| Combined treatment technologies for the removal of carbon, nitrogen, and/or sulfur under anoxic/anaerobic conditions have recently received considerable attention due to advantages such as cost and space reduction. Nitrate is released by various industrial and agricultural activities. Biological nitrate reduction has been successfully used for the removal of nitrate from wastewater. Nitrate reduction occurs in two distinct pathways: dissimilatory nitrate reduction to nitrogen gas (denitrification) and dissimilatory nitrate reduction to ammonia (DNRA). Sulfide, a common constituent in anaerobic digesters, plays an important role in the prevailing pathway of nitrate reduction. It has been reported that nitrate and/or reduced N-oxides, such as nitrite (NO2 -), nitric oxide (NO), and nitrous oxide (N2O), suppress methanogenesis. To date, research dealing with the effect of nitrate reduction on methanogenesis has been conducted either with pure cultures or with soil microcosms. Research involving anaerobic digestion with mixed methanogenic cultures has only investigated the effect of nitrate. In addition, a systematic study on the effect of nitrate and sulfide in the presence of different electron donors on the inhibition of methanogenesis is lacking. Research presented here was conducted to investigate the effect of N-oxides and sulfide on a mixed methanogenic culture, along with the effect of the type of electron donor on the kinetics and pathway of nitrate reduction. The inhibitory effects of nitrate, NO2-, NO, and N2O on a sulfide-free mixed, mesophilic (35°C) methanogenic culture were investigated. Among all N-oxides, NO exerted the most and nitrate exerted the least inhibitory effect on the fermentative/methanogenic consortia. Long-term exposure of the methanogenic culture to nitrate resulted in an increase of N-oxide reduction rates and decrease of methane production rates, which was attributed to changes in the microbial community. The effect of sulfide on nitrate reduction and methanogenesis was investigated in sulfide-free and sulfide-acclimated methanogenic cultures. Sulfide addition to sulfide-free enriched cultures resulted in inhibition of NO2-, NO, and N2 O reduction causing accumulation of these intermediates, which in turn inhibited methanogenesis and fermentation. In the nitrate-amended, sulfide-acclimated cultures, instead of accumulation of N-oxides, nitrate reduction occurred via DNRA and converted NO2- to ammonia; thus, accumulation of N-oxides was avoided and inhibition of methanogenesis was prevented. The effect of different electron donors on the pathway and kinetics of nitrate reduction was investigated in a sulfide-acclimated methanogenic culture. The nitrate reduction rates in the cultures fed with different substrates were as follows in descending order: H2/CO2 > acetate > glucose > dextrin/peptone > propionate. Denitrification was the dominant pathway of nitrate reduction in the propionate-, acetate-, and H2/CO 2-fed cultures regardless of the COD/N ratio value. However, 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 ratio 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. |
