Standard Oxygen Transfer Efficiency Research Articles

Improving the transfer of dissolved oxygen in a biological Fe2+ oxidation process using a venturi jet as an intensive aeration system

The low bio-production of Fe3+ as a leaching agent is one of the main limitations to implementing industrial bio-processes at feasibility conditions. The main limitation of the bio-oxidation process of Fe2+ is the low oxygen transfer to the aqueous phase because of the low oxygen solubility. This study assesses the effectiveness of the venturi jet as an innovative and intensive aeration device for overcoming the oxygen limitation in a continuous ferrous oxidation process in a fixed-bed reactor with immobilized acidithiobacillus ferrooxidans, in contrast to the conventional diffuser aeration device. Firstly, the influence of the airflow and the influence of the medium concentration were determined for the following parameters for both aeration devices; Volumetric mass transfer coefficient (kLa), Standard Oxygen Transfer Rate (SOTR), Standard Aeration Efficiency (SAE), and Standard Oxygen Transfer Efficiency (SOTE). Then, both aeration devices were compared in a continuous bio-oxidation process in an up-flow packed bio-reactor (UFPB). The system performance was assessed by monitoring temperature, pH, oxidation-reduction potential, and dissolved oxygen concentration for 69 days. Findings displayed that when aerating with the diffuser, the ferrous oxidation rate was restricted by the low dissolved oxygen availability, being about 1 ppm (1 mg L−1). Under these oxygen-limiting conditions, the maximum ferrous (Fe2+) oxidation rate was 9.09 g L−1 h−1. However, when aerating with the venturi jet, the dissolved oxygen concentration increased up to 2.70 mg L−1, achieving a maximum of 29.11 g L−1 h−1. So, this study has demonstrated that the change in the aeration device has resulted in an improvement in the process, achieving a 3.5-fold increase in the oxidation rate. Furthermore, the venturi jet offered additional advantages over the diffuser, such as requiring less power to deliver the same amount of air, being unaffected by jarosite precipitates, and not requiring a supply of compressed air.

Nanobubbles produced by nanopores to probe gas-liquid mass transfer characteristics

HypothesisThis study tested the hypothesis of how the nanopore size of membranes and how the surface charge of nanobubbles responds to its pinch-off from the nanopore. This study also tested the hypothesis that nanobubbles that remain in solution after production may increase the dissolved oxygen content in water. ExperimentsThe effect of membrane pore size, hydrodynamic conditions (gas and liquid flow rates), and physicochemical parameters (pH and temperature) on volumetric mass transfer coefficient (kLa) for oxygen nanobubbles formed by the nanopore diffusion technique was investigated. This study experimentally determined the kLa by carefully removing the dissolved oxygen by nitrogen purging from nanobubble suspension to examine the sole contribution of nanobubble dissolution in water to the reaeration. ResultsScaling estimates indicate that the nanobubble pinch-off radius and nanopore radius have a power-law correlation and that nanobubble size declines with the nanopore size. This is in line with our experimental results. The surface charge of nanobubbles delays its pinch-off at the gas-liquid interface. Nanobubbles offered 3-4 times higher kLa than microbubbles. Standard oxygen transfer efficiency in water was found to be 78%, significantly higher than that in microbubbles. However, dissolving stable nanobubbles in water does not considerably increase dissolved oxygen levels.

NETmix technology as ozone gas injection system: Assessment of the gas-liquid mass transfer

This work proposes an innovative high-intensity mixer based on the NETmix technology to promote the contact between ozone (O3) gas and water. The NETmix consists of a network of unit cells comprising mixing cylindrical chambers interconnected by prismatic channels. Its exclusive geometry generates successive contacting and splitting of flows, boosting mass transfer rates and rapidly developing an O3-enriched liquid stream. The gas-liquid mass transfer performance of NETmix was evaluated by measuring the volumetric mass transfer coefficient (kLa), standard oxygen transfer rate, standard oxygen transfer efficiency, and energy expenditure parameters. A co-current continuous flow condition was used to introduce a gas stream (consisting of either O3/oxygen(O2) or synthetic air) and a liquid stream (water), whereby it was varied the direction of co-current fluid flow (downwards or upwards) and the injected gas (QG) and liquid (QL) flowrates. The downward flow of gas and liquid streams enhanced the mass transfer phenomenon by at least 43 %. Moreover, the kLa values increased with QL and QG, reaching 2.46 s−1 at a low specific power consumption (25 kW m−3). The NETmix showed kLa values at least one order of magnitude larger than conventional contactors, maintaining a high-energy utilization efficiency (up to 19.4 kgO3 kWh−1).

