MISTING SCRUBBER ACHIEVES HIGH TOTAL VOC AND LOW HAP REMOVAL EFFICIENCIES

The Washington Suburban Sanitary Commission (WSSC) owns and operated the Montgomery County Regional Composting facility (MCRCF) which is a 400 wet ton per day aerated static pile composting facility. The MCRCF is located in the Washington Metropolitan Area, which is a severe ozone non-attainment area. As such, a Title V operating permit is needed for any major source of volatile organic compounds (VOCs) or Hazardous Air Pollutants (HAPs). To determine if the MCRF is a major source, the WSSC sampled and tested emissions from the composting facilities, biosolids/woodchip mixing facilities, and three stage misting scrubber. The misting scrubber was believed to emit the highest amount of VOCs because it scrubs composting process gas taken directly from the composting piles.The testing results showed that the scrubber emits the vast majority of VOCs and HAPs. They also showed most of the VOC emissions in the scrubber inlet are condensable VOCs. The scrubber is removing approximately 92% of total VOCs. VOC removal appears to be most effective for the condensable fraction of the VOCs. Very few non-condensable VOCs are removed by the misting scrubber. This suggests that the condensable VOCs are condensing at the surface of the liquid droplets inside the misting scrubber.Testing also indicated that the process gas entering and exiting the misting scrubber contained significant quantities of methanol, acetone, methyl ethyl ketone (MEK), and other high boiling point compounds such as terpenes. These compounds are believed to be biological by-products of biosolids and woodchip degradation. Inlet concentrations of these compounds are very consistent and repeatable but outlet concentrations were quite variable. Therefore, the HAP removal efficiency for the misting scrubber was very erratic. Mass balances and equilibrium equations showed that the scrubber was liquid limited.This study shows misting scrubbers are very efficient for removing high boiling point compounds and total VOCs. This is important information for facilities located in ozone nonattainment regions of the United States.