The pretreatment of the wastewater from the edible production based on calcium phosphates precipitation, generates sludge characterized by good sedimenting properties. However, a part of the sludge floats through the surface of the settling tank. Lime added for phosphorus removal simultaneously removes also sulphates to a level of above 2000 mg/l. The liquid associated with floated sludge contains low sulfates concentration – about 370 mg/l. The aim of the research project was to examine the causes of such a low sulphates concentration.
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