High Oxygen Transfer Efficiency Research Articles

Understanding the role of activated sludge in oxygen transfer: Effects of sludge settleability, solids retention time, and nitrification reaction.

The current understanding on the oxygen transfer in activated sludge process is primarily developed based on two-phase systems, focusing only on oxygen transfer from air to water. However, this research demonstrates that activated sludge particles significantly impact oxygen transfer from air all the way to the microorganisms. Three bench-scale complete-mix activated sludge reactors, operated under the same influent loading and dissolved oxygen level but different solids retention times (SRTs), were used to develop oxygen transfer performance data as effects of different sludge property parameters. These reactors were also operated under batch modes to further validate the effect of nitrification reaction on oxygen transfer. Results indicate that high overall oxygen transfer efficiency (OTE) is associated with low mixed liquor viscosity, long SRT, and nitrification reaction. Further analyses suggest that low mixed liquor viscosity, which resulted from high sludge settleability or low settled volume of sludge, reduces the thickness of liquid films at all interfaces and the size of air bubbles. Long SRT results in high active nitrifier population and low specific extracellular polymeric substance (EPS). Nitrification reaction, which serves as the rate-limiting step for oxygen transfer, may increase the oxygen transfer driving force. High active nitrifier population also promotes direct air-sludge contact. All of these factors help facilitate oxygen transfer. This research provides a new approach to improve energy efficiency for wastewater treatment, which is to change the activated sludge property by adjusting treatment plant design and operational parameters. PRACTITIONER POINTS: High sludge settleability reduces viscosity therefore liquid film thickness. Long SRT increases active microorganism population and reduces specific EPS content. Nitrification reaction increases oxygen transfer driving force. Direct air-particle contact adds another pathway for oxygen transfer. Nitrification reaction is the rate-limiting step of the oxygen transfer process.

Recent progress using membrane aerated biofilm reactors for wastewater treatment.

The membrane biofilm reactor (MBfR), which is based on the counter diffusion of the electron donors and acceptors into the biofilm, represents a novel technology for wastewater treatment. When process air or oxygen is supplied, the MBfR is known as the membrane aerated biofilm reactor (MABR), which has high oxygen transfer rate and efficiency, promoting microbial growth and activity within the biofilm. Over the past few decades, laboratory-scale studies have helped researchers and practitioners understand the relevance of influencing factors and biological transformations in MABRs. In recent years, pilot- to full-scale installations are increasing along with process modeling. The resulting accumulated knowledge has greatly improved understanding of the counter-diffusional biological process, with new challenges and opportunities arising. Therefore, it is crucial to provide new insights by conducting this review. This paper reviews wastewater treatment advancements using MABR technology, including design and operational considerations, microbial community ecology, and process modeling. Treatment performance of pilot- to full-scale MABRs for process intensification in existing facilities is assessed. This paper also reviews other emerging applications of MABRs, including sulfur recovery, industrial wastewater, and xenobiotics bioremediation, space-based wastewater treatment, and autotrophic nitrogen removal. In conclusion, commercial applications demonstrate that MABR technology is beneficial for pollutants (COD, N, P, xenobiotics) removal, resource recovery (e.g., sulfur), and N2O mitigation. Further research is needed to increase packing density while retaining efficient external mass transfer, understand the microbial interactions occurring, address existing assumptions to improve process modeling and control, and optimize the operational conditions with site-specific considerations.

A high gas transfer efficiency microfluidic oxygenator for extracorporeal respiratory assist applications in critical care medicine.

