High Dissolved Oxygen Concentration Research Articles

Re-aerobic treatment and dissolved oxygen regulation in full-scale aerobic-hydrolysis and denitrification-aerobic process for achieving simultaneous detoxification and nitrification of coking wastewater

The biological treatment of coking wastewater is a challenge. The application of prepositioned aerobic process has rarely been systematically reported, among which the detoxification and nitrification performance of the prepositioned aerobic unit (O1) is worthy of investigation. Results indicate that O1 achieves stable simultaneous detoxification and nitrification by regulating the dissolved oxygen, effectively maintaining ammonification, nitrosation, and complete nitrification phases. Microbial community structure, metabolic pathways and functional genes showed different preferences at different phases. High dissolved oxygen concentrations (2.20–3.00 mg/L) benefited the enrichment of carbon and nitrogen related major metabolic pathways and functional genes. BOD5/CODCr ratio, dissolved oxygen and toxic pollutants together shaped microbial community structure and nitrogen transformation processes. Based on the principle of DO regulation, it could assemble a biotransformation compartment for nitrogen removal from complex wastewaters through a pollutant detoxification mechanism of rapid microbial proliferation，and provides a promising approach for toxic industrial wastewater.

Comparative studies of the long-term corrosion behavior of Zr-Sn-Fe-Cr-Ni alloys in pure water at 360°C/18.6 MPa with high and low dissolved oxygen content

This study compared the uniform corrosion behavior of Zr-Sn-Fe-Cr-Ni alloys in deionized water at 360°C/18.6 MPa with high and low concentration of dissolved oxygen (DO). It was found that high concentration of DO accelerated the corrosion rate of the Zr-Sn-Fe-Cr-Ni alloys and led to an earlier corrosion transition. Increased DO concentration resulted in a higher content of t-ZrO2 near the metal-oxide interface, which induced greater in-plane compressive stress in the oxide film and a high-level phase transformation from t-ZrO2 to m-ZrO2. This, in turn, led to an earlier occurrence of corrosion transition.

Effect of dissolved oxygen on sulfur autotrophic denitrification and how to address it: An experimental and modelling work

Sulfur autotrophic denitrification (SAD) using elemental sulfur as the electron donor has aroused increasing interest of its application in treating secondary effluent from wastewater treatment plants (WWTPs). However, high influent dissolved oxygen (DO) in secondary effluent would limit the SAD process. This study examined the effect of different DO concentrations on SAD. Results revealed that both low (0–0.5 mg/L) and moderate (2.5–3.5 mg/L) DO concentrations would not harm the nitrate removal rate (NRR) (p > 0.05). However, high DO concentration (5.5–6.5 mg/L) significantly decreased the NRR (p < 0.05) through strong competition over the nitrate for electrons and cutting the relative abundance of sulfur-oxidizing bacteria (SOB). Both modeling and experimental results showed that applying internal reflux could serve as a strategy to mitigate the negative effect of high DO concentration, while keeping an appropriate ratio was crucial. When treating real membrane bioreactor (MBR) effluent with high DO concentration (5.5–6.5 mg/L), an internal reflux ratio of 0.5 boosted the NRR by 1.5 times. This study provided potential reference and strategy for dealing with high DO concentration wastewater by applying SAD technology.

Intense intrusion of low-oxygen waters into mid-Cambrian surface ocean carbonate factories

The Phanerozoic surface ocean is characterized by its high dissolved oxygen content owing to mixing with the atmosphere. However, atmospheric oxygen levels varied in the early Paleozoic and it remains unclear whether the surface ocean was susceptible to significant redox fluctuations in response to extreme environmental events. In this study, we probed the redox structures of shallow middle Cambrian marine depositional environments across the North China Platform, ranging from open tidal flats to relatively deep subtidal environments. We utilized a combination of least diagenetically altered carbonate materials (such as ooid cortices, calcimicrobes, and their fringing cements), as well as in situ element measurement and imaging techniques. By analyzing a set of redox-related elements (e.g., Ce anomaly, Zn/Fe molar ratio, Mn and Cr) and mineralogical proxies (hydrogenetic Fe oxides), we revealed a stratified redox structure in the Drumian surface oceans. Compared to earlier Drumian conditions, late Drumian surface oceans experienced significant intrusions of ferruginous waters, probably reaching into shallow subtidal environments with water depths less than 10 m. Furthermore, we identified shallow subtidal microbial O2-producing factories, characterized by dendritic Epiphyton thalli. These calcimicrobes exhibited more oxygenated signatures (negative Ce anomalies and enrichment of hydrogenetic Fe oxides) relative to contemporaneous less oxic shallower and deeper environments. This finding indicates that they produced oxygen oases or refuges during periods of both normal and poor dissolved O2 conditions. This study has the potential to broaden our understanding of redox conditions and microbial oxygen-producing mechanisms in the surface ocean, particularly during intervals characterized by low atmospheric oxygen levels or episodic anoxic events.

