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Preliminary water treatment by means of coagulation and precipitation

General information

Since natural waters contain impurities with various particle size it makes necessary to treat such waters in several stages. First stage is intended for removal of colloidal and suspended solids, and the subsequent stages - for removal of ion-dispersed substances and dissolved gases. Such system approach to the selected sequence of water treatment methods is aimed at the improvement of technical-and-economic indexes of various treatment stages and allows to automate the work of certain equipment and enhance reliability of water treatment plants in general. For instance, organic substances containing in natural waters may cause loss in performance of the anion-exchange resins in water treatment plants ("ageing" of anion-exchange resins, increase of specific alkali consumption for regeneration), and iron compounds may cause "poisoning" of equipment membranes used in water treatment plants.

Insufficient treatment of make-up water from colloidal and roughly-dispersed solids is one of the reasons for deposits on the heating surfaces and surface corrosion of elements in the turbine wheel space that emphasises the importance of the first stage of water treatment from colloidal and roughly-dispersed impurities (so called preliminary treatment).

The preliminary treatment is based on methods that make some impurities to drop out of the water in the form of flakes with sizes of 0.1-1.1 mm when the special reagents are added to water. The main processes of the preliminary water treatment are coagulation (aggregation) of colloidal impurities and lime treatment that usually are combined in one clarifier to improve overall process efficiency and to reduce costs. Additional water treatment from suspended solids is performed by filtration methods.

Therefore, a removal of initial roughly-dispersed impurities and suspended solids built up during coagulation process (or combined processes of coagulation and lime treatment) is achieved by precipitation in clarifiers and by filtration, i.e. by means of the two-stage clarifying. As a result the suspended solids' content goes down to 8-12 mg/dm3 in the clarifiers, and the clarifying filters reduce finely-dispersed suspended particles that did not precipitate in the clarifier, down to the values lower than 1 mg/dm3.

Combined performance of coagulation and clarification processes enhances water clarity, decoloration, and reduces silica content and acid value due to removal of about 50-70% of colloidal substances.

* The text is taken from:
A.S. Kopylov, V.M. Lavygin, V.F. Ochkov, "Water-treatment in power energetics"
(M. MEI Publishing House, 2003