Standard Aeration Efficiency Research Articles

Selection of aerators for intensive aquacultural pond

Aeration cost is the third largest cost in intensive aquaculture system after post larvae and feed cost representing about 15% of total production cost. Therefore, selection of aerators plays a major role in maximizing the profit in such system. Over the years, various types of aerators have been developed specifically to enhance the production of aquatic species. The performances of these aerators are generally compared in terms of standard aeration efficiency. However, suitability of a particular aerator at different pond sizes and water quality conditions can best be determined in terms of aeration cost per unit time of operation. In the present study, economic performance of five different aeration systems – circular stepped cascade (CSC), pooled circular stepped cascade (PCSC), 1-hp paddle wheel, 2-hp paddle wheel and propeller aspirator pump were evaluated and compared at different pond sizes, initial DO concentrations of pond and operating hours of aerators; assuming a typical Indian major carp (IMC) culture with commonly practiced stocking density and feeding. Both CSC and PCSC aerators were found to be suitable for pond size less than 1000m3. However, for pond sizes more than 5000m3, 1-hp paddle wheel and 2-hp paddle wheel aerators were found to be efficient.

Measurement of air–water oxygen transfer of cylindrical vortex flow regulators at a recirculating system

Hydrodynamic flow regulators are used for diversion and/or limiting excess discharge in wastewater and stormwater systems as a replacement for traditional flow throttling devices. They are highly efficient, reliable and free from common disadvantages of traditional devices. Recent research of the authors indicated that atomization of a liquid by vortex flow regulators accelerates oxygenation and prevents the putrefaction process in wastewater and storm water collection systems. The study presents experimental results of the oxygen transfer measurements for basic designs of cylindrical hydrodynamic flow regulators in a closed-circuit experimental setup at the semi-commercial scale. The oxygen mass transfer coefficient, standard oxygen transfer rate and standard aeration efficiency were determined for the range of tested configurations by the clean water test.

Mass transfer efficiency of a vacuum airlift—Application to water recycling in aquaculture systems

In aquaculture, oxygen transfer and carbon dioxide stripping are the first limiting factors to fish rearing intensification. In this study we measured the O 2 and CO 2 mass transfer coefficient ( K L a) for a vacuum airlift in fresh (<1‰ salinity) and sea water (35‰ salinity) recirculating aquaculture systems (RAS). The airlift was composed of two concentric tubes: an inner riser tube and an external downcomer tube and can be adjusted at three different heights: 2, 4 or 6 m. Several types of air injectors were tested, delivering different sizes of bubble swarms depending on their porosity and functioning conditions (low or high injection pressure), with air flows varying from 0 to 80 L min −1. Experiments were also carried out at different water circulation velocities and with cold (7 °C) and warm water (22 °C). The best transfer coefficient ( K L a) value was obtained at a high air flow rate, a high temperature and with reduced bubble size. Results showed that K L a was not affected by water salinity, but it was slightly affected by water flow ( Q w), airlift inner pipe length and vacuum. The presence of vacuum reduces gas solubility in water and facilitates CO 2 stripping. The comparison between O 2 and CO 2 transfers showed that higher K L a values were obtained for O 2 than for CO 2 in fresh and sea water, probably due to chemical reactions between the CO 2 and water. For RAS, the vacuum airlift provides a Standard Aeration Efficiency (SAE) of 1.13 kgO 2 kW h −1 and a Standard Stripping Efficiency (SSE) of 1.8 kgO 2 kW h −1 or 0.023 kgCO 2 kW h −1. In rearing water, CO 2 and O 2 transfers were negatively affected when feed was added. An empirical model for CO 2 mass transfer coefficient prediction was developed and calibrated. Simulation shows a good correlation between predicted and measured values ( R 2 = 0.87).