Functional and Microbiological Responses of Iron–Carbon Galvanic Cell-Supported Autotrophic Denitrification to Organic Carbon Variation and Dissolved Oxygen Shaking

Iron–carbon galvanic-cell-supported autotrophic denitrification (IC-ADN) is a burgeoning efficient and cost-effective process for low-carbon wastewater treatment. This study revealed the influence of organic carbon (OC) and dissolved oxygen (DO) on IC-ADN in terms of functional and microbiological characteristics. The nitrogen removal efficiency increased to 91.6% and 94.7% with partial organic carbon source addition to COD/TN of 1 and 3, respectively. The results of 16S rRNA high-throughput sequencing with nirS and cbbL clone libraries showed that Thiobacillus was the predominant autotrophic denitrifying bacteria (ADB) in the micro-electrolysis-based autotrophic denitrification, which obtained nitrogen removal efficiency of 80.9% after 96 h. The ADBs shifted gradually to heterotrophic denitrifying bacteria Thauera with increasing COD/TN ratio. DO concentration of 0.8 rarely affected the denitrification efficiency and the denitrifying communities. When the DO concentration increased to 2.8 mg/L, the nitrogen removal efficiency decreased to 69.1%. These results demonstrated that autotrophic denitrification was notably affected by COD/TN and high DO concentration, which could be used to acquire optimum conditions for nitrogen removal. These results provided an in-depth understanding of the influential factors for galvanic-cell-based denitrification and helped us construct a stable and highly efficient treatment process for insufficient carbon source wastewater.

A novel process based on marine heterotrophic nitrification and aerobic denitrification bacteria for treating mariculture wastewater: Performance, biofilm characteristics and microbiome responses to dissolved oxygen

A novel reactor utilizing marine heterotrophic nitrification and aerobic denitrification (MHNAD) bacteria offers an effective approach for treating mariculture wastewater. However, the impact of dissolved oxygen (DO) concentration on its performance has been understudied. This research investigated how varying DO concentrations influence the performance, biofilm characteristics, and microbiome of the reactor when inoculated with MHNAD bacteria. The results demonstrated that reducing the DO concentration from 6 mg/L to 2 mg/L decreased the removal efficiency of chemical oxygen demand (COD) and ammonia nitrogen (NH4+-N) but significantly improved nitrate nitrogen (NO3−-N) removal. Higher DO concentrations (6 mg/L) promoted the production of extracellular polymeric substances (EPS), particularly proteins. Microbial analysis revealed that MHNAD bacteria, particularly the genus Zobellella, consistently dominated the bioreactor, though their abundance decreased at lower DO concentrations. Bugbase analysis indicated that biofilm formation and aerobic bacterial abundance decreased as DO concentration declined. Additionally, metabolic pathway analysis suggested that lower DO levels might inhibit glycolysis and TCA cycle processes, while also altering the primary nitrogen removal pathways.