Advances in microfluidics technologies have spurred the development of a new generation of microfluidic respiratory assist devices, constructed using microfabrication techniques capable of producing microchannel dimensions similar to those found in human capillaries and gas transfer films in the same thickness range as the alveolar membrane. These devices have been tested in laboratory settings and in some cases in extracorporeal animal experiments, yet none have been advanced to human clinical studies. A major challenge in the development of microfluidic oxygenators is the difficulty in scaling the technology toward high blood flows necessary to support adult humans; such scaling efforts are often limited by the complexity of the fabrication process and the manner in which blood is distributed in a three-dimensional network of microchannels. Conceptually, a central advantage of microfluidic oxygenators over existing hollow-fiber membrane-based configurations is the potential for shallower channels and thinner gas transfer membranes, features that reduce oxygen diffusion distances, to result in a higher gas transfer efficiency defined as the ratio of the volume of oxygen transferred to the blood per unit time to the active surface area of the gas transfer membrane. If this ratio is not significantly higher than values reported for hollow fiber membrane oxygenators (HFMO), then the expected advantage of the microfluidic approach would not be realized in practice, potentially due to challenges encountered in blood distribution strategies when scaling microfluidic designs to higher flow rates. Here, we report on scaling of a microfluidic oxygenator design from 4 to 92mL/min blood flow, within an order of magnitude of the flow rate required for neonatal applications. This scaled device is shown to have a gas transfer efficiency higher than any other reported system in the literature, including other microfluidic prototypes and commercial HFMO cartridges. While the high oxygen transfer efficiency is a promising advance toward clinical scaling of a microfluidic architecture, it is accompanied by an excessive blood pressure drop in the circuit, arising from a combination of shallow gas transfer channels and equally shallow distribution manifolds. Therefore, next-generation microfluidic oxygenators will require novel design and fabrication strategies to minimize pressure drops while maintaining very high oxygen transfer efficiencies.

Improvement of transport phenomena and COD removal through the design of manifolds in membrane aerated biofilm reactors for sewage treatment

The membrane aerated biofilm reactor (MABR) has demonstrated a huge potential in wastewater treatment plants (WWTPs) due to biofilm stratification and high oxygen transfer efficiency. However, an inappropriate design can negatively impact hydrodynamic and mass transfer, hampering the reactor performance. This work is intended to enhance these phenomena through a rational design of inlet flow distributors (IFD) using computational fluid dynamics (CFD) and mass-transfer simulations. Different IFDs are analyzed at macro and micromixing levels using the residence time distribution (RTD) test and CFD, respectively. It is found that the computed RTD curves agree with experimental data with an error <5 %. The conical wall distributor generates the most homogeneous velocity in the entire reaction zone (membrane zone). The maximum velocity ratio between the channeling and stagnant zones presents a diminution of ∼70 %, and an increment of ∼11.7 % on the chemical oxygen demand (COD) removal if compared against the MABR without IFD, during the sewage treatment. Results show an acceptable agreement of simulations and experimental data, indicating that this approach is satisfactory to conduct a rational design of this type of reactor.

Acid whey treatment and conversion to single cell protein via aerobic yeast activated sludge

Abstract In this study, a synthetic acid whey was made to mimic acid whey produced during cheese manufacture. A mixed yeast culture, primarily Vanrija albida, was used to degrade the wastewater and produce a single cell protein (SCP). The system was operated in batch mode at high and low loading rates. The COD-use efficiency of the biomass was 93 and 85% at the high and low loading rates, respectively. The pH was maintained at 3.5 to prevent bacterial contamination of the system. High loading and high oxygen transfer efficiency indicate that a full-scale yeast system would probably offer significant cost savings over both aerobic and anaerobic bacterial systems. The biomass produced has the potential to be a valuable commodity. The biomass amino acid profile was good with respect to the FAO protein nutrition guidelines for various farmed livestock species.

