Biofiltration Of The Mixtures Of VOCs And NH₃ From Air Streams

Biofiltration has been recognized as a cost-effective and promising technology for the purification of air streams contaminated with low concentrations of biologically degradable volatile organic compounds (VOCs) because of low costs, operational simplicity and limited negative environmental effects. However, during the long-term operation of biofilters, especially with high VOCs load, available nutrient deficiency and inhibition of VOCs components may be limiting factors for degrading VOCs when VOCs mixtures are removed in biofilters. The major objectives of this work were to investigate the biofiltration of the mixtures of different biologically degradable VOCs and NH₃ with high inlet concentrations, and to resolve the nutrient deficiency for degrading VOCs during the long-term operation of biofilters and seek a convenient and effective method of nutrient addition.In this work, the biofilters packed with the mixtures of compost and polyethylene (PE) were employed to remove the mixtures of n-butyl acetate, p-xylene and NH₃ with high inlet concentrations. The removal efficiencies of n-butyl acetate, p-xylene and NH₃ were investigated with various concentrations at various empty bed residence time (EBRT). The inhibition effect of VOCs components was described. The availability of NH₃ as the nutrient for microorganisms degrading n-butyl acetate and p-xylene was determined. Furthermore, an optimal concentration of NH₃ was obtained by investigating the removal of the mixtures of n-butyl acetate, p-xylene and NH₃ with various NH₃ concentrations in biofilters packed with the mixtures of compost and PE. The nutrients, pH, water content and microbial characteristics in packing media were measured. In addition, the removal of p-xylene with high inlet concentrations was performed in a hybrid biofilter which consisted of a compost-based biofilter and a biofilter packed with PE.The following main conclusions were obtained:(1) p-Xylene was successfully removed in a hybrid biofilter which consisted of a compost-based biofilter and a biofilter packed with PE. At EBRT of 132 s, the removal efficiency was more than 80% and the elimination capacity of 80 g·m^-3·h^-1was obtained with the inlet concentrations of 100-3300 mg·m^-3. At EBRT of 30 s, the maximum elimination capacity was 40 g·m^-3·h^-1.(2) The mixtures of n-butyl acetate, p-xylene and NH₃ could be effectively eliminated in a three-section biofilter packed with the mixtures of compost and PE. At EBRT of 60 s and 30 s, high elimination capacities of 157 g·m^-3·h^-1 and 150 g·m^-3·h^-1 were obtained corresponding to the removal efficiencies of 100% and 96% respectively. When the inlet load of n-butyl acetate was 471 g·m^-3·h^-1, the elimination capacity was 397 g·m^-3·h^-1 in the 1st section of the biofilter at total EBRT of 60 s corresponding to EBRT of 20 s for 1st section of biofilter.At EBRT of 30-90 s, the removal efficiencies of p-xylene were 40-100% with inlet concentrations of 45-827 mg·m^-3. The maximum elimination capacity of p-xylene was 33 g·m^-3·h^-1 at EBRT of 60 s. With the removal efficiency of 100%, the elimination capacities were 30 g·m^-3·h^-1 and 24 g·m^-3·h^-1 at EBRT of 60 s and 30 s respectively. At EBRT of 60 s and 30 s, the removal efficiency of NH₃ was approximate 100% with inlet concentration of 43-150 mg·m^-3.(3) At the simultaneous presence of n-butyl acetate and p-xylene, the removal of n-butyl acetate was more available than that of p-xylene, and the removal ofp-xylene was inhibited by the presence of high concentration of n-butyl acetate.(4) NH₃ was not only removed as a pollutant, but also added as the nitrogen nutrient for degrading VOCs by microorganisms.In the three-section biofilter, due to the feeding of NH₃, the maximal removal concentration of p-xylene with the removal efficiency of 100% was 510 mg·m^-3 at EBRT of 60 s, which wasmore than 400 mg·m^-3 at EBRT of 90 s.(5) The removal of the mixtures of n-butyl acetate, p-xylene and NH₃ was remarkably affected by the variation of NH₃ inlet concentration in biofilters packed with the mixtures of compost and PE. At constant EBRT of 60 s, with the range of NH₃ inlet concentration of 13-193 mg·m^-3, the presence of NH₃ improved the removal of n-butyl acetate and p-xylene, and the elimination capacities of n-butyl acetate and p-xylene increased with the increase of NH₃ inlet concentration. However, when the inlet concentration of NH₃ was in the range of 211-493 mg·m^-3, the elimination capacities of n-butyl acetate and p-xylene did not keep increasing with the increasing of NH₃ inlet concentration and were similar to those obtained with NH₃ inlet concentration of 13-80 mg·m^-3. An optimal inlet concentration of NH₃ was 145-193 mg·m^-3 for good performance of the biofilter under these operation conditions in this biofilter.(6) The removal of p-xylene was sensitive to the variation of NH₃ inlet concentration.(7) pH, water content and nitrogen content of packing media changed in the biofilters where the mixtures of n-butyl acetate, (?)-xylene and NH₃ were continuously removed. The feeding of NH₃ affected pH and nitrogen content of packing media. No acidification occurred in the packing media in during the operation of biofilters. Nitrogen content of packing media gradually decreased without the feeding of NH₃, and after the feeding of NH₃, the nitrogen content of packing media increased.The bacteria and fungi colony counts in packing media increased during the operation period, especially at the inlet section of the biofilter which was due to the high contaminants load at the inlet section of the biofilter.In conclusion, the mixtures of n-butyl acetate, p-xylene and NH₃ with high inlet concentrations were successfully eliminated in a biofilter packed with the mixtures of compost and PE. NH₃ could be used as nitrogen nutrient for degrading the mixture of n-butyl acetate, p-xylene and NH₃ in the biofilter.