Design characteristics of Curved Blade Aerator w.r.t. aeration efficiency and overall oxygen transfer coefficient and comparison with CFD modeling

The main objective of this work is to design a high efficiency curved-blade-surface mechanical aerator for oxidation ditch, which is used to treat municipal and domestic sewage. Aeration experiments were conducted in oxidation ditch made up of mild steel sheets to study the design characteristics of curved blade surface mechanical aerator. The paper critically examines six different configurations of aerators, which were developed, fabricated and tested in the laboratory for its various dynamic parameters, such as diameter of aerators (D), speed (N) and immersion depth (h). Out of the different configurations tested, the curved blade rotor (CBR) emerged as a potential aerator with blade tip angle of 47°. The overall oxygen transfer co-efficient (KLa) was observed to be as high as 10.33 h-1 and the optimum aerator efficiency (AE) was found to be 2.269 kgO2/kWh. The standard aeration efficiency (SAE) of CBR was observed to be higher as compared to other aerators used for oxidation ditch process. Dimensional analysis was used to develop equations that describe the aerator’s behavior. Further, a CFD model is also developed for better understanding of the process that takes place inside the ditch. To prepare it 3D and steady flow, k-e turbulence model of flow was used and the simulation runs were carried out for one phase model to generate the data so as to compare it with experimentally observed values. Keywords: Oxidation ditch, dissolved oxygen, aerator, overall oxygen transfer coefficient, aeration efficiency, CFD

ASPECTS OF HYDRODYNAMICS AND MASS TRANSFER IN DIFFUSED AERATION SYSTEMS

The aim of this paper is the theoretic and experimental study of the hydrodynamics and mass transfer of the swarm of bubbles generated by fine bubble diffusers (FBD) made from ceramics. Thus the hydrodynamic parameters of the dispersed air-water system (pressure loss on FBD, bubble size distribution, global gas hold up) were experimentally determined. Regarding mass transfer, the volumetric mass transfer coefficient Kla, standard oxygen transfer rate (SOTR) and standard aeration efficiency (SAE) were also determined. The results showed that Kla increases with air flow rate and there is an optimum air flow rate (120 180 l/h) that gives maximum aeration efficiency. The maximum value of SAE (4.25 kgO2/kWh) was obtained for Q = 150 l/h, in homogeneous regime. Comparing the ceramic FBD with a sintered glass FBD issued that an increase of 35% of the mass transfer can be obtained by using glass FBD. Still, SAE accomplished by the two types of FBD tested is in the range given by literature or even greater.

Performance evaluation of propeller-aspirator-pump aerator

Aeration experiments were conducted in a brick masonry tank of dimension 5.25m×2.87m×1.57m to study the effects of positional angle of propeller shaft (α), submergence depth and rotational speed of shaft on standard aeration efficiency (SAE) of a propeller-aspirator-pump aerator. Non-dimensional numbers related to standard aeration efficiency (SAE) and wire power (P) termed as E and Ne, respectively, were proposed. To evaluate the optimum geometric condition, i.e., positional angle of propeller shaft (α), aeration experiments were conducted at different values of α: 30°, 45°, 60°, 75° and 90°, keeping the rotational speed N (2130rpm) and submergence depth of propeller shaft d (300mm) as constants. The results showed that SAE becomes maximum at α=75°. In a similar way, keeping the geometric condition constant (α=75°), aeration experiments were further conducted at different rotational speeds of propeller shaft, N (1420, 1775, 2130, 2485 and 2840rpm) and different values of submergence depth, d (140, 220, 300, 380 and 460mm) to evaluate the effect of dynamic conditions on aeration characteristics. It was found that E as well as Ne could be well correlated with Froude number (Fr) and Reynolds number (Re), respectively. Finally, maximum SAE of 0.42kgO2/kWh was obtained at positional angle of 75°, rotational speed of 2840rpm and submergence depth of 0.14m of propeller shaft.