Membrane bubble aeration unit: Experimental study of the performance in lab scale and full-scale systems

The transfer of oxygen is limited at the gas/liquid interface by the low solubility of oxygen in water. Fine bubble diffusers have a standard oxygen transfer efficiency (SOTE) lower than 30% in a full-scale system. Porous membranes can be employed as diffusor to improve the efficiencies taking the advantage of the high exchange interfacial area and the low volume of equipment. The aim of this work is to study the feasibility of membrane systems for aeration in a full-scale system. Several membrane modules were prepared using hollow fibers hydrophobic membranes and compared with the traditional disk diffusor currently employed in wastewater treatment. The mass transfer coefficient (KLa) and SOTE were assessed by carrying out the aeration tests in both lab-scale and full-scale systems exploring different operating parameters such as system flow rate, the geometry of the module and the surface of the device. The advantage and disadvantages of employing membranes in the aeration process were highlighted. The membrane unit showed better overall mass transfer efficiency increasing from 28% at the lab-scale to 34% in the full-scale system. Unlike the conventional disk diffusers, it has been found that the membrane aerator can operate with significantly lower air flow rates, which leads to a doubling of the efficiency. Moreover, the membrane surface generating the bubbles can be easily increased while maintaining the same module footprint, which in turn allowing a further increase of efficiency from 9% to 24%. However, important phenomena of scaling were observed on the membrane surface.

Effects of Environmental Conditions and Bed Configuration on Oxygen Transfer Efficiency in Aerated Constructed Wetlands

This research evaluated the oxygen transfer efficiency in beds to be used as aerated constructed wetlands. The research methods included oxygen transfer efficiency evaluations in several bed configurations using diffused aeration systems. Experiments were conducted at two locations with different environmental conditions: a) Talca (Chile), 120 m above sea level (m.a.s.l.), 0.99 Atm and b) Cajicá (Colombia), 2550 m.a.s.l., 0.76 Atm. A column with only clean water and three bed configurations representing aerated constructed wetlands were evaluated. These configurations included: (a) coarse gravel, (b) coarse gravel with an empty core in the middle (inner container), and (c) fine gravel. Three airflow rates were evaluated: (a) low, 0.7 L/min; (b) medium, 2.5 L/min; and (c) high, 3.6 L/min. The overall oxygen mass transfer coefficient, standard oxygen transfer rate, and standard oxygen transfer efficiency were the variables calculated from the oxygen transfer evaluation tests. The research results indicated that in diffused aeration systems, oxygen transfer efficiency was negatively influenced by environmental conditions, particularly altitude, which limits the driving force for oxygen transfer into water. Furthermore, the results showed that the size of the gravel used in the bed is related to the oxygen transfer efficiency: the larger the gravel size, the higher the oxygen transfer, regardless of the altitude. Finally, research regarding oxygen transfer in aerated constructed wetlands has signaled the need for a standard procedure for aeration testing, and this work suggests a new methodology.

