Low Dissolved Oxygen Conditions Research Articles

Nitrogen removal and carbon reduction of mature landfill leachate under extremely low dissolved oxygen conditions by simultaneous partial nitrification anammox and denitrification

In this study, a SNAD-SBBR process was implemented to achieve ammonia removal and carbon reduction of mature landfill leachate under extremely low dissolved oxygen conditions (0.051 mg/L) for a continuous operation of 266 days. The process demonstrated excellent removal performance, with ammonia nitrogen removal efficiency reaching 100 %, total nitrogen removal efficiency reaching 87.56 %, and an average removal rate of 0.180 kg/(m3·d). The recalcitrant organic compound removal efficiency reached 34.96 %. Nitrogen mass balance analysis revealed that the Anammox process contributed to approximately 98.1 % of the nitrogen removal. Candidatus Kuenenia achieved a relative abundance of 1.49 % in the inner layer of the carrier. In the SNAD-SBBR system, the extremely low DO environment created by the highly efficient partial nitrification stage enabled the coexistence of AnAOB, denitrifying bacteria, and Nitrosomonas, synergistically achieving ammonia removal and carbon reduction. Overall, the SNAD-SBBR process exhibits low-cost and high-efficiency characteristics, holding tremendous potential for landfill leachate treatment.

Nitrous Oxide Emissions from Nitritation Reactors under Hypersaline Conditions

This study focuses on nitrous oxide (N2O) emissions during hypersaline (4 % salinity) nitritation in continuously fed and mixed fixed bed reactors. In the presence of high concentrations of nitrite and ammonium, the percent yield of N2O emissions from ammonium removed decreased with increasing dissolved oxygen (DO). However, N2O production continued even at a high DO of 15 mg/L. Bulk ammonium concentration (not ammonia) was found to be the main controlling factor for N2O emissions under high and low DO during both nitritation and nitrification. Reducing bulk ammonium concentrations below 1 mg N/L in the nitritation reactor under both high and low DO conditions resulted in a reduction of N2O emissions of approximately 90 %. Under full nitrification and low DO, reducing nitrite concentrations below 0.3 mg N/L resulted in a 60 % reduction in N2O emissions. Similar results were observed in a low salinity reactor.

Whole transcriptome analysis of demersal fish eggs reveals complex responses to ocean deoxygenation and acidification

Ocean acidification and deoxygenation co-occur in marine environments, causing deterioration of marine ecosystems. However, effects of compound stresses on marine organisms and their physiological coping mechanisms are largely unknown. Here, we show how high pCO2 and low dissolved oxygen (DO) cause transcriptomic changes in eggs of a demersal fish (Sillago japonica), which are fully exposed to such stresses in natural environment. Overall gene expression was affected more strongly by low DO than by high pCO2. Enrichment analysis detected significant stress responses such as glycolytic processes in response to low DO. Increased expression of a group of glycolytic genes under low DO conditions is presumably because oxygen depletion disables the electron transfer pathway, complementing ATP production in the glycolytic pathway. Contrary to expectations, apparent mitigation of gene expression changes was dominant under combined stress conditions, and may represent an innate fish adaptive trait for severe environments.

Changes of bottom water oxygenation during the last half millennium in the Gulf of Tehuantepec (Eastern Tropical North Pacific): A multiproxy approach

The Gulf of Tehuantepec (GoT), in the Eastern Tropical North Pacific Ocean, is home to one of the largest and more intense Oxygen Minimum Zones (OMZ). To identify the OMZ variability during the last five hundred years in the GoT, we analyzed the temporal variations of benthic foraminiferal (BF) assemblages, enrichment of redox-sensitive elements (Mo, V, Cd, U, and Re) and δ15Nsed, on a laminated sediment core. During the Little Ice Age (LIA; ∼1500 to ∼1860 CE), the BF assemblages showed high dominance, and the most abundant BF species in the sediments were Epistominella sandiegoensis, Takayanagia delicata, and Buliminella tenuata. This assemblage can withstand the lowest dissolved oxygen conditions, indicating an intensified OMZ consistent with the enrichment of Mo, Re, Cd, U, and enhanced denitrification. During the Current Warm Period (CWP; ∼1860 CE to present), the BF assemblages were more diverse, and the most abundant species were Bolivina seminuda, Epistominella sp.1, Gyroidina nitidula, and Suggrunda eckisi, an assemblage less tolerant to low dissolved oxygen conditions, together with lower concentrations of redox-sensitive elements and δ15Nsed, suggest weakening of the reducing conditions within the OMZ. Additionally, our BF data showed evidence of the influence of the Pacific Decadal Oscillation on bottom-water oxygenation. Bottom-water oxygenation changes appear to respond to increased solar forcing caused by Intertropical Convergence Zone migration and strengthened Pacific Walker Circulation, which in turn modulated productivity, organic matter flux to the seabed, and oxygen consumption in the bottom water. Our findings agree with previous studies further north from the study site, indicating these changes' regional scale.

