Dr. Hélène Fruteau de Laclos, Yves Membrez, Julien Dovat, Caroline Tacchini, EREP SA Aclens (Main Applicants); Prof. Christof Holliger, Juan Villegas
Swiss Federal Office for Energy, Swiss Federal Office of Environment, AXPO
Mr. Martin, Puidoux, Switzerland
Ammonia emissions are of increasing environmental concern. They are responsible for atmospheric acidification, over-fertilization of crops and fragile ecosystems, and they also might result in health impact to humans and animals. Anaerobic digestion coupled with biogas utilization could imply advantages in agricultural waste management, regarding infrastructure and cost-effectiveness. Biogas production facilities on farms are increasing in number in Switzerland due to environmental regulations and opportunities in the green energy market. However, while anaerobic digestion improves manure quality and reduces methane emissions, high ammonium content in the liquid effluent is still an issue.
The objective of this study was to evaluate strategies to reduce ammonia (NH3) emissions from liquid effluents of agricultural anaerobic digesters by biologically removing nitrogen. The approach envisioned at the beginning of this work included partial nitrification of the digester effluent and its subsequent recycling into the digester where nitrite can be transformed into nitrogen gas by denitrification or Anammox.
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.
In order to test whether biofilm processes or aerobic denitrification occurring in suspended growth processes were responsible for the nitrogen removal observed, another 15-L bioreactor has been operated as suspended growth process in continuous-culture mode. After optimization of the dilution rate and aeration, nitritation of ammonium was achieved with efficiencies of 80-90% and only small amounts of the nitrite produced has been removed by denitritation. The reactor was then transformed into a moving-bed reactor by the addition of Kaldnes® supports that occupied 50% of the reactor volume. Denitritation increased quite rapidly after support addition and finally reached rates of approximately 75-80% of the nitrogen introduced into the reactor system. These results indicated that microbial processes occurring in biofilms were responsible for the nitrogen removal observed. In addition, it could be shown that one can achieve nitrogen removal in one single reactor that allows avoiding the initially proposed recycling the nitritation-reactor effluent into the anaerobic fermenter. This facilitates considerably the reactor operation making the system much better adapted for agricultural biogas production installations.