Aeration Power Research Articles

Quantitatively Understanding the Performance of Membrane Bioreactors

The membrane bioreactor (MBR) is a special form of activated sludge in which a membrane separator allows perfect solids retention. This offers obvious benefits for effluent COD and attaining a large ratio of solids retention time to hydraulic retention time (SRT/HRT). However, these benefits come with trade-offs. This work explores the trade-offs with a mechanistic model based on the unified theory for the biomass and soluble components in microbiological processes and adapted for the special features of MBRs. In particular, only large biomass-associated products (BAPL) are retained by the membrane, while a high concentration of mixed liquor suspended solids (MLSS) lowers the oxygen-transfer rate and the critical trans-membrane flux. According to the model results, effluent COD is sensitive to the influent COD and to the ability of the membrane to retain BAPL. While the ability of an MBR to achieve high MLSS and volumetric loading has cost benefits, high MLSS increases the required aeration power and decreases the trans-membrane flux. These strong trends point out the areas in which MBR research ought to yield a large benefit.

English

Thorough oxidation and mixing is required for treatment of acid mine drainage on many sites. This is accomplished in the rotating cylinder treatment system (RCTS™) by passing acid mine drainage and a neutralizing agent through a containment cell in which a perforated cylinder rotates. As the cylinder rotates, a thin film of water adheres to the inner and outer surfaces and water bridges across the perforations for additional gas exchange. The agitation is provided primarily by the impact of the perforations with the water flowing through the containment cell. The turbulence that is produced provides efficient mixing, which reduces chemical consumption due to more efficient use of the available alkalinity, and less sludge produced. Metals removal effectiveness, energy requirements, and chemical consumption were characterized in four field tests. In all of these, the RCTS™ effectively precipitated metals and increased pH and did so at a lower cost than conventional systems. At sites that compared the RCTS™ with conventional treatment, the RCTS™ system required substantially less energy, chemical, labor and residence time. A direct comparison with a conventional system at the Leviathan Mine demonstrated that the RCTS system used 69% less power for aeration and mixing and was more effective at oxidizing metals. The system used 41% less lime to achieve a similar discharge pH. In addition, the RCTS™ systems can be mobilized quickly to remote locations where conventional systems cannot easily be installed. System installation time was typically less than one day.

Effects of Sparger Geometry on Power Consumption and Mass Transfer in a Gas-Liquid Agitated Vessel with Disk Turbine

The power consumption, gas dispersion, and mass transfer volumetric coefficients are investigated in a gas–liquid agitated vessel with a disk turbine for various geometries, positions of the sparger, and directions of gas discharge. A sparger larger than the impeller is effective for obtaining high aeration power consumption and a high mass transfer volumetric coefficient. When the sparger is used, the flooding phenomenon does not occur easily. The gas is sufficiently dispersed when the large sparger is used, even if the gas is not blown directly into the impeller.

Solids Separation Coupled with Batch‐Aeration Treatment for Odor Control from Liquid Swine Manure

Previous studies on solids/liquid (S‐L) separation for odor control from swine manure indicated that the practice might not technically be feasible because of the complexity of removing the fine particles, which are usually the major source of the odor problems. This study coupled S‐L separation by sedimentation with an aeration treatment to quickly break down the fine as well as dissolved solids. Results showed that S‐L separation of manure prior to aeration, at the same level of aeration, took only 1.5 days compared to 3 days needed for the control, to bring down volatile fatty acids (VFAs) to the “threshold of unacceptable level”. In addition, it took 2.3 and 5 aeration‐days for VFAs to reach the “acceptable level” for the separated liquid manure and the control, respectively. Results also showed that within the three weeks of post‐aeration storage, the VFAs in the separated liquid manure consistently stayed 13.5 folds below the acceptable level. In the unseparated manure, the VFAs gradually increased upwards from 2.2 folds below acceptable level achieved at the end of aeration treatment, to 1.38 folds below the acceptable level at the end of the third week of storage and looked poised to definitely rise above the acceptable level in a matter of days. A strong relationship (R = 0.99) between pH and the VFAs in the manure suggested that; degradation of VFAs rendered manure more basic as shown by the increase in pH. After only three days of aeration, the oxidation reduction potential (ORP) in the separated liquid manure stabilized at a much higher level of − 15 mV, while the ORP in unseparated manure stabilized at a much lower level of − 200 mV. The S‐L separation treatment thus significantly improves the oxygen transfer efficiency, which in turn significantly reduces the aeration power needed to maintain adequate ORP if prolonged aeration is desired.

