THE POTENTIAL OF COUPLING BIOLOGICAL AND CHEMICAL/PHYSICAL SYSTEMS FOR AIR POLLUTION CONTROL: A CASE STUDY IN THE RENDERING INDUSTRY

Abstract

The promulgation of “Odor Control Rules”, increasing public concerns, and EPA air regulations in non-attainment zones, necessitates the remediation of a wide range of volatile organic compounds (VOCs) generated in the rendering industry. Currently, wet scrubbers using oxidizing chemicals, such as chlorine dioxide or ClO2 and ozone (O3) are utilized to treat VOCs. However, little information is available on the type and concentration of VOCs emitted, and the effectiveness of current air pollution control technologies. In the first part of the project analytical methods were developed using GC/MS units for on-site analysis. Portable gas chromatography units with mass spectrometry detectors (GC/MS) were used to rapidly identify key odor causing compounds and volatile organic compounds in gas samples taken from several rendering plants. Major compounds consistently identified in the emissions from the plant included: methanethiol, octane, hexanal, 2-methylbutanal, and 3-methylbutanal. The two branched aldehydes, 2-methylbutanal and 3-methylbutanal, were by far the most consistent, appearing in every sample and typically the largest fraction of the VOC mixture. Total VOC concentrations in the inlet to high intensity scrubbers and a biofilter ranged from 4 to 90 ppmv. Additional research was performed to determine wet scrubber (plant A) and biofilter (plant B) removal efficiencies for specific compounds. Wet scrubber efficiencies for total VOCs ranged from 23-64% with conversion efficiencies approaching 100% for methanethiol. A bioflter system was used to successfully treat VOCs. In general for the biofilter, total VOC removal efficiencies ranged from 40 to 100% conversion with much higher removal efficiencies for the aldehydes, which were not typically removed to the same levels in the wet scrubber systems. Process temperatures monitored in various unit operations varied significantly during operation, rising as much as 30°C within a few minutes. In one case, temperature changes from 45°C to approximately 60°C were observed in the recirculation liquid of a high intensity scrubber. Temperature changes such as this can reduce wet scrubber efficiency, since gas solubility decreases as liquid temperature increases. However, it is noteworthy that the outlet air temperature of the high intensity scrubber remained relatively constant at 40°C although the inlet air temperature increased 50-65°C during monitoring. These data suggest a hybrid process combining wet scrubbers and biofiltration could be used to improve overall VOC removal efficiencies. Wet scrubbers can be designed to 1) remove particulate matter, 2) remove reduced sulfur compounds and 3) buffer outlet air temperatures. Biofilters have demonstrated high removal rates for aldehydes (compounds not easily removed in the wet scrubber). Removal of reduced sulfur compounds in the wet scrubber will prolong biofilter stability, since pH will not decline and media will not decay.