Effects of aeration and internal recycle flow on nitrous oxide emissions from a modified Ludzak-Ettinger process fed with glycerol

Evaluating biochar and its modifications for the removal of ammonium, nitrate, and phosphate in water

Removal of nitrogen (N) and phosphorus (P) from water through the use of various sorbents is often considered an economically viable way for supplementing conventional methods. Biochar has been widely studied for its potential adsorption capabilities for soluble N and P, but the performance of different types of biochars can vary widely. In this review, we summarized the adsorption capacities of biochars in removing N (NH₄-N and NO₃-N) and P (PO₄-P) based on the reported data, and discussed the possible mechanisms and influencing factors. In general, the NH₄-N adsorption capacity of unmodified biochars is relatively low, at levels of less than 20 mg/g. This adsorption is mainly via ion exchange and/or interactions with oxygen-containing functional groups on biochar surfaces. The affinity is even lower for NO₃-N, because of electrostatic repulsion by negatively charged biochar surfaces. Precipitation of PO₄-P by metals/metal oxides in biochar is the primary mechanism for PO₄-P removal. Biochars modified by metals have a significantly higher capacity to remove NH₄-N, NO₃-N, and PO₄-P than unmodified biochar, due to the change in surface charge and the increase in metal oxides on the biochar surface. Ambient conditions in the aqueous phase, including temperature, pH, and co-existing ions, can significantly alter the adsorption of N and P by biochars, indicating the importance of optimal processing parameters for N and P removal. However, the release of endogenous N and P from biochar to water can impede its performance, and the presence of competing ions in water poses practical challenges for the use of biochar for nutrient removal. This review demonstrates that progress is needed to improve the performance of biochars and overcome challenges before the widespread field application of biochar for N and P removal is realized.

Quantification of nosZ genes and transcripts in activated sludge microbiomes with novel group-specific qPCR methods validated with metagenomic analyses

Substantial N₂O emission results from activated sludge nitrogen removal processes. N₂O-reducing organisms possessing NosZ-type N₂O reductases have been recognized to play crucial roles in suppressing emission of N₂O produced in anoxic activated sludge via denitrification; however, which of the diverse nosZ-possessing organisms function as the major N₂O sink in situ remains largely unknown. Here, nosZ genes and transcripts in wastewater microbiomes were analyzed with the group-specific qPCR assays designed de novo combining culture-based and computational approaches. A sewage sample was enriched in a batch reactor fed continuous stream of N₂ containing 20-10,000 ppmv N₂O with excess amount (10 mM) of acetate as the source of carbon and electrons, where 14 genera of potential N₂O-reducers were identified. All available amino acid sequences of NosZ affiliated to these taxa were grouped into five subgroups (two clade I and three clade II groups), and primers/probe sets exclusively and comprehensively targeting the subgroups were designed and validated with in silico PCR. Four distinct activated sludge samples from three different wastewater treatment plants in Korea were analyzed with the qPCR assays and the results were validated with the shotgun metagenome analysis results. With these group-specific qPCR assays, the nosZ genes and transcripts of six additional activated sludge samples were analyzed and the results of the analyses clearly indicated the dominance of two clade II nosZ subgroups (Flavobacterium-like and Dechloromonas-like) among both nosZ gene and transcript pools.

Spatiotemporal distribution and interaction of denitrifying functional genes in a novel DAS-NUA biofilter used for groundwater nitrate treatment

A newly combined dewatered alum sludge (DAS) and neutralized used acid (NUA) biofilter has been constructed and investigated recently, aiming for improving nitrate (NO₃^--N) removal in simulated groundwater and exploring the spatiotemporal distribution of nirS and nosZ. The biofilter achieved 81.54% and 13.6 g N/ (m³ d) removal efficiency of NO₃^--N during the stabilization period. Spatiotemporal distributions of diversity and composition of nirS and nosZ varied approximately in two media with depths and time. Both DAS and NUA played important roles in attenuating nitrate because of predominant denitrifying genera functions, and the core differences were Rhodanobacter and Rhodobacter in DAS while Halomonas, Pseudogulbenkiania, and Cupriavidus in NUA. Acting as the strongly correlated genera, Magnetospirillum and Halomonas had a significantly positive or negative correlation with other dominant genera. Positive correlations existed among COD, TN, NO₃^--N, NO₂^--N, and both nirS and nosZ in the DAS filter, whereas the correlations were negative in the NUA filter. Particularly, the effluent concentration of NO₃^--N had a significantly negative correlation with the relative abundance of Rubrivivax and Pseudomonas. These results could be useful in adjusting the denitrification of nitrogen contaminants at the genetic level, especially in mitigating the influence of discharge of NO₃^--N on the process of groundwater restoration.

