High Dissolved Oxygen Concentration Research Articles

Effects of dissolved oxygen on the sludge dewaterability and extracellular polymeric substances distribution by bioleaching

Bioleaching is a biological conditioning technology for sludge, which not only improves sludge dewatering performance but also removes heavy metals from sludge. As the bioleaching process is a comprehensive biological and chemical process, it is necessary to explore the effects of dissolved oxygen (DO) concentrations on bioleaching efficiency. Three bioleaching experiments with different DO concentrations (T1: 0.8–3.1 mg/L, T2: 3.1–5.5 mg/L, T3: 5.5–7.5 mg/L) were conducted for five days. The sludge dewatering efficiency was evaluated using capillary suction time (CST) and specific resistance to filtration (SRF). The relationship between sludge dewaterability and extracellular polymeric substance (EPS) fraction distribution was investigated. In the treatment with the highest DO concentration, the minimum values of SRF and CST were 4.31 × 1011 m/kg and 13.5 s, which occurred earlier than the treatment with the lower DO concentrations by approximately 24–48 h. A significant decrease (83.4–93.2%) in tightly bound EPS (TB-EPS) protein (PN) was observed in all treatments, while a positive correlation (r = 0.924, P < 0.01) was observed between SRF and PN content in TB-EPS. A relatively higher abundance of Acidithiobacillus was found with the increase in DO concentration. Additionally, other genera including Metallibacterium, Alicyclobacillus, Acidibacter, Acidocella, and Luteococcus also played important roles in EPS biodegradation. These results revealed that increasing the DO concentration could improve sludge dewatering performance and heavy metal removal by enhancing bioleaching microbial activity, the degradation of PN in TB-EPS, and sludge floc fragmentation, but only if sufficient energy sources were provided.

Comparison of cathodic protection processes of 40% zinc-rich coatings under immersion and atmospheric conditions: Protection for defective areas

The cathodic protection of defective areas of low-content (40 wt%) zinc-rich coatings (ZRCs) was investigated under immersion and atmospheric thin liquid film conditions using a wire beam electrode (WBE). The coatings were characterised by electrochemical impedance spectroscopy, scanning electron microscopy, and surface morphology analysis. Under the two conditions, the ZRCs did not polarise the entire electrode to the protection potential interval specified by the cathodic protection standard. Due to the high dissolved oxygen content in the thin liquid film under the atmospheric condition, the zinc powder on the coating surface was rapidly activated. This resulted in a higher protection current for the steel substrate in the defect area of the coating, and metal corrosion was effectively controlled. However, corrosion in the defect area of the coating persisted under immersion although the zinc powder in the coatings also provided a protection current. In contrast to conventional sheet electrodes, WBE can provide information on surface current density distribution and associated processes such as zinc powder activation, polarisation, and polarity reversal of the substrate metal during the cathodic protection stage of ZRCs.

Effects of temperature, dissolved oxygen, and their interaction on the growth performance and condition of rainbow trout (Oncorhynchus mykiss).

The individual effects of temperature and dissolved oxygen (DO) on rainbow trout (Oncorhynchus mykiss), an important aquaculture species, are clearly established; however, little is known about the interactive effects of these parameters. In this study, the effects of temperature, DO, and their interaction on the growth, antioxidant status, digestive enzyme activity, serum biochemical parameters, and liver IGF-1 expression in rainbow trout were evaluated. Fish (initial weight, 109.98±3.28g) were reared in a recirculating system for 4 weeks and subjected to 6 treatments at three temperatures (13°C, 17°C, and 21°C) and two DO contents (4.2mgL-1 and 9.6mgL-1). Physiological parameters were determined at the end of the trial. Specific growth rate and feed consumption were the highest at 17°C and the lowest at 21°C. Additionally, lysozyme, trypsin, lipase, and amylase activities, serum glucose and serum triglyceride contents, and IGF-1 expression decreased significantly at 21°C and total serum protein and albumin contents were significantly higher at 21°C than at 13°C and 17°C, indicating that high temperature impaired the immunity, digestion, and growth of rainbow trout. However, the adverse effects of high temperature can be alleviated by a high DO content, as evidenced by the smaller increments and decrements of these parameters under hyperoxic conditions than under hypoxic conditions. In response to high temperature stress, an increase in antioxidant enzyme activity led to the removal of oxygen free radicals under hyperoxic conditions; however, this increase was inhibited under hypoxia. Our results indicated that high temperatures have adverse effects on rainbow trout, and these harmful effects can be reduced by a high DO content.