Experimental study on design and operational performance of solar-powered venturi aeration system developed for aquaculture – A semi-floating prototype

Venturi-type injectors have been used usually due to the practical use and modular design that allows for quick configuration, ready-to-use, and low-cost solutions in the last years. This study aims to design a solar-powered venturi aeration system (SVAS) and determine its flow characteristics, standard, and real-time operational performance parameters. SVAS has a design providing the maximum vacuum and air velocity at air inlets and allowing for all technical measurements. Moreover, a solar-powered experimental apparatus having a weather station, a pool station, and a data logger station was constructed. Experiments were carried out using a plastic pool with a capacity of 4000 L, a diameter of 3 m and a depth of 0.75 m, four venturi injectors, and a submersible pump of 0.3 kW operated at the constant water flow regime of 4.1 m3 h-1. According to flow tests, the oxygen mass flow rates ranged from 2.77 to 2.84 kg h−1. The specific energy consumption values are 263.2 kJ per cubic meter for water flow and 380 kJ per kg for oxygen transfer.Standard performance experiments were repeated four times. The maximum overall oxygen mass transfer coefficient obtained during the experiments was 5.58 h−1. The maximum values of standard oxygen transfer rate (SOTR), standard aeration efficiency (SAE), and standard oxygen transfer efficiency (SOTE) were 0.206 kgO2 h−1, 0.681 kgO2 kWh−1, and 7.32%, respectively. The pool conditions were simulated using de-oxygenation and mixing systems to determine the operational performance. For simulation experiments, six procedures and two processes were applied. Throughout operational experiments of four days, real-time dissolved oxygen levels fluctuated between 1.05 mg L−1 and 6.65 mg L−1. The maximum performance values were 0.143 kgO2 h−1 for the actual oxygen-transfer rate, 0.474 kgO2 kWh−1 for aeration efficiency, and 5.1% for oxygen transfer efficiency.

Power Generation and Oxygen Transfer Analyses for Micro Hydro-Turbine Installed in Wastewater Treatment Aeration Tank

Abstract This study targets one of the major energy consumers in the U.S. It suggests a new mechanical system that can recover a portion of the energy in Wastewater Treatment Plants (WWTPs). The proposed system entails a hydro-turbine installed above the air diffuser in the aeration tank to extract the water-bubble current’s kinetic energy and converts it to electricity. Observing the optimum location of the turbine required multiple experiments where turbine height varies between 35% and 95% (water height percentages above the diffuser), while varying the airflow between 1.42 L/s (3 CFM) and 2.12 L/s (4.5 CFM) with a 0.24 L/s (0.5 CFM) increment. Additionally, three water heights of 38.1 cm (15″), 53.4 cm (21″), and 68.6 cm (27″) were considered to study the influence of the water height. It was noticed that the presence of the system has an adverse effect on the standard oxygen transfer efficiency (SOTE). Therefore, a small dual-blade propeller was installed right above the diffuser to directly mix the water in the bottom of the tank with the incoming air to enhance the SOTE. The results showed that the maximum reclaimed power was obtained where the hydro-turbine is installed at 65–80% above the diffuser. A reduction of up to 7.32% in SOTE was observed when the setup was placed inside the tank compared to the tank alone. The addition of the dual-blade propeller showed an increase in SOTE of 7.27% with a power loss of 6.21%, ensuring the aeration process was at its standards.

Optimizing Power Reclamation of Micro Hydro Turbines in WWTPs Aeration Basins

Abstract This study investigates the optimum operating conditions and design configurations that can optimize the power reclaimed by small hydro turbines derived by the rising water-bubble current. The rising current is generated by the compressed air introduced by the diffusers at the bottom of aeration basins of Wastewater Treatment Plants (WWTPs). While optimizing the power production, the standard oxygen transfer efficiency (SOTE) is monitored since it is a significant parameter that cannot be sacrificed in the operation of WWTPs. Using one set of turbine blades, it was found out that the highest velocity is obtained in the upper half of the water column (70% - 80%). In contrast, the lowest velocities were obtained just above the air diffuser and at the water surface. Testing started with using a single turbine (ST) to determine the location of the optimum power reclaimed at each tested airflow (1.18, 1.42, 1.65, and 1.89 L/s). Then using double turbine (DT) and triple turbine (TT) to compare their performance to the ST’s maximum power increased power reclamation. The maximum percentage of increase in power reclamation for DT is 19.59%, while it is 20.24% in the case of TT. At a commonly used airflow in WWTPs (1.42 L/s), the optimum configurations of DTs and TTs were selected to investigate the effect of having the proposed setup on the SOTE. For membrane diffusers, DTs and TTs limited the dispersion of the air bubbles in the tank, therefore, reducing the SOTE (8.3% for DT and 3.7% for the TT). The ceramic and sharp-nub diffusers were also tested versus rubber membrane ones to determine the effect of using the ceramic and sharp-nub diffusers on the power reclamation and SOTE. Ceramic diffusers neither achieve higher power reclamation than the membrane nor increases the SOTE. In contrast, sharp-nub diffusers increase the SOTE for all configurations compared to membranes, but this came into account of power reclamation, where sharp-nub diffusers cause a DT and a TT to produce less power than ST does.