The Effects and Mechanisms of pH and Dissolved Oxygen Conditions on the Release of Arsenic at the Sediment-Water Interface in Taihu Lake.

The pH and dissolved oxygen (DO) conditions are important environmental factors that control the migration of arsenic (As) at the sediment-water interface. This study investigates the distribution differences of reactive iron, manganese, and arsenic at the sediment-water interface under anaerobic and aerobic conditions at different pH levels. The strong buffering capacity of sediment to water pH results in a shift towards neutral pH values in the overlying water under different initial pH conditions. The level of DO becomes a key factor in the release of As from sediment, with lower DO environments exhibiting higher release quantities and rates of As compared to high DO environments. Under low DO conditions, the combined effects of ion exchange and anaerobic reduction lead to the most significant release of As, particularly under pH 9.5 conditions. The formation of amorphous ferrous sulfide compounds under low DO conditions is a significant factor contributing to increased arsenic concentration in the interstitial water. Therefore, the re-migration of endogenous arsenic in shallow lake sediments should consider the combined effects of multiple driving forces.

Response of demersal fishes to low dissolved oxygen events in two eutrophic estuaries

Coastal ecosystems serve as important nursery areas, yet they are facing many challenges. For example, eutrophication-induced hypoxia is one of the major threats to the functioning of coastal ecosystems, particularly estuaries. The Sundays and Swartkops estuaries both experience persistent eutrophic conditions, with frequent phytoplankton blooms (>20 μg Chl-a l−1) that result in instances of bottom-water oxygen depletion (<4 mg/l). Bi-seasonal sampling of both physico-chemical parameters and demersal fishes between February 2018 and September 2019 in the Swartkops and Sundays estuaries allowed for the quantification of low dissolved oxygen (<4 mg/l) and hypoxic (<2 mg/l) events in both estuaries and the impact of low dissolved oxygen (DO) and hypoxic events on overall fish species abundance, richness and distribution. Low DO conditions were typically observed in the bottom waters of the vertically-stratified meso-to polyhaline middle and upper reaches of the Sundays and Swartkops estuaries, respectively. Hypoxic conditions ( DO = 0.5 mg/l) were recorded in the Sundays Estuary during the study, whereas in the Swartkops Estuary the lowest DO recorded was 2.4 mg/l. Low DO events resulted in a decrease in fish species richness and overall abundance in the Sundays Estuary, while in the Swartkops Estuary fish species richness declined during low DO events, with little effect on fish abundance. Fish only completely avoided areas where DO was <1 mg/l or >10 mg/l. As such, the low DO concentrations recorded in the Swartkops Estuary did not have a noticeable impact on fish distribution, although the abundance of Rhabdosargus holubi and Caffrogobius gilchristi declined in the upper reaches of the Swartkops Estuary when DO was <3 mg/l. The results from this study provide much-needed insights into the impact of low DO and hypoxic conditions on the spatial distribution of demersal fish species.

Determining whether an 18/16O technology threshold exists in estimating lake primary productivity and associated metabolism

To determine lake carbon (C) cycling storage, transport, and transformation functions, it is necessary to understand metabolic lacustrine changes and associated drivers, all of which are premised on accurate metabolic estimates. The continuous improvement in conventional metabolic lake methods and the emergence of novel relevant methods in recent years have led to confusion as to their usage. Although 18/16O technology has recently been more widely used, its applicability remains controversial among researchers. The Qinghai–Tibet Plateau (QTP) is an exceptional location for such experimental studies, primarily due to its many different natural lake types. This study used 18/16O technology to investigate 19 lakes of different salinity (0.0 ∼ 196.0 ‰), area (5.0 ∼ 4345.9 km2), and altitude grades (2549.9 ∼ 4541.8 m) on the QTP to determine their specific metabolism. Results show that 18/16O technology is not suitable for use for high saline and low dissolved oxygen lakes (i.e., a salinity level greater than 35‰ and a dissolved oxygen level < 4 mg L-1). Under low dissolved oxygen conditions caused by high salinity, alterations in isotopic fractionation will cause 18/16O technology to fail. These findings can be used as a reference for estimating lake metabolism as well as researching nutrient cycling while also promoting metabolic estimation method improvements.