Transport of dissolved black carbon from marginal sea sediments to the western North Pacific

Pyrogenic carbon, also called black carbon (BC), is produced by biomass burning as well as by fossil fuel combustion and is an important slow-cycling component in the global carbon cycle. BC occurs in a fraction of marine dissolved organic carbon, and this is called dissolved BC (DBC). Marine DBC has been considered the ultimate repository for fluvial DBC; however, the environmental dynamics of marine DBC have not been well documented. In this study, vertical DBC profiles from the surface to the deep layers of the western subarctic Pacific and its marginal seas were analyzed using the benzenepolycarboxylic acid (BPCA) method. DBC concentrations tended to be highest in the surface layer and decreased with increasing depth, except for those in the Bussol’ Strait locatedin the Kuril Islands between the Sea of Okhotsk and the North Pacific. Vertical distributions of the condensation degree index of DBC (i.e., the ratio of B5CA and B6CA to all BPCAs) did not show a general trend with depth. Atmospheric deposition of BC is likely a major source of DBC in the surface layer. In the intermediate layer, the DBC concentration and the condensation degree index in the Bussol’ Strait were higher than those in the western subarctic Pacific. The occurrence of highly condensed DBC observed in the Bussol’ Strait was accompanied by low salinity and a high DO concentration, indicating that DBC is transported from the shelf sediments of the Sea of Okhotsk to the Bussol’ Strait by dense shelf water (DSW) and Okhotsk Sea Intermediate Water (OSIW). The DBC concentration and the condensation degree index in the intermediate layer of the Bering Sea and the Kamchatka Strait (locatedin the Aleutian Islands between the Bering Sea and the North Pacific) were higher than those in the western subarctic Pacific but lower than those in the Bussol’ Strait; this suggests that DBC from the slope sediments in the Bering Sea is transported to the Kamchatka Strait by the East Kamchatka Current and that the DBC flux from the slope sediments of the Bering Sea is lower than that involving the DSW in the Sea of Okhotsk, which accompanies the resuspension of shelf sediments. The results of this study imply that sedimentary BC is an important source of water column DBC in marginal seas and the adjacent open ocean.

Enhanced nitrogen removal from high-strength ammonium wastewater by improving heterotrophic nitrification-aerobic denitrification process: Insight into the influence of dissolved oxygen in the outer layer of the biofilm

This work aims to optimize the process of heterotrophic nitrification-aerobic denitrification (HN-AD) in the membrane-aerated biofilm reactor (MABR) by way of increasing the dissolved oxygen (DO) concentration in the outer layer biofilm to treat high-strength ammonium-nitrogen (NH4+-N) wastewater. The results show that the removal efficiencies of NH4+-N, total nitrogen (TN) and chemical oxygen demand (COD) increased by 42.4%, 65.7%, and 15.2%, respectively, in MABR-1 with high-DO concentrations than in MABR-2 with low-DO concentrations in the outer layer biofilm. High-DO concentrations in the outer layer biofilm increased the thickness of the aerobic layer to encourage HN-AD bacteria enrichment, which paved the way for nitrogen removal enhancement in MABR-1. Moreover, based on the analysis of functional gene expressions predicted by PICRUSt2, the relative abundances of seven key genes (hao, nirK, napA, napB, norB, norC and nosZ) relating to HN-AD bacteria in the MABR-1 significantly increased, indicating that DO increasing in the outer layer biofilm remarkably promoted the occurrence of HN-AD process. From the viewpoint of economic efficiency, the advantage of the novel strategy of increasing DO concentrations in the outer layer biofilm is that this optimized alternative for the MABR provides a superior nitrogen removal process in the high-strength NH4+-N wastewater treatment.

Strategies to Overcome Intermediate Accumulation During in situ Nitrate Remediation in Groundwater by Hydrogenotrophic Denitrification.

