| Developing biology-based technologies for low-concentration waste gas purufication has been a hot research topic in the field of waste gas purification. Due to the fact that formaldehyde is water-soluble and vaporable, it is challenging for using existing biology-based technologies to efficiently purify low concentration formaldehyde waste gas.In the thesis, studies were conducted to overcome the challenge in purifying formaldehyde waste gas with biotrickling filter, which is, due to water solubility of formaldehyde gas, accumulation of formaldehyde in circulating liquid leads to limited formaldehyde purification efficiency. The studies were focused on investigating:feasibility of adding chemicals in a biotrickling filter to enhance formaldehyde purification; effect of sodium sulfite on the microorganisms that degrade formaldehyde; optimal operation parameters in the proposed biotrickling filter; mechanisms and kinetics of formaldehyde degradation in the biotrickling filter. The aim was to develop a new type of biotrickling filter which employs "chemical-reaction enhanced degradation" to purify low concentration formaldehyde waste gas with high efficiency.Experiments showed that when inlet gas flow rate was0.2m3/h, circulating liquid flow rate was5L/h, and inlet formaldehyde gas concentration was50-150mg/m3, concentrations of formaldehyde accumulated in the circulating liquid were found up to8.45～33.00mg/L. Under the condition, sodium sulfite, sodium bisulfite, and ammonium chloride were added, respectively, with Molar ratio of1:1between the chemical concentration and formaldehyde concentration in the circulating liquid. It was shown that adding sodium sulfite led to highest removal efficiency of formaldehyde in the liquid, which was up to68.69%. Experiments also showed that when the concentration of sodium sulfite was increased from0.05M to0.15M, the removal efficiency of formaldehyde in the liquid was up to99.98%. Based on the consideration of concentration level of formaldehyde waste gas from wood-processing industry and economical operation of a biotrickling filter, sodium sulfite of0.05M was used in the studies.Experiments comparing performances of two biotrickling filters (filter1without sodium sulfite and filter2with0.05M sodium sulfite) showed that the maximum concentration of formaldehyde in circulating liquid in filter2was99.2%lower than that in filter1. When the flow rate of circulating liquid and inlet formaldehyde gas concentration were increased, though formaldehyde degradation efficiencies of the two filters were similar in the range of96.6～100%, biochemical degradation of formaldehyde in filter2was increased by factors of0.02and1.5, respectively. Whereas the biochemical degradation of filter1was decreased by factors of1.8and2.2, respectively, which indicated that the amount of formaldehyde accumulated in circulating liquid was increased in filter1. The results showed that adding sodium sulfite led to remarkable removal of formaldehyde in the liquid and it is feasible to use sodium sulfite in a biotrickling filter to enhance purification of formaldehyde waste gas.The affect of sodium sulfite on the dominant bacteria which degraded formaldehyde in the biotrickling filter was studied using PCR-DGGE, PCR amplified16S r RNA, and genomic DNA techniques. The results showed that sodium sulfite accelerated the growth of the bacteria and extended the stable period of the bacteria, and therefore enhanced the degradation of formaldehyde. A new type of bacteria which can degrade formaldehyde, Methylotrophic bacteria-Paracoccus bacteria, was found when sodium sulfite was added. The bacteria grew rapidly under the stimulation of sodium sulfite and became the dominant bacteria, which played an important role in formaldehyde degradation.Comparison experiments were conducted to further examine formaldehyde degradation enhancement caused by sodium sulfite. Sodium-sulfite adapted dominant bacteria were used to hang film and the film was used in a biotrickling filter (filter3). The performance of filter3was compared with that of filter2. Sodium sulfite was added in both filters. The difference between the filters was that filter2did not use sodium-sulfite adapted bacteria for hanging film. With the increase of inlet formaldehyde gas concentration and inlet gas flow rate, the maximum amount of formaldehyde biochemical degradation in filter3were61.1and112.4mg/L-h, which were much larger than those of filter2(5.3and3.08mg/L-h). The results demonstrated significant improvement of formaldehyde degradation with the "chemical enhanced degradation". The optimal operation parameters of filter3were determined based on orthogonal tests:circulating liquid flow rate of5L/h, inlet formaldehyde gas concentration of80mg/m3, and gas flow rate of0.4m3/h. It was found that the product of formaldehyde gas degradation was CO2, and the reaction of sodium sulfite with formaldehyde in the liquid produced CH2(OH)SO3Na,which can be degraded by microorganisms and can be used as carbon source for microorganisms.The mechanism of the biochemical reactions in the formaldehyde degradation and the pathways of degradation were investigated. The results showed that in the biotrickling filter where sodium sulfite was added, the biochemical reaction rate in the biofilm was smaller than the diffusivity. The reaction was first-class slow biochemical degradation reaction. As a result, the biochemical reaction was significantly enhanced. Apparent biochemical reaction rate Ra and first-class reaction constant k1of the biochemical reaction were increased by363.3%and216.8%, respectively, in the biotrickling filter with sodium sulfite, compared to a regular biotrickling filter. It was found that sodium sulfite stimulated the growth of Methylotrophic bacteria-Pseudomonas bacteria, and Paracoccus bacteria, the dominant bacteria which degraded formaldehyde through assimilation and dissimilation. Because the products were CO2and cell energy, it was a complete degradation.The study of the kinetics of formaldehyde degradation in the biotrickling filter where sodium sulfite was added, showed that the kinetics of formaldehyde degradation in the liquid could be described with the Monod model. The model predictions had good correlation with measurements. The correlation coefficient R is0.9852. Due to the effect of sodium sulfite, the formaldehyde concentration was increased by11%, the reaction rate constant was increased by7%, and the maximum degradation rate was increased by15.9%. The results demonstrated that sodium sulfite significantly enhanced formaldehyde degradation in the liquid. The results also showed that the quantities calculated using the "absorption-biofilm" kinetics model had better correlation with experiments (correlation coefficients:0.9297～0.9441) than those calculated using the "adsorption-biofilm" kinetics model (correlation coefficients:0.5982～0.7650). This indicated that the kinetics of the sodium-sulfite enhanced formaldehyde degradation can be described with the "absorption-biofilm" kinetics model.In the thesis, a biochemical method which utilized sodium sulfite to achieve chemical-enhanced degradation in a biotrickling filter, was proposed and studied. The studies showed that with this new technology, formaldehyde waste gas purification was improved remarkably. The technology overcomes the challenge that the efficiency of formaldehyde waste gas purification in a regular biotrickling filter is limited due to that formaldehyde gas is water soluble and is accumulated in the circulating liquid. The study results will have significant impact in the field of formaldehyde waste gas purification and the technology will have wide applications in treatment of low concentration formaldehyde waste gas from industry. |
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