Low Dissolved Oxygen Concentrations Research Articles

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.

Assessment of drinking-water quality in the Dibamba watershed (Douala-Cameroon

Background: Residents of Douala including Dibamba watershed collect water from various points to fulfill their freshwater needs. However, it is known that many microorganisms, including parasites, bacteria, viruses, and even alga, are present in water where they form a complex ecosystem. These microorganisms are held responsible for most of the contamination in drinking water and related diseases that threaten health of many individuals. Objectives: This study aims to determine the bacteriological and physicochemical quality of drinking water used by city dwellers in the Dibamba watershed. Methods: A cross-sectional study was conducted in different communities living in the watershed. A total of 58 drinking water samples were randomly collected from a variety of water resources. The analyses were performed for temperature, Ph, and conductivity; and for bacterial parameters, namely, to trace the presence of organisms and opportunistic pathogens indicative of fecal contamination. Results: Water used for consumption purposes have an acidic pH, low dissolved oxygen content, and shows high levels of microorganisms, testifying water pollution due to domestic wastewater discharged without any prior treatment, and coming from domestic, livestock and agricultural activities carried out in the watershed. These contaminated waters contain pathogenic species (Shigella, Salmonella, E.Coli, etc.) suspected of being the cause of waterborne infectious diseases in the watershed.. Conclusion: The results of our study show that 95% of tested drinking water samples are of low microbiological quality. They neither fulfill requirements of World Health Organization standards, nor fit for human consumption. Therefore, Decision-makers and Stake holders should implement practical actions to facilitate access to safe and sufficient drinking water in the watershed.

Repeated hypoxic episodes allow hematological and physiological habituation in rainbow trout.

Introduction: Under climate change, the increase in temperature in aquatic environments may induce oxygen depletion. In extreme cases, low oxygen may become a limiting factor for fish, thus generating stress. In addition, consecutive hypoxic episodes may complicate the recovery of individuals and hinder their ability to modulate physiological and biochemical responses to maintain homeostasis. Thus, the aim of this study was to determine the hematological and physiological responses of rainbow trout under a condition of repeated hypoxic and manipulation stresses at three different time points. Methods: Every hypoxic episode consisted of exposing the fish to low dissolved oxygen concentrations (2 mgO2/L for 1 h). Following the exposure, the fish were allowed to recover for 1 h, after which they were sampled to investigate hematological and physiological parameters. Results and discussion: The results showed a pattern of habituation reflected by values of hematocrit, hemoglobin, and mean corpuscular volume, indicating a certain ability of rainbow trout to resist this type of repeated hypoxic events, provided that the fish can have some recovery time between the exposures.

Zooplankton of Pronskoye reservoir in August 2023

The aim of this study was to investigate the zooplankton of Pronskoye Reservoir during the summer period of 2023. Sampling and processing of hydrobiological zooplankton samples were carried out using standard methods in August 2023 at six monitoring stations in Pronskoye Reservoir. A total of 15 zooplankton taxa were identified in the samples: 6 taxa of rotifers, 5 taxa of cladocerans, and 4 taxa of copepods. On average, the abundance and biomass of zooplankton (mean values across stations) in Pronskoye Reservoir in August 2023 were 928.1 thousand individuals/m3 and 3.4 g/m3, respectively. Rotifers dominated in terms of abundance (63,5%), while cladocerans dominated in terms of biomass (40,3%). Pronskoye Reservoir exhibited above-average productivity in terms of zooplankton, which is favorable from a fisheries perspective. The abundance and biomass of zooplankton for all major groups (rotifers, cladocerans, and copepods) were higher in the lower part of the reservoir, separated from the middle and upper parts by a narrow neck. The increase in the abundance of the rotifer species Brachionus angularis was particularly noticeable in the lower part of the reservoir. The lower part of the water body is classified as highly productive, while the middle and upper parts has above-average productivity. The lower part of the reservoir is characterized by lower temperatures and lower dissolved oxygen concentrations in the water. The lower part of Pronskoye Reservoir exhibits a more diverse species composition compared to the middle and upper parts. It is evident that abiotic factors, in one way or another, determine the differences in the abundance, biomass, and species composition of zooplankton between the lower and upper parts of Pronskoye Reservoir.

