Pyrolysis of domestic sewage sludge: influence of operational conditions on the product yields using factorial design

This study aims to investigate a sustainable method for sewage sludge (SS) safe disposal and reuse. The study involved exploring the optimum parameters of thermal treatment of SS by pyrolysis to produce biochar. Based on the analysis of the full factorial design, the effects of pyrolysis conditions: temperature, heating rate, and isothermal time on pyrolysis product yields were evaluated. The average yield of biochar was significantly reduced when the pyrolysis temperature was increased from 300 to 500 °C, while the average yields of bio-oil (BO) and non-condensable gases (NCGs) were increased. The yield of biochar was nearly the same when the heating rate was increased from 5 to 35 °C/min, while the yield of BO was increased and the yield of NCGs was decreased. The average yields of biochar and NCGs were reduced when the isothermal time was increased from 45 to 120 min, while the yield of BO was slightly increased. Factorial design methodology revealed all potential interactions between the variables of the pyrolysis process of SS. To predict pyrolysis product yields, first-order regression models were developed based on the effects’ magnitude of the process parameters and their interactions. The models were agreed to the experimental data.