Purification Performance Of Biotrickling Filters On Unsteady State VOCs-containing Waste Gases And Properties Of Biofilm

Volatile organic compounds(VOCs)are one group of the most common pollutants in the atmosphere,which pose a threat to ecosystems and human health.Therefore,treatment of their massive discharge from industrial manufacturing is drawing increasing attention.Biotrickling filter(BTF)as a safe and low-cost approach,has great potentials to treat high-ventilation and low-concentration VOCs without causing a secondary pollution.Inside a BTF,biofilm is the functioning part which is a cell aggregate enclosed by extracellular polymeric substances(EPS)and attaches to the packing materials.Industrial discharge of VOCs is not in homeostasis but is intermittent,composition-and concentration-variable.Under such condition biofilm is difficult to develop and this is the major limit of BTF from its broad industrial application.To break this limit,a full characterization of biofilm formation mechanism inside a BTF under non-homeostatic conditions are required.In combination of lab-scale BTFs and synthetic VOCs,this study first investigated how operating mode would affect biofilm formation,VOC degradation and microbial community development.Then packing materials modified with sodium alginate and polyvinyl alcohol were evaluated too,in terms of the enhancement of biofilm formation,VOC degradation and microbial tolerance against variable VOC composition and concentration.(1)The effects of continuous(BTF1)and discontinuous(BTF2)operating mode on biofilm formation were compared.Been operated for 60 days in discontinuous mode,both biomass and EPS content per gram dry packings decreased,illustrating biofilm formation suffocated.Seen from the extremely low contents of biomass and EPS,the defected biofilm formation was believed the major course of the observed poor VOC removal efficiency(close to zero).Members of genus Mycobacterium became the dominant species,mostly because they were able to remain viable under starvation.Predictive functional profiling of BTF2metagenomics indicated a lower relative abundance of genes involved in biofilm formation but higher of those in degradation of aromatic pollutants.Once switched to continuous mode as how it was in BTF1,biomass and EPS content in BTF2 increased drastically,together with a rapid biofilm formation and the rise of removal efficiencies on all five pollutants(>60%).In BTF1 however,the biofilm well developed in 30 days and better removal efficiencies of all pollutants were recorded(>77.2%).Burkholderia and Comamonas were now the core species,which were widely reported for their high degradation capabilities of numerous aromatic pollutants.The microbial community remained stable when BTF1 was switched to discontinuous operation mode.Biomass and EPS content also remained at relative high levels for the next 30 days.In conclusion,a successfully developed and stabilized biofilm was the key factor affecting removal efficiency of VOCs.(2)Packing material modification with compatible compounds could significantly enhance the performances of biotrickling filters treating VOCs under intermittent inlet loadings.Meanwhile,the addition of different types of carbon sources in modification reshaped the microbial community structures and influenced EPS secretion.BTFs using three different packing materials were progressively evaluated:(i)those modified by 3%sodium alginate and 5%polyvinyl alcohol,(ii)those from(i)plus cell immobilization,and(iii)those of(ii)plus benzoate addition.Biomass and EPS content of the BTF using(i)turned out to have increased significantly,by 2-and 20-fold compared with those using original packing materials.Biomarkers including Sphingopyxis and Hydrogenophaga were selectively adhered to the surface of modified packing materials.It is certain that packing material modification successfully resolved the defected biofilm formation under discontinuous mode and improved the removal efficiencies of all five VOCs(>60%in 20 days).The BTF using(ii)also had increased biomass,EPS content and initial adhesion amounts but had no significant difference in biofilm formation capacity and VOC removal capacity when compared with BTF using(i).The BTF using(iii)had increased biomass but decreased EPS content and changed microbial communities,resulting in a poor performance.Therefore,a simple packing material modification using 3%sodium alginate or 5%polyvinyl alcohol had a better prospect of industrial application,taking operation steps into consideration.(3)Modified packing materials stimulated the attachment of unique microbial biomarkers to surfaces,such as phylum Planctomycetes(3.00%)and genus Flavobacterium(5.56%).Evidences showed that the modifiers(sodium alginate and polyvinyl alcohol)used to modify packing materials could serve as additional carbon source for microbes during the acclimation stage.These biomarkers permanently colonized the BTFs and directly contributed to the final microbial community structures.Biofilm formed since then maintained high stability and EPS secretion activity in order to sustain microbial diversity and evenness,even when modifiers were depleted or the VOCs inlet loading increased.These were the fundamental biological mechanisms accounting for the improved removal efficiency of intermittent VOCs and the microbial resistance to concentration fluctuation.(4)BTFs packed with modified polyurethane foam exhibited an enhanced resistance to VOCs component transformation.Acclimatized by both hydrophilic and hydrophobic VOCs,BTFs accumulated more biomass(51.88�3.24 ug/cm~3)and EPS(34.87�3.03 ug/cm~3)in the surface of packing materials,indicating a well-developed biofilm.Then,the biofilm was damaged to some extent when the complex inlet VOCs shifted to hydrophobic only,seen from the decrease of biomass(to 36.51�4.05 ug/cm~3)and EPS content(to 10.52�3.50 ug/cm~3).But the microbial network complexity remained high and there were still enough biomass and EPS content left to keep the BTF functional,so that the removal efficiencies were not significantly compromised.When the hydrophilic VOCs were re-introduced,microbial community structures changed minor,but the network complexity decreased.Both biomass and EPS content increased,up to 48.13�3.07 ug/cm~3 and 28.89�2.36 ug/cm~3,respectively.These were signs of a full restoration of biofilm formation and removal performance as they were in the acclimatizing stage.In summary,a mature biofilm in the packing materials was the key contributing factor to microbial resistance to VOCs component transformation.