An experimental study of nozzle diameters, aeration depths and angles on standard aeration efficiency (SAE) in a venturi aerator

A 1.1 kW submersible water pump with a venturi air injector was used in this study to examine the effects of three factors including nozzle diameter (14, 17 and 20 mm), aeration depth (20, 40 and 60 cm), and aeration angle (0, 22.5 and 45°) on standard aeration efficiency (SAE). For maximum air entrainment, HT / DT ratio and DT / DN ratio were equal to zero and 2, respectively. Among the first factor experiments, 14 mm nozzle diameter showed the highest SAE value (P&amp;lt;0.05). Although, 60 cm aeration depth and 45° aeration angle had more SAE value among other depths and angles, but there was not a significant difference (P&amp;gt;0.05) in each aeration depths and angles. Only, the determination coefficient for effect of nozzle diameters on SAE value showed a good result (R2= 0.958). The greatest oxygen transfer efficiency in this study has been achieved with using 14 mm nozzle size, 60 cm depth of aeration, and 45° angle of venturi tube in water aeration that was 1.166 kgO2/ kWh. Venturi aerators are very inexpensive in comparison with other aerators and have some other advantages that can be considered as one of the best devices for water aeration.

Performance Evaluation of Double-Hub Paddle Wheel Aerator

Aeration performance of a 500 mm diameter double-hub paddle wheel aerator was tested in a brick masonry tank of 5.9 2.91.6 m 3 . The prime objective of the present study is to evaluate the aeration performance of double-hub paddle wheels at different dynamic conditions. The results showed that the oxygen transfer simulation curve for double hub almost matched that of a single-hub paddle wheel. However, the power consumption simulation curve for double hub distinctly deviated from that of a single-hub paddle wheel due to the fact that if geometric and process conditions remain the same, a double-hub paddle wheel consumes more power than a single hub. Hence, a separate power consumption simulation equation was developed for a double-hub paddle wheel. The optimum dynamic condition producing maximum standard aeration efficiency is also presented along with a sample calculation.

Effect of orifice diameter, depth of air injection, and air flow rate on oxygen transfer in a pilot-scale, full lift, hypolimnetic aeratorA paper submitted to the Journal of Environmental Engineering and Science.

A pilot-scale, full lift, hypolimnetic aerator was used to examine the effect of diffuser pore diameter, depth of diffuser submergence, and gas flow rate on oxygen transfer, using four standard units of measure for quantifying oxygen transfer: (a) KLa20(h–1), the oxygen transfer coefficient at 20 °C; (b) SOTR (g O2·h–1), the standard oxygen transfer rate; (c) SAE (g O2·kWh–1), the standard aeration efficiency and (d) SOTE (%), the standard oxygen transfer efficiency. Diffuser depth (1.5 and 2.9 m) exerted a significant effect on KLa20, SOTR, SAE, and SOTE, with all units of measure increasing in response to increased diffuser depth. Both KLa20and SOTR responded positively to increased gas flow rates (10, 20, 30, and 40 L·min–1), whereas both SAE and SOTE responded negatively. Orifice diameter (140, 400, and 800 µm) exerted a significant effect on KLa20, SOTR, SAE, and SOTE, with all units of measure increasing with decreasing orifice size. These experiments demonstrate how competing design factors interact to determine overall oxygen transfer rates in full lift hypolimnetic aeration systems. The practical application for full lift hypolimnetic aerator design is to maximize the surface area of the bubbles, use fine (i.e., ~140 μm) pore diameter diffusers, and locate the diffusers at the maximum practical depth.

Oxygenation performance of a laboratory-scale Speece Cone hypolimnetic aerator: preliminary assessment

