Models of sewage sludge conditioning are needed in order to assess the type of chemicals to be preferred and the optimal dosages. Laboratory and pilot tests were carried out with organic and inorganic chemicals for determining their influence on sludge parameters and equipment performance. A new parameter, named "Specific Resistance to Centrifugation", for evaluating sludge centrifugability was developed. The pilot tests confirmed that satisfactory solids recoveries can be obtained when the value of this parameter is of 20-30 s. A correlation was also found between the "Specific Resistance to Centrifugation " and dosage, molecular weight and charge density of polyelectrolytes. Other correlations were determined for Specific Resistance to Filtration and compressibility coefficient. In conditioning by inorganic chemicals it was found that ferric chloride mainly influences the specific resistance, while lime the compressibility coefficient. Due to some discrepancies between the classical theory of filtration and actual results of pilot tests, a new parameter called "Average Specific Resistance to Filtration" was proposed. As far as beltpress operation is concerned, a laboratory test was defined thus allowing the drainage phase and the compression one to be simulated. The final cake concentration can be also predicted and it was found a good agreement with the actual experimental values.
