Stripping Of Volatile Organic Compounds Research Articles

Hydrodynamic cavitation effects on advanced oxidation processes and mass transfer: A conceptual model

Advanced oxidation process (AOPs) technologies are the subject of intense research due to the need for treating refractory wastewaters. Among them, hydrodynamic cavitation (HC) is particularly well-studied because of its potential as an AOP and as a means of intensification for other processes, including other AOPs. Understanding HC and its effects is crucial for its development and practical application. This study introduces a conceptual model that integrates the presence of supercritical water (SCW) to interpret HC results. The model was validated by selected experimental scenarios focused on exploring the impact of HC on the viscosity of a soluble polymer solution, the precipitation of an ionic salt from an unsaturated solution, and the stripping of volatile organic compounds (VOCs). The results were analyzed and interpreted using the conceptual model, remarking the scenarios that cannot be explained by the generally accepted mechanisms of radicals’ formation or pyrolysis. Furthermore, the model was then applied to analyze the trends reported in the existing literature regarding the application of HC as an AOP and as a method of intensification. The occurrence of SCW as a key driving force for HC chemical and physical effects represents a novel approach with the potential to enhance the design and operation of HC systems, particularly when tailoring operating conditions to maximize SCW occurrence.

A microfluidic device for open loop stripping of volatile organic compounds

The detection of volatile organic compounds is of great importance for assessing the quality of water. In this contribution, we describe a miniaturized stripping device that allows fast online detection of organic solvents in water. The core component is a glass microfluidic chip that facilitates the creation of an annular-flowing stream of water and nitrogen gas. Volatile compounds are transferred efficiently from the water into the gas phase along the microfluidic pathway at room temperature within less than 5 s. Before exiting the microchip, the liquid phase is separated from the enriched gas phase by incorporating side capillaries through which the hydrophilic water phase is withdrawn. The gas phase is conveniently collected at the outlet reservoir by tubing. Finally, a semiconductor gas sensor analyzes the concentration of (volatile) organic compounds in the nitrogen gas. The operation and use of the stripping device is demonstrated for the organic solvents THF, 1-propanol, toluene, ethylbenzene, benzaldehyde, and methanol. The mobile, inexpensive, and continuously operating system with liquid flow rates in the low range of microliters per minute can be connected to other detectors or implemented in chemical production line for process control.

Dynamic modeling of biodegradation and volatilization of hazardous aromatic substances in aerobic bioreactor

The aerobic biological process is one of the best technologies available for removing hazardous organic substances from industrial wastewaters. But in the case of volatile organic compounds (benzene, toluene, ethylbenzene, p-xylene, naphthalene), volatilization can contribute significantly to their removal from the liquid phase. One major issue is to predict the competition between volatilization and biodegradation in biological process depending on the target molecule. The aim of this study was to develop an integrated dynamic model to evaluate the influence of operating conditions, kinetic parameters and physical properties of the molecule on the main pathways (biodegradation and volatilization) for the removal of Volatile Organic Compounds (VOC). After a comparison with experimental data, sensitivity studies were carried out in order to optimize the aerated biological process. Acclimatized biomass growth is limited by volatilization, which reduces the bioavailability of the substrate. Moreover, the amount of biodegraded substrate is directly proportional to the amount of active biomass stabilized in the process. Model outputs predict that biodegradation is enhanced at high SRT for molecules with low H and with a high growth rate population. Air flow rate should be optimized to meet the oxygen demand and to minimize VOC stripping. Finally, the feeding strategy was found to be the most influential operating parameter that should be adjusted in order to enhance VOC biodegradation and to limit their volatilization in sequencing batch reactors (SBR).

Optimal Design of a Rotating Packed Bed for VOC Stripping from Contaminated Groundwater

Groundwater pollution by volatile organic compounds (VOCs) is a serious environmental concern, and several techniques have been suggested and employed to strip off these harmful compounds. Since groundwater is classified as a distributed scale system, an efficient VOC stripping system should be portable and economically competitive. The focus of the current study is to remove trichloroethylene (TCE) from contaminated groundwater in a rotating packed bed (high gravity or HiGee). Although industrial applications of HiGee do exist, studies on optimal design of the same is scarce in the literature. The present study optimizes the design of an industrial-scale HiGee stripping process with conflicting objectives such as total annual cost (TAC) and total VOC removal under consideration. The synergy effect of heating and rotation is studied, and several inferences from this study are listed. Pareto-optimal solutions obtained provide a wide range of optimized design alternatives, one of which can be chosen and emp...