Hydrodynamic behavior and oxygen transfer performance of a new device utilizing wave energy to transport surface water to deep ocean zones

Dissolved oxygen (DO) concentration is a significant indicator of marine environment and ecology, and hypoxia occurs frequently in deeper seawater. We present a new wave energy utilization device to transport superficial oxygen-rich water to deeper hypoxic zones. A series of experiments were conducted and a simple 3-D CFD model was developed to investigate the hydrodynamic behavior and oxygen transfer performance of the device for waves. With increases in wave heights or periods, the response amplitude (RA) of heaving components and the average flow rate in inlet tube increased. However, RA decreased when distance between the entrance of inlet tube and the flume bottom increased. For relatively small wave period scenarios, relative RA increased with increasing wave steepness; however, for relatively large wave period scenarios, it increased to a peak and then decreased with increasing wave steepness. With the increase in equivalent damping coefficient or buoy relative height, relative RA of heaving buoy decreased. The oxygen transfer coefficient on the wave surface increased with increasing wave steepness. The DO concentrations and oxygen transfer coefficients were higher in the scenarios with the device than in those without the device, indicating that a high oxygen transfer efficiency could be accomplished using the device.

Start-up and operational performance of the partial nitrification process in a sequencing batch reactor (SBR) coupled with a micro-aeration system

Partial nitrification (PN) of ammonia to nitrite is investigated in a lab-scale sequencing batch reactor (SBR) coupled with both a microporous aeration system and a mechanical agitation system at a moderate temperature of (27 ± 1 °C). The SBR has a high actual oxygen transfer efficiency (AOTE) of 2.0% and dynamical efficiency (DE) of 20.0%. Alkalinity consumption declined with the decreasing ratios of HCO3− to NH4+-N in the influent from 2.57, 1.96, 1.91 to 1.66, while the pH of the effluent is constantly maintained at 7.5 ± 0.1. The SBR is successfully operated for 195 days at a nitrogen loading rate (NLR) of up to 2.82 kg·m−3.d−1, achieving a nitrite accumulation rate (NAR) of over 90%. The high-throughput sequencing shows that the ratio of Nitrosomonas, the dominant species, is up to 29.83%.

Mixing Ability of Aeration Pipe with High Oxygen Transfer Efficiency and Application Example for Wastewater Treatment in the Food Industry

食品加工施設の排水は腐敗しやすく，槽底に滞った汚泥から臭気や硫化水素などが発生し，排水処理施設の能力を低下させる．これに対し，高流速で空気を注入する方式のエアレータを用いて攪拌し，好気性処理を安定させる方法を提案した．地下水槽（幅2m×水深1.75m）の底部にエアレータを設置し，曝気中の流速を3軸電磁流速計で測定した．3次元的な循環流が観察され，循環流量は約400 L/min と推定された．また，溶存酸素量（DO）の測定から，良好な酸素溶解効率（OTE）が得られた．さらに，実際の食品加工排水に適用した場合の処理状況について報告した．

Influence of temperature on the single-stage ATAD process predicted by a thermal equilibrium model

Autothermal thermophilic aerobic digestion (ATAD) is a promising biological process that will produce an effluent satisfying the Class A requirements on pathogen control and land application. The thermophilic temperature in an ATAD reactor is one of the critical factors that can affect the satisfactory operation of the ATAD process. This paper established a thermal equilibrium model to predict the effect of variables on the auto-rising temperature in an ATAD system. The reactors with volumes smaller than 10 m3 could not achieve temperatures higher than 45 °C under ambient temperature of −5 °C. The results showed that for small reactors, the reactor volume played a key role in promoting auto-rising temperature in the winter. Thermophilic temperature achieved in small ATAD reactors did not entirely depend on the heat release from biological activities during degrading organic matters in sludges, but was related to the ambient temperature. The ratios of surface area-to-effective volume less than 2.0 had less impact on the auto-rising temperature of an ATAD reactor. The influence of ambient temperature on the auto-rising reactor temperature decreased with increasing reactor volumes. High oxygen transfer efficiency had a significant influence on the internal temperature rise in an ATAD system, indicating that improving the oxygen transfer efficiency of aeration devices was a key factor to achieve a higher removal rate of volatile solids (VS) during the ATAD process operation. Compared with aeration using cold air, hot air demonstrated a significant effect on maintaining the internal temperature (usually 4–5 °C higher).