Bioremediation of polluted groundwater is one of the most difficult actions in environmental science. Nonetheless, the clean-up of nitrate polluted groundwater may become increasingly important as nitrate concentrations frequently exceed the EU drinking water limit of 50 mg L–1, largely due to intensification of agriculture and food production. Denitrifiers are natural catalysts that can reduce increasing nitrogen loading of aquatic ecosystems. Porous aquifers with high nitrate loading are largely electron donor limited and additionally, high dissolved oxygen concentrations are known to reduce the efficiency of denitrification. Therefore, denitrification lag times (time prior to commencement of microbial nitrate reduction) up to decades were determined for such groundwater systems. The stimulation of autotrophic denitrifiers by the injection of hydrogen into nitrate polluted regional groundwater systems may represent a promising remediation strategy for such environments. However, besides high costs other drawbacks, such as the transient or lasting accumulation of the cytotoxic intermediate nitrite or the formation of the potent greenhouse gas nitrous oxide, have been described. In this article, we detect causes of incomplete denitrification, which include environmental factors and physiological characteristics of the underlying bacteria and provide possible mitigation approaches.

Upper Ocean Stratification in the Eastern Pacific During the SPURS‐2 Field Campaign

AbstractThe region between a shallow mixed layer and a deep isothermal layer resulting from salinity stratification is called a barrier layer (BL). Since BLs hinder the surface heat and momentum exchange with the ocean subsurface, they play an important role in air‐sea interaction. Synoptic features and formation of BLs and associated temperature inversions (TIs) in the Eastern Pacific Fresh Pool (EPFP) were investigated using shipboard observations. BLs and TIs were found in the high‐precipitation EPFP, a frontal transition zone (FTZ) and in the sea surface salinity (SSS) front during a 2016 boreal summer cruise. During a 2017 boreal autumn cruise, BLs and TIs were found from the southern part of the EPFP to the SSS front. The BLs in the SSS front and the FTZ were meridionally displaced from the precipitation maximum during both the 2016 and 2017 cruises. Freshening and cooling by tilting of both the isohalines and isotherms of the SSS front occurred via Ekman and geostrophic flow during both cruises, and also within the EPFP during 2016. The SSS front was a dominant contributor to the density gradient in the horizontal plane. A high dissolved oxygen content observed within BLs confirmed that the surface flow plays an important role in the BL and TI formation through the tilting process. Patchy distribution of freshening and cooling by tilting implies an intermittent BL and TI formation with short time scales.

Limnological Features of the Southern Part of Gharaf River and the Impacts of Floodplain Period on its Characteristics

The current study was conducted on the southern part of Gharaf River in order to define the physical-chemical properties and the impact of the Tigris River’s floodplain water on its ecological characteristics during 2019. Four sites were selected along the southern part of the river. The findings showed a strong connection between the temperature of air and water at all sites. A value ranging from 4.85 - 84.65 NTU was reported for turbidity. The water was found to be oligohaline, low alkaline, and well ventilated due to high dissolved oxygen concentrations. Gharaf River is considered to be of questionable clean water, according to the results of BOD5. The total alkalinity values were recorded to be in the range of 92-292.8 mg /L, which are higher than the acceptable limits of 20-200 mg /L CaCO3 for the Iraqi and international water standards. It was also found that Gharaf River water was very hard, but still within the allowable natural water limits (200 mg /L Ca and 150 mg /L Mg). Concentrations of sulphate ranged 50-200 mg / L, while levels of bicarbonate ranged 140-230 mg /L. On the other hand, ranges of 3354-855 mg / L and 3-85 mg / L, respectively, were recorded for TDS and TSS. In addition, nitrates values were found to be in the range of 0.04-5.14 mg / L, being below the permissible limits for Iraqi water (15 mg / L). Other values observed for phosphates were 0.004-0.085 mg / L. Overall, the results demonstrated different effects on the properties of Gharaf River water during and after the floodplain period.

Role of dominant macrophytes to treat Nambul river, the main polluter of Loktak – a dying Ramsar site in the Indo Burma hot spot (Manipur, India)