Effect of Aeration on the Quality of Effluent from UASB Reactor Treating Sewage

The treatment of effluent of pilot- and full-scale upflow anaerobic sludge blanket (UASB) reactors operating at steady state was studied in an aeration-settling system. The fine pore submerged diffusers were used to aerate the effluent of UASB reactors under different operating conditions. Forty to 55% of the biochemical oxygen demand (BOD) and the chemical oxygen demand (COD) removal efficiencies were achieved by the direct aeration of the UASB effluent in the laboratory. The maximum removal efficiencies were achieved at 30 min hydraulic retention time (HRT) and a dissolved oxygen (DO) of 5–6 mg/L or high KLa (vigorous aeration). Batch experiments on nitrogen purging and the aeration of sulfides, volatile organic compounds (VOCs), and nonpurgeable organic carbons (NPOCs) were performed to ascertain the mechanism of BOD/COD removal. During aeration, BOD and COD were reduced by the stripping of H2S and VOCs and by the chemical oxidation of total sulfides and organic carbon. The stripping and chemical oxid...

Using radon-222 as indicator for the evaluation of the efficiency of groundwater remediation by in situ air sparging

A common approach for remediation of groundwater contamination with volatile organic compounds (VOCs) is contaminant stripping by means of in situ air sparging (IAS). For VOC stripping, pressurized air is injected into the contaminated groundwater volume, followed by the extraction of the contaminant-loaded exhaust gas from the vadose soil zone and its immediate on-site treatment. Progress assessment of such remediation measure necessitates information (i) on the spatial range of the IAS influence and (ii) on temporal variations of the IAS efficiency. In the present study it was shown that the naturally occurring noble gas radon can be used as suitable environmental tracer for achieving the related spatial and temporal information. Due to the distinct water/air partitioning behaviour of radon and due to its straightforward on-site detectability, the radon distribution pattern in the groundwater can be used as appropriate measure for assessing the progression of an IAS measure as a function of space and time. The presented paper discusses both the theoretical background of the approach and the results of an IAS treatment accomplished at a VOC contaminated site lasting six months, during which radon was applied as efficiency indicator.

Steam Stripping of Bromine from Sea Brine: Mass-Transfer Analysis of Randomly Packed Columns

Mass transfer data have been collected on stripping of bromine from aqueous solution at different flow rates of liquid and steam using Rasching and Pall Ring packings randomly packed in glass columns (6.25 to 90 cm diameter) and tower constructed by using granite slabs having square cross-section (25 cm × 25 cm). In the present study, the model proposed by Wagner et al. [1] for distillation considering gas phase resistance as the controlling resistance is modified wherein liquid phase resistance is considered as the controlling resistance for mass-transfer. The modified model and model proposed by Castillo et al. [2] for stripping of volatile organic compounds (VOCs) from water have been successfully applied to the experimental data on debromination of sea brine. Average deviation between observed and calculated value of height of transfer unit based on the modified model of Wagner et al. [1] is 20.9 and 10.7% for bromide-bromate solution and bittern, respectively. The average deviation between observed and calculated overall volumetric liquid phase mass transfer coefficient is 16.62, 15.13 and 84.19% for debromination of bittern using pilot plant data, commercial bromine plants, and sodium bromide-bromate solution, respectively, based on correlations given by Wagner et al. [1]. Similar results based on Castillo et al. [2] model are 34.43, 49.69 and 26.80%, respectively. Volumetric liquid phase mass transfer coefficient for stripping of VOCs from water is in the range of 0.001-0.008 (1/s) whereas in the present study it varies from 0.001 to 0.017 (1/s). In other words, steam stripping of bromine from aqueous systems appears comparatively easier than stripping of VOCs from water.

Modeling of air stripping from volatile organic compounds in biological treatment processes

The removal of volatile organic compounds from biological treatment processes occurs through several mechanisms. These include biodegradation, adsorption onto solids, and air stripping or volatilization to the atmosphere. Volatilization results in fugitive emissions to the atmosphere, which is largely uncontrolled. Recent regulations have called for increased evaluation and control of inadvertent volatile organic compounds emissions from treatment processes. The use oxygen as a parallel volatile compound is extremely useful for prediction of volatile organic compounds removal by air stripping. In this study, the simultaneous biodegradation and air stripping of volatile organic compounds, based on steady state mass balance are examined and a general approach to estimating the dominant removal mechanism is developed. A Monte Carlo simulation technique was used to estimate air stripping over a wide range of operating conditions. Several volatile organic compounds were selected for this study. The results showed the values drived from the model correspond with the experimental data for benzene, toluene, methylene chloride, trichloroethylene, and methyl isobutyl ketone.

