Air Sparging Research Articles

Comprehensive evaluation of a cost-effective method of culturing Chlorella pyrenoidosa with unsterilized piggery wastewater for biofuel production

BackgroundThe utilization of Chlorella for the dual goals of biofuel production and wastewater nutrient removal is highly attractive. Moreover, this technology combined with flue gas (rich in CO2) cleaning is considered to be an effective way of improving biofuel production. However, the sterilization of wastewater is an energy-consuming step. This study aimed to comprehensively evaluate a cost-effective method of culturing Chlorella pyrenoidosa in unsterilized piggery wastewater for biofuel production by sparging air or simulated flue gas, including algal biomass production, lipid production, nutrient removal rate and the mutual effects between algae and other microbes.ResultsThe average biomass productivity of C. pyrenoidosa reached 0.11 g L−1 day−1/0.15 g L−1 day−1 and the average lipid productivity reached 19.3 mg L−1 day−1/30.0 mg L−1 day−1 when sparging air or simulated flue gas, respectively. This method achieved fairish nutrient removal efficiency with respect to chemical oxygen demand (43.9%/55.1% when sparging air and simulated flue gas, respectively), ammonia (98.7%/100% when sparging air and simulated flue gas, respectively), total nitrogen (38.6%/51.9% when sparging air or simulated flue gas, respectively) and total phosphorus (42.8%/60.5% when sparging air or simulated flue gas, respectively). Culturing C. pyrenoidosa strongly influenced the microbial community in piggery wastewater. In particular, culturing C. pyrenoidosa enriched the abundance of the obligate parasite Vampirovibrionales, which can result in the death of Chlorella.ConclusionThe study provided a comprehensive evaluation of culturing C. pyrenoidosa in unsterilized piggery wastewater for biofuel production. The results indicated that this cost-effective method is feasible but has considerable room for improving. More importantly, this study elucidated the mutual effects between algae and other microbes. In particular, a detrimental effect of the obligate parasite Vampirovibrionales on algal biomass and lipid production was found.

Numerical and experimental analyses of a stirred vessel for a large volumetric flow rate of sparged air

Abstract Computational fluid dynamics (CFD) and experimental analyses of some of the basic characteristics of air sparging in a tall stirred vessel equipped with a three-stage impeller are presented. The impeller was assembled from a radial ABT impeller as the lower, a turbine 6PBT45 as the middle and an axial Scaba-type 3SHP1 impeller as the upper. All the impellers were of the same diameter, i.e., 225 mm, while the vessel diameter was 450 mm. The impeller's rotational speed was 178 r·min−1. The aeration regime was established with an air volumetric flow rate of 28.3 m3·h−1. To the best of our knowledge, this study is the first to consider the very high gassing rate by means of CFD in a tank stirred by three-stage axial/radial impellers. The numerical simulation was performed using the ANSYS Fluent (R17.2, 2016) code for solving the governing equations of fluid dynamics in single- and multi-phase systems. While discussing the bubble size distribution, a discrete population balance model (PBM) was used. Adopting CFD, the stirring power and the total void fraction (the total gas holdup) were calculated. The results were in good agreement with the measured values using a laboratory experimental device.

Oxidative dissolution of nickel matte in dilute sulfuric acid solutions

Nickel matte (Ni3S2) is the most common feedstock for producing nickel oxide and nickel metal in associated refineries. Nickel matte is produced from sulfide or laterite ores, and contains around 73 wt% nickel. This study investigated the dissolution parameters of nickel matte in dilute sulfuric acid media in the presence of air as oxidant with the goal of presenting a cost-effective process for leaching nickel matte. The dissolution experiments were carried out to examine the following effects: air flow rate, stirring speed, S/L ratio, acid concentration, reaction temperature, reaction time and the amount of ferrous sulfate added. Nickel extraction of 90% and cobalt extraction of 80% were achieved using dilute H2SO4 and sparging of air as a source of oxygen. To increase the dissolution percentage of the matte, ferrous sulfate addition was studied and the oxidation and reduction potential (ORP) was measured to investigate the effect of ferrous ions on nickel and cobalt recovery. The highest nickel dissolution percentage (95%) was observed with the initial addition of 1200 mg/L Fe (II). Characterization of samples has been carried out with quantitative X-ray diffraction (XRD) and scanning electron microscopy, along with an energy dispersive system (SEM-EDS).