As a part of the monitoring process of the main polluting tributary of Loktak, the paper analyzed the water characteristics of the main polluted stretch of Nambul river i.e. Imphal Urban region. The results showed low pH (6.23 ± 0.32), high electrical conductivity (1373 ± 50.33 µSm−1), high turbidity (25.40 ± 0.90 NTU), poor dissolved oxygen content (0.53 ± 0.24 mgL−1), high nitrate, and phosphate concentrations (9.27 ± 0.40 and 2.05 ± 0.10 mgL−1, respectively). The Water Quality Index values indicate that Nambul river is unfit for drinking. Moreover, the Sodium Percentage, Sodium Adsorption Ratio, and Kelly’s Ratio revealed the unsuitable nature of the river for irrigation. Therefore, in order to clean the river water, lab scale microcosm experimentation was carried out to highlight the potential of the dominant macrophytes viz. Eichhornia crassipes (Mart) Solms and Pistia stratiotes (L.) as phytofilters. The results showed increased pH (7.6 ± 0.11 and 7.5 ± 0.1), higher dissolved oxygen content (5.87 ± 0.30 and 3.28 ± 0.30 mgL−1), less turbidity (0.60 ± 0.25 and 0.40 ± 0.15 NTU) and reduced nitrate and phosphate concentrations (0.37 ± 0.03 and 0.52 ± 0.01 mgL−1, and 0.534 ± 0.025 and 0.346 ± 0.01 mgL−1, respectively). Novelty statement The present study showed the deteriorating water quality of Nambul river, the main polluting source of Loktak – a Ramsar site in the Montreux records. Water Quality Index calculated was above the unsuitable level for drinking at all the sampling locations in the Imphal Urban region. Sodium percentage, sodium adsorption ratio and Kelly’s ratio were above the desired limit, making the river water unfit for irrigation. Further, the microcosm experimentation showed that the dominant macrophytes viz. Eichhornia crassipes (Mart) Solms and Pistia stratiotes (L.) can control the pollution level of Nambul.

Corrosion Risk and Repassivation of Duplex Stainless Steel UNS S82551 in Treated Seawater Injection Service

In seawater injection wells, the available well tubing materials are generally low alloy steel, glass-reinforced epoxy-lined low alloy steel or corrosion-resistant alloys such as super duplex stainless steel. However, in treated seawater the corrosion risk can be controlled and lower-grade alloys (low alloy steel) can be considered. But for long well lifetime designs (20 y plus), then low alloy steel tubing can be challenged. In this respect, recent efforts have focused attention on better dissolved oxygen control which permits the investigation and on the possible use of more cost-effective materials such as the duplex stainless steels UNS S82551 and UNS S82541 (the latter is a higher strength version, but same PRENw). Full-scale testing of tubes joined together with a proprietary premium threaded connection (pipe-coupling premium connection [PCPC] couplings) was performed in controlled seawater loops simulating service conditions at 30°C. The flow rate and dissolved oxygen were controlled at 5 m/s and &amp;lt;20 ppb, respectively. Weekly dissolved oxygen excursions corresponding to 24 h at 100 ppb followed by 1 h at 300 ppb were performed during the 5 months exposure. Corrosion results of UNS S82551/S82541 tubing were compared to UNS S31803 and UNS S39274. In parallel, laboratory exposures of creviced coupons for parametric study were performed in dissolved oxygen-controlled cells, allowing the measurement of electrochemical potentials as function of dissolved oxygen content and the related corrosion resistance. The results showed that dissolved oxygen content should be properly controlled below critical values to avoid crevice corrosion of the lesser alloyed duplex stainless steels. The ability of UNS S82551/S82541 to recover or repassivate after prolonged exposures to high dissolved oxygen concentrations (DOC) was also determined with both the use of full-sized PCPC test cells and electrochemical testing involving a remote crevice assembly. The repassivation potential was investigated after different active crevice corrosion durations. The results of the study allowed to precisely define the limits of use of UNS S82551/S82541 in treated seawater, i.e., the critical DOC conditions for corrosion initiation and for repassivation of UNS S82551/S82541. For all tested conditions, the UNS S82551/S82541 showed a rather good ability to repassivation when normal service conditions (i.e., low dissolved oxygen) are recovered.

Effect of Dissolved Oxygen on Removal of Benzotriazole from Co during a Post-Co CMP Cleaning

A systematic study of the adsorption of benzotriazole on Co surface and its removal in aqueous solutions was carried out for post-CMP cleaning application. Static etch rate (SER) measurements and electrochemical impedance spectroscopy (EIS) were employed. The experimental results show that BTA adsorbed well on Co surface when it exposed to BTA solution at neutral and alkaline pH. BTA did not adsorb due to active Co dissolution at acidic pH. The effect of dissolved oxygen (DO) concentration in de-ionized (DI) water on the removal of Co-BTA complex layer was investigated. At DI water rinse process after BTA treatment, Co-BTA layer was maintained on Co surface when DO concentration of DI water was low. Interestingly, BTA was removed by DI water with high DO concentration.

Seasonal variation of nitrogen biogeochemical processes constrained by nitrate dual isotopes in cascade reservoirs, Southwestern China.

