There is little information to describe how a biofilter responds biologically and chemically to the dynamic loading of complex mixtures of contaminants that are present in autothermal thermophilic aerobic digestion (ATAD) offgases. This paper presents the results of a full-scale study that is the first stage in a longer project to evaluate the use of biofilters for treatment of ATAD offgases. In this 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 a full-scale biofilter and the concentrations of these compounds in the treated air were measured on an hourly basis for a 24-h period, which was the feeding frequency for the ATAD system. The leachate from the biofilter was characterized for ammonium (NH4+), sulfate (SO42–), nitrate (NO3–), and nitrite (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 biofilter were more than 99% for hydrogen sulfide and methyl mercaptan and more than 94% for dimethyl sulfide and dimethyl disulfide. The total elimination rates of RSC and NH3 were 12.4 g-S m–3 d–1 and 6.5 g-N m–3 d–1, respectively. Almost 88% of the total mass 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. Key words: biofilter, ATAD, dynamic loading, hydrogen sulfide, methyl mercaptan, dimethyl sulfide, dimethyl disulfide, ammonia.

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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 (NH 3 ) 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 NH 4 + , SO 4 2-, NO 3 -and NO 2 -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 NH 4 + -N while the remaining 12% was in the form of NO 3 --N and NO 2 --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.

The results of pilot scale testing that was performed to evaluate the use of biofilters for treatment of ATAD offgases under controlled conditions are presented in this paper. The biofilter temperature, source of post-humidification water, bed pH and pollutant loading were varied to assess their impact on the removal of a mixture of reduced sulphur compounds (RSC's) and NH 3 . Sampling events consisted of hourly measurements of the RSC and NH 3 in the biofilter influent and effluent over a 24 hour, which was the interval between feeding of the ATAD. The leachate from the biofilter was also characterized with respect to pH, NH 4 + , SO 4 2-, NO 3 -and NO 2 -concentrations to allow for mass balances on sulfur and nitrogen and to identify the removal mechanisms that were active in the biofilter.Temperature was observed to have a substantial impact on nitrification in the biofilter but had little impact on the removal of RSC's. The source of post-humidification water (potable vs. plant) did not appear to influence the performance of the biofilter. Increasing the leachate pH in the biofilter did not appear to affect either the rate of nitrification or the rate of oxidation of reduced sulfur compounds. The elimination capacity of RSC's did not appear to be exceeded over the range of loadings examined in this study. The loadings of NH 3 appeared to be higher than the biofilters elimination capacity for this substance.