A prototype laboratory-scale Speece Cone hypolimnetic aerator was used to examine the effect of oxygen input rate and outlet port water velocity on oxygen transfer, using four standard units of measure for quantifying oxygen transfer: (i) the oxygen transfer coefficient at 20 °C, KLa20 (h–1); (ii) the standard oxygen transfer rate (SOTR) (g O2·h–1); (iii) the standard aeration efficiency (SAE) (g O2 kW·h–1); and (iv) the standard oxygen transfer efficiency (SOTE) (%). The maximum inlet velocity (i.e., 70 cm·s–1) was only 23% of the recommended design velocity (i.e., 305 cm·s–1), and the two-phase bubble swarm did not properly develop inside the cone, but remained as a gas pocket at the top of the cone, resulting in a drastically reduced bubble surface area to water ratio. Therefore, all of the performance measures from this prototype Speece Cone were much lower than would be expected with the recommended design inlet velocity of 305 cm·s−1. Despite this difference, the system was still capable of oxygen transfer efficiencies of about 61%, under low gas flow rates, which is still higher than any full-lift design hypolimnetic aerator operating on air. Future research efforts are focused on building a pilot-scale Speece Cone, with as close to the correct inlet and outlet velocities, hydraulic residence time, and physical dimensions as possible, such that a two-phase bubble swarm could be generated. Once this experimental data is collected and analyzed, it can be properly compared with predictive models.

Evaluation of Ceramic and Membrane Diffusers under Operating Conditions with the Dynamic Offgas Method

The aeration systems of two full-scale, activated-sludge basins were compared during a period of three years, under the same operating conditions, using dynamic offgas testing. Only the material of the diffuser was different (membrane versus ceramic-tube diffusers). The investigation has shown that, although the membrane diffusers have higher initial standard-oxygen-transfer efficiency (alphaSOTE) and standard-aeration efficiency (alphaSAE), these decreased over time, while the alphaSAE of the ceramic diffusers started lower, but increased slightly over the whole period. A cost comparison makes clear how important it is to evaluate the aeration system under process conditions. The operating costs were the dominant factor (approximately 10x higher than capital costs), and operating costs were approximately 20% higher for membrane versus ceramic diffusers. The poor performance of the membrane-tube diffusers under process conditions could be explained on the basis of the actual alphaAE values in the basin, not the standardized values.

Design Characteristics of Single Hub Paddle Wheel Aerator

Aeration experiments, based on dimensional analysis, were conducted in brick masonry tanks of dimensions 2.9×2.9×1.6 m3 and 5.9×2.9×1.6 m3 to study the design characteristics of a single hub paddle wheel aerator. A generalized equation has been developed to estimate the optimum volume of water to be aerated at a given paddle wheel diameter (d) and speed (N) . A simulation equation with E [nondimensional number characterizing standard aeration efficiency (SAE)] and P* (nondimensional number characterizing power input per unit volume) provided a correlation better than that with E and F (Froude number) and E and R (Reynolds number) only. A simplified generalized equation correlating E and P* was developed which can be used to predict aeration efficiency for P* ⩽6.56 . Similarly, a simulation equation with Ne [(power number)(nondimensional number charcterizing power consumption)] and P* provided a correlation better than that with Ne and F and Ne and R only. The optimum dynamic condition at which maximum SAE...

Comparison of the efficiency of large-scale ceramic and membrane aeration systems with the dynamic off-gas method

The aeration systems of two full-scale activated sludge basins were compared over 2.5 years under the same operating conditions using dynamic off-gas testing. Only the material of the diffuser was different, membrane vs. ceramic tube diffusers. The experimental design took the complexity and dynamics of the system into consideration. The investigation has shown that, although the membrane diffusers have higher initial standard oxygen transfer efficiency (SOTE) and standard aeration efficiency (SAE), these decreased over time, while the SAE of the ceramic diffusers started lower, but increased slightly over the whole period. Measurement of air distribution in the basins along with dissolved oxygen concentration profiles have provided important information on improving process control and reducing energy costs. The results show that dynamic off-gas testing can effectively be used for monitoring the aeration system and to check design assumptions under operating conditions. The information can be used to improve the design of new aeration systems or in retro-fitting existing basins.