Simulation of in situ biodegradation of 1,4-dioxane under metabolic and cometabolic conditions

Bioaugmentation is an option for aerobic remediation of groundwater contaminated with 1,4-dioxane. One approach uses microbes that cometabolize 1,4-dioxane following growth on a primary substrate (e.g., propane), whereas another uses microbes (e.g., Pseudonocardia dioxivorans CB1190) capable of using 1,4-dioxane as a sole substrate. The relative merits of these approaches are difficult to distinguish based on field data alone, and theoretical analyses of these processes have yet to be published. The objective of this study was to compare these remediation options using a transport model that incorporates advection, dispersion and biodegradation reactions described by multi-substrate Monod kinetics and co-inhibition effects. The transport model was coupled to an approximate steady-state air sparging simulation used to estimate gas (propane and oxygen) distribution at the field scale. The model was calibrated with field data for 1,4-dioxane and propane concentrations from a previously reported pilot study. The two remediation approaches were evaluated under different conditions that vary the initial concentration of 1,4-dioxane and the loading rates of oxygen, propane, and biomass. The metrics used to evaluate the remediation success were the time to reach an average 1,4-dioxane concentration of 1 μg L−1 and the percent of 1,4-dioxane biodegraded after 10 years of simulation. Results indicate that the initial concentration of 1,4-dioxane strongly influences which remediation approach is more effective. When initial concentrations were <10 mg L−1, propane-driven cometabolism led to faster remediation; whereas metabolic biodegradation was faster when initial concentrations were 10 mg L−1 or higher. Below 0.25 mg L−1, the viability of metabolic biodegradation improved, although cometabolism by propanotrophs still required less time to reach 1 μg L−1. Biomass injection rates had a strong effect on the rate of metabolism but not cometabolism because continuous input of primary substrate supported growth of propanotrophs. The performance of both cultures was negatively affected by a decrease in oxygen injection rate. The endogenous decay coefficient and the dispersion rate for biomass had a significant impact on cometabolic and metabolic biodegradation of 1,4-dioxane. The maximum specific rate for cometabolism of 1,4-dioxane, the dispersion rate for 1,4-dioxane, and effective porosity also had significant effects on the time to achieve remediation with propanotrophs.

Effects of crystalline nanocellulose on wastewater-cultivated microalgal separation and biomass composition

Microalgae are renewable and promising feedstock rich in biochemicals for biofuel and bioenergy production. The viability of this technology relies on the energy- and cost-efficient cultivation (culture medium used) and harvesting (coagulant applied) processes. The natural coagulant of crystalline nanocellulose modified with 1-(3-aminopropyl) imidazole (CNC-APIm) was demonstrated as a green and recyclable coagulant for microalgal harvesting. However, optimisation is still needed to ensure its applicability to microalgae cultivated on wastewaters, no effect on biomass composition, as well as cost-effective harvesting. In this study, microalgal growth and nutrient removal capacity of Chlorella vulgaris (C. vulgaris) were first investigated on two types of municipal wastewaters. C. vulgaris grew well on both primary and 30% (v/v) diluted centrate wastewaters with biomass productivities of 0.071 ± 0.005 and 0.062 ± 0.006 g/(L·d), respectively. High nitrogen and phosphorus removal efficiencies (91.1–100%) were obtained. Subsequently, the wastewater-cultivated C. vulgaris was harvested using a novel natural coagulant of crystalline nanocellulose modified with 1-(3-aminopropyl) imidazole (CNC-APIm). Based on the optimization results of the Design of Experiments driven response surface methodology approach, the optimal conditions for maximum HEs (86.5%) and RCs (38.5 g-algae/g-CNC) responses were determined for C. vulgaris under the following conditions: 0.02 g-CNC-APIm/g-algae of mass ratio, 5 s of CO2 sparging time, 8 min of air sparging time, and 50 ml/min of air flow rate. Moreover, no statistically significant differences were observed in the contents of carbohydrate, protein, lipid and fatty acids in the biomass harvested by centrifugation and CNC-APIm, respectively, suggesting that CNC-APIm would not impact the downstream microalgal application. According to the rough technical and economical estimation, CNC-APIm will be an alternative to conventional coagulants for commercial microalgal harvesting application.

