Nitrogen Removal Pathway Research Articles

Nitrogen removal performance and microbiological characteristics for the landfill leachate treatment in a three-stage vertical flow constructed wetlands system

Landfill leachate (LL) has the characteristics of high NH 4 + -N and low C/N ratio, increasing the cost and difficulty of efficient treatment. Constructed wetlands system (CWs) has proven to be feasible for the treatment of LL. A constructed three-stage series of vertical flow CWs (termed CW1, CW2, and CW3) based on effluent recirculation (T-VF) was used to treat LL, and nitrogen removal effect and microbiological characteristics was investigated. The results show that the nitrogen removal efficiency showed a increased trend from CW1 to CW3. The average removal efficiencies of TN, NH 3 + -N, and NO 3 − -N in T-VF were 91.43%, 94.19%, and 98.11%, respectively, which showed good nitrogen removal performance. Furthermore, application of the combined CW1, CW2 and CW3 increased diversity of substrate bacterial communities and decreased the relative abundances of the dominant nitrifying(e.g. Nitrosomonas , Nitrobacter ) and heterotrophic denitrifying bacteria (e.g. Thauera ). The relative abundances of autotrophic denitrification (e.g. Thiobacillus ) increased. Pathway of nitrogen metabolism and functional genes analysis showed that the abundance of enzymes associated with the denitrification and dissimilatory nitrate reduction increased and that the abundance of enzymes associated with the nitrification decreased. Application of the combined CW1, CW2 and CW3 could be respectively formed corresponding nitrogen removal microbial community, influent quality and pathways of nitrogen to enhance the nitrogen removal effect of T-VF to treat LL. • The removal rate of nitrogen was over 90% in vertical flow constructed wetlands. • Nitrosomonas and Nitrobacter were the dominant nitrification bacteria. • Denitrifying bacteria Thauera , Thiobacillus , and Lautropia predominated.

Nitrogen removal and metabolic pathway of Enterobacter cloacae DK-6

A heterotrophic nitrifier and aerobic denitrifier was isolated from chaff bedding of a pig farm and identified as Enterobacter cloacae DK-6. The strain DK-6 showed efficient nitrogen removal abilities under aerobic condition, with the ammonium-N (105.56mg/L), nitrate-N (101.70mg/L) and nitrite-N (86.19mg/L) removal efficiency of 86.98, 94.66 and 100% under aerobic condition, respectively. Further experiments demonstrated that the nitrate-N removal efficiency was improved and strain DK-6 preferred to utilize nitrite-N when ammonium-N, nitrate-N and nitrite-N were used as mixed nitrogen sources. Nitrogen balance showed ammonium-N removal was mainly achieved by assimilation in the process of nitrification, while nitrate-N removal was mainly realized through N 2 loss and hardly no greenhouse gas ( N 2 O) was generated during denitrification. Efficient nitrate-N removal occurs at sodium succinate as carbon source, carbon/nitrogen ratio of 8–10, dissolved oxygen concentration of 4.3–5.2 mg/L, pH of 6–7, temperature at 25–35 °C and initial nitrate-N concentration of 50–200 mg/L. Above results suggested strain DK-6 has significant potential involving in multiple nitrogen pollutants treatment of wastewater under aerobic condition and lower carbon/nitrogen ratio. • Enterobacter cloacae DK-6 was isolated from chaff bedding. • DK-6 had higher capability of multiple nitrogen pollutants removal. • Hardly no N 2 O was generated by stain DK-6 during nitrogen removal. • Nitrogen removal by strain DK-6 was mainly realized through N 2 loss.