Removal of Organic Pollutants and Metabolic Adaptation of Microorganisms by Micro-aeration

Aerobic systems are high rate processes for wastewater treatment, but they consume much power for aeration. Several low-cost anaerobic systems have been developed for mid- and low-income countries. The removal efficiencies of nutrients, particulate matter, and pathogens are not satisfactory for the anaerobic systems. Micro-aeration is a type of low-cost aerobic application which enhances oxygen transfer through water surface. Trickling was found the most efficient method of micro-aeration and batch experiments were performed to evaluate the effectiveness. When soluble organic carbon was removed, ORP7 (redox potential rectified to pH at 7) were around −120 or +260 mV. When soluble organic carbon increased due to organic overload, ORP7 was around +30 mV. At +30 mV, carbon fraction of biomass increased to 0.54 (higher than average value of 0.46). It was presumable that the ORP7 of −120, +260, and +30 mV were anaerobic, aerobic, and internal storage mode, respectively. Maximum reaction rate of aerobic condition was four times as high as that of anaerobic condition. At internal storage mode, the rate was halved from aerobic mode. It is concluded that microorganisms adapt the metabolic systems to conform to the redox environment, which can be evaluated by ORP. Organic overload results metabolic shift to internal storage mode which retards mineralization of organic matter.

Design and operation of unbaffled aerated agitated vessels with unsteadily forward–reverse rotating impellers handling viscous Newtonian liquids

AbstractDesign and operation of unbaffled aerated agitated vessels with multiple unsteadily forward–reverse rotating impellers (AJITERs) for viscous Newtonian liquids were studied. The effects of operating conditions such as gas sparging rate, agitation rate and the number of impeller stages, geometrical conditions such as the diameters of vessel and impeller, and the physical properties of liquids on the drag and added moment of inertia coefficients, necessary to predict the average and maximum power consumptions of the impellers in AJITERs, were evaluated and the empirical relationships which estimate values of each of these coefficients are presented. The effects of operating conditions, geometrical conditions and liquid physical properties on the gas hold‐up, ϕgD, and volumetric oxygen transfer coefficient, kLaD, were evaluated in relation to the total power input which is the sum of the average power consumption of impellers, ie average agitation power input, and aeration power input. Empirical relationships, useful for design and operation of AJITERs, were obtained for each viscosity range, where the dependences of ϕgD and kLaD on the specific total power input and superficial gas velocity differed, to predict ϕgD and kLaD respectively as a function of the specific total power input, superficial gas velocity and liquid physical properties.© 2003 Society of Chemical Industry

Solid-Liquid Mass Transfer in Gas-Liquid-Solid Agitated Vessel.

The solid-liquid mass transfer coefficients kL in gas-liquid-solid agitated vessels with new type impellers were measured to be compared with those in vessels with conventional impellers (e.g. propeller, paddle, Rushton turbine). kL for the new type impellers are larger than those for the conventional type impellers at a low impeller rotational speed and have the same value at the same total power consumption per unit volume. For the conventional impellers the rotational speed for the complete suspension NJS changes with the impeller height and the gas flow rate. In the case of the new type impellers for which the aeration assists the suspension of the particles, kL is correlated well with the sum of the power consumption per unit volume under aeration and the aeration power per unit volume. However, in the case of the Rushton turbine the aeration dose not affect the suspension, then kL under aeration is correlated just with the power consumption per unit volume. kLs for the conventional impellers are affected with the impeller position for the complete suspension.

