Abstract
The influence of sludge from precipitation purification on the dewatering behaviour of sludges was examined in four comparative tests carried out in municipal sewage treatment plants. The same pilot plants were used in all cases for sludge dewatering: hydrostatic bag, belt filter and filter press. Pure precipitation sludge from postprecipitation is difficult to dewater. A mixture (1:1) of precipitation sludge and conventional digested sludge displays the same dewatering behaviour as natural mixed sludge, e.g. from simultaneous precipitation. Introduction of precipitation purification in the form of simultaneous precipitation increased the total solids content of the sludge and with that the absolute flocculant consumption; the specific consumption only increased in the belt filter in spite of the reduced volume of sludge produced. Iron III precipitants had a more favourable effect on flocculant consumption and dry matter content achieved than aluminium precipitants. This was not confirmed by the other dewatering units. Pre and simultaneous precipitation displayed similar results in sludge dewatering. Simultaneous precipitation is to be given preference on account of the smaller amount of sludge produced.
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