Stream nitrogen uptake associated with suspended sediments: A microcosm study

Despite significant advances in our understanding of nitrogen (N) removal pathways along river networks, the role of water column processes remains largely understudied. This knowledge gap not only limits our capacity to determine N transport and retention in mid-to-large rivers but also hampers our understanding of N removal processes in smaller streams during stormflow conditions, in which significant increases in suspended sediment concentrations (SSC) typically occur. High SSC in the water column can provide abundant substrate for microbial growth and water column N uptake. However, storms of different size mobilize different quantities of sediment of varying properties and sizes, which can ultimately modulate water column N uptake rates in the stream during stormflows. To assess water column N uptake associated with suspended sediment particles of different sources and sizes, we quantified assimilatory and dissimilatory N uptake rates in a set of microcosms representing a gradient of sediment properties (organic matter, N content, and microbial activity) and surface area (fine vs. coarse size) availability. Water column assimilatory uptake (Used) ranged from 12.7 to 187.8 µg N [g sediment]−1 d−1 across all sediment sources and size fractions, and was higher on average than denitrification rates (DNsed) in agricultural and stream bank sediments but not in streambed sediments (mean DNsed = 240.9 ± 99 µg·N [g sediment]−1·d−1). Sediment-bound C in suspended sediment varied among sediment sources and was directly related to Used rates, but not to DNsed rates, which were less predictable and more variable. Overall, our results showed a positive nonlinear relationship between water column N uptake and SSC, while indicating that water column N uptake may scale differently to SSC depending on sediment source, and to a lesser degree, particle size. Because low, moderate, and large storms can mobilize different quantities of sediment in the watershed of different sources and sizes, it is likely that storm size will ultimately modulate the contribution of water column uptake during storm events to whole-reach N retention.

Bioaugmentation performances with a powerful strain for nitrogen removal without N2O accumulation

N2O is regarded as an inevitable intermediate during nitrogen removal, especially for wastewater treatment plants where good operating conditions would be required to mitigate N2O releasing, which generally causes a high treatment cost. In this study, a novel bacterium capable of removing nitrogen without N2O accumulation was isolated and identified as Citrobacter freundii XY-1. The nitrogen removal characteristics, nitrogen removal pathway, bioaugmentation in different reactors as well as microbial diversity were investigated. Results showed that 99.42% of NH+ 4-N and 95% of total organic carbon could be removed within 48 h with the corresponding removal rates being 4.03 mg/(L·h) and 39.42 mg/(L·h), respectively. It was inferred that traditional denitrification and N2O generation do not exist in the pathway of removing nitrogen by XY-1 based on isotope analysis and functional genes detection. Bioaugmentations of XY-1 in both sequencing batch reactor and biological aerated filter significantly promoted the performances of nitrogen removal. The microbial diversity indicated that the relative abundance of strain XY-1 ranged from 45% to 66%, predominating throughout the running period. Overall, XY-1 could become an incredibly important candidate for the upgrading of wastewater treatment plants.

Mainstream anammox driven by micro-oxygen nitrification and partial denitrification using step-feed for advanced nitrogen removal from municipal wastewater

This study developed a novel integrated nitrification, partial denitrification (PD), and anammox (INPDA) process in micro-aerobic sequencing batch reactor using step-feed strategy treating municipal wastewater. The results showed that the INPDA could achieve a 94.5 ± 1.1% total inorganic nitrogen (TIN) removal efficiency with effluent TIN concentration as low as 2.3 mg/L. The micro-aerobic with low dissolved oxygen concentration of 0.36 ± 0.01 mg/L allowed complete ammonium oxidation to nitrate. The applied step-feed by optimizing influent distribution ratio to 5:3 provided sufficient organics for nitrate reduction to nitrite. According to mass balance analysis, anammox was the dominant nitrogen removal pathway (87.9%). Microbial analysis indicated that aerobic nitrifiers Nitrosomonas (1.2%) and Nitrospira (0.9%) ensured full nitrification, while PD bacteria Thauera (27.1%) conducted labor metabolism coordination with anammox bacteria Candidatus Brocadia (1.3%) for nitrogen removal. These findings demonstrate that the INPDA has a good prospect for energy-efficient municipal wastewater treatment.

Quantitatively deciphering the roles of sediment nitrogen removal in environmental and climatic feedbacks in two subtropical estuaries

Sedimentary denitrification and anaerobic ammonium oxidation (anammox) are two microbially-mediated nitrogen removal pathways with distinct climatic feedbacks. Estuaries receive large fluxes of anthropogenic nitrogen and serve as hotspots for nitrogen loss. Applying 15N isotope pairing technique and sediment intact core incubation in two subtropical estuaries, the Yangtze River Estuary (YRE) and Jiulong River Estuary (JRE), we show that denitrification predominates the sedimentary nitrogen loss with a minor contribution (8.6 ± 7.5%) from anammox. Particulate organic matter degradation sustains the sedimentary nitrogen removal linking the nitrogen transformations between water column and sediment. Our results indicate that estuarine sediments exhibit high areal nitrogen removal rate, but play a relatively weak role in eliminating the nitrogen inputted from river basin due to the limited area. The riverine excess nitrogen will eventually enter into the adjacent continental shelf and be removed via phytoplankton assimilation-sedimentation-degradation-coupled nitrification-denitrification. In addition, sedimentary denitrification causes 1.8 ± 2.2% of nitrogen flow towards nitrous oxide (N2O) production and the derived N2O release flux accounts for 59% and 65% of the daily sea-air N2O emission in the YRE and JRE, respectively. These findings contribute to a better understanding of estuarine sedimentary nitrogen removal and associated climate feedbacks, and to the parameterization of Earth system models.