Struvite Phosphorus Recovery from Aerobically Digested Municipal Wastewater

Small, municipal wastewater treatment plants (WWTPs) that use aerobic digestion treat approximately 40% of the discharged wastewater in the USA, and yet they are an overlooked source of recoverable P. There are no known reports of small, aerobic WWTPs recovering P through struvite (MgNH4PO4·6H2O) precipitation for repurposing as a mineral fertilizer, even though some large WWTPs with anaerobic digestion are. Four small WWTPs in north Florida, USA, with treatment capacities from 371 to 2650 m3 wastewater d−1 and incoming P loads from 2 to 14 kg d−1 were investigated for their potential to produce struvite from digester filtrates. A chemical equilibrium model was used to predict the feasibility of struvite production and the results compared with actual WWTP filtrate measurements. Filtrates from aerobic digesters were able to form struvite if solution pH was increased by ≤1 pH unit. Depending on the WWTP, P recovery in filtrates through struvite precipitation ranged from 27–57% by mass at pH 8.5, via NaOH additions or air sparging. Increasing filtrate Mg concentrations improved P recovery up to 97%. Based upon these results, small WWTPs using aerobic digestion will be able to lower their P waste output through recovery as struvite fertilizer.

Comparison of physical cleaning techniques for membrane distillation: temperature reversal, air sparging, and ultrasonication

Comparison of physical cleaning techniques for membrane distillation: temperature reversal, air sparging, and ultrasonication

Removal of non-aqueous phase liquids (NAPLs) from TPH-saturated sandy aquifer sediments using in situ air sparging combined with soil vapor extraction

Contamination in coastal aquifer plains is of great concern in many countries given that non-aqueous phase liquids (NAPLs) have polluted numerous sites through accidental oil spills or improper disposal. We have developed a method to remove pollutants such as NAPLs from sandy sediment samples collected from the Mandol area of Gomso Bay in western South Korea. The sediments were collected from around the diffuser in a two-dimensional (2D) acrylic reaction apparatus, and these contained a total petroleum hydrocarbon (TPH) concentration of 89.3 ppm (mg/kg media). The maximum perchloroethylene (PCE) concentration was 398.51 ppm in the unsaturated zone and 0.77 ppm in the saturated zone. Volatile organic compounds (VOCs) were detected between 20 and 44 hour. However, non-volatile contaminants remained in the sediments after treatment. In situ air sparging (IAS) combined with soil vapor extraction (SVE), transformation from sorbed and nonaqueous phases to the vapor phase, is incomplete when treatment is performed using a pervasive air flow for sediments such as the sand of Mandol. During air transformation, the groundwater flow conditions increased the rate of contaminant removal. Although pilot-scale testing in the field site was fluctuated due to the heterogeneous of sediments condition, this 2D study found that the proposed method can alter the measurable geophysical properties of NAPLs. These findings demonstrate that IAS combined with SVE in the saturated zone is an effective technology for aquifer remediation high applicability of sandy coastal sediments contaminated by NAPLs.

Effective bioreactor pH control using only sparging gases.

pH control is critical in bioreactor operations, typically realized through a two-sided control loop, where CO2 sparging and base addition are used in bicarbonate-buffered media. Though a common approach, base addition could compromise culture performance due to the potential impact from pH excursions and osmolality increase in large-scale bioreactors. In this study, the feasibility of utilizing control of sparge gas composition as part of the pH control loop was assessed in Chinese hamster ovary (CHO) fed-batch cultures. Fine pH control was evaluated in multiple processes at different setpoints in small-scale ambr®250 bioreactors. Desired culture pH setpoints were successfully maintained via air sparge feedback control. As part of the pH control loop, air sparging was increased to improve CO2 removal automatically, hence increase culture pH, and vice versa. The effectiveness of this pH control strategy was seamlessly transferred from ambr®250 to 200 L scale, demonstrating scalability of the proposed methodology. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 35: e2743, 2019.