The increase of affected river reaches by reservoirs has drastically disturbed the original hydrological conditions, and subsequently influenced the nutrient biogeochemistry in the aquatic system, particularly in the cascade reservoir system. To understand the seasonal variation of nitrogen (N) behaviors in cascade reservoirs, hydrochemistry and nitrate dual isotopes (δ15N-NO3- and δ18O-NO3-) were conducted in a karst watershed (Wujiang River) in southwest China. The results showed that NO3--N accounted for almost 90% of the total dissolved nitrogen (TDN) concentration with high average concentration 3.8 ± 0.4 mg/L among four cascade reservoirs. Higher N concentration (4.0 ± 0.8 mg/L) and larger longitudinal variation were observed in summer than in other seasons. The relationship between the variation of NO3--N and dual isotopes in the profiles demonstrated that nitrification was dominated transformation, while assimilation contributed significantly in the epilimnion during spring and summer. The high dissolved oxygen concentration in the present cascade reservoirs system prevented the occurrence of N depletion processes in most of the reservoirs. Denitrification occurred in the oldest reservoir during winter with a rate ranging from 18 to 28%. The long-term record of surface water TDN concentration in reservoirs demonstrated an increase from 2.0 to 3.6 mg/L during the past two decades (~ 0.1 mg/L per year). The seasonal nitrate isotopic signature and continuously increased fertilizer application demonstrated that chemical fertilizer contribution significantly influenced NO3--N concentration in the karst cascade reservoirs. The research highlighted that the notable N increase in karst cascade reservoirs could influence the aquatic health in the region and further investigations were required.

Photosynthetic performance of Chlorella vulgaris R117 mass culture is moderated by diurnal oxygen gradients in an outdoor thin layer cascade

Dissolved oxygen concentration is a critical point for microalgae in large scale cultivation systems. Highly productive cultures inevitably generate a build-up of oxygen gradients along the reactor which can affect photosynthetic performance. In this study, a fast-growing strain of Chlorella vulgaris R117 was cultured outdoors in a thin-layer cascade during a one-week trial reaching a biomass productivity of 4 g DW L−1 d−1 (27 g DW m−2 d−1). High photosynthetic activity led to oxygen oversaturation of up to 400% in some parts along the culture units at midday. The aim was to examine the effect of high dissolved oxygen concentration on diurnal changes in the photosynthetic performance and growth of the Chlorella culture using multi-technique approach. Photosynthetic activity of Chlorella R117 culture was estimated in situ and ex situ using oxygen production and in vivo Chl a fluorescence measurements, which showed good correlation. The rates of electron transport and of oxygen production were related, but the values of the μmolelectrons/μmolO2 ratio was higher than predicted, suggesting the probable involvement of electron and oxygen consuming processes such as photorespiration and Mehler reaction. These processes probably function as photoprotective mechanisms, since no photodamage was observed in the Chlorella R-117 cultures. Depression (down-regulation) of photosynthetic activity due to the exposition to high dissolved oxygen concentration along the cascade area over time was observed. The usefulness of on-line measurements was demonstrated to obtain immediate and in-situ information on the physiological status of the culture. This data can be used in models of operation control for large-scale microalgae production units.

Core nitrogen cycle of biofoulant in full-scale anoxic & oxic biofilm-membrane bioreactors treating textile wastewater

Core nitrogen cycle within biofoulant in full-scale anoxic & oxic biofilm-membrane bioreactor (bMBR) treating textile wastewater was investigated. Wastewater filtered through membrane with biofoulant had elevated NH4+-N and NO2−-N concentrations corresponding to decreased NO3−-N concentrations. Nevertheless, total nitrogen concentrations did not change significantly, indicating negligible nitrogen removal activities within biofoulant. Metagenomic analysis revealed a lack of genes, such as AmoCAB and Hao in biofoulant, indicating absence of nitrification or anammox populations. However, genes encoding complete pathway for dissimilatory nitrate reduction to ammonium (DNRA) were discovered in 15 species that also carry genes encoding both nitrate reductase and nitrite reductase. No specie contained all genes for complete denitrification pathway. High temperature, high C:N ratio, and anoxic conditions of textile wastewater could favorite microbes growth with DNRA pathway over those with canonical denitrification pathway. High dissolved oxygen concentrations could effectively inhibit DNRA to minimize ammonia concentration in the effluent.

