||Ammonia, a colorless gas with a characteristic pungent odor, is produced by various industrial and agricultural activities. Emissions of ammonia into the atmosphere not only cause a nuisance in the vicinity of the sources, but also have various environmental effects, such as eutrophication and acidification of terrestrial and aquatic ecosystems, and visibility problems resulting from the formation of aerosols. The traditional treatment of ammonia emissions is based on physical and/or chemical processes, both of which are expensive and produce secondary pollutants. Biological methods are effective and economical for biodegradable odorants and VOC contaminants. This study used fixed-film bioreactors, a biofilter and a biotrickling filter, to remove and oxidize gas-born ammonia.|
Firstly, a pilot-scale biofilter consisted of two columns (40 cmW × 40 cmL × 70 cmH acrylic column) arranged in series. A medium consisting solely of fern chips, on which biofilms were cultivated, was used as a packing material. The biofilter was tested continuously for 110 days, measuring the removal efficiency, empty bed residence time (EBRT), removal capacity, pressure drop, moisture content and pH. Most of ammonia was eliminated in the first biofiltration column and the removal efficiency increased with the increase in EBRT. Complete removal of the influent ammonia (20-120 ppm) was obtained with an ammonia loading as high as 5.4 g N kg-1 dry media d-1 during the experiment. The Michaelis-Menten equation was tested to be adequate for modeling the ammonia elimination kinetics in the biofilter and the maximum removal rate (Vm) and the half-saturation constant (Ks) were estimated to be 28.2 g N kg-1 dry media d-1 and 129 ppm, respectively.
Secondly, a pilot-scale reactor, consisting of a set of two-stage-in-series biotrickling filters, an influent gas supply system and a liquid recirculation system, was utilized to treat ammonia in an air stream. Each stage of the biotrickling filter was constructed from a 20 cm × 200 cm (inner diameter × height) acrylic column packed with cokes (average diameter = 3.0 cm, specific area = 150 m2/m3) of 125 cm height. Experimental results indicate that a time of 30 days is required for development of biofilms for nitrification of the absorbed ammonia from the gas. Long-term (187 days) experimental results show that, in the conditions of EBRT (empty bed gas retention time) = 7.25 s, “circulation liquid/gas” flow rate ratio = 7.7 L m-3, and liquid pH = 6.65, the level of ammonia in the influent gas was reduced from 230 to 4.0 ppm. With the volumetric ammonia loading of less than 7.37 g NH3-N m-3 hr-1, the system could achieve ammonia removal and nitrification efficiencies of 98 and 94%, respectively, without supplementary glucose as a carbon source. However, with a loading of 13.1 g NH3-N m3 h-1, both decreased gradually due to a lake of carbon source and an accumulation of ammonium and nitrite ions in the recirculation liquid.