Sulfate-reducing consortium HQ23 stabilizes metal(loid)s and activates biological N-fixation in mixed heavy metal-contaminated soil

Sulfate-reducing bacteria (SRB) are used in the remediation of mine pollution; however, the mechanism of stabilizing multiple heavy metal(loid)s by the SRB consortium under low oxygen conditions needs further study. Indigenous microflora were extracted from non-ferrous metal-contaminated soil co-inoculated with enriched SRB consortium and assembled as the HQ23 consortium. The presence of Desulfovibrio (SRB) in HQ23 was confirmed by 16S rRNA sequencing and qPCR. The effects of culture media, dissolved oxygen (DO), SO42¯, and pH on the HQ23 growth rate, and the SO42¯-reducing activity were examined. Data indicates that the HQ23 sustained SRB function under low DO conditions (3.67 ± 0.1 mg/L), but the SRB activity was inhibited at high DO content (5.75 ± 0.39 mg/L). The HQ23 can grow from pH 5 to pH 9 and can decrease mobile or bioavailable Cr, Cu, and Zn concentrations in contaminated soil samples. FTIR revealed that Cu and Cr adsorbed to similar binding sites on bacteria, likely decreasing bacterial Cu toxicity. Increased abundances of DSV (marker for Desulfovibrio) and nifH (N-fixation) genes were observed, as well as an accumulation of nitrate-N content in soils suggesting that HQ23 stimulates the biological N-fixation in soils. This study strongly supports the future application of SRB for the bioremediation of heavy metal-polluted sites.

Integration of down-flow hanging sponge reactor to oreochromis niloticus − Brassica oleracea aquaponics system

Aquaponics is a promising solution for addressing food security concerns. Nonetheless, an effective water-purification system is necessary to achieve high and stable yields of fish and vegetables. This study aimed to evaluate the nitrification and oxygen transfer performance of a laboratory-scale down-flow hanging sponge (DHS) reactor with a Brassica oleracea aquaponics system to treat water in an Oreochromis niloticus closed-aquaculture system. The DHS reactor showed a higher oxygen transfer coefficient (KLa) than the conventional aerator and provided an adequate dissolved oxygen (DO) concentration of approximately 5.5 mg/L essential for O. niloticus growth throughout the experimental period. The evaluated DHS-based aquaponic system maintained high water quality in an aquaculture tank, with a survival rate of 97%. The O. niloticusgrew at a low feed conversion ratio of 1.5–2.1 and a low feeding rate of 0.5% at high stocking densities of 17.5–22.2 kg-fish-weight/m3. 16S rRNA gene sequencing indicated that the DHS sponge carrier effectively retained nitrifying bacteria such as Nitrosomonas and Nitrospira. This study demonstrated that the DHS reactor provided a high DO concentration and that a simultaneous DHS reactor with a hydroponic tank provided a low-cost aquaponic system that could be applied for food production in the aquaculture industry.

Influence of water quality and seasonal variations on freshwater macroinvertebrate diversity and community structure in wastewater treatment ponds, Phetchaburi Province, Thailand

Wastewater originating from the Phetchaburi municipality undergoes treatment in a series of five distinct stages at the King’s Royally Initiated Leam Phak Bia Environmental Research and Development Project (LERD) in Phetchaburi province, Thailand. These stages involve a sedimentation (pond 1), three oxidation ponds (ponds 2 to 4), and a final stabilization pond (pond 5). These ponds serve as habitats for macroinvertebrates; consequently, their diversity and composition might be influenced by fluctuations in water quality and seasonal variations. The primary aim of this research was to analyze the diversity and species composition of macroinvertebrate communities concerning varying levels of organic contamination across the five wastewater treatment ponds at LERD. This investigation spanned three seasons: cold season (December 2019), rainy season (July 2020), and hot season (April 2021). The findings revealed that the diversity and species composition of macroinvertebrate communities displayed distinct alterations across multiple environmental gradients, especially identifying the significant influence of organic loading levels observed in ponds 1 to 5. The macroinvertebrate communities exhibited two distinct groupings, with the Chironomidae and Candonidae or ostracods prevailing prominently in ponds 1 and 2 (heterogenous environments). This prevalence was attributed to the high levels of detrital food and the robust resilience of chironomid larvae and ostracods to organic pollution, thriving even in environments characterized by low dissolved oxygen levels. Conversely, the prevalence of snails from the Thiaridae family in ponds 3 to 5 (homogenous environments) indicated improved water quality conditions, notably lower organic matter levels, and a higher dissolved oxygen content. In addition, the study identified seasonal variations in macroinvertebrates, likely influenced by the differing organic loading and environmental conditions. Thus, this research provided insights into the factors shaping macroinvertebrate communities in a wastewater treatment system.

