Usually water treatment is settled in a reasonably large area, not necessarily near the power plant. The area required is due to the volume of tanks and ponds necessary to treat and stock water. Raw water has to be conditioned to be used in many forms and each treatment stage produces one kind of utility water. A regular treatment sequence may include screening to remove big bodies like trash, leaves, fish etc. Coagulation is provided by addition of chemicals aiming to aggregate small particles into bigger ones, and a slow mixing called flocculation, provokes collision of particles, making larger ones called flocs. These flocs tend to aggregate, becoming heavier than water and settled in a slow flow tank, known as sedimentation tank or clarifier. Additional removal of small particles is provided by pressure filters using sand and gravel as filtering media. All water for industrial use is extracted from this stage, but for boiler use, further treatment is needed.
The additional boiler water treatment, to remove hardness and non-hardness salts, is called demineralization. This is achieved by using a 'cation' resin, which exchanges cations in water with hydrogen ions, producing hydrochloric, sulfuric and carbonic acid. Carbonic acid is removed in a degassing tower in which air is blown through the acid water. In sequence, water passes through an 'anion' resin, which exchanges anions with a mineral acid, like sulfuric acid, and forms water. Continuous use saturates resins with ions and regeneration of cations and anions is regularly necessary using mineral acid and caustic soda, respectively. Complete removal of silica can be achieved by correct choice of anion resin. For high pressure boilers, water quality is very restrictive, so a supplementary ions removal is demanded, called polishing, using basically the same process of resin beds, but with a higher efficiency and deeper capture capacity.
Best practices related to energy efficiency are not around the water treatment unit itself, although some significant pumping energy requirements are in this unit. The main actions to reduce energy consumption are in condensate recovery and avoiding water losses in general, even as steam. They allow lower water makeup, fewer chemicals and less energy consumption, allied to a reduced environmental impact.
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