Effect of Media Nitrogen Concentration on Biofilter Performance.

A pilot-scale biofilter was used to determine important design and operational parameters related to biofiltration of volatile organic compounds (VOCs). The importance of nitrogen availability in terms of biofilter performance was determined. Results showed that microbially accessible nitrogen (ammonia and nitrate) in the areas of highest microbial activity were depleted when toluene loading rates were 30 g/m-h or greater. This depletion led to a marked reduction in performance in terms of the VOC elimination rate of the biofilter. The amount of nitrogen available to microorganisms can be depleted by microbial uptake of soluble nitrogen to make new biomass, stripping of ammonia, denitrification of nitrate, and leaching. Nitrogen is made available by mineralization of biomass, mineralization of organic nitrogen to soluble nitrogen, and addition of nitrogen fertilizers. Mineralization of biomass to ammonia results in the recycle of nitrogen through the system. Even though organic nitrogen in the media was present at a high concentration, its mineralization to ammonia was not fast enough to keep the media from becoming nitrogen limited during extended periods of high toluene loading. Therefore, if biofilters are designed to operate at high VOC loading rates and over long periods of time, care must be taken to assure adequate amounts of microbially available nitrogen. The threshold amount of soluble nitrogen that is needed to avoid nitrogen limitations is discussed.