| Compared with traditional physical and chemical methods, biofiltration technique has significant advantages in the treatment of volatile organic compounds (VOCs) with high flow rate and low concentration. However, the traditional biofiltration technique has some drawbacks such as requiring a large area of land, being prone to clogging, and having low efficiency in the removal of less soluble and acid-produced substances, which limit the wider application of biofiltration technique. As a novel type of bioreactor, membrane biofilm reactor (MBfR) combines the advantages of both biofiltration and membrane techniques, which is helpful for enhancment of the removal of VOCs.Six kinds of membranes were tested and compared including three different types of flat PVDF/PET compound membranes and three capillary type microporous membranes made of PVDF, PS and PP in this study. The flat compound membrane of PVDF/PET with pore size of0.03μm was used in the non-biological mass transfer experiment of mixed xylene. It was found that in the gas-gas experiment, when gas residence time (GRT) was20s, the removal efficiency was higher than95%; however, the lower removal efficiency was observed when the same membrane was used in the gas-liquid experiment. At the final stage of liquid-phase domestication of microorganisms, the effluent water was in good quality. The protozoa and metazoan such as rotifers were monitored which indicated that the sludge was successful. In order to promote biofilm formation, the operating conditions of flat MBfR in the start-up were modified until the degradation efficiency (DE)/elimination capacity (EC) was stable.The influence of different nutrients on the performance of the reactor was studied, and the results showed that NO3ˉwas a more suitable nitrogen source than NH4+to maintain long-term operational stability in the reactor. In addition, similar to the performance of traditional bioreactor, the inlet gas concentration and GRT could directly affect the performance of the MBfR. The DE of mixed xylene decreased with an increase in inlet gas concentration, while it increased with the increasing of GRT. The flat MBfR was used to remove mixed xylene in the experiment. When GRT was20s, the DE attained the highest level. With an inlet gas concentration lower than938mg·m-3, the xylene concentration in the outlet was below the allowable value of the "Integrated emission standard of air pollutants".This study compared the degradation effect of six pollutants in MBfR, The DE of acetone was the highest, followed by ethyl acetate, n-butyl alcohol, toluene, n-hexane and dichloromethane. This was directly related to the biodegradability and solubility of the substances. For the degradation of binary system in MBfR, the inhibition effect of mixed xylene on ethyl acetate was negligible, on the contrary, the presence of ethyl acetate significantly inhibited the degradation of mixed xylene. The n-hexane and mixed xylenes could inhibit the degradation mutually, which lead to a lower DE. In addition, the degradation of mixed xylene under most loading conditions were severely inhibited because of the toxicity of dichloromethane; the removal efficiency of dichloromethane was promoted under low loading condition due to the co-metabolism of these substances, while it was still inhibited by mixed xylenes under high loading conditions. For the degradation of the isomers of mixed xylene, inhibition effect was observed. The inhibition effect on o-xylene was more significant than on/w-xylene and p-xylene. In terms of the degradation effect, m-xylene was superior to p-xylene and o-xylene.In the gas phase domestication process, the DE of DMS in MBfR was significantly improved when low concentration methanol was added to the circulating nutrient solution. The degradation effect of DMS in MBfR was alse directly impacted by inlet concentration and GRT. The increased inlet concentration resulted in a decrease in the DE of DMS in reactor, while an increased GRT caused an increase in DE. The DE of DMS in the MBfR was not ideal when methanol was not added to the circulating nutrient solution, with anECv,max value of65g·m-3·h-1. The promotion and inhibition were synchronously existed in the DMS degradation in MBfR when methanol was added to the circulating nutrient solution. The best ratio of DMS to methanol was1500mg·Nm-3to1000mg·L-1when GRT was20s.The mixed xylene degradation effect of the three flat MBfR with PVDF/PET membranes of different pore sizes were examined in this study. The flat MBfR with0.03μm PVDF/PET membrane exhibited the best degradation effect. Compared with the flat MBfR, the structure of capillary MBfR was more complex, while its larger surface area improved gas-liquid mass transfer and promote the biofilm formation in the reactor. Thus, the start-up time required for capillary MBfR was shorter than flat MBfR, and degradation effect in capillary MBfR was better. Similarly, the hollow fiber MBfR has more advantages over capillary MBfR. MBfR has an excellent adaptability to the intermittent operation and transient loads in actual conditions. For long-term operation, the coupling of ozone with low concentrations could not only effectively improve the EC, but also inhibit the membrane fouling due to the excessive growth of microorganisms.The production of biomass was calculated by the carbon balance method, and the result was compared with estimated values from bioenergetics approaches in this study. Five kinetic models, namely the first order kinetics models in plug flow and completely mixed flow, the Michaelis-Menten kinetics models in plug flow and completely mixed flow, and the Stover-Kincannon kinetics model were examined. The results showed that the Michaelis-Menten kinetics model in plug flow could predict the toluene degradation in the flat MBfR accurately. The flow patten of gas pollutants in the above mentioned six types of MBfR were simulated by CFD method, and the results provide fundamental basis for optimizing the structure of the MBfRs. |
PDF Full Text Request