Integrated Effects of Site Hydrology and Vegetation on Exchange Fluxes and Nutrient Cycling at a Coastal Terrestrial‐Aquatic Interface

AbstractThe complex interactions among soil, vegetation, and site hydrologic conditions driven by precipitation and tidal cycles control the biogeochemical transformations and bi‐directional exchange of carbon and nutrients across the terrestrial–aquatic interfaces (TAIs) in coastal regions. This study uses a highly mechanistic model, Advanced Terrestrial Simulator (ATS)‐PFLOTRAN, to explore how these interactions affect exchanges of materials and carbon and nitrogen cycling. We used a transect in the Chesapeake Bay region that spans zones of open water, coastal wetland, transition, and upland forest. We designed several simulation scenarios to parse the effects of the individual controlling factors and the sensitivity of carbon cycling to reaction rate parameters derived from laboratory experiments. Our simulations reveal an active zone for carbon cycling under the transition zones between the wetland and the upland. Evapotranspiration is found to enhance the exchange fluxes between the surface and subsurface domains, resulting in a higher dissolved oxygen concentration in the TAIs. The transport of organic carbon derived from plant leaves and roots provide an additional source of organic carbon needed for the aerobic respiration and denitrification processes in the TAIs. The variability in reaction rate parameters associated with microbial activities is also found to play a dominant role in controlling the heterogeneity and dynamics of the simulated redox conditions. This modeling‐focused exploratory study enabled us to better understand the complex interactions among soil, water and microbes that govern the hydro‐biogeochemical processes at the TAIs, which is an important step toward representing coastal ecosystems in larger‐scale Earth system models.

Changes in fronts regulate nitrate cycling in Zhanjiang Bay: A comparative study during the normal wet season, rainstorm, and typhoon periods

Typhoons and rainstorms (>250 mm/day) are extreme weather events changing hydrological characteristics and thus nitrogen (N) cycle in coastal waters. However, responses of N cycle to rainstorms and typhoons and their underlying mechanisms need to be elucidated. In this study, we conducted an analysis of a comparative dataset encompassing concentrations of nitrate (NO3−), ammonium (NH4+), dissolved oxygen (DO), chlorophyll a (Chl a), hydrological parameters, dual isotopic composition of NO3− (δ15N-NO3− and δ18O-NO3−) in Zhanjiang Bay during three distinct periods: the normal wet season, rainstorm, and typhoon periods. After the rainstorm, the salinity front in Zhanjiang Bay was more weakened and steadier than that during the normal wet season, mainly because onshore wind and a large amount of freshwater was inputted into the ocean surface. This weakened and steady salinity front strengthened water stratification and provided a favorable condition for phytoplankton blooms. Correspondingly, evident NO3− deficits coincided with elevated δ15N-NO3− and δ18O-NO3− values indicated that sufficient NO3− sustained phytoplankton blooms, leading to NO3− assimilation during the rainstorm period. By contrast, due to the onshore wind induced by the typhoon, the salinity front in Zhanjiang Bay was more intensified and unsteady after the typhoon than the normal wet season. The salinity front after the typhoon was unsteady enough to enhance vertical mixing in the water column. Relatively high DO concentrations suggested that enhanced vertical mixing after the typhoon support freshly organic matter decomposition and nitrification via oxygen injection from the air into the water column. In addition, NO3− deficits coincided with elevated δ15N-NO3− values and δ18O-NO3− values demonstrated the coexistence of NO3− assimilation during the typhoon period. This study suggests that the changing processes involved in NO3− cycling after typhoons and rainstorms are associated with the stability and intensity of the salinity front altered by these weather events.

Earlier ice melt increases hypolimnetic oxygen despite regional warming in small Arctic lakes

AbstractAlthough trends toward earlier ice‐out have been documented globally, the links between ice‐out timing and lake thermal and biogeochemical structure vary spatially. In high‐latitude lakes where ice‐out occurs close to peak intensity of solar radiation, these links remain unclear. Using a long‐term dataset from 13 lakes in West Greenland, we investigated how changing ice‐out and weather conditions affect lake thermal structure and oxygen concentrations. In early ice‐out years, lakes reach higher temperatures across the water column and have deeper epilimnia. Summer hypolimnia are the warmest (~ 11°C) in years when cooler air temperatures follow early ice‐out, allowing full lake turnover. Due to the higher potential for substantive spring mixing in early ice‐out years, a warmer hypolimnion is associated with higher dissolved oxygen concentrations. By affecting variability in spring mixing, the consequences of shifts in ice phenology for lakes at high latitudes differ from expectations based on temperate regions.

