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Alkalinity of some industrial effluent may have to be increased in order to maintain a pH of approximately 7 in the reactor.

Alkalinity increase is applied to raw water preferably in the form of soda or sometimes lime (case of the anaerobic contact process), encouraging sludge flocculation and settling, and possibly, depending on the characteristics of the effluents to be treated, a combination of the 2.

On the other hand, with sludge blanket or fluidised bed systems, due to the risk of scaling, the use of the lime is to be avoided (scaling). Sodium hydroxide or sodium bicarbonate is recommended instead.