Effects of System Parameters on the Physical Characteristics of Bubbles Produced through Air Sparging

Air sparging is a relatively new, cost-effective technology for the remediation of soil and groundwater contaminated with volatile organic compounds (VOCs). While the method has met with reasonable success at a large number of field sites, implementation of the technique is restricted to relatively coarse-grained soils with large values of air permeability, which significantly limits its applicability. An understanding of the fundamental parameters that control the formation and distribution of air in the sparging process is useful for optimizing the system implementation and extending its range of applicability. This work presents the results of an experimental investigation into the effect of process control parameters on the size and size distribution of air bubbles produced in aqueous and idealized saturated porous media systems. The experiments used digital image analysis to image and quantify the physical characteristics of the bubbles generated in a bench scale test cell. Results demonstrated that the average bubble size and range of size distribution increased as the injection pressure and size of the injection orifice were increased. Larger diameter bubbles with wider size distributions were produced in the presence of particles when compared to aqueous systems. As the particle size was decreased, the size of bubbles produced increased. Finally, the presence of trace quantities of the surfactant Triton X100 led to uniformly small diameter bubbles under all experimental conditions. The presence of the surfactant coating produced bubbles with physical characteristics that are more suited to in situ stripping of VOCs than the bubbles produced in the absence of a surfactant.

Design of Steam-Stripping Columns for Removal of Volatile Organic Compounds from Water Using Random and Structured Packings

Mass-transfer data at different feed and steam rates, using structured and random packings, have been collected to develop a model for the design or analysis of packed columns for stripping of volatile organic compounds (VOCs) from water. The steam-stripping experiments were conducted in a stainless steel column of 0.245-m internal diameter, with 2.8 m of packed section. The packings used were Sulzer BX gauze structured packing, Mellapak 250Y structured packing, 1-in. Flexirings, and 1-in. Fleximax random packing. The VOCs were chloroform (CH3Cl) and toluene (C7H8). The model considers the simultaneous occurrence of mass transfer and hydraulic phenomena with the same expressions for liquid and vapor mass-transfer coefficients and the same expression for effective interfacial area with one constant for each packing. The average deviation for the measured and calculated volumetric mass-transfer coefficient and effective height of packing is 0.29 and 0.23, respectively.

Volatile Organic Compound Stripping at Clarifier Weirs

Volatile organic compound (VOC) stripping at clarifier weirs was investigated for both clean water and primary wastewater using a pilot-scale model with a cross-sectional geometry similar to clarifier weirs. Drop height, type of flow regime over the weir, and weir shape were identified as important parameters influencing mass transfer. It was also observed that VOC stripping from free-fall flow could be correlated with Henry's law constant. A model that accounts for liquid- and gas-phase mass transfer was developed to predict VOC stripping from clean water. The relatively low and consistent gas- and liquid-phase mass-transfer coefficients used in the model suggest that a representative value may apply for all weir conditions. Incorporating a transition coefficient between clean water and wastewater made it possible to predict VOC transfer in primary wastewater based on clean water data. Experimental results and modeling efforts described in this paper could serve as a first step in estimating VOC emissions for flows over clarifiers at wastewater treatment plants.

Oxygen uptake at Parshall flumes

Parshall flumes are frequently used to measure flow at municipal wastewater treatment plants. With the flume causing turbulent flow, there is the potential for the emission of volatile organic compounds. To assess the extent of the emissions, laboratory and field measurements at Parshall flumes were completed, using oxygen as a surrogate. The laboratory measurements showed that the most important parameters influencing oxygen uptake were total energy loss and downstream water depth. Satisfactory results were also obtained using drop height. Field results from three municipal wastewater treatment plants showed that oxygen uptake correlated strongly with drop height and only slightly with discharge rate. This is beneficial as downstream water depth is difficult to measure in the field. Findings suggest that the use of an appropriate weir model would allow the estimation of oxygen uptake and volatile organic compounds stripping at Parshall flumes.Key words: oxygen uptake, volatile organic compounds, flumes, wastewater.

