Elimination of an undesirable compound in water treatment may be performed by precipitation of an insoluble salt. Nevertheless, because of the low concentration of the undesirable compound, the solid concentration in the reactor is low, and the precipitation reactor volume is consequently very large.

The paper shows some principal methods for the drastic decrease of the precipitator volume, based on the recycling and/or in situ separation of the solid phase. The detailed mass and population balances of the installation prove that the efficiency depends on two main parameters, recycling ratio and solid-liquid separation factor in the thickener. Some quantitative relations are obtained to estimate the reactor volume saving. These methods can be successfully used to improve the functioning of existing plants and/or to design new plants having reduced dimension.

The perfectly mixed precipitation zone has a relatively low elimination rate due to the low supersaturation corresponding to the outlet limit concentration of the undesirable compound. To avoid this disadvantage, a fluidized bed column can be used. The fluidized bed precipitator combined with a classifier can be largely used for the treatment of waste water containing undesirable compounds in low or very low concentrations. It represents the most efficient device for the process intensification in the environmental engineering or other purification industrial activities.