Xueqing Zhu

Effect of substrate Henry's constant on biofilter performance

Abstract Butanol, ether, toluene, and hexane, which have Henrys constants ranging from 0.0005 to 53, were used to investigate the effects of substrate solubility or availability on the removal of volatile organic compounds (VOCs) in trickle-bed biofilters. Results from this study suggest that, although removal of a VOC generally increases with a decrease in its Henrys constant, an optimal Henrys constant range for biofiltration may exist. For the treatment of VOCs with high Henrys constant values, such as hexane and toluene, the transfer of VOCs between the vapor and liquid phases or between the vapor phase and the biofilm is a rate-determining step. However, oxygen (O2) transfer may become a rate-limiting step in treating VOCs with low Henrys constants, such as butanol, especially at high organic loadings. The results demonstrated that in a gas-phase aerobic biofilter, nitrate can serve both as a growth-controlling nutrient and as an electron acceptor in a biofilm for the respiration of VOCs with low Henrys constants. Microbial communities within the biofilters were examined using denaturing gradient gel electrophoresis to provide a more complete picture of the effect of O2 limitation and denitrification on biofilter performance.

The effect of nitrate on VOC removal in trickle bed biofilters

Abstract In response to the growing concern over volatile organic compounds (VOCs), biofiltration is becoming an established economical air pollution control technology for removing VOCs from waste air streams. Current research efforts are concentrating on improving control over key parameters that affect the performance of gas phase biofilters. This study utilized diethyl ether as a substrate, nitrate as the sole nutrient nitrogen source within two co-currently operated trickle-bed biofilters, for over 200 days. The two pelletized medium biofilters were operated at a low empty bed contact time of 25 s, inlet gas flow rates of 8.64 m 3 /day, nutrient liquid flow rates of 1 liter/day, and COD loading rates of 1.8 and 3.6 kg/m 3 per day, respectively. Operational parameters including contaminant concentration in the gas phase, nutrient nitrate concentration in the aqueous phase, and the frequency of biomass removal were considered. Special attention was given to the effect and the role of nitrate on VOC removal. Throughout the experiment, nitrate persisted in the liquid effluent and the ether removal efficiencies improved with increasing influent nitrate concentration, which suggest that the nitrate diffusion into the biofilms is rate determining. By increasing the concentration of oxygen in the feed to this biofilter from 21% (ambient air) to 50 and 100%, while maintaining an influent ether concentration of 133 ppmv and a feed nitrate concentration of 67 mg-N/liter, the performance of the biofilter was not significantly affected. These results suggest that nitrogen was rate limiting as a growth nutrient rather than as an electron acceptor for the respiration of ether. The results also indicated that removal of excess biomass is necessary to maintain long-term performance. However, the required frequency of biomass removal depends on operating parameters such as loading.

Effect of gas empty bed contact time on performances of various types of rotating drum biofilters for removal of VOCs

The effects of gas empty bed contact time (EBCT), biofilter configuration, and types of volatile organic compounds (VOCs) were evaluated to assess the performance of rotating drum biofilters (RDBs), especially at low EBCT values. Three types of pilot-scale RDBs, a single-layer RDB, a multi-layer RDB, and a hybrid RDB, were examined at various gas EBCTs but at a constant VOC loading rate. Diethyl ether, toluene, and hexane were used separately as model VOC. When EBCT increased from 5.0 to 60s at a constant VOC loading rate of 2.0kgCOD/(m(3)day), ether removal efficiency increased from 73.1% to 97.6%, from 81.6% to 99.9%, and from 84.0% to 99.9% for the single-layer RDB, the multi-layer RDB, and the hybrid RDB, respectively, and toluene removal efficiency increased from 76.4% to 99.9% and from 84.8% to 99.9% for the multi-layer RDB and the hybrid RDB, respectively. When hexane was used as the model VOC at a constant loading rate of 0.25kgCOD/(m(3)day), hexane removal efficiency increased from 31.1% to 57.0% and from 29.5% to 50.0% for the multi-layer RDB and hybrid RDB, respectively. The single-layer, multi-layer, and hybrid RDBs exhibited, respectively, the lowest, middle, and highest removal efficiencies, when operated under similar operational loading conditions. Hexane exhibited the lowest removal efficiency, while diethyl ether displayed the highest removal efficiency. The data collected at the various EBCT values correlated reasonably well with a saturation model. The sensitivity of removal efficiency to EBCT varied significantly with EBCT values, VOC properties, and biofilter configurations. Process selection and design for RDB processes should consider these factors.

Performance of rotating drum biofilter for volatile organic compound removal at high organic loading rates

Uneven distribution of volatile organic compounds (VOCs) and biomass, and excess biomass accumulation in some biofilters hinder the application of biofiltration technology. An innovative multilayer rotating drum biofilter (RDB) was developed to correct these problems. The RDB was operated at an empty bed contact time (EBCT) of 30 s and a rotational rate of 1.0 r/min. Diethyl ether was chosen as the model VOC. Performance of the RDB was evaluated at organic loading rates of 32.1, 64.2, 128, and 256 g ether/(m3 x h) (16.06 g ether/(m3 x h) approximately 1.0 kg chemical oxygen demand (COD)/(m3 x d)). The EBCT and organic loading rates were recorded on the basis of the medium volume. Results show that the ether removal efficiency decreased with an increased VOC loading rate. Ether removal efficiencies exceeding 99% were achieved without biomass control even at a high VOC loading rate of 128 g ether/(m3 x h). However, when the VOC loading rate was increased to 256 g ether/(m3 x h), the average removal efficiency dropped to 43%. Nutrient limitation possibly contributed to the drop in ether removal efficiency. High biomass accumulation rate was also observed in the medium at the two higher ether loading rates, and removal of the excess biomass in the media was necessary to maintain stable performance. This work showed that the RDB is effective in the removal of diethyl ether from waste gas streams even at high organic loading rates. The results might help establish criteria for designing and operating RDBs.

Mathematical model for the biodegradation of VOCs in trickle bed biofilters

The objective of this work is to develop a fundamental mathematical model that describes the biodegradation of volatile organic compounds (VOCs) in gas phase trickle-bed biofilters, and to estimate the unknown model parameters. The mathematical model considers a three-phase system (biofilm, water, and gas phase), non-uniform bacterial population, and one limiting substrate. Two pilot-scale trickle-bed biofilters were operated to remove the VOC diethyl ether from a waste gas stream. Experimental results from this system were used to estimate the unknown parameters of the steady-state model: the maximum rate of substrate utilization μ m X f / Y , the Monod constant, K s , and the ether biofilm/water diffusivity ratio, r d . While the value of μ m X f / Y was uniquely determined, the values of K s and r d were highly correlated. High values of the Monod constant and the ether diffusivity in the biofilm gave similar predictions to those corresponding to low values of both parameters. Batch studies were used to estimate the value of K s without mass transfer limitations showing that K s < 1 mg/L=2.6 mg COD/L. Using this information and biofilter operating data the true values of K s and r d were determined.