Simultaneous nitrification and denitrification (SND) at a microbial fuel cell (MFC) Biocathode

The relationship between energy production and simultaneous nitrification and denitrification via bioelectric derivation of microbial fuel cells at different anode numbers

In this study, three microbial fuel cells (MFCs) with different numbers of anodes (i.e., 1A, 3A, and 5A) were constructed to study the effects of a multi-anode (MA) system on power generation performance and nitrogen (N) removal from low carbon (C)/N wastewater. The maximum power density of 3A-MFC was 236.7 mW m^-3, which was 2.6-fold and 1.2-fold that of 1A-MFC and 5A-MFC, respectively. The 3A-MFC system produced the highest total energy output in one cycle, approximately 41.7 mW h, which was 1.5-fold and 1.3-fold that of 1A-MFC and 5A-MFC, respectively. 3A-MFC also had the highest total N (TN) removal efficiency (71.1 ± 3.9%) and simultaneous nitrification and denitrification (SND) rate (93.5 ± 2.4%). An analysis of electron flow distribution in the 3A-MFC biocathode showed that electro-autotrophic denitrification accounted for 19% of the total denitrification in the last 135 h. Thereafter, the relationships between TN removal, anode number, and bioelectricity were systematically evaluated. TN removal efficiency had a good linear relationship with energy production (R² = 0.97539); TN removal was mainly dependent on SND. Generally, the MA-MFC configuration proposed in this study produced more electrical energy and improved TN removal by enhancing nitrification and heterotrophic and electro-autotrophic denitrification of the biocathode. The proposed method is therefore effective for enhancing N removal.

Simultaneous removal of concentrated organics, nitrogen and phosphorus nutrients by an oxygen-limited membrane bioreactor

Simultaneous removal of organics, nitrogen and phosphorus was achieved in a bench-scale oxygen-limited membrane bioreactor (OLMBR). Due to the limited dissolved oxygen (~ 0.2 mg/L equilibrium concentration) and the increased sludge concentration associated with the hollow fiber membrane, the OLMBR was endowed with an excellent performance on the removal of multi-pollutants. The optimized removal efficiencies of COD, nitrogen (N), and total phosphorus (TP) were approximately 95.5%, 90.0% and 82.6%, respectively (COD/N/P = 500:10:1, influent loading = 5.0 kg COD·m^-3·d^-1, 35°C). Mass balance and bacterial community analysis indicated that the removal of organic carbon was mainly achieved by the methane production process (67.6%). Short-cut nitrification-denitrification (SCND) was observed as the primary denitrification process in the OLMBR, in which the concentrated organic compounds served as the electron donors for denitrification. Nitrosomonas was observed to be the predominant ammonium-oxidizing bacteria, while nitrite-oxidizing bacteria were almost absent in the microbial community as revealed by the high-throughput sequencing technique. In addition, Euryarchaeota and Candidatus, which were well associated with the process of denitrifying anaerobic methane oxidation, were also detected. Sludge absorption was the main route for TP removal in the OLMBR, and the production of PH₃ gas also accounted for 19.4% of TP removal. This study suggested that the interception effect of hollow fiber membrane provided higher sludge concentration, therefore offering more bacteria for pollutant removal. The OLMBR can be used for simultaneous removal of highly concentrated organics and nutrients in livestock and poultry breeding wastewater.

Bacterial community structure of autotrophic denitrification biocathode by 454 pyrosequencing of the 16S rRNA gene

Few studies have been conducted to explore the community composition in denitrifying biocathode. Herein, the microbial communities of denitrifying biocathodes yielding current of 1 mA (reactor C1) and 1.5 mA (reactor C2) were characterized by 454 pyrosequencing. The nitrate removal efficiencies in C1 and C2 were about 93 and 85%, respectively. The optimization of data generated high-quality sequences of 18509 in C1 and 14857 in C2. Proteobacteria was the predominant phylum, and Bacteroidetes, Chloroflexi, and Planctomycetes were the subdominant groups. Classes of Alphaproteobacteria, Anaerolineae, and Phycisphaerae may benefit the performance of current production and nitrate removal. Twenty-nine dominant operational taxonomic units (OTUs) accounted for 64 and 65% of sequences in C1 and C2, respectively. A denitrifying pathway was constructed based on the phylogenetic analysis and function inferring of the dominant OTUs. Obviously, the 454 pyrosequencing provided a high-resolution profile of bacteria community in denitrifying biocathode.

Dissimilatory nitrate reduction to ammonium as an electron sink during cathodic denitrification

Nitrate reduction to ammonium is shown as a competitive pathway during cathodic denitrification at low potential, and is dependent on biofilm age and electron uptake capacity.