Enhancing Groundwater Remediation in Air Sparging by Changing the Pulse Duration

AbstractAn important operation parameter in the design of a pulsed air sparging (PAS) system is the pulse duration (PD). To study the effect of the PD on the remediation process, a series of laboratory experiments and numerical simulations were performed. The experimental apparatus was a cylindrical tank, packed with fine sand and partially filled by water contaminated with toluene. Toluene concentrations in water and in effluent air were measured over time during the application of PAS, which was applied with three different PD. Next, the T2VOC model, an extension of the TOUGH2 simulation program, was used to simulate the two‐phase flow and transport processes for these cases. The simulation model was calibrated to the experimental results, and then run with a range of PD values. Results showed that there exists an optimal PD which yields the highest remediation efficiency. Next, it was shown that this PD may be obtained by performing a PAS pilot test and measuring the groundwater pressure response in a monitoring well. The characteristic time which describes the exponential decay of the pressure response was shown to provide an adequate estimate for the optimal PD. The estimation improved by taking a number of injection cycles.

Photocatalytic decolorization of methylene blue using TiO2/UV system enhanced by air sparging

In this work, the photocatalytic degradation of wastewater containing an organic dye (Methylene Blue) was investigated in a slurry-type reactor enhanced by air sparging. Commercial TiO2 was used as the photocatalyst which was activated using ultraviolet irradiation. The effects of initial dye concentration, photocatalyst loading, solution pH and air’s superficial velocity on the degradation efficiency were evaluated. The degradation efficiency of the dye increased with increasing the pH of the solution and decreasing the initial dye concentration. Moreover, increasing the catalyst loading up to 1 g/l showed an improvement in the degradation efficiency. However, the efficiency deteriorated with a further increase in the catalyst loading. Introducing air into the reaction medium in the form of micro-bubbles significantly improved the degradation efficiency by providing oxygen molecules required for the oxidation reaction. Furthermore, these air bubbles created turbulence in the reaction medium, which resulted in the enhancement of the mass transfer rate of dye molecules from the solution bulk to the catalyst surface with a subsequent increase in the degradation efficiency. A complete decolorization of 10 ppm dye solution was achieved in the neutral solution (pH = 7) with a catalyst loading of 1 g/l and an air superficial velocity around 2 cm/s. The effect of the air superficial velocity on the degradation reaction kinetics was investigated and the reaction was found to follow the pseudo second order rate law.

Electromagnetic waves' effect on airflow during air sparging

Air sparging is a popular, yet slow, remediation technology for soil and groundwater contaminated with volatile organic compounds (VOCs). This paper theoretically and experimentally studies the effect of electromagnetic (EM) waves on air-channel formation within a glass-bead medium—used as an analogy to soil—during air-sparging experiments. The impact of EM waves on cleanup is not the focus of this paper, and the impact on airflow may or may not positively impact resulting cleanup process using air sparging to remove VOCs through volatilization. The hypothesis is that dielectrophoretic forces by EM waves can be used to alter airflow. Air injection was performed at different pressures, in the presence of EM waves (referred to as EM-stimulated) of various power and frequencies and the absence of EM waves (referred to as unstimulated). Digital images of the airflow patterns were collected, processed, and analyzed for all tests. The shape of the zone of influence (ZOI) was observed, and the radius of the zone of influence (ROI) was measured, which showed a 16% increase in ROI due to EM stimulation. An experimentally validated numerical simulation of the electric-field component of EM waves was developed. The correlation between EM-wave and air sparging characteristics were then studied using the numerical simulation and acquired digital images of the airflow to investigate and validate that the dielectrophoretic mechanism is behind the EM effect on airflow.

Influence of air and nitrogen sparging on flux during ultrafiltration of hemicelluloses extracted from wheat bran