Nest-site selection in a fish species with paternal care

Fish that perform paternal care may increase their fitness by choosing nest sites that enhance survival and development of embryos. We studied nest-site choice with respect to dissolved oxygen concentration and water temperature in males of the three-spined stickleback (Gasterosteus aculeatus), a small fish species with exclusive male parental care that usually breeds in the littoral zone of freshwaters of the Northern hemisphere. Fathers oxygenate the embryos by fanning movements of their pectoral fins. We expected choice for conditions at potential nest sites that would benefit offspring development, i.e., higher temperature and higher dissolved oxygen concentration. In the laboratory, we offered males a choice between two potential nest sites that differed in dissolved oxygen concentration or water temperature. Males preferred to build a nest at sites with a higher dissolved oxygen level or higher temperature and thus chose sites that would promote embryo development.

Hydrodynamics and mass transfer in spinner flasks: Implications for large scale cultured meat production

Cultured meat has emerged as a potential alternative source of animal proteins and fat. Commercially viable cultured meat production likely requires proliferation bioreactors much larger than those used in the pharmaceutical industry today. However, most lab-scale process developments are carried out in spinner flasks that cannot be readily scaled-up using conventional, semi-empirical methods. We systematically investigated the hydrodynamics and mass transfer characteristics of a 250 mL spinner flask under 30−120 rpm stirring speeds and 50 or 100 mL filling volumes, as well as 0−30 g/L microcarriers, to facilitate scale-down experiment design hence the transfer of lab-generated knowledge to large scale bioreactors. It was found that meat cells are typically cultured under operating conditions that give a combination of <10 s mixing time and about 1 mW/kg energy dissipation rate, but with high dissolved oxygen concentrations. Computational fluid dynamics moelling was performed for both the spinner flask and a generic cell culture reactor of 20 m³ working volume. Simulations suggested that it might not be possible to reproduce the most favourable conditions in large scale reactors. In particular, even when operated at a stirring speed close to the Njs, the shear exerted by the impellers alone could cause cell detachment from microcarriers, while even more hydrodynamic stress is introduced via sparging.

Effect of COD:N ratio on biological nitrogen removal using full-scale step-feed in municipal wastewater treatment plants

This study investigated the effect of low and high chemical oxygen demand (COD):N ratios on biological nitrogen removal and microbial distributions in full-scale step-feed (SF) municipal wastewater treatment plants (WWTPs) in Thailand (SF1) and Taiwan (SF2). The SF1 WWTP had a low COD:N (4:1) ratio, a long solids retention time (SRT) (> 60 d), and low dissolved oxygen (DO) conditions (0.2 mg L− 1 in anoxic tank and 0.9 mg L− 1 in aerobic tank). The total nitrogen (TN) removal efficiency was 48%. The SF2 WWTP had a high COD:N (10:1) ratio, a short SRT (7 d), and high DO (0.6 mg L− 1 in anoxic tank and 1.8 mg L− 1 in aerobic tank). The TN removal efficiency was 61%. The nitrification and denitrification rates from these two plants were inadequate. Using a quantitative polymerase chain reaction (qPCR) technique, the populations of ammonium oxidizing bacteria (AOB) and ammonium oxidizing archaea were quantified. Measurement of ammonia monooxygenase (amoA) gene abundances identified these AOB: Nitrosomonas sp., Nitrosospira sp., Nitrosoccus sp. and Zoogloea sp. Higher amounts of the archaeal-amoA gene were found with long SRT, lower DO and COD:N ratios. Abundance of Nitrobacter sp. was slightly higher than Nitrospira sp. at the SF1, while abundance of Nitrobacter sp. was two orders of magnitude greater than Nitrospira sp. at the SF2. More denitrifying bacteria were of the nirS-type than the nirK-type, especially at higher COD:N ratio. Most bacteria belong to the phyla Acidobacteria, Actinobacteria Bacteroidetes, Chloroflexi, Proteobacteria. The results from this work showed that insufficient carbon sources at the SF1 and high DO concentration in anoxic tank of SF2 adversely affected nitrogen removal efficiencies. In further research work, advanced techniques on the next generation sequencing with different variable regions should be recommended in full-scale WWTPs.

Metabolic flexibility of SUP05 under low DO growth conditions.