Weak electric field effect of MFC biocathode on denitrification

Microbial fuel cells (MFCs) have been reported to enhance denitrification. However, it is difficult to investigate the denitrification enhancement mechanism under weak electric field (WEF) due to the multi-parameter variation in MFCs. In this study, a series of potentiostatic bioelectrochemical systems (BESs) for wastewater with a low COD/N ratio were constructed by simulating the MFC biocathode. The denitrification effect differences of the BESs under different constant potentials (−400, −200, +200 and +400 mV vs. Ag/AgCl) were investigated. The nitrogen removal pathway was identified by exploring the source and the flow direction of electrons in the biocathode. The electron transfer method in the biocathode and the influencing mechanism of the WEF on microorganisms was elucidated by electrochemical analysis, high throughput sequencing and PICRUSt2. The results showed that the WEF in MFCs could enhance the denitrification effect by increasing the biomass, the abundance of denitrifying functional bacteria (Thauera, SM1A02, Plasticicumulans, Ignavibacterium, Methylopolylis), the expression of functional genes (nasA, narGH, narI, narV, nirk, hcp, nosZ), and Nar and Nir enzyme activities, thereby providing electrons for the electroautotrophic denitrifying bacteria (Thauera). The electron transfer method between the biofilm of denitrifying bacteria and the solid cathode electrode mainly involved direct electron transfer, and there were at least two electroactive components, which might be related to Nar and Nir activities. These results have important theoretical significance and positive support for the application of microbial electrochemical systems and the realization of the advanced treatment of wastewater with a low COD/N ratio.

Anoxic-Aerobic-Anoxic sequencing batch reactor for enhanced nitrogen removal

An anoxic-aerobic-anoxic process was established to achieve simultaneous removal of organic carbon and nitrogen from wastewater in a sequencing batch reactor. The optimum conditions were attained at a DO of 1.5 mg/L with 1 h pre-anoxic and post-anoxic periods. TOC, NH4+-N, and TN removal efficiencies were 98.76 ± 0.95 %, 98.52 ± 0.48 %, and 88.23 ± 0.62 %, respectively, at optimum conditions. Breakpoints in the pH, DO, and ORP curves provided a clear understanding of biochemical reactions happening in the reactor. Inhibition studies showed that 27.69 % of NH4+-N was removed through nitrogen removal pathways such as heterotrophic nitrification or direct conversion, and 20.55 % of TN was removed through aerobic denitrification. Microbial community analysis confirmed the presence of heterotrophic nitrifiers and aerobic denitrifiers. This study highlighted that the varied redox conditions offered by limited aeration would be beneficial for nitrogen removal, thereby reducing the energy usage and operating costs.

A half-century of research on microalgae-bacteria for wastewater treatment

The use of microalgae-bacteria consortia (MBC) for wastewater treatment have received increasing attention because of its high capacity for nutrients and organic matter removal, using sunlight as an energy source, through self-sufficient oxygen production provided by photosynthesis . This review addresses the research hotspots and main challenges of this emerging technology, using bibliometric analysis . The evolution of MBC-related literature from 1970 to 2021 was analyzed, identifying of the main research topics explored, based on the frequency of the keywords. This document develops the following topics identified as research hotspots: 1) The factors that influence the complex interactions between microalgae and bacteria (e.g., pH, incident light, dissolved oxygen), and the removal mechanisms of nutrients and organic matter, which are still relevant issues still requiring significant research efforts. 2) The new enhanced pathways of total nitrogen and phosphorus removal , such as short cut nitrogen removal, algammox, and phosphorus luxury uptake, which have been explored during the last decade. 3) The application of mathematical models to represent the behavior of MBC, as a tool for prediction and optimization of MBC systems. And 4) The biomass retention technologies, such as biofilms, membrane bioreactors , and granular sludge; that allows the uncoupling of HRT from SRT , a condition required for high-rate wastewater treatment. • Research trends found using bibliometric analysis were consistent with the experiences found in the literature. • Nitrogen removal mechanisms and biomass retention are relevant research trends in the field. • Mathematical models are a powerful tool to describe and simulate complex biological processes such as MBC. • Further research is required on combining the MBC with biomass retention technologies such as biofilm or membranes. • Research and applications of MBC system in Latin American countries should be promoted.