Ultrafiltration can be used to concentrate arabinoxylan isolated from wheat bran. Prefiltration with diatomaceous earth and air sparging during ultrafiltration of the alkaline hemicellulose solution were both found to increase the flux, while nitrogen sparging had no effect. The flux of the untreated solution was 51 L/m2 h, while the flux after prefiltration of the solution with diatomaceous earth was 62 L/m2 h. After 5 h of air sparging during ultrafiltration of the prefiltered solution the flux was 230 L/m2 h. However, sparging the prefiltered solution with nitrogen gas had no effect on the flux. This shows that the increase in flux was not due to the reduction of concentration polarization by sparging per se. Size-exclusion chromatography showed that sparging with air caused a reduction in the size of hemicelluloses >100 kDa to 10–100 kDa, which is believed to be due to oxidative degradation of hemicelluloses. This size reduction of the hemicelluloses is likely the main cause of the increased flux. The retention of hemicelluloses during ultrafiltration with a ceramic membrane with a nominal cut-off of 10 kDa was 96% in all experiments, except at the end of the experiment with the prefiltered, air-sparged solution, when the retention decreased to 93% due to the size reduction of the hemicelluloses. Air sparging seems to be an efficient flux enhancing method with limited negative influence on the molecular size of hemicelluloses.

Aerating static mixers prevent fouling

To mitigate fouling and reduce concentration polarization at membrane surfaces, various measures have been investigated. One approach is to alter the hydrodynamics in a tubular membrane by inserting static mixers, which disturb the flow, enhance particle back-transport and increase the shear rate at the membrane surface. Another approach widely applied in wastewater treatment, but not limited to this application, is air sparging. Combining both measures - air sparging and static mixers - yields a novel device to mitigate fouling in tubular membrane filtration: aerating static mixers.Aerating mixers are static mixers with an incorporated air channel. They can be fabricated in one piece by means of 3D-printing. Thanks to the incorporated air channel, aerating mixers are able to direct air bubbles straight towards the membrane surface. That way, the fouling mitigation potential of static mixers is enhanced by the application of two-phase flow. We demonstrate that aerating mixers reduce fouling to a greater extent than static mixers or air sparging alone. Furthermore, we show that the novel aerating Kenics is superior to a Kenics mixer combined with air sparging from an external source. With the aerating Kenics, the total membrane resistance stayed nearly constant for the investigated range of fluxes, which reached up to 353LMH. Due to the significantly reduced TMP required to obtain the desired permeate fluxes, the systems where a static mixer was applied showed the lowest specific energy consumption. In case of the systems with air, the energy to compress the air contributed less than 13% to the total specific energy consumption.

Special Issue: Diagnostic Tools to Assess In Situ Remediation System Performance

Special Issue: Diagnostic Tools to Assess In Situ Remediation System Performance

Application of Diagnostic Tools to Evaluate Remediation Performance at Petroleum Hydrocarbon‐Impacted Sites

AbstractIn situ treatment technologies for petroleum hydrocarbon‐impacted sites (e.g., multiphase extraction, air sparging, soil vapor extraction, or in situ chemical oxidation) usually rely on a specific chemical, microbial, or physical contaminant removal process. However, target contaminant concentrations can also vary due to other co‐occurring processes (e.g. delivery of remedial fluids, natural variability), which can confound the ability to demonstrate treatment efficiency. This technical note proposes a methodology that integrates several diagnostic tools to assess treatment performance. Stable isotope methods and biomarkers were selected because they provide process‐specific and, often, also compound‐specific information on contaminant removal. The isotope tools include compound‐specific isotope analysis that can be used to discriminate between a broad range of removal processes, and isotope analysis of oxidants and degradation end products to assess overall transformation of hydrocarbons. The biomarkers cover characteristic metabolites and functional genes on a mRNA rather than DNA level to understand biological activity more carefully. This technical note integrates information from laboratory and field studies, especially controlled‐field experiments where the tools have been evaluated side‐by‐side for different treatment methods. A tiered approach is proposed to deploy the tools in a stepwise manner until sufficient information is obtained to confidently identify the mass removal processes of interest and demonstrate efficacy of the intended treatment mechanism. The order of tool application considers the type of information that can be gained, the level of certainty, and the ease of implementation. The objective of this technical note is to enable widespread use of these diagnostic tools with the motivation to improve the efficacy of in situ treatment systems.

CFD analysis of the flow dynamics of microorganisms in dilute cultures in stirred tank photobioreactors

The objective of the present work is to investigate the influence of the key factors such as the impeller speed in revolutions per minute and frequency of crossover of the particles between the light and the dark zones on the growth of microalgae in a stirred tank photobioreactor. To that end, the numerical simulations are performed for the culture in a stirred tank photobioreactor using computational fluid dynamics. The culture medium and the sparged air are considered to be two interpenetrating continua. Based on the light intensity profiles within, the reactor is divided into light and dark zones. Individual microorganisms are tracked. The fractional time spent by them in the dark zone, and the average frequency of crossover between the zones, are determined. The critical impeller speed for the maximum cell growth is compared against the experimental observation reported in literature and the maximum tolerable shear stress is identified.