Manipulating dissolved oxygen concentration to control the partitioning between carbon oxidation and redirection in the A-stage high rate activated sludge process

The adsorption/bio-oxidation (A/B) process has emerged as a remedy to the significant energy use from wastewater treatment plants. In such a process, the A-stage's objective is to balance maintaining high COD redirection towards recovery streams with achieving high effluent quality to facilitate energy savings in the B-stage. While SRT was intensively investigated to control A-stage performance, this study investigates the use of dissolved oxygen (DO) concentration under two adverse conditions: short and long residence times (SRT and HRT). The findings indicate that excessive oxidation is impeded at short residence times, meanwhile, high DO concentration (>2 mgO2/L) is required for enhanced substrate utilization and high removal efficiencies. Conversely, low DO concentration (< 0.5 mgO2/L) is needed at long residence times to control the system's oxidative status and partition more COD towards redirection instead of oxidation. Whereas, high removal efficiency is governed by the long residence times. Notably, this study implies that the low COD removal efficiencies in existing A-stage systems can be referred to the need to deliver oxygen at rates beyond the maximum achievable oxygen transfer rates in full-scale systems. Novelly, this study demonstrates that systems operated at lower rates coupled with low DO concentration can achieve higher COD removal efficiencies while maintaining high COD diversion.

The Evaluation of a Novel Denitrifying Woodchip Bioreactor: Fairmont, MN, USA

The risk of nitrate contamination became a reality for Fairmont in Minnesota, when water rich in NO3-N exceeded the drinking water standard of 10 mg/L. This was unexpected because this city draws its municipal water from a chain of lakes that are fed primarily by shallow groundwater under row-crop land use. Spring soil thaw drives cold water into a subsurface pipe where almost no NO3-N reduction occurs. This paper focuses on NO3-N reduction before the water enters the lakes and no other nitrogen management practices in the watershed. A novel denitrifying bioreactor was constructed behind a sediment forebay, which then flowed into a chamber covered by a greenhouse before entering a woodchip bioreactor. In 2022 and 2023, water depth, dissolved oxygen, and temperature were measured at several locations in the bioreactor, and continuous NO3-N was measured at the entry and exit of the bioreactor. The results showed better performance at a low water depth with lower dissolved oxygen and higher water temperature. The greenhouse raised the inlet temperature in 2022 but did not in 2023. The forebay and the greenhouse may have impeded the denitrification process due to the high dissolved oxygen concentrations in the influent and the stratification of dissolved oxygen caused by algae in the bioreactor.

Denitrification regulates spatiotemporal pattern of N2O emission in an interconnected urban river-lake network

Urban rivers are hotspots of N2O production and emission. Interconnected river-lake networks are constructed to improve the water quality and hydrodynamic conditions of urban rivers in many cities of China. However, the impact of the river-lake connectivity project on N2O production and emission remains unclear. This study investigated dissolved N2O and emission of the river-lake network in Wuhan City, China from March 2021 to December 2021. The results showed that river-lake connection greatly decreased riverine Nitrogen (N) concentration and increased dissolved oxygen (DO) concentration compare to traditional urban rivers. N2O emissions from the urban river interconnected with lakes (LUR: 67.3 ± 92.6 μmol/m2/d) were much lower than those from the traditional urban rivers (UR: 467.3 ± 1075.7 μmol/m2/d) and agricultural rivers (AR: 20.4 ± 15.3μmol/m2/d). Regression tree analysis suggested that the N2O concentrations were extremely high when hypoxia exists (DO < 1.6 mg/L), and TDN was the primary factor regulating N2O concentrations when hypoxia does not occur. Thus, we ascribe the low N2O emission in the LUR and AR to the lower N contents and higher DO concentrations. The microbial process of N2O production and consumption were quantitatively estimated by isotopic models. The mean proportion of denitrification derived N2O (fbD) was 63.5 %, 55.6 %, 42.3 % and 42.7 % in the UR, LUR, lakes and AR, suggested denitrification dominated N2O production in the urban rivers, but nitrification dominated N2O production in the lakes and AR. The positive correlation between logN2O and fbD suggested that denitrification is the key process to regulate the N2O production and emission. The abundance of denitrification genes (nirS and nirK) was much higher than that of nitrification genes (amoA and amoB), also evidenced that denitrification was the main N2O source. Therefore, river-lake interconnected projects changed the nutrients level and hypoxic condition, leading to the inhibition of denitrification and nitrification, and ultimately resulting in a decrease of N2O production and emission. These results advance the knowledge on the microbial processes that regulate N2O emissions in inland waters and illustrate the integrated management of water quality and N2O emission.