The formation and adsorption mechanism studies of 3D hydrangea-like ZnFe-LDHs/FeOOH for the highly efficient removal of phosphate

Phosphate in water can cause a series of problems such as overgrowth of algae and other aquatic organisms, foul-smelling, turbid water and lower dissolved oxygen concentrations, thereby posing a serious threat to human health and environment. To efficiently remove phosphate from water, three-dimensional hydrangea-like ZnFe-layered double hydroxides/FeOOH composites (3D hydrangea-like ZnFe-LDHs/FeOOH) were designed and prepared by a facile urea hydrothermal method. The maximum adsorption capacity of phosphate by 3D hydrangea-like ZnFe-LDHs/FeOOH reached about 200 mg·g−1, much higher than that of ZnFe-LDHs prepared by co-precipitation method (53 mg·g−1) and other reported adsorbents, which confirmed that 3D morphology contributed to the improvement of adsorption capacity. Time-dependent experiments were designed to explore the formation mechanism of 3D hydrangea-like structure and the results indicated directional aggregation and epitaxial growth were two main processes. Batch experiments were conducted to investigate adsorption behavior of phosphate. The adsorption kinetics and isotherms showed that the adsorption process followed the quasi-first-order kinetics and Langmuir model. Besides, 3D hydrangea-like ZnFe-LDHs/FeOOH could be regenerated effectively and recycled at least three times. 3D hydrangea-like ZnFe-LDHs/FeOOH possessed fascinating performance in phosphate removal from real river samples and laundry detergent samples, with the removal rates of 66–100 % and 80–100 %, respectively. Furthermore, 3D hydrangea-like ZnFe-LDHs showed good adsorption performance towards other anionic dyes, including Congo Red and Orange Yellow II, with the maximum adsorption capacity of 867 mg·g−1 and 225 mg·g−1, indicating its widespread applicability. The adsorption mechanism of phosphate by 3D hydrangea-like ZnFe-LDHs was furtherly investigated, mainly containing metal coordination, anion exchange between layers and electrostatic interactions.

Root-zone aeration improves fruit yield and quality of tomato by enhancement of leaf photosynthetic performance

Heavy irrigation and soil compaction cause hypoxic stress in plant roots, which limits crop growth. This issue can be solved by root-zone aeration, but the photophysiological responses of crop plants to the dissolved oxygen (DO) concentration in irrigation water remain unclear. Here, a greenhouse plot experiment was conducted to investigate the changes in leaf photosynthesis and plant growth of tomato (Solanum lycopersicum L. cv. ‘Omanda 3′) under aerated irrigation with various DO concentrations. Plants were treated with three different levels of aeration: 5 mg L–1 DO (conventional subsurface irrigation as a non-aeration control), 15 mg L–1 DO (low aeration treatment), and 30 mg L–1 DO (high aeration treatment). Compared with the control, the aeration treatments promoted electron transport from the primary to secondary plastoquinone acceptors (QA to QB) of photosystem II (PSII) and increased leaf net photosynthetic rate in plants at the seedling, fruit expansion, and maturation stages. The opposite effects of aeration treatments were observed at the flowering and fruit-setting stage. The enhancement of leaf photosynthetic performance contributed to improved plant growth, fruit yield, and quality of tomato as a result of increased oxygen supply in the root zone. The aeration treatments additionally facilitated the biosynthesis and transport of photosynthetic carbon assimilates in plant tissues, as evidenced by increased starch and sucrose contents in the leaves and sucrose content in the roots. However, under the high aeration level, excessive transport of sucrose from leaves to roots hindered further improvements in tomato yield and biomass at the maturation stage. Based on plant photophysiological and yield performance, aerated irrigation with a low concentration of DO (15 mg L–1) is recommended for greenhouse tomato crops during the seedling, fruit expansion, and maturation stages, and there is no need of root-zone aeration at the flowering and fruit-setting stage.