A hybrid of vapor permeation and membrane-based absorption-stripping for VOC removal and recovery from gaseous emissions

A hybrid process that combines a vapor permeation process with a recently developed membrane-based absorption and stripping process using an inert essentially nonvolatile absorbent oil is described here for a very high degree of removal of volatile organic compounds (VOCs) from vented gas streams. The combined and continuous process of this study uses the same utility, a vacuum pump, and achieves a very high level of VOC removal, for example, 99.57–99.97% from a N 2 feed stream containing 6,000 ppmv of methylene chloride. The performance is illustrated here using hollow fiber membrane modules of two kinds: microporous polypropylene hollow fibers having an ultrathin nonporous silicone coating used for vapor permeation first and then later for regenerating the VOC-loaded absorbent by VOC stripping via vacuum; micreoporous polypropylene hollow fibers without any nonporous coating for nondispersive absorption of VOCs from the waste gas stream.

Oxygen uptake and VOC emissions at enclosed sewer drop structures

Municipal wastewater is currently receiving regulatory scrutiny as an emissions source for volatile organic compounds (VOCs). In response, the fate of VOCs at drop structures has been studied. The study described herein involved two major components. First, 64 pilot experiments were conducted to study the effects of drop height, liquid flow rate, and tail water depth on oxygen transfer. Second, 15 additional experiments were completed to determine the effects of the same parameters on the stripping of a cocktail comprising 10 volatile tracers. Headspace ventilation rates, chemical properties, and the differences between clean water and raw wastewater on VOC stripping were studied. Results suggest that oxygen is a useful surrogate for VOC measurements as both oxygen and VOC mass transfer were observed to be highly dependent on drop height and less on liquid flow rate. Stripping efficiencies for VOCs were observed to vary between 0.5 and 32% depending upon specific compounds and experimental conditions. VOC stripping was also observed to be greater from clean water as compared to wastewater. Gas‐liquid mass transfer of VOCs was seen to be a strong function of Henry's law coefficient, suggesting the Occurrence of gas‐phase mass transfer resistance.

Predicting Oxygen Uptake and VOC Emissions at Enclosed Drop Structures

Drop structures used during wastewater collection and treatment are sources for volatile organic compound (VOC) emissions. To assist in the reduction of such emissions, pilot-scale experiments were completed using municipal wastewater to study the effects of drop height, liquid flow rate, and tailwater depth on oxygen transfer, and to evaluate the effects of the same parameters on the stripping of 10 VOC tracers. Results were used to develop predictive models for oxygen and VOC transfer. Oxygen uptake at the pilot drop structure suggests that the drop height is the most important parameter influencing oxygen uptake at enclosed drop structures. Tailwater depth had little effect on oxygen transfer at the drop structure. Stripping of VOCs at drop structures was seen to be a strong function of Henry's law coefficient. This sensitivity was related to gas-phase resistance in mass-transfer and/or VOC accumulation in the air bubbles. Incorporating gas-phase resistance and an appropriate a factor for wastewater into the model allowed the prediction of VOC deficit ratios and estimation of VOC stripping at drop structures for both clean water and wastewater.

A field scale evaluation of the airstripping of volatile organic compounds by surface aerators

The stripping of volatile organic compounds (VOCs) by surface aerators was evaluated in experiments performed at field scale. Each experiment consisted of dosing a tank containing tapwater with a selection of VOCs, activation of the aerator, and subsequent monitoring of the disappearance of the compounds. The tank contents were deoxygenated before the experiment thereby allowing concurrent measurement of the uptake of oxygen. Experiments were performed with two aerators with differing power inputs and aeration patterns and over a range of temperatures from 3 to 22°C. Overall mass transfer coefficients were calculated for each compound in each experiment. The impacts of temperature and aerator horsepower on the mass transfer coefficients of the candidate compounds were examined and relationships were calibrated with the experimental data. The fundamental gas and liquid mass transfer coefficients were calibrated and the ratio of gas to liquid phase mass transfer coefficient was estimated to range from 16 to 24. The estimated ratio of kg/kl was found to be relatively insensitive to variations in the Henry's Law coefficient over the range typically observed in the literature. The observed ratio was lower than that previously reported in the literature and would suggest a greater impact of gas phase resistance than previously thought. The lower value of the ratio may be attributed to mass transfer limitations caused by entrained air.