Framework for Energy Neutral Treatment for the 21st Century through Energy Efficient Aeration

Aeration is commonly identified as the most significant energy use in the treatment of wastewater and therefore garners significant focus in reducing energy use in the ultimate aspirational goal of achieving net energy neutrality for water resource recovery. This research establishes the mechanistic link between biofilm DNA concentration and reduced aeration efficiency caused by biofilm fouling. Although the connection between biofilm growth and fouling has been implicit in discussions of diffuser fouling for many years, this research provides directly measured evidence of the connection between biofouling and reduced diffuser efficiency. This was clearly established by studying systematically the evolution of aeration diffusers' efficiency over time, concurrently with the biological study of the biofilm fouling the diffusers in order to understand the major factors contributing to diffuser fouling. While all diffusers foul eventually, some novel materials exhibited fouling resistance. Fouling mitigation techniques, such as chemical cleaning and membrane reverse-flexing were studied. However, during the course of this study the research could not conclude that these techniques were always effective in curbing effects of fouling. By linking bacterial growth with aeration efficiency and calculating the aeration process energy requirements from the latter, the research was able to show quantitatively the causal connection between bacterial fouling and energy wastage during aeration. The methodology applied here can be extended to perform parallel tests during the initial phases of aeration design to yield site-specific data that increase the robustness of design. Fine-pore diffused aeration is the most commonly used aeration system for new activated sludge treatment processes due to its high oxygen transfer efficiency. However, fouling of finepore diffusers has the double-negative effect of greatly diminishing their oxygen transfer efficiency and increasing the back-pressure on the blower, which decreases its operating efficiency. Development of a fundamental understanding of the science of fouling and its impact on fine-pore diffusers facilitates the development of mitigation strategies and guidelines. Overall, application of the findings of this research will result in reduced energy usage at water resource recovery facilities (WRRFs) that will enable them to close the gap between energy use and energy generation to ultimately reach the goal of energy neutrality. This title belongs to WERF Research Report Series ISBN: 9781780406794 (eBook)

Impact of Groundwater Salinity on Bioremediation Enhanced by Micro-Nano Bubbles.

Micro-nano bubbles (MNBs) technology has shown great potential in groundwater bioremediation because of their large specific surface area, negatively charged surface, long stagnation, high oxygen transfer efficiency, etc. Groundwater salinity, which varies from sites due to different geological and environmental conditions, has a strong impact on the bioremediation effect. However, the groundwater salinity effect on MNBs’ behavior has not been reported. In this study, the size distribution, oxygen transfer efficiency and zeta potential of MNBs was investigated in different salt concentrations. In addition, the permeability of MNBs’ water through sand in different salt concentrations was studied. The results showed that water salinity has no influence on bubble size distribution during MNBs generation. MNBs could greatly enhance the oxygen transfer efficiency from inner bubbles to outer water, which may greatly enhance aerobic bioremediation. However, the enhancement varied depending on salt concentration. 0.7 g/L was found to be the optimal salt concentration to transfer oxygen. Moreover, MNBs in water salinity of 0.7 g/L had the minimum zeta potential. The correlation of zeta potential and mass transfer was discussed. The hydraulic conductivities of sand were similar for MNBs water with different salt concentrations. The results suggested that salinity had a great influence on MNBs performance, and groundwater salinity should be taken into careful consideration in applying MNBs technology to the enhancement of bioremediation.

Performance of coal fly-ash based oxygen carrier for the chemical looping combustion of synthesis gas

The performance of coal fly-ash based oxygen carriers for chemical looping combustion of synthesis gas has been investigated using both a thermogravimetric analyser and a packed bed reactor. Oxygen carriers with 50wt% active metal compounds, including copper, nickel and iron oxides, supported on coal fly-ash were synthesised using the deposition–precipitation method. Copper oxide and nickel oxide supported on fly-ash showed high oxygen transfer efficiency and oxygen carrying capacity at 800°C. The fly-ash based nickel oxide was effective in reforming hydrocarbons and for the conversion of carbon dioxide into carbon monoxide; a nickel complex with silicate was identified as a minor phase following the reduction reaction. The fly-ash based iron oxide showed various reduction steps and resulted in an extended reduction time. The carbon emission at the oxidation stage was avoided by reducing the length of the exposure to the reduction gas.

