Biological removal of gaseous ammonia in biofilters: Space travel and earth-based applications

Gaseous NH3 removal was studied in laboratory-scale biofilters (14-L reacto r volume) containing perlite inoculated with a nitrifying enrichment cultur e. These biofilters received 6 L/min of airflow with inlet NH3 concentratio ns of 20 or 50 ppm, and removed more than 99.99% of the NH3 for the period of operation (101, 102 days). Comparison between an active reactor and an a utoclaved control indicated that NH3 removal resulted from nitrification di rectly, as well as from enhanced absorption resulting from acidity produced by nitrification. Spatial distribution studies (20 ppm only) after 8 days of operation showed that nearly 95% of the NH3 could be accounted for in th e lower 25% of the biofilter matrix, proximate to the port of entry. Period ic analysis of the biofilter material (20 and 50 ppm) showed accumulation o f the nitrification product NO3- early in the operation, but later both NO2 - and NO3- accumulated. Additionally, the N-mass balance accountability dro pped from near 100% early in the experiments to similar to 95 and 75% for t he 20- and 50-ppm biofilters, respectively. A partial contributing factor t o this drop in mass balance accountability was the production of NO and N2O , which were detected in the biofilter exhaust.