Pilot Study of Oxic–Anoxic Process under Low Dissolved Oxygen for Nitrogen Removal from Low COD/N Tropical Wastewater

Conventionally, nitrification in biological nitrogen removal (BNR) requires high dissolved oxygen (DO) concentrations (&gt;2 mg L−1), making the process energy intensive. Recent studies have shown that efficient ammonium removal and energy reduction can be realized by operating the nitrification at low DO concentrations (&lt;1 mg L−1). In this study, the low-DO oxic anoxic (low-DO OA) process was operated in a pilot-scale sequencing batch reactor (SBR) over 218 days to evaluate the feasibility of nitrogen removal from low chemical oxygen demand-to-nitrogen ratio (COD/N) tropical municipal wastewater. The results revealed that the low-DO OA process attained high removal efficiency for ammonium (97%) and total nitrogen (TN) (80%) under an average DO concentration of 0.6 mg L−1. The effective TN removal efficiency is attributed to the occurrence of simultaneous nitrification–denitrification (SND) under low DO conditions. Further batch tests revealed that slowly biodegradable COD (sbCOD) in tropical wastewater can support denitrification in the post-anoxic phase, resulting in a high TN removal rate. Compared with high DO concentrations (2 mg L−1), low DO conditions achieved 10% higher TN removal efficiency, with similar ammonium and COD removal efficiency. This study is crucial in promoting the energy efficiency and sustainability of wastewater treatment plants treating low COD/N wastewater.

Climate Change and the Sea: A Major Disruption in Steady State and the Master Variables.

Since the beginning of the industrial revolution, humans have burned enormous quantities of coal, oil, and natural gas, rivaling nature's elemental cycles of C, N, and S. The result has been a disruption in a steady state of CO2 and other greenhouse gases in the atmosphere, a warming of the planet, and changes in master variables (temperature, pH, and pε) of the sea affecting critical physical, chemical, and biological reactions. Humans have also produced copious quantities of N and P fertilizers producing widespread coastal hypoxia and low dissolved oxygen conditions, which now threaten even the open ocean. Consequently, our massive alteration of state variables diminishes coral reefs, fisheries, and marine ecosystems, which are the foundation of life on Earth. We point to a myriad of actions and alternatives which will help to stem the tide of climate change and its effects on the sea while, at the same time, creating a more sustainable future for humans and ecosystems alike.

Influence of key cations and anions on phosphate removal by Fe(0) electrocoagulation

The electrocoagulation technology is efficient for the removal of phosphate. The removal efficiency of phosphate in the secondary effluent is higher than that of in the synthetic wastewater. In order to identify the key factors for promoting the removal of phosphate in Fe(0) electrocoagulation (EC), this study investigated typical coexisted substances in the secondary effluent, including Ca2+, Mg2+, NH4+, HCO3−, SO42−. The results indicated that Ca2+ and Mg2+ were the key factors promoting the removal of phosphate by Fe(0) EC both under normal (5–6 mg/L) and low dissolved oxygen (DO) (≤ 1 mg/L) conditions. Under low and normal DO conditions, the phosphate removal efficiency increased from 28 % to 98 % ± 2 % (in 5 min) at 10 mA/cm2 with the addition of Ca2+ (0.2–20 mM) or Mg2+ (0.2–2 mM) but decreased to approximately 70 % with the addition of a Mg2+ of 20 mM. Under low and normal DO conditions, the aggregation time of iron oxide flocs decreased from 10 min to 5 min when the concentration of Ca2+ increased from 0 to 20 mM and Mg2+ increased from 0 to 2 mM. The aggregation stability of flocs was not sufficient when the Mg2+ concentration was 20 mM at 5 mA/cm2. Our results confirmed that Fe(0) EC was suitable for the removal of phosphate in the secondary effluent, especially containing moderate amounts of Ca2+ and Mg2+ regardless of the concentration of DO.