The Changing and Distribution Laws of Oxygen Transfer Efficiency in the Full-Scale IFAS Process

The integrated fixed-film activated sludge (IFAS) process has been widely used in the upgrading of wastewater treatment plants (WWTPs). The oxygen transfer efficiency (αOTE) is of great significance to the design and operation of the IFAS process. The carrier filling ratio (CFR) and aeration type are two critical factors affecting αOTE and standard oxygen transfer efficiency (αSOTE). However, the distribution and changing laws of αOTE and αSOTE in the full-scale IFAS process areunclear. To optimize the operation of a WWTP and to improve the αOTE of the aeration systems, several off-gas tests were conducted under different aeration types and different CFRs. The results show that for the aerobic tank investigated (the ratio of length and width was 8:1), the αOTE and the αSOTE of the middle of the aeration systems were higher than those of the other two sides. However, the reason for the low αOTE at the beginning and the end of the tank may be different. Coarse-bubble aeration systems had a lower αOTE and almost the same oxygenation capacity (αSOTE) as the fine-bubble aeration systems under constant CFR (43%). The average αSOTE (18.7–28.9%) of the hybrid aeration systems increased with increasing CFR (7.7–57.7%), and different locations exhibited different degrees of change. The results reveal the distribution and changing law of the αOTE of aeration systems in the IFAS process, and attention should be paid to the improvement of the OTE of the plug-flow IFAS process.

Evaluating the performance of fine bubble diffused aeration systems in cylindrical aeration tanks by fuzzy c-means algorithm.

In order to make a comprehensive evaluation of the performance of fine bubble diffused aeration systems in cylindrical aeration tanks, the following parameters are considered: distribution of DO concentration in the horizontal direction of the different water depth of an aeration tank, distribution of DO concentration in the vertical direction of the aeration tank, distribution of DO concentration in all the points of the aeration tank and ratio of total mass increment of DO in the aeration tank to total mass of oxygen in aeration air. The aeration characteristic criterion (ACC) is proposed to synthesize these parameters, and weighted sums of the similarity degrees derived from the extensions of fuzzy c-means (FCM) are used to construct ACC. The results show that compared with total volumetric oxygen transfer coefficient (KLα) and specific standard oxygen transfer efficiency (SSOTE), ACC is demonstrated to be more marked and sensitive for the performance evaluation of the fine bubble diffused aeration systems equipped with fine-pore aeration tubes. Moreover the performance of aeration systems equipped with different layouts of fine-pore aeration tubes is comparatively studied, and their performance from best to worst is ring-type diffuser > square-type diffuser > parallel-lines-type diffuser > cross-type diffuser.

Experimental study on aeration efficiency in a pilot‐scale decelerated oxidation ditch equipped with fine bubble diffusers and impellers

AbstractThis work is an experimental study on the oxygen transfer capability and efficiency in a pilot‐scale decelerated oxidation ditch equipped with fine bubble diffusers and impellers (inducing horizontal liquid flows). Aqueous solutions of carboxymethyl cellulose (CMC) exhibiting shear‐thinning rheological properties are selected to simulate activated sludge in the oxidation ditch process. The effect of air flow rate and horizontal liquid velocity along the loop channel on oxygen transfer capability and aeration efficiency is examined in tap water and CMC aqueous solutions. The standard volumetric mass transfer coefficient KLa20, standard oxygen transfer efficiency (SOTE), and standard aeration efficiency (SAE) are introduced to study the oxygen mass transfer performance and energy consumption efficiency in the oxidation ditch. The results show that KLa20, SOTE, and SAE in CMC aqueous solutions follow similar trends observed in tap water. Due to the effect of shear‐thinning rheological properties, KLa20, SOTE, and SAE in CMC aqueous solutions are smaller than those in tap water. Two dimensionless numbers, Froude number and Ohnesorge number are introduced to study the oxygen transfer efficiency through the consideration of the combined influence of aeration, crossflow, gas bubble size, and physical properties of the liquid phase.

