Biofiltration of High-Strength Exhaust from Biosolids Composting: Lessons Learned and Demonstration of Successful Treatment

The Solid Waste Authority of Palm Beach County, Florida, operates an in-vessel composting facility that processes 30 dry tons/day of dewatered biosolids combined with shredded yard waste. The facility consists of 36 aerated agitated bays in three buildings supplied by International Processes Systems (Now Siemens Water Technology). All exhaust air from the buildings is treated in three 60,000 cfm biofilter cells. New biofilter cells were commissioned in December 2002. Design and construction included some innovative features including: use of locally available limestone for the support plenum, use of locally available finely ground bark mulch in the media, a pre-humidification system with air-atomizing nozzles, clean-outs for every perforated lateral, and a fast-track design-build method of delivery.After start-up the well water used for humidification caused rapid scaling of the laterals, due to the high mineral content and high evaporation efficiency. Pre-humidification was discontinued while a water softening and deionization system was under consideration. Since then the filters have been operated only with surface irrigation. The most recent rounds of performance testing have indicated that surface irrigation, combined with rainfall, were sufficient to maintain optimal moisture content throughout the full depth of media. Without pre-humidification, the irrigation distribution pattern is critical.The filters have been periodically tested for odor and ammonia removal, most recently in 2006 and scheduled again for 2007. The filter media in the different cells have been in continuous use for 2.6 to 3.1 years. Typical inlet conditions were:Temperature: dry-bulb range 96-105 deg. F. wet-bulb range 86-92 deg F.Ammonia: 22 – 40 ppmHydrogen sulfide and methyl mercaptan: neglible or non-detectOdor concentration: 1,500 – 2,400 D/T, using the prEN 13725 olfactometry standard.The biofilters have consistently removed 100 % of ammonia and 93 – 98 % of odor with outlet odor concentrations in the range of 66 to 86 D/T. Thus, the biofilter is showing highly effective performance, despite the high temperature and moisture deficit in the inlet air. As a result, the composting facility is no longer a significant source of off-site odors.Ammonia is an alkaline gas and there little acid gas present. The media has a pH in the range of 4.8 – 6.8. The acidic pH indicates that alkalinity is being consumed as a result of nitrification. A nitrogen mass balance indicated that the biofilters are operating in steady state rather than accumulating nitrogen, and that nitrogen leaves the biofilters by way of leachate and denitrification. Denitrification was demonstrated by the detection of nitrous oxide. Nitrous oxide is of increasing interest due to its effect as a greenhouse gases.