Analysis of greenhouse gas emissions and the energy balance in a model municipal wastewater treatment plant

Abstract

Municipal wastewater treatment plants (WWTPs) with multi-stage activated sludge technology generate significant amounts of greenhouse gases (GHG) in the form of nitrous oxide N₂O, methane (CH₄) and carbon dioxide (CO₂). Although the exact magnitudes of the specific emissions are difficult to estimate, they strongly affect the energy balance of a plant. This article presents a simulation study carried out on a model municipal WWTP. The research aimed to analyse the potential for the reduction in GHG emissions through the operational optimization of some core operational parameters and its effects on the plant’s energy balance. The results showed that the combined effect of optimization of the dissolved oxygen concentration in the aerobic zone, the solids retention time and the ratio of chemical oxygen demand to total nitrogen (COD:TN) in the influent may lead to a reduction in the N₂O emissions by 1,103 kg CO₂ eq/d and also a slight reduction in the CO₂ and CH₄ emissions, by 256 and 87 kg CO₂ eq/d, respectively. This was coupled with an improvement in the plant’s net energy balance by 34 kW through the reduction in energy consumption for aeration of the activated sludge by 18 kW and the increased energy production from biogas by 16 kW.

Keywords:

• Wastewater treatment
• Activated sludge
• Greenhouse gases
• Energy efficiency
• Computer simulation

Acknowledgement

The research was financially supported by the Cracow University of Technology, Department of Environmental Engineering, grant no. Ś-3/169/2014/DS.