Chemoautotrophic bacteria from the SUP05 clade often dominate anoxic waters within marine oxygen minimum zones (OMZs) where they use energy gained from the oxidation of reduced sulfur to fuel carbon fixation. Some of these SUP05 bacteria are facultative aerobes that can use either nitrate or oxygen as a terminal electron acceptor making them ideally suited to thrive at the boundaries of OMZs where they experience fluctuations in dissolved oxygen (DO). SUP05 metabolism in these regions, and therefore the biogeochemical function of SUP05, depends largely on their sensitivity to oxygen. We evaluated growth and quantified differences in gene expression in Ca. T. autotrophicus strain EF1 from the SUP05 clade under high DO (22 μM), anoxic, and low DO (3.8μM) concentrations. We show that strain EF1 cells respire oxygen and nitrate and that cells have higher growth rates, express more genes, and fix more carbon when oxygen becomes available for aerobic respiration. Evidence that facultatively aerobic SUP05 are more active and respire nitrate when oxygen becomes available at low concentrations suggests that they are an important source of nitrite across marine OMZ boundary layers.

Suppression of nitrite-oxidizing bacteria under the combined conditions of high free ammonia and low dissolved oxygen concentrations for mainstream partial nitritation

Abstract The possibility of nitrite-oxidizing bacteria (NOB) suppression in mainstream wastewater (COD/N of 6.0, ammonia of approximately 50 mg N L−1) was investigated by controlling the ratio of dissolved oxygen to total ammoniacal nitrogen (DO/TAN) in a continuous suspended sludge reactor without prior COD removal. It was found that a combination of high free ammonia (FA) of approximately 20 mg N L−1 and low dissolved oxygen (DO) of 0.3 ± 0.1 mg L−1 (DO/TAN ratio of 0.003) established in the start-up period was an efficient way to inhibit the growth of NOB. The activity of NOB was more inhibited as FA concentration increased, particularly at the initial ammonia concentration of 175 mg N L−1 (FA of about 20 mg N L−1) Then, the influent ammonia was lower to 50 ± 2 mg N L − 1 (FA of 4.9 ± 0.2 mg N L−1), a typical nitrogen concentration of mainstream wastewater. After four months of operation, mainstream partial nitritation (PN) became stable with nitrite accumulation ratio over 87% and COD removal of above 94% (effluent COD of 18.4 mg L−1). Besides, the effluent NO 2 − -N/NH3-N ratio was 1.22 ± 0.12, which was suitable for the subsequent anaerobic ammonium oxidation (anammox) reaction. The relative abundance of ammonia-oxidizing bacteria (AOB) (e.g., Nitrosomonas) was 15 times higher than NOB. The abundance (0.3%) of Nitrospira (k-strategist NOB) was insignificant, while Nitrobacter (r-strategist NOB) and Nitrotoga (NOB) were not detected during the steady state of PN under low nitrogen load because NOB was suppressed and washed out of the reactor by the condition of extremely low DO/TAN ratio in the start-up period. Thus, it was noted that NOB suppression for stable PN could be accomplished by providing high FA and low DO concentrations, especially in the early stage.

In-stream oxygenation to mitigate internal loading of phosphorus in lowland streams

Many lowland regions are afflicted with episodic high phosphorus (P) concentrations in streams during low flow periods. It has been hypothesised that these P peaks are due to internal loading after reductive dissolution of ferric iron (Fe(III)) minerals in sediments with a high P/Fe ratio. Here, we experimentally tested that hypothesis by measuring the sediment/water fluxes of P in streams and by testing responses to in-stream oxygenation. A phosphate stock solution with a bromide (Br−) tracer was administered during 3 h in four streams with varying sediment P/Fe ratios during summer (low DO concentration). The experiment was repeated in winter (high DO concentration) in one of the streams. The sediments were either net P sinks (three cases), net unreactive (one case) or a net source (one case), with the net source occurring in the stream with the highest sediment P/Fe and at lowest DO concentration. A direct oxygenation experiment in one stream increased downstream DO concentration with about 1 mg O2 L−1 and decreased the P concentration with, on average, about 0.3 mg P L−1 over a two-week period. The efficiency of oxygenation to reduce P concentrations enhanced when flow rates were lower. In conclusion, this experimental study confirmed the hypothesis that internal P loading in streams is largest in high P/Fe sediments at low DO conditions. Lowering P concentrations in lowland streams should combine reduced P emission and strategies avoiding low DO concentration, i.e. reduced biological oxygen demand (BOD) and nutrient emissions.