Simultaneous nitritation, denitritation and phosphorus removal in an algal-bacterial consortium system treating low-strength mariculture wastewater

Mariculture wastewater is characterized by high salinity and low concentration of pollutants, which poses a great challenge to the conventional wastewater treatment processes. In this study, an algal-bacterial consortium process for simultaneous organic carbon, nitrogen and phosphorus removal from such wastewater was investigated in a red-blue (4:1) LED photo-sequencing batch reactor (PSBR). Simultaneous bacterial ammonium oxidation (70 %) and algal ammonium assimilation (30 %) was obtained in the photo phase. In the dark phase, a weak ammonium assimilation was also observed. In the PSBR with an influent COD/N of 5:1, the nitrogen removal pathway was identified to be simultaneous nitritation and denitritation probably due to low-rate oxygenation of algae at a light luminance of 500 lx. In addition, the average COD and phosphorus removal efficiencies reached 82.4 % and 68.2 % with a removal activity of 0.63 mg PO43−-P/(L·d). High-throughput sequencing results showed that Nitrosomonus and Nitrospina were the dominant nitrifying genera using oxygen released by Cyanobacteria and Bacillariophyta. Therefore, it is feasible to efficiently remove nitrogen and phosphorus from mariculture wastewater by the algal-bacterial consortium, which opens a new window for the treatment of high-salinity and low-strength wastewater.

Enhancement of partial denitrification-anammox pathways in constructed wetlands by plant-based external carbon sources

Insufficient carbon source/electron donor often limits the denitrification efficiency of constructed wetlands (CWs) when treating wastewater characterized by low carbon-to-nitrogen ratios. Hence, the viability of wetland plant-based carbon sources for enhanced nitrogen removal and its mechanisms in horizontal subsurface flow CWs was assessed. Results showed cattails to have a better carbon release capacity than wheat straw and corn cob. Alkali pretreatment of cattails is demonstrated to significantly enhance the release of volatile fatty acids (68.31 ± 1.36 mg/g) dominated by acetic acid to sustain a high denitrification rate (0.62 ± 0.05 mg/(L·h)) over a long duration. External carbon source addition to adjusting C/N ratio to 3.5 achieved the highest removal rates of 75.45%, 91.33% and 76.03%, respectively, for total nitrogen (TN), ammonia-nitrogen (NH4+-N) and nitrate-nitrogen (NO3−-N). High-throughput sequencing showed plant-based external carbon sources to significantly increase the relative abundance of the denitrifying bacteria, as well as the anammox bacteria. Moreover, it elevated the number of nitrate reductase relative to nitrite reductase by 4.65-fold. Plant-based external carbon sources enhanced the partial denitrification-anammox (PDN/AMX) contribution to TN removal by at least 27.29% and improved nitrogen removal efficiency through co-promotion of denitrification and PDN/AMX pathways. Findings from this study support the utilization of plant-based external carbon sources in CWs. Additionally, it provides new insights into enhancement pathways of nitrogen removal with plant-based carbon sources.

Pathways of Nitrogen and Phosphorus Utilization and Removal from Cyanobacteria Wastewater by Combining Constructed Wetlands with Aerobic Reactors

