SLUDGE PRODUCTION MINIMISATION IN SEWAGE COAGULATION WITH LIME

Sewage coagulation with lime may increase COD removal efficiency in primary treatment from typical 30-35% up to 65-70%. However, in spite of the low cost, worldwide availability, and easy dosage control of lime, its use may be hampered by an excessive production of sludge, even when a low lime dosage (pH 9) is used. The results of this work suggest that excess sludge production is most likely due to incomplete dissolution of the added lime slurry. The study shows that lime dissolution rate is highly dependent on the particle size. Even 10 minutes of residence time in the coagulation tank may not be sufficient to assure complete dissolution of particles larger than 50 ?m, which are a large fraction of the commercial lime. In addition, even though CaCO3 precipitation does not typically occur at pH 9, precipitate formation may take place in localised supersaturation conditions due to improper mixing conditions. This work suggests that excess sludge production may be minimised in batch treatment processes, where residence time and mixing conditions are more easily controlled with respect to continuous flow reactors.