Key Techniques For Treating A Complex VOC Mixture In Airlift Bioreactors

Volatile organic compounds(VOCs)from extensive solvents usages in industry largely contribute to haze pollution and photochemical O3 production in China.Biotechnology is a perfect alternative due to high flow rates and low concentrations of VOC emissions from the industry.Biofiltration is still regarded as the most cost-effective biotechnology.Unfortunately,deterioration of removal performance and additional maintenance costs are still inevitable in such system because of excess biomass accumulation,heterogeneous distribution of nutrient,and non-optimal moisture content after a long-term operation.Bioscrubbers have no such problems,and are especially advantageous for hydrophilic-compound removal.However,complex VOC mixtures from the industry have made bioprocesses complicated due to the different hydrophobicity,volatility,biodegradability and biocompatibility of VOCs.Two-phase partitioning bioreactors(TPPBs)with non-aqueous chases can enhance mass transfer of hydrophobic VOCs and protect microbes from substrate toxicity,environmental fluctuations and operational perturbations.They have been successfully used for the removal of single VOC.However,the applications of TPPBs for the treatment of complex VOC mixtures from the industry are rare.Continuous stirred tank bioreactors have been widely used for the VOC treatment.However,their applications have been limited by excessive energy consumption.In this study,a one-phase partitioning airlift bioreactor(1P-ALR)and a two-phase partitioning airlift bioreactor(2P-ALR)were developed for the treatment of a complex VOC mixture.The complex VOC mixture was composed of ethyl acetate,1,2-dichloroethane(1,2-DCA),dichloromethane(DCM)and toluene.Isolation of a hydrophobic mixed culture,performance evaluation under steady-state and transient conditions,mathematical modeling and microbial information analysis were conducted.A hydrophobic mixed culture with 79%of cell surface hydrophobicity was obtained from an activated sludge of a resin wastewater treatment plant.The hydrophobic mixed culture was primarily composed of Xanthobacter genus(62%).It was able to efficiently degrade 1,2-DCA as sole carbon and energy sources at a wide range of 1,2-DCA concentrations(114.1—1141.5 mg·L-1)with a dechlorinating rate of 92%.The values of μmax,Ks and Ki from Haldane-Andrews model were 0.247 h-1,4.7 g·m-3 and 10.4 g·m-3,respectively.The kinetic parameters were superior to previous reports,and matched the ideal values of microbes inoculated into the TPPBs.The hydrophobic mixed culture was also able to degrade ethyl acetate,DCM and toluene.Silicone oil with a volumetric ratio of 7%was enough for high enhanced ratios of dissolved 1,2-DCA and oxygen,microbial activity.The hydrophobic mixed culture and 7%of silicone oil was able to improve the total elimination capacity of the ALR.The mineralization rate,dechlorinating rate and cell yield of the 2P-ALR were 60%、39%and 0.19 gDCW-gvocs-1,respectively,which were all lower than those of the 1P-ALR.However,the 2P-ALR was more stable and resilient under transient conditions.At steady-state operation,removal efficiencies(REs)of ethyl acetate for both ALRs were 100%,which were not influenced by empty bed residence times(EBRTs).The RE of moderately hydrophobic toluene was increased by one time because of silicone oil.In both ALRs,the RE of toluene was higher than that of 1,2-DCA.The removal performance of 1,2-DCA and DCM was not enhanced with silicone oil,although their solubility was improved.Compared maximum volumetric mass transfer rates with maximum elimination capacities,proposed firstly in this study,it indicated that the removal of ethyl acetate was limited by mass transfer,while the removal of 1,2-DCA,DCM and toluene was limited by biodegradation.Any microbial dysfunction and metabolic disturbances were caused by silicone oil.The individual VOC removal in both ALRs was controlled by dominant species with specialized enzymes for VOC degradation,especially for recalcitrant-compound degradation,like chlorinated alkenes.The 2P-ALR was superior at the removal of hydrophobic VOCs,like aromatics,profiting from hydrophobic microbes.Deterioration of biodegradation kinetics for individual VOC would be triggered by complex interactions among VOC mixtures,particularly competitive enzyme inhibition.This study provides a valuable reference for operational strategies to optimize functions of engineered microbial ecosystems and VOC removal performance of the bioreactors.