Abstract

Abstract
Estimation of Greenhouse Gas Emissions in Wastewater Treatment Plant of Pulp & Paper Industry

Omid Ashrafi, Ph.D.
Concordia University, 2012

Greenhouse gas (GHG) emission and energy consumption in wastewater treatment plants (WWTPs) of the pulp-and-paper industry were estimated by using an elaborate mathematical model. The steady-state and dynamic models were used for the development of mass and energy balances. Significant changes were observed in the magnitude of GHG generation in response to variations in operating conditions, demonstrating the limited capacity of steady-state models in predicting the time-dependent emissions of these harmful gases, thus justifying the use of dynamic model. Aerobic, anaerobic, and hybrid - anaerobic/aerobic - biological processes were used as the main treatment processes. In addition, anaerobic digestion for sludge treatment, nitrification and denitrification processes to remove excess nitrogen in the effluent, and chemical coagulation/flocculation process for the removal of color, residual BOD and suspended solids were incorporated in the model. The generated biogas 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 required energy. Carbon dioxide, methane and nitrous oxide were considered as the major generated GHGs. The impact of pertinent operating parameters including reactor temperature, solid retention time, primary clarifier underflow rate and BOD concentration on GHG emission and energy consumption were investigated, leading to the identification of controlling operating parameters and adequate strategies to reduce GHG emission and energy consumption.
The overall GHG generation by using the steady-state model was equal to 3152, 6051, and 6541kg CO2-equivalent/day by the three examined systems. The results showed considerably higher generation of sludge by the aerobic treatment system, amounting to 376 kg/day, compared to that produced by the anaerobic and hybrid treatment systems. 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. The results of the dynamic model during 140 days of operation showed that the daily variations of GHG emissions were changed up to ±30%, ±19%, and ±17% in the examined systems. The estimated energy consumption amounted to 4028, 2017 and 3084 MJ/day in the aerobic, anaerobic and hybrid systems. The results showed that the produced energy by the recovery and combustion of biogas could exceed the energy demands of treatment plants examined in this study. The variations of process variables caused variations in energy generation from biogas recovery by ±16%, ±17%, and ±14% in the three examined systems. The lowest fluctuations of GHG emission and energy generation were observed in the hybrid system, showing the stability of this particular process design. Parametric studies using the steady-state model indicated that the best strategy to reduce GHG emission and energy consumption would result from a 12% increase in the bioreactor temperature in the aerobic system, a 10% increase of the bioreactor temperature and a 5 days increase of SRT in the anaerobic system, and a 10% increase of temperature and a 5 days reduction of SRT in the anaerobic bioreactor of the hybrid system. Additional reductions in the GHG emission and energy consumption would result from a 50% increase of the primary clarifier underflow rate.