Portland's in-Vessel Biofilter System for Odor Control

Abstract

The City of Portland, Bureau of Environmental Services, uses a few different technologies to control odors at its main wastewater treatment plant. The most recent additions to the odor control facilities at the Columbia Boulevard Wastewater Treatment Plant are biofilters that treat odorous air from the plant's solids processing areas. Three large-scale biofilters were constructed by reusing existing composting vessels. The system treats up to 49,000 cfm and began operation in November of 2002.Prior to proceeding with design of the full-scale biofilters, pilot plant testing was conducted during the fall of the year 2000. The pilot testing was conducted on two different types of organic media. Biofilter design variables evaluated included airflow or empty bed retention time, H2S removal, and pressure loss. An odor panel was assembled to evaluate the failure threshold.The biofiltration process was selected in part because it does not rely on chemicals. Each biofilter uses a locally supplied organic media. Part of the media may consist of wood waste from local parks. Biofiltration was also selected because it was compatible with reuse of existing tanks and material handling equipment in the composter.The solids processing building facilities were modified to include tank covers for waste sludge, thickened waste sludge and filtrate storage tanks, gravity belt thickener covers, air supply, exhaust and odorous air collection systems, and foul air fans. The composting system was modified by converting composting vessels to biofilters and adding a screw conveyor to move the media into the existing material handling system. The bottom of each vessel was modified to accommodate a new odorous air distribution system. Humidification systems and associated monitors and controls were also added. Process variables monitored include total odorous airflow and its distribution between the biofilters, H2S concentrations in and out, and media moisture content.The paper presents an overview of the pilot testing program, highlights of the construction project, and up-to-date system performance data. Information to be presented includes monitoring and control of the full-scale system, odor removal and pressure loss trends, the improved environment of the solids processing building, and lessons learned.