Nitrogen removal from digested manure to decrease ammoniac emissions from agriculture

Christof Holliger
2007
Discussion par affiche

Résumé

Biogas production transforms organic matter into methane, carbon dioxide and ammonia (NH3). The aim of this study was reducing NH3 emissions from liquid effluents of agricultural anaerobic digesters. An alternative to classic nitrification-denitrification was tested: partial nitrification of the ammonium-rich effluent and its recycling into the digester for nitrite conversion into dinitrogen gas (N2) via denitrification or anaerobic ammonium oxidation (Anammox). Two laboratory-scale bioreactors were set-up to this effect. In the first reactor, short residence time and oxygen-limited conditions were guaranteed. The second was filled with digested manure simulating the final stage of a plug-flow anaerobic digester. The feed was artificially amended with nitrtite (NO2-), simulating the effluent of the first reactor. Ammonium (NH4+) removal efficiencies higher than 80% were reached in the aerobic reactor, producing a NO2--rich effluent. However, mass balance showed a large percentage of “nitrogen losses”. This can either be explained by denitrification or Anammox occurring in anoxic zones inside the reactor or by aerobic denitrification, an interesting but not yet fully understood process. The anaerobic reactor removed about 90% of nitrite fed and during a certain period also approximately 80% of ammonium was degraded. Using FISH analysis, Anammox bacteria were identified in the microbial community of this reactor.

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https://infoscience.epfl.ch/record/162153?ln=fr

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A process based on partial nitrification and recirculation into the anaerobic digester was studied to remove nitrogen from digested manure and thus reduce enhanced gaseous ammonia emissions due to on-farm biogas production. An anaerobic reactor representing an anaerobic manure digester was fed with a nitrite solution and digested manure liquor. Nitrite was efficiently removed from the influent and ammonium formation was observed first. Ammonium was subsequently eliminated up to a maximum of 90% of the influent concentration indicating anaerobic ammonium oxidation activity. This activity however decreased again and was lost at the end of the 4 months operation period. In an 1.5-L aerobic CSTR that was fed with digested manure liquor, ammonium was efficiently removed from the influent. Nitrite and nitrate formation was observed but mass balances indicated significant N-removal. Accumulation of suspended solids was observed at the end of the experiment suggesting presence of oxygen-free environments. In a second test in a 15-L CSTR where suspended solids sedimentation could be avoided, low N-removal rates were observed in the absence of biofilm carrier elements whereas high N-removal rates were achieved in their presence. A simple one-stage process based on immobilized biomass could therefore be installed downstream of agricultural anaerobic digesters in order to mitigate undesirable gaseous ammonia emissions

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