THE EFFECT OF NITRATE ON VOC REMOVAL IN TRICKLE-BED BIOFILTERS

In response to the growing concern over volatile organic compounds (VO Cs), 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 param eters that affect the performance of gas phase biofilters. This study utilized diethyl ether as a substrate, nitrate as the sole nutrient ni trogen 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 parame ters including contaminant concentration in the gas phase, nutrient ni trate concentration in the aqueous phase, and the frequency of biomass removal were considered. Special attention was given to the effect an d the role of nitrate on VOC removal. Throughout the experiment, nitra te persisted in the liquid effluent and the ether removal efficiencies improved with increasing influent nitrate concentration, which sugges t 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 e ther concentration of 133 ppmv and a feed nitrate concentration of 67 mg-N/liter, the performance of the biofilter was not significantly aff ected. These results suggest that nitrogen was rate limiting as a grow th nutrient rather than as an electron acceptor for the respiration of ether. The results also indicated that removal of excess biomass is n ecessary to maintain long-term performance. However, the required freq uency of biomass removal depends on operating parameters such as loadi ng. (C) 1997 Elsevier Science Ltd.