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Increased nitrous oxide (N 2 O) in the atmosphere is of global concern. Biofiltration has been studied for gaseous nitrogen treatments in drinking water environments; however, the molecular mechanisms mediating N 2 O accumulation in denitrification biofilters have not been quantified. Five denitrification biofilters were developed in this study and all achieved high removal efficiencies for total nitrogen (TN: 72.4–92.7%), nitrate nitrogen (NO 3 − -N: 88.6–98.2%) and chemical oxygen demand (COD: 79.3–90.0%) across a low-temperature gradient (5–25 °C). Denitrification coupling with anaerobic ammonium oxidation (ANAMMOX) and dissimilatory nitrate reduction to ammonium (DNRA) processes yielded the presently robust treatment performance. Nitrite availability limited TN removal at 5–15 °C. Our findings indicate that temperature affected N 2 O accumulation indirectly by controlling the balance of nitrite versus N 2 O reductase carrying microorganisms. In addition, our results demonstrated that genetic association was an important index reflecting the relative intensity of N 2 O accumulation at low temperatures. [ABSTRACT FROM AUTHOR]

Heterotrophic denitrification is usually inhibited by insufficient carbon sources; however, the underlying mechanisms responsible for nitrous oxide (N 2 O) accumulation within denitrification at low carbon:nitrogen (C/N) ratios have not been quantified from a molecular level. In this study, five denitrification biofilters were developed and exhibited efficiency (total nitrogen: 18.5%–92.2%; nitrate nitrogen: 42.9%–99.5%; chemical oxygen demand: 50.5%–93.7%) in remediating micro-polluted water with C/N ratios ranging between 0.65 and 3.0. A combined analysis revealed that the coupling of anaerobic ammonium oxidation (ANAMMOX) and denitrification accounted for N 2 O accumulation in the biofilters, and the key drivers of the N 2 O accumulation rates were qnorB/nirK , nosZ/(narG + napA) , amx/(nirS + nirK) , narG/bacteria and qnorB/bacteria . Our study demonstrated that genetic association was indicative of microbial processes relative to nitrogen cycling and reflected N 2 O flux within denitrification biofilters at low C/N ratios. [ABSTRACT FROM AUTHOR]

The present study explored the distribution patterns of nitrogen micro-cycle genes and the underlying mechanisms responsible for nitrogen transformation at the molecular level (genes) in a biotrickling filter (biofilter). The biofilter achieved high removal efficiencies for ammonium (NH 4 + -N) (80–94%), whereas nitrate accumulated at different levels under a progressive NH 4 + -N load. Combined analyses revealed the anammox, nas , napA , narG , nirS , and nxrA genes were the dominant enriched genes in different treatment layers. The presence of simultaneous nitrification, ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) were the primary factors accounted for the robust NH 4 + -N treatment performance. The presence of DNRA, nitrification, and denitrification was determined to be a pivotal pathway that contributed to the nitrate accumulation in the biofilter. The enrichment of functional genes at different depth gradients and the multi-path coupled cooperation at the functional gene level are conducive to achieving complete nitrogen removal. [ABSTRACT FROM AUTHOR]

The multimedia biofilter achieved high and stable removal efficiencies for chemical oxygen demand (COD, 62–98%) and NH 4 + (68–98%) without costly aeration. Results revealed that lower CL (less than 13.9 g COD/m 3 d) and ACL (less than 2.8 g NH 4 + -N/m 3 d) or a C/N ratio exceeding five was required to reduce NO 3 − -N accumulation and NO/N 2 O emission. Integrated analyses indicated that the coupling of simultaneous nitrification, anammox and denitrification processes (SNAD) were the primary reason accounted for the enhanced NH 4 + -N treatment performance. NH 4 + -N removal pathways can be ranked as follows: nitrification ( amoA , archaeal) (54.6%) > partial denitrification ( nirS , nirK ) and anammox (37.8%) > anammox and partial denitrification ( narG , napA ) (12.6%). Specifically, NH 4 + -N removal was significantly inhibited by NO 2 − -N accumulation in the system (−21.6% inhibition). Results from stepwise regression analysis suggested that the NH 4 + removal rate was collectively controlled by amoA , archaeal, anammox, nirS , nirK , narG and napA . [ABSTRACT FROM AUTHOR]

An aerobic denitrification biofilter (ADB) for groundwater remediation was developed with high removal efficiencies (total nitrogen (TN): 82.3–95.8%; NO 3 − -N: 93.2–98.2%). Nitrate (NO 3 − -N) transformation rates stabilized between 21.0 and 23.4 g/(m 3 h), whereas nitrite (NO 2 − -N) and ammonium (NH 4 + -N) transformation rates remained less than 6.0 g/(m 3 h) as the dissolved oxygen (DO) level increased from 1.0 mg/L to 6.0 mg/L. Nitric oxide (NO) and nitrous oxide (N 2 O) accumulated with great fluctuations (NO: 0–1.6 × 10 −3 g/(m 3 h); N 2 O: 0.1–1.1 g/(m 3 h)) throughout the experiment. This study suggested that gene associations reflect quantitative relationships with aerobic denitrification rates and can provide useful information regarding aerobic denitrification processes in groundwater. Especially, the qnorB / nosZ ratio acts as the main driver for NO 3 − -N and NH 4 + -N transformation, while the qnorB / nosZ ratio followed by the ( nirS + nirK )/ nosZ ratio serve a dominant role in the accumulation of N 2 O and NO. [ABSTRACT FROM AUTHOR]

The present study explored the treatment performance and nitrogen removal pathways in a multimedia biofilter treating micro-polluted source water under hydraulic loading rate (HLR) constraints, ranging from 0.5 to 3.0 m 3 /m 2 d. High and stable chemical oxygen demand (COD) (97.7–99.1%) and ammonium (NH 4 + -N) (76.3–90.9%) removal efficiency were simultaneously achieved. Results showed that an HLR exceeding 2.5 m 3 /m 2 d was required to achieve complete denitrification without NO 3 − -N accumulation in the biofilter. Molecular biological analyses showed that nitrification and anaerobic ammonium oxidation were the dominant NH 4 + -N removal pathways in the biofilter. Quantitative analysis demonstrated that the key functional gene groups for NH 4 + -N transformation process rate were amoA /archaea, nxrA /archaea, and ( nirS + nirK )/anammox. Furthermore, nxrA /archaea contributed the most to the NH 4 + -N transformation rate, followed by ( nirS + nirK )/anammox, and amoA /archaea. The results support that archaea potentially play vital roles in the nitrogen removal pathways, and nitrogen transformation pathways are coupled at the molecular level (i.e., the functional gene level). [ABSTRACT FROM AUTHOR]

Abstract: Real-time PCR was fist used to quantitatively study the spatial distribution of the absolute abundance, relative abundance, and relative richness of amoA, nxrA, narG, napA, nirK, qnorB, nosZ, nas, and nifH genes and anaerobic ammonium oxidation (anammox, or ANO) bacteria in four multimedia biofilters. The results showed that these multimedia biofilters could provide NH4 + and total nitrogen (TN) average removal efficiencies of 88%. Except for nifH, ANO and the other nitrogen removal functional genes all were dominantly enriched in different zones. In fact, ANO, napA, qnorB, nosZ, and nxrA were enriched by 5.6–330.1 times; and nirK, narG, amoA, and nas were enriched by 1.4–2.1 times. In addition, ANO, napA, qnorB, and nosZ showed partially or mutually beneficial cooperation. The nxrA and nirK genes showed protocooperation, and the amoA and narG genes showed partially beneficial cooperation. [Copyright &y& Elsevier]