Greenhouse gas emission by wastewater treatment plants of the pulp and paper industry – Modeling and simulation

Greenhouse gas (GHG) emission and energy consumption in wastewater treatment plants (WWTPs) of the pulp and paper industry were modeled and estimated. Aerobic, anaerobic, and hybrid biological processes were used for the removal of contaminants. In addition to the removal of carbonaceous compounds, anaerobic digestion of the produced sludge and the removal of excess nitrogen in the effluent of treatment plants by nitrification/denitrification processes were incorporated in the model. Carbon dioxide, methane, and nitrous oxide were the major GHGs generated during the biological treatment, combustion, energy generation, and transportation. The generated biogas from the anaerobic processes was assumed to be recovered and used as a source of energy for the treatment plant, in an effort to reduce GHG emissions while decreasing the total energy needs of the WWTP. The established kinetic relationships of wastewater treatment processes along with mass and energy balances were employed for the simulation of different treatment systems and estimation of GHG emissions. Various sources of GHG emission were divided into on-site and off-site sources to simplify the modeling and simulation procedure. The overall GHG generation in the presence of biogas recovery was equal to 1.576, 3.026, and 3.271 kg CO2-equivalent/kg BOD by the three examined systems. The energy produced by the recovery and combustion of biogas could exceed the energy demands of all different treatment plants examined in this study and reduce off-site GHG emission. The generation of GHGs from aerobic and hybrid processes increased by 27% and 33.2%, respectively, when N2O emission from nitrogen removal processes was taken into consideration.