Power Consumption and Gas-Liquid Mass Transfer Volumetric Coefficient in a Mechanically Agitated Vessel with Wire-Gauze Impeller.

Power consumption and gas-liquid mass transfer volumetric coefficient in a mechanically agitated vessel with wire-gauze impeller were measured over a wide range of Reynolds number. The power consumption of the wire-gauze impeller did not decrease so much with aeration and was almost independent of the gas flow rate in the turbulent flow region, contrast to that of conventional impellers such as turbine and paddle. The agitation power correlation of the wire-gauze impeller was proposed in the operating condition of the gross circulation of the gas back into the impeller. The mass transfer volumetric coefficient was empirically correlated with both the physical properties of liquid and the agitation and aeration power input per unit liquid volume in the same manner as that by Nishikawa et al. (1981b), where it depended on the void fraction of the wire-gauze blade.

Influence of impeller type on power input in fermentation vessels

Prior investigations comparing radial flow Rushton impellers with axial flow hydrofoil impellers (Maxflo T and A315) were extended at the pilot scale. Six types of impellers (disk-style Rushton, Prochem Maxflo T hydrofoils of three diameters pumping downwards and A315 hydrofoils pumping upwards and downwards) were compared for qualitative differences in power number behavior with Reynolds' number, single versus double impeller power draw, gassed power reduction with aeration number and gas hold-up. Power measurements were obtained using watt transducers which, although limited in accuracy and prone to interferences, were able to provide useful qualitative monitoring results. Measurements were conducted using three model liquid systems: water, glycerol and Melojel (soluble starch). Apparent viscosities for actual Streptomyces cultivations were estimated using measured gassed power values and the experimental relationships obtained for gassed/ungassed power to aeration number and power number to Reynolds' number for the glycerol model system. Results confirmed the lower power number and lower shear environment for hydrofoil impellers, yet suggested useful trends for various process parameters and process fluids.

Practical experiences with aerobic biofilters in TMP (thermomechanical pulping), sulfite and fine paper mills in Canada

An industrial aerobic biofilter technology has been piloted extensively in Canada on several types of pulp and paper mill effluents, resulting in one full scale operational facility at one Canadian fine paper mill. In addition two industrial scale pilot plant tests were performed with the same technology, one in an integrated sulfite pulp and paper mill and one in a TMP mill. In the case of the fine paper mill, the full scale plant has two cells (8 m × 8 m × 6 m height of the media) and treats an unclarified but screened effluent with a loading of 3500 kg BOD/d. The system produces low quantities of biosludge (0.28 kg TSS/kg BOD removed), has a rapid rate of recovery after shock loadings and is very energy efficient (150 kW installed power for aeration). Pilot plant treatability studies were performed at one integrated Canadian sulfite mill using a combination of a biofilter-aerated lagoon system. The biofilter treated the most concentrated stream (sulfite pulp mill effluent) and this stream was then combined with the remaining wastewaters of the mill and polished in an aerated stabilization basin (ASB). The high quality ASB effluent (50 mg BOD/L/30 mg TSS/L) was non toxic to rainbow trout, Daphnia magna and photoluminescent bacteria. Integrated TMP mill effluents were treated in a 5 m3 aerobic biofilter filled with two types of media (10 to 20 mm and 20 to 30 mm). Under average loading conditions of 3.5 kg BOD/d.m3 media, the system produced effluents meeting regulation limits for BOD, TSS as well as for toxicity.