Depuration in aerated ponds of citrus processing wastewater with a high concentration of essential oils

Citrus processing wastewater was treated in aerated pilot plants in order to evaluate the following: (a) energy efficiency under different air flow rates and times; and (b) limits of spontaneous microflora in adapting to essential oils. In comparison to permanent air flow, night aeration for 12 hours determined an increase of up to 12% of the monthly removal rate of chemical oxygen demand (COD) and a consequent reduction by 10% of energy consumptions per unit of COD removed from 0.63 to 0.57 kWh/kgCOD. Lowering night aeration from 14 to 7 l/m3/h reduced by only 10% the removal rate of COD; the energy consumption per unit of COD removed (0.32 kWh/kgCOD) was consequently reduced by more than 40%. Dissolved oxygen was maintained at very low level, rarely exceeding 0.2 ppm, with no bad smell. The consequent high oxygen deficit of 98–99% of saturation induced high oxygen transfer efficiency. The microbial population was characterized mainly by aerobic bacteria; only 5–8% of bacteria were strictly anaerobic. In the deep tank layer under the air diffuser a small amount of sludge settled (0.03–0.04 kg of dry matter per kg of COD removed), containing only 3% of total organic matter detected at the end of the depuration process. The fact that the concentration of essential oils could be progressively increased up to 1400 ppm without noticeably slowing down the biological processes demonstrated the remarkable microbial adaptation.

Energy efficient aeration of wastewaters from the pulp and paper industry

More than 50% of the electrical power needed to treat pulp and paper industry effluents is used for aeration in biological treatment stages. A large share of the oxygen that passes through the wastewater is not consumed and will be found in the off-gas. Energy can be saved by aerating under conditions where the oxygen transfer is most efficient, for example at low concentrations of dissolved oxygen Consider the sludge as an energy source; electricity can be saved by avoiding sludge reduction through prolonged aeration. High oxygen transfer efficiency can be retained by using the oxygen consumption of biosolids. Quantified savings in the form of needed volumes of air while still achieving sufficient COD reduction are presented. The tests have been made in a bubble column with pulp mill process water and sludge from a biological treatment plant. These were supplemented with case studies at three pulp and paper mills.

Carrier effects on oxygen mass transfer behavior in a moving‐bed biofilm reactor

AbstractThis study investigates the carrier effects on the oxygen mass transfer behavior of a gas–liquid biofilm surface, and aims to provide evidence for parameter optimization in the practical operation of a moving‐bed biofilm reactor (MBBR) during the coking‐plant wastewater process. By using the dynamic oxygen dissolution method, the volumetric oxygen mass transfer coefficient KLa was measured by varying the suspended carrier stuffing rate and the intensity of aeration. Within the range of fluidizable flow rate, the efficiency of oxygen mass transfer increased with suspended carrier stuffing rate, and KLa reached its peak value when the stuffing rate was 40%. KLa has an increasing trend with an increase of the aeration intensity, but high aeration intensity was not favorable for reactor operation. Better oxygen mass transfer effect and higher oxygen transfer efficiency could be achieved when the aeration intensity was 0.3 m3 h−1 and the suspended carrier stuffing rate was 30–50%. The possible mechanisms that can account for carrier effects on oxygen mass transfer are the changes in the gas–liquid interfacial area. The ammonia nitrogen removal performance of the coking‐plant wastewater in MBBR was satisfied by using the above‐suggested conditions. Copyright © 2009 Curtin University of Technology and John Wiley &amp; Sons, Ltd.