Due to its low C/N ratio and high concentrations of nitrogen and phosphorus, the effluent of anaerobic cyanobacteria fermentation cannot be directly discharged without further treatment. To effectively reduce nutrient loads and utilize the nutrient resources of biogas slurry generated from the anaerobic digestion of stored algae, two different aerobic treatment units (AUs) were combined with an ecological treatment unit (EU) to create two different treatment systems. The two AUEU systems paired a constructed wetland (CW) with either a cascade biological contact reactor (CBCR) or a carrousel oxidation ditch reactor (CODR). In this paper, the water quality characteristics of biogas slurry were measured, and comprehensive experiments on the two trial-treatment systems were carried out to validate their performance in removing pollutants and utilizing resources. Furthermore, the pollutant removal efficiencies of the combined systems, along with the removal mechanisms and utilization of the nitrogen and phosphorus in the CWs, were also investigated. The results showed that the CWs, with aquatic vegetation, took up the majority of removed nitrogen and phosphorus by absorption, which effectively reduced the concentration of pollutants in the effluent and enabled the nitrogen and phosphorus to be reused in plants. Biomass assimilation by the absorption by vegetation took up 75.8%, 66.1%, 70.3%, and 86% of the removed NH4+-N, NOx−-N, TN, and TP, respectively.

Successional dynamics of low C/N activated sludge system under salinity shock: Performance, nitrogen removal pathways, microbial community, and assembly

Limited carbon (low C/N) and salinity stress affect the stability of wastewater treatment plants. However, the effect of salinity shock on activated sludge systems with low C/N ratio wastewater remains unclear. An anaerobic/aerobic/anoxic sequencing batch reactor treating low C/N wastewater was established to investigate the effects of salinity shock on system performance, nitrogen removal pathways, microbial community, interactions, and assembly. The results showed that the effluent COD concentration could maintain a stable level, and the average COD removal efficiency was 94.9%. However, total nitrogen removal was significantly inhibited. With the addition of salinity, efficiencies of total nitrogen removal and simultaneous nitrification and denitrification decreased from 91.4 to 73.8% to 86.7 and 39.7%, respectively; however, nitrite reduction capacity increased by 25.4%. After removing salinity, ammonia oxidation capacity further deteriorated, evidenced by the increase in effluent NH4+-N from 8.0 to 11.8 mg/L. During the salinity shock, partial nitrification became the main nitrogen removal pathway because of the inhibition of Nitrospira and high nitrite accumulation ratio (>99.0%). Molecular ecological network analysis indicated that increased competition, decreased total modules, and disappearance of keystone taxa were related to the deterioration of ammonia oxidation capacity and simultaneous nitrification and denitrification. Moreover, the abundant denitrification module and increased denitrifiers contributed to the increase in nitrite reduction capacity. Salinity shock under low C/N conditions resulted in a stronger stochastic community assembly. This study provided information that can help enable stable operations for treating low C/N wastewater.

Lab‐scale data and microbial community structure suggest shortcut nitrogen removal as the predominant nitrogen removal mechanism in post‐aerobic digestion (PAD)

Implementing an aerobic digestion step after anaerobic digestion, referred to as "post aerobic digestion" (PAD), can remove ammonia without the need for an external carbon source and destroy volatile solids. While this process has been documented at the lab-scale and full-scale, the mechanism for N removal and the corresponding microbial community that carries out this process have not been established. This research gap is important to fill because the nitrogen removal pathway has implications on aeration requirements and carbon demand, that is, short-cut N-removal requires less oxygen and carbon than simultaneous nitrification-denitrification. The aims of this research were to (i) determine if nitrite (NO2 - ) or nitrate (NO3 - ) dominates following ammonia removal and (ii) characterize the microbial community from PAD reactors. Here, lab-scale PAD reactors were seeded with biomass from two different full-scale PAD reactors. The lab-scale reactors were fed with biomass from full-scale reactors and operated in batch mode to quantify nitrogen species concentrations (ammonia, NH4 + , NO2 - , and NO3 - ) over time. Experimental results revealed that NO2 - production rates were several orders of magnitude greater than NO3 - production rates. Indeed, nitrite accumulation rate (NAR) was greater than 90% at most temperatures, confirming that shortcut nitrogen removal was the dominant NH4 + removal mechanism in PAD. Microbial community analysis via 16S rRNA sequencing indicated that ammonia oxidizing bacteria (AOB) were much more abundant than nitrite oxidizing bacteria (NOB). Overall, this study suggests that aeration requirements for post-aerobic digestion should be based on NO2 - shunt and not complete simultaneous nitrification denitrification. PRACTITIONER POINTS: AOB are a key feature of PAD microbial communities NOB are present, but in much lower abundance than AOB High nitrite accumulation ratio suggests shortcut nitrite as the main mechanism for nitrogen removal Nitritation in PAD reactors is sustained at temperatures as high as 40°C No ammonia oxidation occurred at 50°C implying different mechanisms of nitrogen removal including ammonia stripping.

