DE3725386

1. A method of treating waste water from flue-gas purification plants, constituents being precipitated by adding precipitation and floculating agents and the sedimented sludge being separated from the supernatant water and subsequently dewatered in a separate stage and salts such as calcium chloride or sodium sulphate dissolved in the supernatant water are converted to usefull salts, e.g. sodium chloride, in batches by adding suitable double decomposition substances, e.g. sodium carbonate or calcium chloride after the sludge separation, characterised in that the batchwise double decomposition process is carried out in two steps, i.e. a first step in which 80 to 90% of the amount necessary for double decomposition and calculated by a first analysis is added through a volumetric metering system, after which a second analysis of the waste water is made and subsequently, in a second step, the remaining amount necessary for complete double decomposition is added through a gravimetric metering system.