Research On Performance And Mechanism Of Tubular Biofiltration

Emmisions of volatile organic compounds (VOCs) have severely adversely affected the environment and human health, which have been paid close attention. Due to its cost-effectiveness and environmental friendliness, biofiltration is bocoming an established technology for the control of VOCs and odor emissions from waste gas streams. However, some problems still commonly exist for traditional biofilters and biotrickling filters. Among the problems are large footprints and low removal efficiencies when a biofilter is operated over a long period at high organic lodaings. Innovative rotating drum biofilters (RDBs) can effectively overcome the above problems. However, the construction and operation costs are still high. Therefore, develop of new bioreactors with high performances and low costs are particularly essential.Based on the hypothesis of biomass accumulation patterns and the surface biofiltration process, a novel biofilter-tubular biofilter (TBF) containing the tubular type medium was developed and operated on the mode of air circulation from outside to inside. And then, toluene, isobutyl alcohol, diethyl ether and methyl isobutyl ketone was used as model VOC individually to startup the bioreactor at the settled reference conditions. Furthermore, TBF performance was evaluated at different operating conditions such as different organic loadings, gas empty bed contact times (EBCTs) or air flux over a long period of continuous operation. Gas choromatograph and other standard analytical methods were used for determining the VOCs and the other parameters. Microbiology in the TBF was also primarily researched when running over a long period. Finally, mathematical modeling of rotating drum biofiltration and tubular biofiltration process for VOCs removal were carried out to understand the process mechanisms, respetively.The TBF can successfully startup at the reference conditions settled at organic loading of 1.4 kg COD m-3 d-1 and gas EBCT of 15 s when toluene was used as model VOC. At the startup stage, the removal efficiency was less than 50% firstly and then increased gradually, and over 95% on day 52. When toluene loadings increased to 2.8, 5.6 and 11.2 kg COD m-3 d-1 but kept the other operating conditions unchanged, the removal efficiencies were immediately decreased to 54.1%,37.4% and 11.4%, and then gradually increased to the maximum removal efficiencies of 89.7%,78.4% and 55.6% separately after 30 days operation. When gas EBCTs changed to 10,7.5,5 and 30s but kept the other operating conditions unchanged, toluene removal efficiencies could stabilized over 97% in a short period operation (less than 3 days). Maintaning the unchanged inlet toluene concentration but increasing the inlet flow rate, the maximum average removal efficiency was decreased but the elimination capacity increased. A long-term stable operating performance was also obtained at a high toluene loading of 5.6 kg COD m-3 d-1 in which the maximum average removal efficiency was 74.9% and the elimination capacity was 4.2 kg COD m-3 d-1. Some interesting phenomema were observed. A lager insect like cockroach was emerged and survived over 3 months. Biomass appeared in yellow-brown corlor and distributed evenly on the medium. The TBF can successfully startup and provide a long-term, stable and high performence for toluene removal.When isobutyl alcohol used as model VOC, the TBF could sucessfully startup and the removal efficiency was over 98% in the first 60 days operation. The isobutyl alcohol removal efficiencies were larger than 97% at all the different organic loadings of 2.0,2.8,3.0 and 4.0 kg COD m-3 d-1. Only a small volume of biomass accumulated in the medium when dismantling the bioreactors on day 339. A kind of small insect was found in the space between the outer tube of the reactor and the tubular medium and was primarily classified into Chironomidae. A large number of the small insects were appeared when operating on middle temperature. Furthermore, the disturbances of TBF performance were occurred simutaneously. In conclusion, TBF is suitable for isobutyl alcohol removal at the normal operating conditions.When the inlet diethyl ether concentration fluctuated strongly, removal efficiency was incresed slowly at the startup stage but still less than 80% in the whole 77 days operation. Changed the model VOC of diethyl ether to methyl isobutyl ketone (MIBK) on day 78, the removal efficiency was stably increased and over 80% after 7 days operation. The MIBK removal efficiencies were decresed with increased organic loadings. A large amount of yellow biomass accumulated in the outerlayer, bottom and innerlayer of the media after continuouly operated over a long period.Experimental results show that biodegradability of the three VOCs by TBFs is: isobutyl alcohol> MIBK> toluene and that some different experimental phenomena are appearred for each VOC removal.The results of mathematical modeling of rotating drum biofiltration shows that the temporal and spatial changes of toluene removal efficiencies, biomass accumulation rates and porosities of the media are varied with organic loadings and gas EBCTs. Calculation results of toluene removal efficiencies and biomass accumulation rates in the outerlayer media are larger than that in the innerlayers, that implys the existence of surface pattern in biomass accumulation and the surface biofiltration process. On the basis of modeling of RDBs, process models of TBF for toluene removal were developed. Mathematical modeling results suggest that the removal efficiency and biofilm thickness in the TBF medium are changed with time and spatial position.This dessertation researched on the modeling of rotating drum biofiltration, experimental evaluation of the performance of TBF for differenct VOCs removal over a long period and the modeling of tubular biofiltration process. The reseach results would provide a feasible way to solve the difficulties including the large footprints and the low removal efficiencies existing in the traditional biofilters and biotrickling filters when running over a long period at high organic lodaings, and is helpful for understanding the mechanisms of rotating drum biofiltration and tubular biofiltration process. This study provides scientific foundation and theriotical basis on the development and application of tubular biofilter.