Nitrous Oxide Emission from Full-Scale Anammox-Driven Wastewater Treatment Systems.

Wastewater treatment plants (WWTPs) are important contributors to global greenhouse gas (GHG) emissions, partly due to their huge emission of nitrous oxide (N2O), which has a global warming potential of 298 CO2 equivalents. Anaerobic ammonium-oxidizing (anammox) bacteria provide a shortcut in the nitrogen removal pathway by directly transforming ammonium and nitrite to nitrogen gas (N2). Due to its energy efficiency, the anammox-driven treatment has been applied worldwide for the removal of inorganic nitrogen from ammonium-rich wastewater. Although direct evidence of the metabolic production of N2O by anammox bacteria is lacking, the microorganisms coexisting in anammox-driven WWTPs could produce a considerable amount of N2O and hence affect the sustainability of wastewater treatment. Thus, N2O emission is still one of the downsides of anammox-driven wastewater treatment, and efforts are required to understand the mechanisms of N2O emission from anammox-driven WWTPs using different nitrogen removal strategies and develop effective mitigation strategies. Here, three main N2O production processes, namely, hydroxylamine oxidation, nitrifier denitrification, and heterotrophic denitrification, and the unique N2O consumption process termed nosZ-dominated N2O degradation, occurring in anammox-driven wastewater treatment systems, are summarized and discussed. The key factors influencing N2O emission and mitigation strategies are discussed in detail, and areas in which further research is urgently required are identified.

Removal efficacy and mechanism of nitrogen and phosphorus by biological aluminum-based P-inactivation agent (BA-PIA)

In this study, aluminum-based P-inactivation agent (Al-PIA) was used as a high-efficiency microbial carrier, and the biological Al-PIA (BA-PIA) was prepared by artificial aeration. Laboratory static experiments were conducted to study the effect of BA-PIA on reducing nitrogen and phosphorus contents in water. Physicochemical characterization and isotope tracing method were applied to analyze the removal mechanism of nitrogen and phosphorus. High-throughput techniques were used to analyze the characteristic bacterial genus in the BA-PIA system. The nitrogen and phosphorus removal experiment was conducted for 30 days, and the removal rates of NH4+-N, TN and TP by BA-PIA were 81.87%, 66.08% and 87.97%, respectively. The nitrogen removal pathways of BA-PIA were as follows: the nitrification reaction accounted for 59.0% (of which denitrification reaction accounted for 56.4%), microbial assimilation accounted for 18.1%, and the unreacted part accounted for 22.9%. The characteristic bacteria in the BA-PIA system were Streptomyces, Nocardioides, Saccharopolyspora, Nitrosomonas, and Marinobacter. The loading of microorganisms only changed the surface physical properties of Al-PIA (such as specific surface area, pore volume and pore size), without changing its surface chemical properties. The removal mechanism of nitrogen by BA-PIA is the conversion of NH4+-N into NO2−-N and NO3−-N by nitrifying bacteria, which are then reduced to nitrogen-containing gas by aerobic denitrifying bacteria. The phosphorus removal mechanism is that metal compounds (such as Al) on the surface of BA-PIA fix phosphorus through chemisorption processes, such as ligand exchange. Therefore, BA-PIA overcomes the deficiency of Al-PIA with only phosphorus removal ability, and has better application prospects.

Achieving deep autotrophic nitrogen removal in aerated biofilter driven by sponge iron: Performance and mechanism