Predominance of comammox bacteria among ammonia oxidizers under low dissolved oxygen condition

Although low-oxygen nitrification can significantly cut down the aeration demand in wastewater treatment plants, little is known about the community dynamics of relevant microorganisms under different oxygen concentrations. Here, by conducting a series of bioreactors with oxygen concentrations of 0%, 2%, 5%, 10%, 20%, 40%, and 70%, we provided a comprehensive investigation on the behaviors and performances of comammox bacteria (CMX), ammonia-oxidizing bacteria (AOB) and archaea (AOA) during the nitrification process. Quantitative PCR analysis demonstrated that CMX was the dominant ammonia-oxidizer under low oxygen condition (10%) after the four-month operation with the abundance increased by 8.65 times higher than the initial operation, whereas the growth of AOA and AOB was inhibited. Moreover, Nitrospira nitrosa dominated the CMX species (relative abundance >96%) in low dissolved oxygen concentrations, while Nitrospira nitrificans (3.39%) seemed to prefer high oxygen conditions. Our study indicates the long-term effects of oxygen concentrations on the niche differentiation of ammonia oxidizers, and highlights the significance of CMX in low-oxygen nitrification for reducing global carbon emission and energy consumption.

Simultaneous and Efficient Removal of Linear Alkylbenzenesulfonate and Nitrogen in a Membrane Biofilm Reactor under Low Dissolved Oxygen Conditions

Simultaneous and Efficient Removal of Linear Alkylbenzenesulfonate and Nitrogen in a Membrane Biofilm Reactor under Low Dissolved Oxygen Conditions

Mechanisms of NO and N2O production by enriched nitrifying sludge in a sequencing batch reactor: Effects of hydroxylamine

NO and N2O as important greenhouse gases andtheir production mechanisms during nitrification are not completely understood. This study aimed to analyze the effect of hydroxylamine (NH2OH) on NO and N2O produced by nitrifying bacteria from activated sludge in a sequencing batch reactor (SBR). Experimental results showed that when nitrite (NO2−) accumulated during aerobic ammonia (NH4+) oxidation, N2O was the main product. The total amount of NO and N2O produced by NH2OH oxidation was positively correlated with dissolved oxygen (DO) levels. The imbalance of NH4+ oxidation caused by NH2OH addition was more conducive to the generation of NO and N2O under high DO conditions. When NH2OH was added into the reactor with NO2− as the substrate, the production of NO and N2O under high DO levels was mainly related to NH2OH oxidation. Under low DO conditions, NO and N2O from the biotic/abiotic hybrid pathways were more significant in the reactor of the coexistence of NO2− and NH2OH, which could be mainly caused by the pathways of nitrifier denitrification and abiotic reaction. Besides, limited amount of NO and N2O was generated by heterotrophic denitrification pathway during autotrophic nitrification. The implications for the above results are important for understanding the production of NO and N2O under NH2OH stress in nitrifying sludge reactor.

Response of antibiotic resistance genes and microbial niches to dissolved oxygen in an oxygen-based membrane biofilm reactor during greywater treatment

Linear alkylbenzene sulfonates (LAS) in greywater (GW) will simulate antibiotic resistance genes (ARGs) production in the biofilm-based system. Our study emphasizes the dissolved oxygen (DO)-dependent ARGs accumulation and microbial niches succession in an oxygen-based membrane biofilm reactor (O2-MBfR) treating GW, as well as revealing the key roles of EPS. Changing DO concentrations led to significant differences in ARGs production, EPS secretion and microbial communities, as well as the organics and nitrogen removal efficiency. Increasing DO concentration from 0.2 to 0.4 mg/L led to improved organics (> 90%) and nitrogen removal, as well as less EPS (especially for proteins and carbohydrates) and ARGs accumulation (e.g., intI-1, korB and sul-2) in the biofilm; the high-DO-concentration accumulated microbial niches, including Flavobacteriaceae and Cyanobacteria that revealed by LEfSe analysis, contributed to both nitrogen reduction and organics biodegradation. While, the inefficient electron acceptor at low DO conditions (0.2 mg/L) reduced the organics and nitrogen removal efficiency, as well as the improved accumulation of EPS in biofilm; high EPS enabled the capture of residual LAS from the liquid phase, which stimulated the production of ARGs by the distinct microbial community compositions. These findings suggested the DO-based ARGs reduction regulation strategy in the O2-MBfR treating GW.