Prediction of aeration performance of paddle wheel aerators

Aeration experiments were conducted in brick masonry rectangular tanks of dimensions 2.9×2.9×1.6 and 5.9×2.9×1.6 m to study the effect of geometric and dynamic variables on aeration process based on dimension analysis. Non-dimensional numbers relating to standard aeration efficiency (SAE), effective power ( P) and theoretical power per unit volume ( P/ V) termed as SAE′, Ne and X, respectively, are proposed. An optimal geometric similarity of various linear dimensions was established. It has been established that neither the Reynolds criterion nor the Froude criterion is singularly valid to simulate either SAE′ or Ne, simultaneously for different sizes of aerators, even though they are geometrically similar. Occurrence of scale effects due to the Reynolds and the Froude laws of similitude on both SAE′ and Ne are also evaluated. Simulation equations uniquely correlating SAE′, Ne and X were developed which can predict the aeration performance of paddle wheel aerators having the optimised geometric dimensions as established.

Comparison of Submerged Aerator Effectiveness

ABSTRACT The effectiveness of three different submerged aerators (diffusers), a membrane diffuser, a coarse bubble diffuser, and a soaker hose, is compared by analyzing their performance parameters at varying gas flow rates and water depths of up to 10 meters. The type of diffuser chosen for a specific application depends on aeration characteristics such as the total mass transfer coefficient, standard oxygen transfer rate (SOTR), standard oxygen transfer efficiency (SOTE), and standard aeration efficiency (SAE). From experimental data, the effects of the gas flow rate and submerged depth on the total mass transfer coefficient and SOTR are determined up to a 10 meter depth. As the flow rate and diffuser depth are increased, the mass transfer coefficient and the SOTR increase for all three diffusers. At low gas flow rates, the segment of soaker hose transfers oxygen at the highest rate. The coarse bubble diffuser transfers oxygen at a lesser rate than the soaker hose but is capable of performing at higher gas flow rates. The membrane diffuser performance resembles the high transfer of a soaker hose before approaching the transfer behavior of the coarse bubble diffuser at higher flow rates. Relationships are also defined for the SOTE and SAE. Non-dimensional parameter correlations quantify the dependence of the performance parameters on the gas flow rate and depth and provide a means of predicting diffuser behavior for specified system applications.

Deep Water Installation of a Diffused-Air Aeration System in a Shallow Pond

A 33-cm diameter air diffuser was installed in the bottom of a 3.28-m deep bore hole in the bottom of a 350-m2 pond. Water depth above the diffuser surface was 4.34 m. Air was supplied to the diffuser at 158 L/minute and a pressure of 4.86 m H20. The apparatus was highly effective in transferring oxygen to the water. Standard oxygen transfer efficiency was 28.7%, and the standard aeration efficiency, based on air delivered power, was 6.37 kg 02/kWµhr.

Factors Affecting the Performance of Diffused-Air Aeration Systems for Aquaculture

A diffused-air aeration system which operated at low gauge pressure (1.1 to 4.3 m H20) was operated with six or twelve diffusers at five depths between 0.91 and 3.35 m and six air flow rates between 28 and 255 l/minute. The standard oxygen transfer rate (SOTR) or amount of oxygen transferred per hour increased markedly with increasing diffuser depth and air flow rate, but the power required to deliver air to the diffusers also increased as air flow rates and diffuser depths increased. Best standard aeration efficiency (SAE) values (2.0 to 2.5 kg O2/kW·hr) were obtained at air flow rates of 28 and 57 1/minute. With six diffusers and 28 and 57 air/minute, peak SAE occurred at 2.13 and 2.74 m, respectively. With twelve diffusers, peak SAE was achieved at 0.91 m for 28 1 air/minute and at 2.74 m for 57 1 air/minute. In order to efficiently deliver air into water at a high rate, a large number of diffusers is needed.