Reduction of energy demand by use of air sparging during ultrafiltration of alkali-extracted wheat bran hemicelluloses

The flux during membrane filtration can be enhanced by the use of a two-phase gas–liquid flow. This has been shown to be an energy-efficient alternative to increasing the cross-flow velocity. In this work, air sparging was used to increase the flux during ultrafiltration of alkali-extracted wheat bran hemicelluloses. Batch filtration was performed in a pilot unit with a ceramic ultrafiltration membrane with a nominal cut-off of 10kDa. Parametric studies with and without air sparging were performed at temperatures of 30°C, 50°C and 80°C and cross-flow velocities of 1, 3, 5 and 7m/s. The limiting flux was not affected by air sparging at 30°C, while a slight increase was observed at 50°C and a considerable increase was obtained at 80°C. Air sparging reduced the energy demand per m3 permeate produced during dead-end batch ultrafiltration at 80°C and 1m/s from 0.96kWh/m3 to 0.51kWh/m3.

Comparison of physical technologies for biomass control in biofilters treating gaseous toluene

ABSTRACTExcessive accumulation of biomass within gas-phase biofilters often results in the deterioration of removal performance. Compared with chemical and biological technologies, physical technologies are more effective in removing biomass and inducing less inhibition of the biofilter performance. This study applied different physical technologies, namely, air sparging, mechanical mixing, and washing with water at various temperatures, to remove excess biomass in biofilters treating toluene. Filter pressure drop, removed dry biomass, biofilter performance, and microbial metabolic characteristics were analyzed to evaluate the effectiveness of the methods. Results showed that air sparging was inefficient for biomass removal (1 kg dry biomass/m3 filter), whereas mechanical mixing significantly inhibited removal efficiencies (<30%). Washing of the packing with fluids was feasible, and hot fluids can remove a large amount of biomass. However, hot fluids reduce microbial activity and inhibit removal performance. Washing of the packing with either 20°C or 50°C water showed efficiency as >3 kg dry biomass/m3 filter can be removed at both temperatures with removal efficiencies at approximately 40% after treatment. Finally, different technologies were compared and summarized to propose an optimized strategy of biomass control for industrial biofilters.Implications: This study is to apply different physical technologies, namely, air sparging, mechanical mixing, and washing with water of different temperatures, to remove the excess biomass in biofilters treating toluene. The filter pressure drop, removed dry biomass, biofilter performance, and microbial metabolic characteristics were all analyzed to evaluate the effectiveness of the methods. The results of this study provide useful information regarding biomass control of industrial biofilters.

Diagnostic Tools to Assess Mass Removal Processes During Pulsed Air Sparging of a Petroleum Hydrocarbon Source Zone

AbstractDuring remediation of contaminated aquifers, diagnostic tools can help evaluate whether an intended mass removal process was successfully initiated and acted on specific contaminants of concern. In this study, several diagnostic tools were tested in a controlled‐release in situ air sparging experiment that focused on the treatment of target hydrocarbons (e.g., benzene, toluene, ethylbenzene, and xylenes). The tools included compound‐specific isotope analysis (CSIA), expression of functional genes (mRNA), and metabolites characteristic of aerobic and anaerobic biodegradation. Total and compound‐specific mass balances were established and used, along with traditional monitoring parameters, to validate the results from the various tools. CSIA results indicated biodegradation as the main process contributing to benzene and toluene removal. Removal process‐specific isotope shifts were detected in groundwater as well as in the system effluent gas. CSIA, metabolite, and mRNA biomarkers consistently indicated that both aerobic and anaerobic biodegradation of benzene and toluene occurred, but that their relative importance evolved over time and were related to the treatment system operation. While the indicators do not allow quantification of the mass removed, they are particularly useful to identify if a removal process has been initiated, and to track relative changes in the predominance of in situ contaminant attenuation processes resulting from remediation efforts.