Precipitation, in a strictly chemical sense, is the transition of a substance from the dissolved state to the non-dissolved state by the addition of other reagents that lead to the formation of precipitates.

Most nitrogen compounds are, unfortunately, readily dissolved in water, which implies that precipitation cannot be used as an easy solution to the problem of nitrogen removal. Nitrogen removal by the use of precipitation may, however, be carried out by the two processes shown as equations (11.33) and (11.34) in Chapter 11. The nitrogen needs to be in form of either proteins or ammonium.

The application of precipitation in the removal of nitrogen requires a three-step plant. Addition of chemicals is the first step. The second step is flocculation and as the third step follows some sort of separation process to separate the suspended matter (precipitate) from the clear water phase.

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