Optimization of Nitrogen Removal in Small Activated Sludge Plants

In France, many small sewage treatment plants are of the activated sludge/extended aeration type and, generally, only receive a part of their nominal organic load. They are mostly equipped with surface aerators whose Standard wire Aeration Efficiency is in the range 1.3-1.9 kg 02 kwh−1. Consequently the sludge age and the supply of oxygen are sufficient to obtain a large elimination of nitrogen from domestic waste water even when the sludge temperature is low. However the concern is with the optimizing of the nitrogen treatment by the nitrification of ammonia in the aeration basin whilst avoiding any parasitic denitrification in the clarifier. Amongst other investigations, 4 small domestic wastewater treatment plants were studied over several months. A number of adjustment modifications were imposed on them in order to optimize the elimination of nitrogen : The final concentrations to be reached were : N-NH4− lower than 5 mg.l−1 and N-NO3− lower than 3 mg.l−1. The daily operating time of the aerators depends on the received load, the sludge concentration and the oxygenation capacity. When the works are under-loaded, the non-operational periods of the aerators should be as long as 1 1/2 and 2 hours in order to achieve the nitrates reduction. When the load is higher, the time required to attain anoxia after the shutdown of the aerators is shorter and the length of the non-operational periods can be reduced. These field experiments have allowed an evaluation of the tolerances around an optimum adjustment of the aeration operation : a reduction of 5 to 10 % of the daily aeration time may bring about an increase in the residual concentration of Kjeldahl nitrogen which reaches 10 rag.1−1. Conversely, sludge losses may take place in the clarifiers if the daily aeration time is increased by 5 to 10 % in the case of completely mixed basins equipped with slow vertical shaft aerators, and more than 15 % in the case of oxidation ditches (better denitrification probably due to the continuation of the current after stopping the rotors). The sludge concentration should be kept within fairly tight limits (± 0.5 g.1−1). It is however essential to rectify the aeration adjustments during the seasons : a rise in the sludge temperature of 10°C generates an increase in the total oxygen demand by 3 to 5 %. The particular case of plants with anoxic zone ahead is finally discussed.

Modeling the effects of serial off-gas reuse on the performance of a hooded surface oxygen absorption system

A numerical model was used to evaluate the performance of a surface agitation system designed to contact commercial oxygen with water. The modeled system was unique in that oxygen-rich off-gas, normally discharged to the atmosphere, was directed in serial reuse through additional contact stages receiving untreated water. A correlation between the agitator mass-transfer coefficient and power demand, needed to calibrate the model, was established using a single-stage (37 W) contactor of 1·18 m 3 capacity. Additional tests, conducted with both single and three-stage equipment, verified model assumptions and performance predictions. Simulation runs indicated oxygen flow or power input required to meet a given effluent dissolved gas criterion can be substantially reduced by the off-gas reuse step; for example, to achieve an effluent dissolved oxygen of 24·1 mg/litre with a single stage agitator the oxygen feed rate needed was 61·5% greater than that required by a six-stage system receiving the same total power input (standard aeration efficiency, 0·5 kg/kW h; water flow rate, 100 litre/min; influent dissolved oxygen, 9·08 mg/litre at 15°C). The savings achieved increased with (1) greater target effluent dissolved oxygen concentrations, (2) lower oxygen feed rates, (3) higher input power levels, and (4) number of contact stages.

Nightly Aeration to Increase the Efficiency of Channel Catfish Production

Abstract Channel catfish (Ictalurus punctatus) ponds were stocked at 10,000 fish/hectare and fed to a maximum daily rate of 53.2 kg/hectare. Three ponds were aerated 6 h/night from 30 May to 12 October at a rate equivalent to 6.25 kW/hectare with aerators that had a standard aeration efficiency of 1.34 kg O2/kWh. Three ponds served as unaerated controls, but emergency aeration was occasionally applied in these ponds. Dissolved oxygen concentrations were always above 4 mg/L in aerated ponds, but concentrations below 2 mg/L often were recorded in unaerated ponds. Harvest weight of fish averaged 4,813 kg/hectare in aerated ponds and 3,659 kg/ hectare in unaerated ponds. The same quantity of feed was applied to all ponds. Feed conversions (weight of feed applied/net weight gained by fish) were 1.32 and 1.75 in aerated and unaerated ponds, respectively. Production data were expanded to larger ponds for a budget analysis. Net returns to land, management, and equity capital were US$1,719/hectare for aerated pond...