Enrichment, Isolation, and Characterization of High-Affinity N2O-Reducing Bacteria in a Gas-Permeable Membrane Reactor

The recent discovery of nitrous oxide (N₂O)-reducing bacteria suggests a potential biological sink for the potent greenhouse gas N₂O. For an application toward N₂O mitigation, characterization of more isolates will be required. Here, we describe the successful enrichment and isolation of high-affinity N₂O-reducing bacteria using a N₂O-fed reactor (N₂OFR). Two N₂OFRs, where N₂O was continuously and directly supplied as the sole electron acceptor to a biofilm grown on a gas-permeable membrane, were operated with acetate or a mixture of peptone-based organic substrates as an electron donor. In parallel, a NO₃^- -fed reactor (NO₃FR), filled with a nonwoven sheet substratum, was operated using the same inoculum. We hypothesized that supplying N₂O vs NO₃^- would enhance the dominance of distinct N₂O-reducing bacteria. Clade II type nosZ bacteria became rapidly enriched over clade I type nosZ bacteria in the N₂OFRs, whereas the opposite held in the NO₃FR. High-throughput sequencing of 16S rRNA gene amplicons revealed the dominance of Rhodocyclaceae in the N₂OFRs. Strains of the Azospira and Dechloromonas genera, canonical denitrifiers harboring clade II type nosZ, were isolated with high frequency from the N₂OFRs (132 out of 152 isolates). The isolates from the N₂OFR demonstrated higher N₂O uptake rates (V_max: 4.23 × 10^-3-1.80 × 10^-2 pmol/h/cell) and lower N₂O half-saturation coefficients (K_m,N2O: 1.55-2.10 μM) than a clade I type nosZ isolate from the NO₃FR. Furthermore, the clade II type nosZ isolates had higher specific growth rates on N₂O than nitrite as an electron acceptor. Hence, continuously and exclusively supplying N₂O in an N₂OFR allows the enrichment and isolation of high-affinity N₂O-reducing strains, which may be used as N₂O sinks in bioaugmentation efforts.

Startup strategy for nitrogen removal via nitrite in a BAF system

A biological aerated filter (BAF) pilot plant consisting of two reactors (aerobic and anoxic one) was used to determine a strategy to remove nitrogen via nitrite. RNA/DNA analysis was performed to assess microbial activity and support chemical results. In less than 13 days the pilot plant was able to remove COD and suspended solids. Nitrogen removal via nitrite pathway could not be observed until day 130 when the empty bed contact time (EBCT) was set at 0.71 h. Nitrite was detected in the aerated BAF effluent but never nitrate. qPCR of amoA gene from RNA and DNA extracts of the aerobic biofilm confirmed that ammonia oxidizing bacteria (AOB) were present from the beginning of the operation but not active. AOB activity increased with time, reaching stability from operational day 124. The combination of both, low EBCT together with high OLR, has been demonstrated to be a feasible strategy to startup a BAF to achieve nitrogen removal via nitrite.

Counter-diffusion biofilms have lower N2O emissions than co-diffusion biofilms during simultaneous nitrification and denitrification: Insights from depth-profile analysis

The goal of this study was to investigate the effectiveness of a membrane-aerated biofilm reactor (MABR), a representative of counter-current substrate diffusion geometry, in mitigating nitrous oxide (N₂O) emission. Two laboratory-scale reactors with the same dimensions but distinct biofilm geometries, i.e., a MABR and a conventional biofilm reactor (CBR) employing co-current substrate diffusion geometry, were operated to determine depth profiles of dissolved oxygen (DO), nitrous oxide (N₂O), functional gene abundance and microbial community structure. Surficial nitrogen removal rate was slightly higher in the MABR (11.0 ± 0.80 g-N/(m² day) than in the CBR (9.71 ± 0.94 g-N/(m² day), while total organic carbon removal efficiencies were comparable (96.9 ± 1.0% for MABR and 98.0 ± 0.8% for CBR). In stark contrast, the dissolved N₂O concentration in the MABR was two orders of magnitude lower (0.011 ± 0.001 mg N₂O-N/L) than that in the CBR (1.38 ± 0.25 mg N₂O-N/L), resulting in distinct N₂O emission factors (0.0058 ± 0.0005% in the MABR vs. 0.72 ± 0.13% in the CBR). Analysis on local net N₂O production and consumption rates unveiled that zones for N₂O production and consumption were adjacent in the MABR biofilm. Real-time quantitative PCR indicated higher abundance of denitrifying genes, especially nitrous oxide reductase (nosZ) genes, in the MABR versus the CBR. Analyses of the microbial community composition via 16S rRNA gene amplicon sequencing revealed the abundant presence of the genera Thauera (31.2 ± 11%), Rhizobium (10.9 ± 6.6%), Stenotrophomonas (6.8 ± 2.7%), Sphingobacteria (3.2 ± 1.1%) and Brevundimonas (2.5 ± 1.0%) as potential N₂O-reducing bacteria in the MABR.