High concentrations of dissolved organic nitrogen and N-nitrosodimethylamine precursors in effluent from biological nutrient removal process with low dissolved oxygen conditions

The presence of dissolved organic nitrogen (DON) in biological nutrient removal (BNR) effluent has led to increased concern about its adverse effects on wastewater discharge and reuse applications. Previous studies have demonstrated efficient biological inorganic nitrogen removal in BNR under low dissolved oxygen (DO) conditions; however, information on DON is scarce. This study investigated low-DO effects on DON and N-nitrosodimethylamine (NDMA) precursor concentrations in BNR effluents. Identical BNR reactors consisting of an external real-time DO intelligent control system were operated at three different DO concentrations (0.3, 1.0, and 4.0 mgO2/L). Surprisingly, significantly higher values of effluent DON (p<0.05, t-test) and NDMA precursors (p<0.01, t-test) were observed at lower DO levels. Ultrahigh-resolution mass spectrometry analysis showed that molecules produced by microbes at low-DO levels exhibited high proteins/amino sugars-like and low normal oxidation state of carbon characteristics, which possibly acted critical roles in NDMA formation. Furthermore, path analysis by partial least-squares path modeling suggested that NDMA formation potential had strong associations with microbe-DON network stability of microbe-DON co-occurrence interactions (r=0.979, p<0.01). These results highlight the necessity of reconsidering the feasibility of BNR systems operating at low-DO concentrations considering the adverse effects of DON on wastewater discharge and reuse applications.

Efficacies of Nitrogen Removal and Comparisons of Microbial Communities in Full-Scale (Pre-Anoxic Systems) Municipal Water Resource Recovery Facilities at Low and High COD:TN Ratios

At a low COD:TN ratio (≤5) in influent, maintaining a longer HRT (≥9 h) and longer SRT (≥30 d) are suggested to improve higher N removal efficiency in case of operation at low DO (Dissolved oxygen) level (0.9 ± 0.2 mg-O2/L). However, in case of operation at high DO level (4.0 ± 0.5 mg-O2/L), short HRT (1 h) and typical SRT (17 d) make it possible to achieve nitrogen removal. On the other hand, at a high COD:TN ratio (≥8.4), a typical HRT (9–15 h), SRT (12–19 d), and DO level (1.3–2.6 mg-O2/L) would be applied. Microbial distribution analysis showed an abundance of AOA (Ammonia-oxidizing archaea) under conditions of low DO (≤0.9 mg-O2/L). Nitrosomonas sp. are mostly found in the all investigated water resource recovery facilities (WRRFs). Nitrosospira sp. are only found under operating conditions of longer SRT for WRRFs with a low COD:TN ratio. In comparison between abundances of Nitrobacter sp. and Nitrospira sp., abundances of Nitrobacter sp. are proportional to low DO concentration rather than abundance of Nitrospira sp. A predominance of nosZ-type denitrifiers were found at low DO level. Abundance of denitrifiers by using nirS genes showed an over-abundance of denitrifiers by using nirK genes at low and high COD:TN ratios.

Analyzing the sludge characteristics and microbial communities of biofilm and activated sludge in the partial nitrification/anammox process

The implementation of anammox based process remains a challenge in treating low ammonium wastewater. In present research, partial nitrification/anammox (PN/A) performance was investigated in the integrated fixed-film activated sludge (IFAS) system. The influent of the IFAS reactor was simulated residential domestic sewage. The nitrite oxidizing bacteria (NOB)'s proliferation was observed when dissolved oxygen (DO) concentrations were 0.65 and 0.35 mg/L. By conducting intermittent aeration (aerobic 1 min/anoxic 5 min) and low DO condition (0.20 mg/L) strategy, the IFAS reactor performed good nitrogen removal performance. The average total nitrogen (TN) removing efficiency and TN removing rate reached 80.6% and 0.13 kg N/(m3·d), respectively. In long term operation, extracellular polymeric substance (EPS) concentration of activated sludge was largely increased, which favored the formation of granules. The maximum frequency diameter of sludge flocs was increased to 1000 μm. The anammox bacteria and ammonium oxidizing bacteria (AOB) were both enriched within activated sludge and biofilm. High throughput analyzing result implied that anammox bacteria and AOB's abundances were 2.01% and 5.04% in activated sludge, while 2.43% and 2.45% in biofilm, respectively.

