Effect of Nitrate Reduction on the Methanogenic Fermentation: Process Interactions and Modeling
Tugtas, Adile Evren
MetadataShow full item record
Combined treatment technologies for the removal of waste carbon, nitrogen, and/or sulfur under anoxic/anaerobic conditions have recently received considerable attention. It has been reported that nitrate and/or reduced N-oxides, such as nitrite (NO2-), nitric oxide (NO), and nitrous oxide (N2O), which are products of denitrification, suppress methanogenesis. Research was conducted to investigate the effect of N-oxides and sulfide on mixed, mesophilic (35oC) methanogenic cultures, along with the effect of the type of electron donor on the kinetics and pathway of nitrate reduction. Among all N-oxides tested, NO exerted the most and nitrate exerted the least inhibitory effect on the fermentative/methanogenic consortia. Long-term exposure of a methanogenic culture to nitrate resulted in an increase of N-oxide reduction and a decrease of methane production rates. Sulfide addition to sulfide-free enriched cultures resulted in inhibition of NO2-, NO, and N2O reduction causing accumulation of these intermediates, which in turn inhibited methanogenesis and fermentation. In nitrate-amended, sulfide-acclimated cultures, nitrate reduction occurred via dissimilatory nitrate reduction to ammonia (DNRA); thus, accumulation of N-oxides was avoided and inhibition of methanogenesis was prevented. The nitrate reduction rates in cultures fed with different electron donors followed the descending order: H2/CO2 > acetate > glucose > dextrin/peptone > propionate. Denitrification was observed in the propionate-, acetate-, and H2/CO2-fed cultures regardless of the COD/N value. Both denitrification and DNRA were observed in the dextrin/peptone- and glucose-fed cultures and the predominance of either of the two pathways was a function of the COD/N value. Nitrate reduction processes were incorporated into the IWA Anaerobic Digestion Model No. 1 (ADM1) in order to account for the effect of nitrate reduction processes on fermentation and methanogenesis. The extended ADM1 described the experimental results very well. Model simulations showed that process interactions during nitrate reduction within an overall methanogenic system cannot be explained based on only stoichiometry and kinetics, especially for batch systems and/or continuous-flow systems with periodic, shock nitrate loads. The results of this research are useful in predicting the fate of carbon-, nitrogen-, and sulfur-bearing waste material, as well as in understanding microbial process interactions, in both natural and engineered anoxic/anaerobic systems.