Three decades of oxygen transfer tests in clean water in a pilot scale test tank with fine-bubble diffusers and the resulting conclusions for WWTP operation

Abstract We summarized the experience from three decades of oxygen transfer testing and aeration research at the Technical University of Darmstadt to validate the oxygen transfer efficiency of modern fine-bubble diffusers. A total of 306 oxygen transfer tests in clean water of 65 different fine-bubble diffusers, carried out in the same test tank under identical test conditions, were analysed and compared with previous results. As a result, we could show that the performance of fine-bubble aeration systems has increased by 17% over the last three decades. Therefore, modern well-designed and operated aeration systems can achieve specific standard oxygen transfer efficiency (SSOTE) values between 8.5 and 9.8% · m−1. Additionally, a comparison of various diffuser types and diffuser densities was done. Based on the new results, an exemplary cost/benefit analysis for a 100,000 PE WWTP shows the calculation of an optimized diffuser density with respect to investment and operating costs.

Oxygen transfer performance of a supersaturated oxygen aeration system (SDOX) evaluated at high biomass concentrations

Oxygen transfer in wastewater treatment is significantly influenced by the mixed liquor suspended solids (MLSS). The effect is more pronounced at MLSS concentrations higher than 20g L−1 when supplying air by conventional diffused aeration systems. The oxygen transfer performance of a supersaturated oxygenation technology (i.e., the supersaturated dissolved oxygen (SDOX) system) was evaluated in clean water and in activated sludge with MLSS concentrations from 4 to 40gL−1 as a promising technology for uncapping such limitation. The evaluation was carried out at the laboratory facilities of the faculty of food technology and biotechnology at the University of Zagreb. The sludge was collected from a full-scale conventional activated sludge (CAS) wastewater treatment plant (WWTP) operated at a solid retention time (SRT) of approximately 5 days. The evaluation was carried out using a laboratory-scale setup consisting of a bench-scale SDOX system (2.75L) supplying pure oxygen to a 5L biological reactor. The SDOX exhibited oxygen mass transfer rate coefficient (KLa) values (2.6h−1) in clean water lower than for fine bubble diffusers (11h−1). However, higher oxygen transfer rate (OTR) values and alpha factors (mass transfer ratio of process-water to clean-water) as a function of the MLSS concentration were observed. A standard oxygen transfer efficiency (SOTE) of approximately 100 % in clean water was reported. The SDOX technology can be presented as a promising alternative for supplying dissolved oxygen (DO) into mixed liquor solutions; particularly, at the high MLSS concentrations required by high-loaded membrane bioreactor (HL-MBR) systems and aerobic digesters.

Fouling characterization and aeration performance recovery of fine-pore diffusers operated for 10 years in a full-scale wastewater treatment plant

Fouling characterization and aeration performance recovery of fine-pore diffusers operated for 10 years in a full-scale wastewater treatment plant were investigated to elucidate fouling mechanisms and develop cleaning strategy. The performance decline of diffusers was observed with dynamic wet pressure increased by 3.2 times and standard oxygen transfer efficiency dropped to 73%, which contributed to 15.0% increase in total energy consumption. Oxygen-affinity, filamentous and extracellular polymeric substances secreting bacteria tended to accumulate on the diffuser surface. External (mainly biofilm growth), internal (organic and inorganic matters) and irrecoverable (mainly material aging) foulants accounted for 34.1%, 45.4% and 20.1% of total fouling, respectively. HCl cleaning failed to restore aeration efficiency because it eliminated structural support formed by inorganics, leaving organic foulants broken into smaller fragments and distributed more dispersed. NaClO showed better cleaning efficiency by effectively removing organic foulants. Sequential cleaning by NaClO and HCl, which achieved the best recovery, was recommended.

