence of stirring of the liquid the effect is higher. This process is called coagulation-flocculation, and the substances used are called coagulants. The choice of the optimal coagulant for a particular water to be treated is determined on the basis of laboratory tests.