Reduction of Air Toxics from Digester Gas-Fired IC Engines at WWTPs

Major sources of emissions at a wastewater treatment plant typically include 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 toxic air contaminants (TACs). Plant 1 has three, 2.5 megawatts (MW) IC engines and Plant 2 has five, 3 MW IC engines. The IC engines are fueled primarily by digester gas and supplemented by natural gas. TAC, namely formaldehyde, emitted by the IC engines was found to be the major contributor to health risk to neighborhood residents and businesses.In order to address compliance with California Air Resource Board (CARB) and South Coast Air Quality Management District (SCAQMD) air toxics regulations, a facility wide health risk assessment was performed. The results of the risk assessment determined formaldehyde from the IC engines as the major contributor to the facilities' health risk to surrounding communities. Therefore, control of the air toxics from the gas-fired IC engines would result in the greatest reductions in risk levels from both plants.Alternatives for reducing emissions from the gas-fired IC engines were identified and evaluated. Two alternatives to reduce the health risk levels at Plant 1 and Plant 2 were pilot tested: 1) Pre-combustion chamber (PCC) timing and air-to-fuel control optimization for pilot gas system; and 2) post combustion control using a catalytic oxidizer. The purpose of the pilot testing program was to evaluate the long term effectiveness of the proposed alternatives and to determine the technical and economic feasibilities of the alternatives for reducing air toxics emissions. This paper presents results of an emission evaluation for IC engines, the health risks, and details of the pilot testing programs.