Removal Of VOC Mixtures Of High Concentrations From Air Emissions

Biofilters can be used for the treatment of dilute emissions of odors, volatile organic compounds (VOCs) and hazardous air pollutants. The large majority of biofilters installed to date treat off-gases containing organic carbon compounds at concentration of less than 0.5 gm-3. Corresponding VOC loads and elimination capacities rarely exceed 50 grams of organic carbon per cubic meter of biofilter material per hour (g m-3 h-3). The primary goal of this study was to investigate and resolve several obstacles that are usually met when biofilters are applied to highly polluted air streams.Compost-based biofilters, which were consisted of multiple sections, were employed to treat air emissions containing VOCs of high concentrations. The effluences on the biofilter performance of acclimations approaches, initial media water content and soluble nitrogen amount addition to the initial media were investigated. During the biofiltration process, the variation of media pH and microorganism available nutrient content was focused on to obtain the appropriate conditions for removing the mixtures of ethyl acetate, isopropanol and toluene.The following main conclusions were obtained:(1) Compared with the biofilter acclimated with toluene, the biofilter acclimated with ethyl acetate had a much high elimination capacity (EC) for ethyl acetate (the maximum EC is 200 and 400 g m-3 h-1, respectively) and showed equivalent EC for toluene (50 g m-3 h-1 of pure toluene) without the effect of ethyl acetate.At the same conditions, hexane removal efficiencies by the biofilter acclimated with ethyl acetate could achieve about 90 %, however, the removal efficiencies by the biofitersacclimated with hexane/air were about 80 %.(2) The presence of high concentration of ethyl acetate in the biofiter system significantly reduced its removal capacity of toluene. However, the removal efficiency of ethyl acetate was not affected by the presence of toluene in the air stream.(3) During the biofiltration of the mixtures of ethyl acetate, isopropanol and toluene at the high loads of ethyl acetate and isopropanol, the removal efficiency for ethyl acetate and isopropanol was nearly 100 % at the gas retention time from 45 to 90 sec with the total inlet load about 300 g m-3h-1. Toluene removal efficiency was in the range of 40 % to 100 % with the inlet concentration about 0.5 g m-3 due to the inhibition of ethyl acetate and isopropanol.The optimal range of media pH for removal the mixtures of ethyl acetate, isopropanol and toluene is 5.4 to 7.4; the microorganism nitrogen content of the media should be higher than 0.3 mg g-1.(4) Media pH decreased in the area where ethyl acetate were biodegraded, whereas, pH increased slightly in the area where toluene were biodegraded.(5) The loss of microorganism available nitrogen in the media occurred during the biofiltration process and resulted to the deterioration of the performance of the biofilter.The soluble nitrogen content in the media could be maintained around 400-500 mg kg-1 (dry media) when peanut shell was used as one of the component of the packing media. The pH of the media decreased during the biofiltration process of the mixture of ethyl acetate and isopropanol, however, it could increase to some extent after about one-week run without the presence of pollutants.