Nutrients removal from integrated multi-trophic aquaculture (IMTA) water using waste stabilization ponds (WSP)

Nutrients enrichment due to waste from Keramba Jaring Apung (KJA) aquaculture is one of the main causes of eutrophication in Indonesian Lakes and Reservoirs. Integrated multi-trophic aquaculture (IMTA) is one of environmentally friendly aquaculture that currently being developed at the Research Center (RC) for Limnology - National Research and Innovation Agency (BRIN). This aquaculture would re-use its water, along with fish waste and leftover fish pellets, to grow Duckweed (Lemna sp.) in its ponds system as additional feed. However, due to closed water recirculation in IMTA ponds system, the water quality would eventually deteriorate which would be marked with high turbidity of organic solid waste and low dissolved oxygen (DO). This study aims to improve water quality from aquacultures activity, especially in nutrients (nitrogen and phosphorus) reduction, using a constructed wetland (CW) system. This system consists of waste stabilization ponds (WSP) and compartments of CW and adsorbent. During this initial stage, IMTA water was treated in the WSP for then observed for its improvements in the nutrients and other water quality parameters. This research was conducted from July to September 2019 in the Prototype Laboratory of RC for Limnology-BRIN. Parameters of DO, conductivity, pH, oxidation reduction potential (ORP), and salinity were observed using a calibrated water quality checker (WQC) Horriba©, while nutrients parameters in the form of N-NH4, N-NO2, N-NO3, P-PO4, and dissolved organic matters (DOM) were measured in the laboratory referring to the standard method of the American Public Health Association (APHA). Results showed that average values of turbidity and electrical conductivity parameters were reduced from 102 to 76 NTU and 0.21 to 0.14 mS/cm, respectively. Average nutrients reductions were also found from N-NO2, N-NO3 and P-PO4 parameters of 85.5%, 44.3% and 37%, respectively. Significant changes were observed in parameters of DO and N-NH4. DO was increased from 4.63 mg/L to 7.44 mg/L, while N-NH4 were reduced for 81%. Even though the experiments were not conducted during low DO conditions of water from IMTA ponds, improvements from the vital water quality parameters after treatment in the WSP were observed.

A Novel Step-Feed Integrated Nitrification, Partial Denitrification, and Anammox Process in Single-Stage Reactor for Advanced Nitrogen Removal from Municipal Wastewater Under Low Dissolved Oxygen Condition

A Novel Step-Feed Integrated Nitrification, Partial Denitrification, and Anammox Process in Single-Stage Reactor for Advanced Nitrogen Removal from Municipal Wastewater Under Low Dissolved Oxygen Condition

Pilot-scale comparison of biological nutrient removal (BNR) using intermittent and continuous ammonia-based low dissolved oxygen aeration control systems.

Sensor driven aeration control strategies have recently been developed as a means to efficiently carry out biological nutrient removal (BNR) and reduce aeration costs in wastewater treatment plants. Under load-based aeration control, often implemented as ammonia-based aeration control (ABAC), airflow is regulated to meet desired effluent standards without specifically setting dissolved oxygen (DO) targets. Another approach to reduce aeration requirements is to constantly maintain low DO conditions and allow the microbial community to adapt to the low-DO environment. In this study, we compared the performance of two pilot-scale BNR treatment trains that simultaneously used ABAC and low-DO operation to evaluate the combination of these two strategies. One pilot plant was operated with continuous ABAC while the other one used intermittent ABAC. Both processes achieved greater than 90% total Kjehldal nitrogen (TKN) removal, 60% total nitrogen removal, and nearly 90% total phosphorus removal. Increasing the solids retention time (SRT) during the period of cold (∼12 °C) water temperatures helped maintain ammonia removal performance under low-DO conditions. However, both processes experienced poor solids settling characteristics during winter. While settling was recovered under warmer temperatures, improving settling quality remains a challenge under low-DO operation.