PILOT AND FULL SCALE STUDIES OF THE TREATMENT OF ATAD OFFGASES WITH BIOFILTERS

There is little information to describe how a biofilter responds biologically and chemically to the dynamic loading of complex mixtures of odorous contaminants that are present in autothermal thermophilic aerobic digestion (ATAD) offgases. This paper presents the results of pilot and full –scale testing that has been performed to evaluate the use of biofilters for treatment of ATAD offgases. In the full-scale study the composition and flow of offgases from an ATAD system located in McMinnville, Oregon were characterized. The loading of reduced sulfur compounds (RSC) and ammonia (NH3) to the full-scale biofilter and the concentrations of these compounds in the treated air were measured on an hourly basis for a 24 hour period, which is the feeding frequency for the ATAD system. The leachate from the biofilter was characterized for NH4 +, SO4 2−, NO3 − and NO2 − concentrations to allow for mass balances on sulfur and nitrogen and to identify the biodegradation processes that were active in the biofilter. The removal efficiencies in the full-scale biofilter were more that 99% for hydrogen sulfide and methyl mercaptan and more than 94 % for dimethyl sulfide and dimethyl disulfide. Almost 88% of the total of N in the leachate left from the biofilter as NH4 +-N while the remaining 12% was in the form of NO3−-N and NO2−-N.The pilot scale studies employed an ATAD/biofilter pilot plant that was located at the City of Ottawa's waster water treatment plant to evaluate the treatment of ATAD offgases in biofilters under more controlled conditions. The properties of the offgases from the pilot scale ATAD were varied by adjusting the aeration of the digester. These parameters impact on the frequency and magnitude of the cyclic production of odorous compounds. The biofilter parameters that were examined included the impact of residence time and pollutant loading on treatment efficiency and contaminant fate. In the pilot scale studies the gas and liquid streams were monitored regularly to determine the removal of the odorous compounds and to identify the mechanisms of contaminant removal.