Short-term effects of additives on sludge sedimentation characteristics

The effect of different concentrations of additives on the settling properties of two types of activated sludge were evaluated by means of a Settlometer. In addition to the evaluation of the immediate positive effect of the additive, a method consisting of consecutive cycles of resuspension and evaluation of the settling properties, is proposed to study the stability of the beneficial effect of the additive. First, a sludge from industrial origin was supplemented with talc and with a polymer. Subsequently, a sludge from domestic origin was tested. Both additives were successful in improving the sedimentation characteristics of the sludges. The effect of talc was stable over a period of at least 14 h. However, the positive effect of the polymer was transient because after 4 h the beneficial effect disappeared. The experimental results from the talc addition were analysed by means of a physical model. The latter revealed that the observed increase of the initial settling velocity is smaller than but close to the increase related to the specific weight of the floc.