Effects of different layouts of fine-pore aeration tubes on sewage collection and aeration in rectangular water tanks

Sewage collection and dissolved oxygen (DO) management have always been the focus of industrialized recirculating aquaculture. However, there are still no reports on how to achieve efficient sewage collection and increase DO in rectangular water tanks. In the present study, the performance of sewage collection and aeration in rectangular water tanks were compared among three fine-pore aeration tubes (disc-type diffuser, four corner-type diffuser and distribute-type diffuser) layouts at three airflow rates, 6, 18, and 30 m3/h. The results of computational fluid dynamics (CFD) modeling and bait collection tests revealed that sewage collection using the four corner-type diffuser was better than using the disc-type diffuser and the distribute-type diffuser. Using the four corner-type diffuser, a strip-shaped sewage collector could be set up at the bottom of the tank to collect sewage at an airflow rate of 18 m3/h and a disc-shaped sewage collector could be set up at the bottom of the tank to collect sewage at an air flow rate of 30 m3/h. In terms of aeration, volumetric oxygen transfer coefficient (KLa20), standard oxygen transfer rate (SOTR), and standard oxygen transfer efficiency (SOTE) of the distribute-type diffuser were all the highest at the three airflow rates, followed by four corner-type diffuser, and finally the disc-type diffuser, indicating that the aeration performance in the distribute-type diffuser was optimal. However, there were no significant differences in KLa20, SOTR, and SOTE between the four corner-type diffuser and the distribute-type diffuser at both 18 and 30 m3/h airflow rates. Therefore, four corner-type diffusers were the optimal choice for recirculating aquaculture, considering the sewage collection and aeration requirements. The results of the present offer novel insights on the application of CFD in recirculating aquaculture, in addition to basic data and theoretical guidance on how to achieve efficient sewage collection and aeration in recirculating aquaculture.

Study on the Standard Oxygen Transfer Efficiency Monitoring System in the Aeration Tank for Reuse and Discharge of Wastewater

Study on the Standard Oxygen Transfer Efficiency Monitoring System in the Aeration Tank for Reuse and Discharge of Wastewater

Design and automation of a solar-powered floating-type aeration system (SPFTAS) for fish ponds

Aeration helps improve water quality and make it favorable to fish. SPFTAS was designed based on the environment and characteristics of freshwater ponds. SPFTAS consist of floating platform, power source, sensors, signaling and aeration system. It functions to disperse and increase dissolved oxygen (DO). It possessed capability to monitor real time condition of water, store data and send it through short messaging service (SMS). The SPFTAS oxygen transfer rate was 0.07 kg of oxygen using four diffusers and the standard aeration efficiency of 0.2710 kg oxygen/hp-hr. The standard oxygen transfer efficiency was 0.36 at 18 kg of oxygen transferred. SPFTAS increased DO level to 1.32 ppm and rated highly effective during the test. DO levels using 1, 2, 3 and 4 diffusers at different depths, distances and time results were significantly different. The DO level using different diffusers at time intervals was highest at 7.16 ppm in three hours operation. The total cost of automated SPFTAS was ₱ 170, 385.75. The cost analysis of using the machine indicated a breakeven point of 1, 440 hours per year. The annual operation expected save ₱ 32, 737.39 and expected recover the investment in 5.33 years. Finally, machine was found beneficial to increase DO concentration in fishponds.