Practical experiences with aerobic biofilters in tmp (thermomechanical pulping), sulfite and fine paper mills in Canada

An industrial aerobic biofilter technology has been piloted extensively in Canada on several types of pulp and paper mill effluents, resulting in one full scale operational facility at one Canadian fine paper mill. In addition two industrial scale pilot plant tests were performed with the same technology, one in an integrated sulfite pulp and paper mill and one in a TMP mill. In the case of the fine paper mill, the full scale plant has two cells (8 m × 8 m × 6 m height of the media) and treats an unclarified but screened effluent with a loading of 3500 kg BOD/d. The system produces low quantities of biosludge (0.28 kg TSS/kg BOD removed), has a rapid rate of recovery after shock loadings and is very energy efficient (150 kW installed power for aeration).Pilot plant treatability studies were performed at one integrated Canadian sulfite mill using a combination of a biofilter-aerated lagoon system. The biofilter treated the most concentrated stream (sulfite pulp mill effluent) and this stream was then combined with the remaining wastewaters of the mill and polished in an aerated stabilization basin (ASB). The high quality ASB effluent (50 mg BOD/L/30 mg TSS/L) was non toxic to rainbow trout, Daphnia magna and photoluminescent bacteria.Integrated TMP mill effluents were treated in a 5 m3 aerobic biofilter filled with two types of media (10 to 20 mm and 20 to 30 mm). Under average loading conditions of 3.5 kg BOD/d.m3 media, the system produced effluents meeting regulation limits for BOD, TSS as well as for toxicity.

A fluid dynamic study of the retrofitting of large agitated bioreactors: Turbulent flow

Studies were conducted in three 19-m(3) fermentors (14 m(3) working volume, aspect ratio = 3:1), one fitted with four Rushton turbines (D/T = 0.35), one with three Lightnin' A315 hydrofoil impellers (D/T = 0.46). The power drawn under the same aerated conditions relative to the unaerated ones was always greater with the hydrofoils, which gives them the potential for enhanced mass transfer rates under practical operating conditions. However, the power draw was also sensitive to the magnitude of the unaerated power. Indeed, at low unaerated specific power ( approximately 0.6 W-kg) and high air flow rates ( approximately 1vvm), the relative power draw with the hydrofoils could be even greater than 1. The hold-up with each of the impellers was broadly similar at the same aeration rate and power input, though the later had a much smaller impact in these large vessels than has been reported in the literature based on smaller scale work. As usual, repressed coalescence caused increased hold-up, and, with the hydrofoils, this increase was associated with a lower power draw. Because of the greater mechanical vibration of the reactors with the hydrofoils, vibration characteristics of the vessels were measured and they were very similar. The results showed that provided care is taken in the mechanical design of the system, such impellers can operate reliably in large-scale fermentations with the potential for enhanced biological performance. (c) 1994 John Wiley & Sons, Inc.

Investigation of hypothesized anaerobic stabilization mechanisms in biological nutrient removal systems

ABSTRACT: Anaerobic stabilization (AnS) is denned as difference between actual and theoretical oxygen use in activated sludge systems with anaerobic selectors. AnS-related oxygen savings translate into potentially lower aeration power costs. A comprehensive electron balance approach was developed for more accurate determination of AnS, along with procedures for in-situ oxygen uptake rate (OUR) measurement and analysis of dissolved gases by vacuum stripping.Lab-scale anaerobic/oxic (A/O) and anaerobic/anoxic/oxic (A2/O) systems operated under various conditions yielded AnS values of 15-55% of the theoretical oxygen requirement. Hydrogen and methane production together explained less than 1% of AnS, except when the A/O feed was supplemented with formate, in which case methane production explained about 19% of AnS. Stripping of reduced volatiles explained less than 1% of AnS in both systems. Kinetic limitations of the chemical oxygen demand (COD) test were not found to be significant in explaining AnS, but thermodynamic limitations were identified as potentially capable of explaining a significant fraction of AnS. Mechanisms are hypothesized that can partially explain AnS based on the results of this study.