Plant development alters the nitrogen cycle in subsurface flow constructed wetlands: Implications to the strategies for intensified treatment performance

Plant development greatly influences the composition structure and functions of microbial community in constructed wetlands (CWs) via plant root activities. However, our knowledge of the effect of plant development on microbial nitrogen (N) cycle is poorly understood. Here, we investigated the N removal performance and microbial structure in subsurface flow CWs at three time points corresponding to distinct stages of plant development: seedling, mature and wilting. Overall, the water parameters were profoundly affected by plant development with the increased root activities including radial oxygen loss (ROL) and root exudates (REs). The removal efficiency of NH4+-N was significantly highest at the mature stage (p < 0.01), while the removal performance of NO3−-N at the seedling stage. The highest relative abundances of nitrification- and anammox-related microbes (Nitrospira, Nitrosomonas, and Candidatus Brocadia, etc.) and functional genes (Amo, Hdh, and Hzs) were observed in CWs at the mature stage, which can be attributed to the enhanced intensity of ROL, creating micro-habitat with high DO concentration. On the other hand, the highest relative abundances of denitrification- and DNRA-related microbes (Petrimonas, Geobacter, and Pseudomonas, etc.) and functional genes (Nxr, Nir, and Nar, etc.) were observed in CWs at the seedling and wilting stages, which can be explained by the absence of ROL and biological denitrification inhibitor derived from REs. Results give insights into microbial N cycle in CWs with different stages of plant development. More importantly, a potential solution for intensified N removal via the combination of practical operation and natural regulation is proposed.

Utilização de um gerador de bolhas ultrafinas para oxigenação de tanques de criação de tilápia do Nilo (Oreochromis niloticus) em sistema de recirculação

The objective of this work was to evaluate the use of an ultrafine bubble generator of own manufacture for oxygenation of Nile tilapia (Oreochromis niloticus) breeding tanks in a recirculating water system. The research was divided into two steps: 1) oxygen saturation test; 2) application of ultrafine bubble production technology for the breeding of Nile tilapia. In the first step, the water of a 2.0 m3 test tank was completely deoxygenated and the ultrafine bubble generator was turned on for 60 min. In the second step, the generator was connected to the water recirculating system for breeding of Nile tilapia to compare the overall performance of this system with other under conventional aeration system. The ultrafine bubble generator could reach 100% oxygen saturation in the test tank (27.8 °C) in approximately 21 min and, at the end of 60 min, the concentration was 21.8 mg L-1 (277.52% saturation). The results showed significant difference (p&lt;0.05) between the mean dissolved oxygen concentration in the treatment with ultrafine bubbles (9.80 ± 3.68 mg L-1) and the treatment with conventional aeration (3.47 ± 0.88 mg L-1). No significant difference (p&gt;0.05) was found for the zootechnical performance parameters evaluated. The conclusion was that the ultrafine bubble generator is more efficient to maintain a high dissolved oxygen concentration in the recirculating water in Nile tilapia breeding tanks than the conventional aeration system.

