Emissions Reduction for Digester Gas-fired Internal Combustion Engines at Wastewater Treatment Plants

Major sources of emissions at a wastewater treatment plant are typically combustion sources (internal combustion engines, boilers, flares, emergency diesel generators and turbines). At the two wastewater treatment plants owned and operated by the Orange County Sanitation District (OCSD), natural and digester gas-fired internal combustion (IC) engines are the major emitters of criteria pollutants (oxides of nitrogen (NOx), carbon monoxide (CO), volatile organic compounds (VOC's), and particulate matter (PM)) and air toxics. Toxic Air Contaminants (TAC), namely formaldehyde and acrolein, emitted by the engines were found to be major contributors to health risk to neighborhood residents and businesses.Alternatives for reducing emissions from the gas-fired IC engines were identified and evaluated. Based on technical and economic feasibilities, three alternatives to reduce the health risk levels at Plant 1 and Plant 2 for the spark-ignited IC engine were pilot tested: 1) Modification of turbocharger 2) Pre-combustion chamber (PCC) timing and air-to-fuel control optimization for pilot gas system; and 3) oxidative catalyst control with digester gas pretreatment.This paper discusses the regulatory drivers for the IC engines, steps used to develop the air toxics emissions inventory from large IC engines at wastewater treatment plants and the corresponding health risk assessment. Different alternatives for emissions reduction are considered. Preliminary results of the study of performance of post-combustion emission reduction system together with the catalyst-poisoning cleaning equipment (digester gas pretreatment) are also included.