Identification of key microbial communities and intracellular metabolic pathways to response micro-aerobic environment for enhancing degradation of N, N-dimethylformamide in refractory membrane-making wastewater

Enhanced sludge hydrolysis in micro-aerobic environment with dissolved oxygen (DO) regulation could degrade stubborn refractory N, N-dimethylformamide (DMF) in membrane-making wastewater. However, investigations on how micro-aerobic condition affects DMF conversion and microbial metabolism in biological system are still inadequate. Herein, the responses of microbial community to micro-aerobic environment, and the consequent variations of metabolic pathways in DMF degradation were unraveled. Results indicated that low concentration of DO (0.2–0.4 mg/L) could significantly enhance the ammonification rate (from 21.8 % to 52.8 %) and COD removal efficiency (from 65.2 % to 82 %) by breaking C–N and CO bonds of amide group in macromolecular organics, which help it degraded into micromolecular organics. 16S rRNA and metagenomic analysis further revealed that low DO contents prompted the changes of microbial community structure. Specifically, toxicity-tolerant fermentation bacteria (e.g. Desulfomicrobium and unclassified_c_ Deltaproteobacteria) gradually became the dominant species in micro-aerobic environment, showing positive connections with COD removal and ammonification functions. Additionally, functional genes analysis further revealed the intracellular metabolic pathways of DMF, where the upregulation of tricarboxylic acid (TCA) cycle and oxidative phosphorylation pathway was observed in micro-aerobic environment. Such regulation led to the massive synthesis of adenosine triphosphate (ATP) and NADH, which are needful in oxidative demethylation process, and thus promoting the conversion from DMF to formamide, CH4 and NH3 in Methanolobus step-by-step. Therefore, this study provided vital insights into the responses of microbial community structure and DMF intracellular metabolic pathways to micro-aerobic environment, and profoundly discussed their contributions to the efficient treatment effect on membrane-making wastewater.

Degassing Dissolved Oxygen through Bubbling: The Contribution and Control of Vapor Bubbles

An innovative yet sustainable approach for industrial deaeration is proposed, with demonstrated results and analyses, to contribute to finding solutions to improve energy efficiency in this field. Vacuum bubbling deaeration, sharing the same working principles of solubility control and the mass diffusion through vapor (or steam) with conventional thermal deaeration processes, works, however, at lower vacuum pressures. It neither resorts to heating nor requires any third-party materials such as membranes or gases, achieving orders of magnitude of reduction in the expected energy consumption in a simple and concrete way. In this study, the mechanisms of vapor bubble generation and retention were discussed by employing a vacuum bubbling model based on the experimental apparatus at Kongju National University, which uses a venturi-nozzle bubbler. The four parameters influencing vapor bubble generation and retention were identified as vessel pressure p1, nozzle depth Δh, nozzle performance p4−p3, and water temperature Tw. A series of deaeration experiments using the present approach for a tap water sample of 360~400 L were conducted under four different conditions to investigate the effects of the water temperature, vessel pressure, and bubbler nozzle depth. Final dissolved oxygen (DO) concentrations close to zero could be achieved with a vessel pressure of p1=1 kPa, with different bubbling times to reach a zero mg/L reading of DO concentration (case 2 and 3), which demonstrates the vital roles of the vapor bubble generation condition of (psat−p3)  and retention condition of (p4−psat) in achieving the lowest DO concentration. Analysis of the test results, based on the discrete-bubble model with the measured DO concentrations and degassing rates, showed promising results in reproducing the experimental data. Though the potential of vacuum bubbling deaeration is demonstrated, for the first time, to its full extent, further research efforts are encouraged in many areas, including more case-specific validation test cases with optimum operating conditions along with the study of more detailed modeling for performance prediction, including energy analysis.

Environmental influence on summer survival of Manila Clam Ruditapes philippinarum: A case study in an aquaculture bay

The aquaculture of Manila clam (Ruditapes philippinarum) is under threat due to its high mortality in summer. To investigate the environmental influence on mortality of the species, we have conducted field surveys in the aquaculture area in Laizhou Bay, during the summers of 2019 and 2020. Environmental and biological data were collected. The results of data analysis have shown that in 2020, large-scale mortality events were not recorded and spatial variations of environmental variables were no significant within the survey area. However, in August 2019, significant variations of environmental variables were detected in the middle tidal area. This area was charactered with relatively high-water temperature (with maximum temperature of 31.49 °C and mean value of 26.17 ± 1.62 °C), elevated hydrogen sulfide concentrations (with a maximum concentration of 24.72 μmol/L and a mean concentration of 7.89 ± 4.14 μmol/L), and low dissolved oxygen concentrations (with a minimum concentration of 3.35 mg/L and a mean concentration of 6.27 ± 0.42 mg/L). Correspondingly, high mortality of the clam was recorded in the area, the abundance of live Manila clams has decreased by 80%. No significant abnormalities of environmental variables and clam growth were observed in other regions in 2019. Glycogen content and condition index of the clam were analyzed to investigate the relationship between environmental stress and the health of the clam. Compared to the condition index, glycogen content is a more sensitive indicator of the health status of the clam because changes in glycogen content appeared earlier than changes in condition index and mortality. A principal component analysis further indicated that the high mortality of the clam corresponds with a few environmental variables, including elevated temperature, hypoxia and the concentration of hydrogen sulfide. The simultaneous presence of these multiple environmental stressors could have triggered alterations in the physiological responses of the clam.