Different from anammox, the combination of Fe (III) reduction coupled to anaerobic ammonium oxidation (Feammox) and nitrate/nitrite dependent ferrous oxidation (NDFO) do not require to control nitrite accumulation. Furthermore, sponge iron can avoid continuous iron supplementation in practice and is a good iron source for the occurrence of Feammox and NDFO in wastewater treatment. Therefore, a biofilter using sponge iron as carrier treating low nitrogen wastewater was built. In this study, the performances of nitrogen removal were explored under different hydraulic retention times (HRT) and gas-water ratios in sponge iron biofilter. And the pathways of nitrogen removal were analyzed by activity tests. The results showed ammonia removal efficiency reached 94.1% and total inorganic nitrogen removal efficiency was up to 70.6% at HRT of 19 h and gas-water ratio of 18. Compared to nitrogen removal by adsorption under non-aeration, the activity tests showed that total inorganic nitrogen loss was caused by Feammox and NDFO after aeration. The results of microbial communities showed that appearances of nitrifier-Nitrosomonadaceae, Feammox bacteria-Clostridiaceae and NDFO bacteria-Gallionellaceae resulted in deep nitrogen removal after aeration, in which Nitrosomonadaceae and Clostridiaceae contributed to ammonia removal and Gallionellaceae contributed to nitrite/nitrate reduction to nitrogen gas. Therefore, it was feasible to achieve deep autotrophic nitrogen removal and Fe (II) and Fe (III) cycle in sponge iron biofilter.

Effects of flow mode on the performance and microbial community in tidal flow constructed wetland treating mariculture wastewater

This study aimed to investigate the influence of flow mode (down-flow and up-flow) on the removal of organic matter and nitrogen, and their transformation processes in two tidal flow constructed wetlands (TFCWs) fed with simulated mariculture wastewater with 2.4% salinity and wastewater without adding salt (0.0% salinity). Down-flow mode was beneficial for COD removal (98.28–99.62%), while up-flow mode improved the denitrification capacity and benefited the removal of total inorganic nitrogen (TIN) (81.46–88.26%). High salinity decreased the NH4+-N removal (70.42–73.05%) compared with 0.0% salinity (80.57–86.04%), and lead to higher NO2−-N accumulation. High-thought sequencing indicated flow mode, salinity and depth along the TFCWs together alter the composition and abundance of functional bacteria in the wetlands. Higher abundance of ammonia oxidizing bacteria (AOB) was observed under down-flow mode (0.3–3.7%) compared with up-flow mode (0.2–0.9%) in two TFCWs. Denitrifying bacteria (DNB) enriched in 38–43 cm depth layer in saline system and 0–5 cm depth layer in freshwater system, with the relative abundance of 18.3–63.8%. High salinity inhibited the growth of AOB and nitrite oxidizing bacteria (NOB), while DNB could adapt to salinity stress and maintained high abundance in saline system. The enrichment of halotolerant/halophilic bacteria and diverse nitrogen removal pathways provided a solid foundation for the efficient nitrogen removal in TFCW treating mariculture wastewater.

The intrinsic relevance of nitrogen removal pathway to varying nitrate loading rate in a polycaprolactone-supported denitrification system.

Up to date, the intrinsic association of nitrate loading rate (NLR) with treatment performance of solid-phase denitrification (SPD) systems is still ambiguous. To address this issue, three continuous up-flow bioreactors were configured. They were packed with polycaprolactone (PCL) under a filling ratio of 30%, 60% or 90% and were operated under a varying NLR of 0.34 ± 0.01-3.99 ± 0.12 gN/(L·d). Results showed that the denitrification efficiency was high (RE > 96%) and stable except the case with the highest NLR, which was mainly attributed to the lack of available carbon sources. At the phylum or genus level, most of the detected dominant bacterial taxa were either associated with organics degradation or nitrogen metabolism. The difference in bacterial community structure among the three stages was mainly caused by NLR rather than the filling ratio. Moreover, as the NLR got higher, the Bray-Curtis distance between samples from the same stage became shorter. By the results of gene or enzyme prediction performed in PICRUSt2, the main nitrogen metabolism pathways in these reactors were denitrification, dissimilatory nitrate reduction to ammonium (DNRA), assimilatory nitrate reduction to ammonium (ANRA) and nitrogen fixation. Moreover, aerobic and anaerobic nitrate dissimilation coexisted in the systems with the latter playing a dominant role. Finally, denitrification and DNRA occurred under both high and low NLR conditions while nitrogen fixation and ANRA preferred to occur under low NLR environments. These findings might help guide practical applications.

Treatment of Ammonium-Nitrogen–Contaminated Groundwater by Tidal Flow Constructed Wetlands Using Different Substrates: Evaluation of Performance and Microbial Nitrogen Removal Pathways

Treatment of Ammonium-Nitrogen–Contaminated Groundwater by Tidal Flow Constructed Wetlands Using Different Substrates: Evaluation of Performance and Microbial Nitrogen Removal Pathways