Model Experiments on Waves and Nearshore Currents around Floating Wave Power Plant in Coastal Waters in Comparison with Detached Breakwater

The role of a floating wave power plant in the environmental control of coastal waters for coastal fisheries is discussed. An experimental investigation was carried out into the physical characteristics, i.e. waves and wave induced nearshore current around a model of a floating wave power plant on an inclined bottom in comparison with a model of a detached breakwater. The results are summarised as follows: (1) The floating wave power plant can be utilised for environmental conservation and the progress of coastal fisheries to create calm sea areas, improve water quality, control nearshore currents and provide power for aeration and dipping out of bottom waters. (2) The calm water area behind the floating power plant model where wave transmission ratio is less than 0.5 in three dimensional condition is almost the same as behind the detached breakwater. (3) The calm water area becomes wider near the shore line as the bottom slope becomes more gentle. (4) The reflected wave height at the front of the floating power plant model is much smaller than the detached breakwater. (5) Circulating current on free surfaces from the centre of the model to the outer side of the floating power plant model is dominant near water surface, it is dominant from the outer side to the centre of the model of the detached breakwater. In particular, strong currents toward the shore line just behind the floating wave power plant model was observed. (6) Water exchange near the floating wave power plant model is larger than the detached breakwater. It becomes larger as the bottom slope becomes more gentle for both models.

Hydrodynamics, mass transfer, and yeast culture performance of a column bioreactor with ejector

A bubble column fitted with an ejector has been tested for its physical and biological performance. The axial diffusion coefficient of the liquid phase in the presence of electrolytes and ethanol was measured by a stimulus-response technique with subsequent evaluation by means of a diffusion model. In contrast to ordinary bubble columns, the coefficient of axial mixing is inversely dependent on the superficial air velocity. The liquid velocity acts in an opposite direction to the backmixing flow in the column. The measurement of volumetric oxygen transfer coefficient in the presence of electrolytes and ethanol was performed using a dynamic gassing-in method adapted for a column. The data were correlated with the superficial air and liquid velocities, total power input, and power for aeration and mixing; the economy coefficient of oxygen transfer was used for finding an optimum ratio of power for aeration and pumping. Growth experiments with Candida utilis on ethanol confirmed some of the above results. Biomass productivity of 2.5 g L(-1) h(-1) testifies about a good transfer capability of the column. Columns fitted with pneumatic and/or hydraulic energy input may be promising for aerobic fermentations considering their mass transfer and mixing characteristics.

Design and characterization of a multistage, mechanically stirred column absorber

A multistage, mechanically stirred column absorber has been designed and built with a modular construction, based on preliminary experiments with a test column. The column has been characterized as a gas-liquid contactor by its gas holdup, gas and liquid axial dispersion, mixing times, oxygen transfer coefficients and power consumptions, determined as a function of gas velocity, liquid velocity and impeller speed for one and two impellers per stage. Gassed power was correlated with ungassed power, gas rate and impeller speed. The gas phase axial mixing was essentially plug flow and the liquid phase axial mixing varied between 5 and 12 equivalent stages. Oxygen transfer coefficients were correlated with power consumptions and aeration rates by the equation K L a γ (P/V) a (υ sg ) b . The oxygen transfer coefficients with single stiffer stages were 25% above those for the double stirrer stages for equal power consumption and gas rates. Except for the low aeration and high power consumption extremes, the column showed superior oxygen transfer performance. in comparison to tubular loop and tank fermenters.

Enhanced treatment of livestock wastewater: III. Enhanced aerobic treatment of swine waste by anaerobic pre-conditioning

A slurry of screened hog waste, diluted fivefold (1 : 5), was pre-treated anaerobically for periods of several days prior to the beginning of aerobic treatment. The rates of COD and TS reduction in the aerobic stage were enhanced by the anaerobic pretreatment. Thus, in systems designed for remote oxidative treatment, possible savings in aeration power requirements can be realized.