A Physically Based Model for Air‐Lift Pumping

A predictive, physically based model for pumping water from a well using air injection (air‐lift pumping) was developed for the range of flow rates that we explored in a series of laboratory experiments. The goal was to determine the air flow rate required to pump a specific flow rate of water in a given well, designed for in‐well air stripping of volatile organic compounds from an aquifer. The model was validated against original laboratory data as well as data from the literature. A laboratory air‐lift system was constructed that consisted of a 70‐foot‐long (21‐m‐long) pipe, 5.5 inches (14 cm) inside diameter, in which an air line of 1.3 inches (3.3 cm) outside diameter was placed with its bottom at different elevations above the base of the long pipe. Experiments were conducted for different levels of submergence, with water‐pumping rates ranging from 5 to 70 gallons/min (0.32–4.4 L/s), and air flow ranging from 7 to 38 standard cubic feet/min (0.2–1.1 m3 STP/min). The theoretical approach adopted in the model was based on an analysis of the system as a one‐dimensional two‐phase flow problem. The expression for the pressure gradient includes inertial energy terms, friction, and gas expansion versus elevation. Data analysis revealed that application of the usual drift‐flux model to estimate the air void fraction is not adequate for the observed flow patterns: either slug or churn flow. We propose a modified drift‐flux model that accurately predicts air‐lift pumping requirements for a range of conditions representative of in‐well air‐stripping operations.

Stripping of VOC's from dissolved air flotation

AbstractA pilot scale study was performed to assess gas phase emissions of volatile organic compounds (VOCs) from the dissolved air flotation process. A high degree of mass closure was observed in experiments using tapwater dosed with a cocktail of VOCs, indicating that techniques employed to characterize the pilot plant were valid. Subsequent dosed wastewater experiments examined volatilization from a wastewater matrix that contained oils and suspended solids as well as investigating the impacts of hydraulic loading and recycle rate on the fate of the VOCs in the dissolved air flotation unit. Emissions of the dosed candidate compounds, calculated as a percentage of the influent mass flow, ranged from 0.2% of 1,1,2,2‐tetrachloroethane to 9.9% of tetrachloroethylene. Hydraulic loading and compound type had a statistically significant effect on the emissions of VOCs, however, a high degree of interaction between parameters was observed. Effluent recycle had a greater effect on emissions at the higher hydraulic loading than at the lower loading. A model which incorporated stripping by bubbles, volatilization from a quiescent oil‐free surface and equilibrium partitioning to oil was developed. The model was able to simulate the candidate compound response under all experimental conditions except the case with low hydraulic loading and low recycle rate. The results suggest that the surface volatilization model may underestimate emissions. It is hypothesized that the presence of a float in the form of a foamy layer with a high surface area tends to increase liquid‐gas mass transfer of the candidate compounds over that assumed in the surface volatilization model.

Stripping volatile organic compounds and petroleum hydrocarbons from water

Volatile organic compounds (VOCs) and petroleum products are ubiquitous groundwater contaminants. Petroleum products, for example, diesel fuel, contain a wide array of volatile, semivolatile, and large‐chain hydrocarbon compounds. This research sought to determine whether air stripping can provide a site‐specific treatment solution for petroleum‐contaminated groundwaters and to document the abilities and limitations of tray‐type (ShallowTray™ air‐stripping technology. Full factorial experimental trials were conducted to determine the influence of inlet water flow rate and temperature on trichloroethylene (TCE), perchloroethylene (PCE), and total petroleum hydrocarbon (TPH) removal. As expected, TPH removal controlled air stripper performance, and liquid temperature affected removal more than flow rate. The mass‐transfer rate of TCE and PCE from water to air was controlled by the compound's volatility, whereas the TPH mass‐transfer rate was controlled by the compound's concentration gradient. Results indicate that economical air stripping of VOC and TPH compounds can be achieved using low liquid flow rates (20 to 75 L/min), high air/water ratios (225 to 898), and medium liquid temperatures (16°C to 28°C) in tray‐type air strippers.

A multi‐parameter analysis of volatile organic compound emissions from sewers

A wide range of parameters affect the extent of volatile organic compound (VOC) stripping from municipal and industrial sewers. These parameters can generally be classified as VOC physicochemical properties, wastewater fluid properties, wastewater flow conditions, and sewer gas flow conditions. However, because of the continuous spectrum of possible parameter combinations, previous studies related to VOC emissions from sewers have been based on highly simplifying assumptions that represent only a small subset of parameter values. A computational model was used to simulate greater than 2.5 million parameter combinations. A frequency array approach was used to facilitate a semiquantitative assessment of parameter combinations that lead to high VOC stripping efficiencies. Results were interpreted in terms of their practical significance with respect to industrial discharge control and VOC emissions reduction.