Modeling re-oxygenation performance of fine-bubble–diffusing aeration system in aquaculture ponds

Fine-bubble-diffusing (FBD) aeration system is widely used in aquaculture ponds. To maximize its re-oxygenation capability, it is needed to have a quantitative understanding of the reoxygenation performance. In practice, two indexes, namely oxygen volume mass transfer coefficient (KLa) and standard oxygen transfer efficiency (E), are commonly used to measure the re-oxygenation performance. However, few mathematical models are available to accurately predict these two indexes. The objective of this regard was to develop such a model driven by commonly available data. In this regard, the results from 54 group laboratory tests were regressed on four independent variables, including air flow rate (Qg), aeration tube length (L), submerged water depth of the diffuser (hd), and plane-view tank area (Acs). The regression revealed that both KLa and E are negatively related to hd and Acs, but they are positively related to L. In addition, KLa was found to be positively related to Qg, whereas E was found to be negatively related to Qg. Two regression models, one for KLa while another for E, are expected to be effective tools for operating FBD aeration system in practice to maximize its re-oxygenation capability though they may need to be further verified using field data.

Development of a Model to Determine the Baseline Mass Transfer Coefficient in Bioreactors (Aeration Tanks).

Over 60 percent of all wastewater treatment plants in the developed countries use the activated sludge process as their secondary treatment system. About 50 to 85 percent of the total energy consumed in a biological wastewater treatment plant is in aeration. The activated sludge process, the most common such process, is performed in large aeration basins to provide air for microorganisms to remove nutrients and pollutants through biodegradation. Designs for the air supply often fail to meet sustained peak organic loading, which may lead to unsatisfactory treatment performance and even plant failure. On the other hand, excessive safety factors used in design for the air supply may similarly lead to unsatisfactory treatment performance and energy wastage. Improper sizing of the aeration system is primarily due to inability to estimate the mass transfer coefficient (KL a) correctly for different tank depths, leading to improper blower design and to inappropriate operation, among other things. In general, the efficiency of porous fine-bubble diffuser systems varies from 10 to 30 percent or more, depending on tank depth. The term KL a is used in ASCE/EWRI Standard 2-06 to define the apparent volumetric mass transfer coefficient in a non-steady-state clean water test in an aeration tank. This parameter is a function of tank depth, due to the variation of oxygen gas depletion with depth. The objective of this paper is to introduce a baseline oxygen mass transfer coefficient (KL a0 ), a hypothetical parameter defined as the oxygen transfer rate coefficient at zero depth, and to develop new models relating KL a to the baseline KL a0 as a function of temperature, system characteristics (e.g., the gas flow rate, the diffuser depth Zd ), and oxygen solubility (Cs ). Results of this study on data extracted from the literature indicate that a uniform value of KL a0 that is independent of tank depth can be obtained experimentally. Using the baseline, a family of rating curves for KL a20 (the standardized KL a at 20 °C) can thus be constructed for various gas flow rates applied to various tank depths. The new model relating KL a to the baseline KL a0 is an exponential function, and (KL a0 )T is found to be inversely proportional to oxygen solubility in water (Cs )T to a high degree of correlation. Using a predetermined baseline KL a0 , the new model predicts oxygen transfer coefficients KL a20 for any tank depth to within 1-3% error compared to observed measurements, and similarly for the standard oxygen transfer efficiency. The discovery of a standard baseline (KL a0 )20 determined from shop tests is important for predicting the KL a20 value for any other aeration tank depth and gas flowrate, and this finding can be used in the development of energy optimization strategies for wastewater treatment plants. This work may also improve the accuracy of aeration models used for aeration system evaluations.

Oxygen transfer study and modeling of plunging hollow jets

Experiments were performed to study the effect of jet angle of hollow jet aerators on oxygen transfer of water. Three jet angles of 30°, 45° and 60° were used to produce hollow jets impingement on the water in the pool. Results from experimental study showed a significant effect of hollow jet angle on volumetric oxygen transfer coefficient (KLa) at higher jet velocities. Standard oxygen transfer efficiency (SOTE) of 60° hollow jet aerators is observed higher with low-kinetic-power jets, but at higher kinetic power, 30° hollow jet aerator is observed to be the best in terms of SOTE. Moreover, modeling performance of the experimental data is evaluated by training and testing of the models, and empirical equations derived from multiple linear and multiple nonlinear regression techniques are compared with artificial neural network approach. The results of the present study are also compared with the previous studies of plunging jet aerators.