Environmental implications of phosphate-based fertilizer industrial waste and its management practices.

During the green revolution in the mid-twentieth century, the consumption of inorganic phosphorous and phosphate-based fertilizers (P-fertilizers) in the developing world skyrocketed, resulting in a proliferation of P-fertilizer industries. Phosphate-based fertilizer industries are ranked among the most environment-polluting industries. The worldwide phosphorus market, which was 68.5 million metric tons in 2020, is expected to increase at a compound annual growth rate (CAGR) of 2.5% to 81 million metric tons by 2027. The release of untreated hazardous pollutants from these fertilizer industries into the soil, water, and atmosphere has resulted in severe environmental health issues. Excessive surface runoff of phosphorus from agricultural fields and its deposition in water promote the growth of algae and macrophytes and lower dissolved oxygen concentration through eutrophication, which is detrimental to aquatic life. Fluorides (F-) and sulfur dioxide (SO2) and/or heavy metals (potentially toxic elements, PTEs) are also detected in the emissions from these fertilizer industries. The main solid waste generated from the phospho-gypsum plant produced up to 5 tons of di-hydrogen phosphate (H2PO4), including PTEs and radioactive substances. Phosphates and fluorenes from these industries are usually disposed of as sludge in storage ponds or trash piles. Humans inhaling poisonous gases released from the P-fertilizer industries can develop hepatic failure, autoimmune diseases, pulmonary disorders, and other health problems. The objectives of this review are to provide guidelines for eliminating the bottleneck pollutions that occur from the phosphate-based fertilizer industries and explore the management practices for its green development.

Effects of Lotus (Nelumbo nucifera Gaertn.) on the Methane Emission in Littoral Zones of a Subtropical Lake, China

Freshwater lakes represent a potential source of methane (CH4) emission into the atmosphere. However, the CH4 emission contribution to the total emission in the littoral zones of lakes, especially emergent macrophytes (e.g., lotus), is poorly known. Lotus has been cultivated in almost all provinces in China; it is not only an aquatic plant, but also a kind of vegetable. In this study, two sampling zones (lotus plant and open water) were established in the lake of the middle reaches of the Yangtze River. The CH4 emission was measured using a floating opaque chamber and gas chromatography between April and December in the years 2021 and 2022. The results indicated that the flux of CH4 emissions ranged from 0.10 to 59.75 mg m−2 h−1, with an average value of 5.61 mg m−2 h−1, in the open water, while ranging from 0.19 to 57.32 mg m−2 h−1, with an average value of 17.14 mg m−2 h−1, in the lotus plant zone. The maximal CH4 emissions occurred in July and August for the open water, which was highly related to the air and water temperature, whereas it happened in September for the lotus plant zone, possibly due to the high vegetation biomass, indirectly enhancing the high soil organic carbon content, plant-mediated CH4 emission, as well as the lower dissolved oxygen concentration, thus strengthening the production and emissions of CH4. Considering the carbon emissions (both CH4 and CO2) and plant productivity, although greater CH4 emission occurred in the lotus plant zone, it could still represent a potential carbon sink (213 g m−2 yr−1) compared to the open water.

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.

