Mechanical and thermal lysis of waste activated sludge for anaerobic digestion optimization

In view of enhancing the efficiency of the anaerobic digestion process performed on waste activated sludge (WAS) produced in a WWTPs, several pretreatments were performed using mechanical and thermal systems. Samples of WAS were prepared at different total solids (TS) content, between 2% and 9% w/w, with the focus of investigating the influence of this parameter on the performance of the chosen pretreatment. The evaluation of the disintegration rate (DR), referred to the COD solubilization due to the pretreatment, permitted to compare the results obtained even though coming from different lysis systems and working conditions. The mechanical pretreatment, performed with a high shear mixer, permitted to obtain a COD disintegration rate dependent on the TS content of the WAS with a maximum DR equal to 1,7%. Thermal pretreatments were performed with a thermostatic bath (343K), an oven (343K) and an autoclave (393K at 1.2 atm overpressure): no influence of TS content was observed, while the choice of the lysis system and the time of treatment played a fundamental role towards COD solubilization. The higher DR values obtained were between 14% and 28% making thermal pretreatments more suitable for the subsequent WAS anaerobic digestion optimization. Moreover, rheological tests demonstrated the effectiveness of pretreatments towards viscosity decrease of sludge which has gained, as a consequence, an improved pumpability and handling.