Gas holdup and oxygen transfer in an aerobic granule-based sequencing batch reactor

Aerobic granules with mean radius of 0.25 mm were cultivated in a sequencing batch reactor (SBR) fed with soybean-processing wastewater, and the gas holdup and oxygen transfer were investigated for this granule-based reactor. At the discontinuous point in the variation of gas holdup with superficial gas velocity at 1.73 cm/s, bubble flow regime was changed into churn-turbulent flow one. Gas holdup of the granule-based SBR was greater than that of a floc-based SBR, indicating that higher oxygen transfer efficiency could be achieved in the former. The volumetric gas–liquid oxygen transfer coefficient increased with increasing superficial gas velocity, but declined with increasing solid fraction. Observed Thiele modulus and effectiveness factor were effectively used to describe the efficiency of oxygen diffusion in the granules and the oxygen utilization rate of the reactor. With the porous pellet model, the oxygen concentration profiles and effectiveness factor for aerobic granules with different sizes were predicted. Effectiveness factor of 0.98 for the granules with radius of 0.25 mm suggests that the oxygen diffusion limitation could be neglected. For the granules with radius of 1.00 mm, a severe oxygen limitation occurred, as indicated by an effectiveness factor of 0.55.

308 超微細気泡散気装置による地球温暖化抑制効果

The total power consumption of sewage treatment plants in Japan consists of 0.7% of that of Japan as a whole. Therefore, from a greenhouse effect prevention point of view, it is important to reduce the energy consumption of blowers in sewage treatment plants to reduce CO_2 emission. The Ultra Fine Bubble Diffuser is made of a special membrane punched with many small holes, which enables the generation of ultra-fine bubbles. This enables the Diffuser to have a very high oxygen transfer efficiency, leading to the reduction of required air for the activated sludge process and the reduction of power consumption of the blower. In this paper, the structure and features of the Ultra Fine Bubble Diffuser, along with a case study of an actual application, are introduced.

Evaluation of oxygen transfer efficiency and alpha‐factor on a variety of diffused aeration systems

ABSTRACT: Because the aeration system in an activated sludge wastewater treatment plant typically represents more than 50% of total plant energy requirements, designers and operators can substantially reduce overall plant energy costs by using accurate oxygen transfer information to make the aeration system as energy efficient as possible. This paper presents data from 65 off‐gas analysis tests performed at 21 wastewater treatment plants. The effect of various factors on oxygen transfer was evaluated by comparing pairs of tests in which all factors but one were held relatively constant. Factors evaluated were diffuser type, diffuser layout, diffuser age, solids retention time, and level of nitrification. Flexible membrane diffusers had more than a 30% higher oxygen transfer efficiency than coarse bubble diffusers. Grid layouts had higher oxygen transfer efficiencies than spiral roll layouts. Flexible membrane diffusers that were 3.5 years old had a 20% lower oxygen transfer efficiency than new flexible membrane diffusers. Systems at low solids retention times that were not nitrifying had lower oxygen transfer efficiencies than nitrifying systems at high solids retention times. For some wastewater systems, use of this information on solids retention time in plant design and operation can decrease overall energy costs despite the additional oxygen required to meet the nitrogenous oxygen demand.

Cobalt interference in the “non-steady state clean water test” for the evaluation of aerator equipment—I. Causes and mechanisms

Water soluble and insoluble oxidizing compounds are formed during the non-steady state clean water test in certain natural water systems causing interference with Winkler dissolved oxygen determination. This results in pseudo high oxygen transfer efficiency of aerator devices. The interfering precipitate consists mainly of hydrated cobaltic oxide. Evaluation of the electrochemical-oxygen equivalent measurements made the presence of hydrated cobalt dioxide in the precipitate plausible. The presence of hydrogen peroxide in the filtrate suggested the formation of peroxodi- and peroxomono-sulfuric acid during the reaeration step. The pH along with the buffering capacity of a test water are the most critical water quality parameters directly affecting the magnitude of the cobalt interference. At pH values of 6.9 and below the cobalt and peroxide interference in the non-steady state clean water tests can be eliminated. Maximum cobalt interference was found to occur at pH values of 10–11.