Kinetics study of the methane production from experimental recycled pulp and paper sludge by CSTR technology

The aims of this study were to perform the anaerobic digestion of recycled pulp and paper sludge (RPPS) at laboratory scale, using a continuously stirred tank reactor (CSTR) digester, to determine the methane yield of this type of waste, and to control the physico-chemical parameters to maintain its stability during the experiment. Recycled pulp and paper sludge has an important organic matter with a COD/BOD5 ratio of 1.94, which is easily degradable. The result of anaerobic digestion experiment has given a methane yield coefficient of 159 N mL CH4/g VS, while biodegradability was found around 77% (in VS). The results obtained from the kinetic models studied in this work, mainly the modified Gompertz, first-order, Richards, and Logistic kinetic models show that the difference between the predicted and measured methanogenic potentials was higher in the modified Gompertz kinetic model (0.25–12.94%) than in the Logistic model (0.18–2.69%). The Logistic model showed the best fit for the substrates used. The Logistic, the modified Gompertz, Richards models fitted adequately to cumulative methane production (R2 was 0.9744; 0.9597 and 0.9655, respectively). Specifically, the Logistic model predicted the methane production volume like the experimental within loads added of 2.5 g VS/L.