Monsoon-driven seasonal hypoxia along the northern coast of Oman

Dissolved oxygen and current observations from a cabled ocean observatory in the Sea of Oman show that the annual recurrence of coastal hypoxia, defined as dissolved oxygen concentrations ≤63 μM, is associated with the seasonal cycle of local monsoon winds. The observations represent the first long-term (5+ years) continuous moored observations off the northern Omani coast. During the summer/fall southwest (SW) monsoon season (Jun-Nov), winds in the Sea of Oman generate ocean currents that result in coastal upwelling of subsurface waters with low dissolved oxygen concentrations. The source of the poorly oxygenated water is the oxygen minimum zone (OMZ) in the Arabian Sea, a layer approximately 1000-m thick within the 100 to 1200 m depth range, where dissolved oxygen values approach anoxia. During the winter monsoon season (Dec-Feb), the Sea of Oman winds are from the northwest, forcing strong and persistent southeast currents. These winds generate oceanic downwelling conditions along the coastal ocean that ventilate waters at depth. Possible impacts of the monsoon-driven seasonal hypoxia on local fisheries and implications due to climate change are also discussed in this study.

Influence of Anthropogenic Activities on the Water Quality of an Urban River in an Unplanned Zone of the Amazonian Coast

The database on water quality in Amazonian rivers located in unplanned urbanized regions along the Brazilian Amazon Coast is still quite limited. This study addresses these concerns, and the tested hypothesis was that the water quality of the Cereja River has deteriorated in recent years, despite the efforts of government authorities to mitigate anthropogenic impacts. To assess changes in water quality, seven campaigns were conducted, collecting data at six fixed points during two different periods. High-resolution satellite images were used to document unplanned occupation. Unfortunately, the number of houses along the Cereja River has increased, in violation of the law. This has contributed to the river’s intense trophic condition, lower dissolved oxygen concentrations, higher concentrations of pathogenic bacteria, and loss of vegetation cover. According to national water quality standards, the Cereja is unsuitable for any human use. This is in stark contrast to the scenario a few decades ago when the river was used for leisure, fishing, and other activities. The results obtained confirm the initial hypothesis and can support potential management strategies and decision-making by authorities. The observed scenario can be extrapolated to other rivers located in urban areas in the Amazon region that have similarly regrettably experienced relatively uncontrolled growth.

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.

Enhancement of nutrient removal in an activated sludge process using aerobic granular sludge augmentation strategy with ammonium-based aeration control

To enhance nutrient removal from low-strength municipal wastewater in a continuous-flow activated sludge (CFAS) process using aerobic granular sludge (AGS) augmentation strategy, a pilot-scale demonstration was configured with a mainstream reactor (anaerobic/aerobic process) and a sidestream sequencing batch reactor for AGS production. The aeration of the mainstream reactor was controlled based on dissolved oxygen (DO) and ammonium concentrations during Phases I and II–III, respectively. During Phase III, an anoxic zone was created in the mainstream aerobic tank. Throughout the demonstration period, excellent sludge settleability in the mainstream reactor (SVI30 ≤ 80 mL g−1) under long sludge retention time conditions (≥12 d) allowed the maintenance of a high mixed liquor suspended solids concentration (≥3000 mg L−1). The total nitrogen (TN) removal ratio improved significantly during Phases II and III (49.3 ± 4.1% and 50.1 ± 10.2%, respectively) compared to Phase I (43.2 ± 5.5%). Low DO concentration (< 0.5 mg L−1) by the ammonium-based aeration tended to increase the simultaneous nitrification and denitrification efficiency (> 40%), enhancing TN removal (> 50%). The reduction of DO and nitrate concentrations in the returning sludge liquor can stabilize phosphorus removal (approximately 80% of the 25th percentile). In addition, the aeration efficiency during Phase III decreased by 26–29% compared to Phase I. These results suggest that the introduction of ammonium-based aeration control to the CFAS using the AGS augmentation strategy could contribute to superior sewerage treatment, including nutrient removal and a low carbon footprint.

Effect of dissolved oxygen concentration on performance and mechanism of simultaneous nitrification and denitrification in integrated fixed-film activated sludge sequencing batch reactors

Integrated fixed-film activated sludge (IFAS) is a superior system for achieving simultaneous nitrification and denitrification (SND), however, the impact of dissolved oxygen (DO) has not been fully elucidated. Therefore, this study investigated the effect of DO concentration on performance and mechanism of SND in IFAS system. Results showed that IFAS outperformed control systems and achieved optimal SND performance at a DO concentration of 0.5 mg/L, with an SND efficiency of 88.51% and total nitrogen removal efficiency of 82.78%. Typical cycles analysis demonstrated limited-DO promoted SND performance. Further analysis implied biofilms exhibited high biomass and denitrification activity with decreasing DO. Microbial community analysis revealed low DO concentrations were responsible for abundant functional groups and genes associated with SND and promoted unconventional nitrogen removal pathways. Moreover, co-occurrence network analysis elucidated microbial interactions, responses to DO, and keystone genera. This study helps understanding the roles of DO for enhanced SND in IFAS.