Achieving partial nitrification: A strategy for washing NOB out under high DO condition

This study investigated the effect of high DO concentrations on PN. The experimental setup involved operating at high DO concentrations (1.5–2.5 mg/L) and environmental temperatures (15–20 °C) over a period of 180 days. Through a sludge enrichment process, the kinetic parameters of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were determined. Surprisingly, contrary to conventional reports, it was observed that NOB exhibited a stronger affinity for DO compared to AOB. As a result, high DO concentrations were necessary to provide favorable conditions for the growth of AOB. In order to establish PN, strategies including intermittent aeration, free ammonia (FA), and controlled sludge retention time (SRT) were employed. The successful PN was achieved with a specific ammonia oxidation rate of 24 mg N/g MLVSS/h and a specific nitrite oxidation rate below 0.10 mg N/g MLVSS/h. The nitrite accumulation rate (NAR) was maintained at 100% during stable operation. The abundance of Nitrosomonas, a typical genus of AOB, was found to be 68.62%, which surpasses previous studies in similar research. A slightly higher DO concentrations may increase energy consumption, but achieve higher efficiency and stability in PN. This study provided new insights into the application of PN in wastewater treatment.

Response of the tomato rhizosphere bacterial community to water–oxygen coupling under micronanobubble oxygenated drip irrigation in mildly saline soil

AbstractThe deterioration of the soil water–air environment due to salinization can reduce soil bacterial activity and cause crop yield reduction. Micronanobubble oxygenated drip irrigation can significantly improve the nonsaline soil water–air environment and enhance bacterial activity, but its effect on salinized soil bacteria is unknown. In this study, we used a tomato cultivation experiment in mildly saline soil to evaluate the responses of the rhizosphere bacterial community to the soil microenvironment created by micronanobubble oxygenated drip irrigation. The results indicated that the soil environmental factors (including soil electrical conductivity, i.e., EC) (r = 0.46) and root length (r = −0.86) had important regulatory effects on bacterial community function, which had significant direct effects on tomato yield (r = 0.46). The moderate dissolved oxygen concentration treatment significantly reduced the EC by 63.22%–73.32% compared with CK (noncultivated soil treatment) and generally increased the root length; however, only the combination of a moderate dissolved oxygen concentration and excess irrigation (A15W1.2) significantly increased tomato yields by 190.90% compared with A0W0.8 (the combination of a low dissolved oxygen concentration and insufficient irrigation) because the irrigation water volume was the limiting condition affecting the benefits of the root length increase. The high dissolved oxygen concentration treatment significantly reduced the EC by 60.36%–92.31% and increased the bacterial community diversity by 4.08%–6.16% compared with CK; these aforementioned changes favored the increase in tomato yield. However, similar to the result of the moderate dissolved oxygen treatments, the tomato yields were significantly higher and among the highest of all treatments only when a high dissolved oxygen concentration was combined with sufficient irrigation or excess irrigation (A30W1 or A30W1.2) due to the limiting effect of the irrigation water volume. Therefore, accounting for the changes in EC, soil bacterial community, and tomato yields, A15W1.2, A30W1, and A30W1.2 are recommended for tomato production in mildly saline soils.

Influence of dissolved oxygen on phosphorus removal by polyphosphate-accumulating organisms biofilm: Performance and metabolic response

The present study focused on investigating the effect of dissolved oxygen (DO) concentration on phosphorus (P) transformation and metabolism in the polyphosphate-accumulating organisms (PAOs)-biofilm system. Two biofilm sequencing batch reactors (BSBRs) with high-DO (6 mg/L, BSBR-H) versus low-DO (2 mg/L, BSBR-L) concentrations were typically operated for 90 days. Collectively, the high DO concentration in aerobic phase would force PAOs to preferentially and rapidly perform phosphate metabolism and be beneficial for them to establish the dominant ecological niche. Clear differences in stoichiometry and kinetics indicated a glycolytic pathway in BSBR-L, while it shifted to polyphosphate hydrolysis in BSBR-H. The expression of functional genes suggested that the high-affinity P transport system was mainly responsible for P transformation in BSBR-H while the low-affinity P transport system was accountable for BSBR-L. Moreover, the BSBR-H showed relatively higher abundances of Pseudomonas while Tetrasphaera were the most dominant PAOs in BSBR-L. Dechloromonas contributed substantially to poly-P metabolism by occupying the largest proportion of ppk1/2 and ppx genes in both BSBRs.