Effect Of Ozone On The Long Term Performance Of Biofilter And Characeristics Of Microorganisms For Gaseous VOC Removal

Biofiltration is one of the promising techniques for volatile organic compounds?VOC? gas treatment. Recently, it was found that continuous ozone injection can improve the performance of biofilters by controlling the excess biomass in a short-term operation. However, the effect of ozone injection on long-term biofiltration performance and the microbial community still need further investigation. In this study, two lab-scale biofilters using porous perlite as packing media were operated to treat toluene gas for 160 d. The empty bed residence time was controlled at 40 s, and inlet toluene concentration was around 800-1300 mg/m3. During day 45-160, one biofilter was injected with 200mg/m3 gaseous ozone continuously. The performance of two biofilters and their microbial characteristics were analyzed during the operation.During the 160 days, the toluene removal efficiencies of two biofilters were both maintained in 50-90 % and did not show obvious difference, which verified that ozone injection will not disrupt the biofilter performance in toluene degradation. In addition, the pressure drops of biofilter with ozone exposure were 20-60 mmH₂O while those of control biofilter were 40-120 mmH₂O at the end of operation when superficial velocity was 16-32 m/h. This indicated that the ozone injection helps in maintaining a stabler biofilter system in long-term operation.Accordingly, the wet biomass in the biofilter with ozone injection kept stable at 275 g while the wet biomass accumulated to 450 g in the control biofilter by the end of operation. In the meantime, the carbon balance results showed that the biofilter with ozone injection had higher ratios in CO₂ production and leachate carbon output, which explained the lowered biomass. Further study about modeling of biomass accumulation revealed that ozone may react with the inert biomass and enhance the conversion of inert biomass to CO₂ and soluble microbial products.Based on the observation of biofilm in the two biofilters by Confocal Laser Scanning Microscope?CLSM? coupled with a ViaGram staining kit for 6 weeks, the reduction in biofilm thickness by ozone injection was also verified. For the biofilter without ozone injection, the cell viability ratio?0.51⁰.89? and the Gram positive bacteria ratio?0.20⁰.60? both decreased within 4 weeks of cultivation. A continuous injection of 200 mg/m3 ozone was capable to significantly induce the ratio of viable cells to 0.77⁰.97 and lead to the dominance of Gram positive bacteria in the biofilms.Similar to CLSM results, PMA-qPCR results indicated that the microbial viability was higher in the biofilter with ozone injection. The variations of the microbial community structures in the two biofilters were analyzed by Miseq highthroughput sequencing technique. The results showed that two biofilters were both dominated by bacteria in the phylum of Proteobacteria, Actinobacteria, and Bacteriodes along the operation. However, the dominance of Rhodanobacter was found in the biofilter with ozone while the predominant genus in the control biofilter was Pseudomonas. According to 96-well Biolog ECO plate culturing results, the ozone injection did not change the metabolic characteristics too much after the biofilters underwent a long-term operation.All the experimental results above provide a better understanding on how ozone affects the biofilter performance in the long-term operation and verified that continuous ozone injection is a promising biomass control technique.