Assessment of Water Quality in a Coastal Region of Sea Dike Construction in Korea and the Impact of Low Dissolved Oxygen Concentrations on pH Changes

To investigate the factors affecting water quality in coastal regions with sea dike constructions, surface water outside a sea dike was monitored for six years from 2015 to 2020 in the Saemangeum region of Korea. Statistical analyses of the six years of high-frequency measurements revealed that the water quality in this system was predominantly governed by natural processes followed by pollutant inputs as the secondary influencing factor. Severe dissolved oxygen (DO) depletion was observed in the surface waters during warm periods, probably owing to the advection of DO-depleted water from elsewhere to the surface layer. Based on the apparent oxygen utilization (AOU)–pH relationship (r = 0.52, n = 1837), the maximum AOU (180 µM) led to a pH decrease from 8.04 to 7.50, which was considerably lower than the estimated value of 7.72. This extra pH drop was probably due to a reduction in the buffering capacity associated with increased CO2 in the water column originating from the atmosphere and in situ production, as well as local water column redox reactions associated with benthic inputs of reduced chemical species. Overall, persistent DO depletion with ongoing eutrophication/hypoxia could accelerate ocean acidification in Korean coastal waters, which could be more acute in coastal regions with artificial coastal constructions.

Non-uniform dissolved oxygen distribution and high sludge concentration enhance simultaneous nitrification and denitrification in a novel air-lifting reactor for municipal wastewater treatment: A pilot-scale study

Achieving simultaneous carbon and nitrogen removal with sludge-liquid separation in a single reactor offers a solution to land shortages and improves treatment efficiency in municipal wastewater treatment plants of megacities. This study proposes a novel air-lifting continuous-flow reactor configuration with an alternative-aeration strategy that creates multi-functional zones for anoxic, oxic, and settlement processes. The optimal operating conditions for the reactor include a long anoxic hydraulic retention time, low dissolved oxygen (DO) in the oxic zone, and no specific reflux for external nitrifying liquid, which exhibit a high nitrogen removal efficiency of over 90% in treating real sewage with C/N < 4 in the pilot-scale study. Results show that a high sludge concentration and a low DO concentration facilitate simultaneous nitrification and denitrification, and a well mixing of sludge and substrate in different reaction zones promotes mass transfer and microbial activity. The long-term operation enriches functional microbes for carbon storage and nutrient removal.

Analysis of chemical variables in the water of the Córrego do Galante Watershed – SP

Population growth accompanied by inefficiencies in the management of sanitary effluents has been triggering adversities in relation to water quality. In view of the above, this work aims to analyze the concentrations of dissolved oxygen and biochemical oxygen demand in the water of the Córrego do Galante Hydrographic Microbasin - SP. Six strategic points were selected to carry out the research, each with its own characteristic, totaling eight collections of water samples, between the months of December/2021 to July/2022. Taking into account the results of the work, it was necessary to carry out an analysis of two chemical variables, which followed specific methodologies. Regarding the Dissolved Oxygen (DO), it was determined by the spectrophotometry methodology of HANNA Instruments, as for the Biochemical Oxygen Demand (BOD), it followed the method described in the Standard Methods for Examination of Water and Wastewater. The lowest DO concentrations obtained were recorded in the Dec/2021 and Jan/2022 periods. In general, over the period studied, the BOD resulted, for the most part, in values ​​outside the limits established by legislation. The period Feb/2022 for point 1 – east – resulted in a concentration of 181 mg.L⁻¹. It is concluded that the Córrego do Galante Watershed is an example of an environment that has been supporting constant anthropogenic impacts, being able to confirm this state through the OD and BOD variables. Among the negative factors, those that most influence the variations in concentrations are the release of sanitary effluent and urban drainage.