Mechanical dewatering and thermal drying of residual sludge

The optimal management of a residual sludge dewatering unit using mechanical dehydration and thermal drying needs a good evaluation of the interactions between those two processes. In particular it is necessary to appreciate the dependence between the mechanical step and the drying behaviour. This influence is estimated by the comparison of drying rate curves fitted on cakes obtained with different filtration-compression running conditions (various applied pressures, compression time, conditioning). Laboratory experiments were carried out on several residual sludges (organic, mineral, biological). The results showed that the drying step was not significantly influenced by the compressed cake quality (structure more or less squeezed). This result is of real industrial interest; it shows that in many cases it is desirable to eliminate the maximum amount of water by the mechanical method as this is energetically cheaper.