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The part that our eye can actually see (visible light, from 400-700 nm), is also where plants utilize the spectrum.
Pigment molecules, such as chlorophyll, inside the thylakoids of chloroplasts absorb light energy from the sun. This is an illustration of Photosystems II and I, embedded in the thylakoid membrane using chlorophyll a and b. When electrons leave the chlorophyll, water molecules are split (called hydrolysis) into hydrogen and oxygen atoms. Though aerobic respiration is much more efficient, making up to 36-38 total ATPs per glucose molecule, some organisms live in environments where oxygen is not abundant. Given the limited capabilities of conventional pump-and-treat systems and the large number of contaminated sites, a substantial market exists for innovative ground water cleanup technologies. This chapter evaluates the capabilities of innovative subsurface cleanup technologies and reviews why application of these technologies has been limited. In this chapter, the committee has divided innovative technologies into two categories: enhanced pump-and-treat systems and alternative technologies. For each of these technologies, the importance of thorough site characterization and field tests prior to implementation and of process monitoring after implementation cannot be overstated.
Conventional pump-and-treat systems extract relatively large volumes of water with relatively low contaminant concentrations.
The committee has divided enhancements to pump-and-treat systems into two categories: demonstrated technologies and technologies in development. The following technologies are all close to being accepted or are already accepted for site cleanups. Soil vapor extraction (SVE) is one of the few innovative technologies that has gained wide use. As shown in Figure 4-1, an SVE system usually consists of one or more extraction wells, vacuum pumps or air blowers, and a treatment system for the extracted vapors. SVE has proven effective for removing substantial quantifies of certain volatile organic contaminants from the unsaturated zone at a variety of sites in the United States and abroad.
Conceptually, contaminant removal efficiency for SVE systems depends on the physical state of the contaminant. Vapor extraction facility that treats highly concentrated gasoline vapors from a large free-product plume.
Based on this conceptual model, it is apparent that the efficiency of SVE depends strongly on contaminant and soil properties. Flushing the subsurface with air, either injected or induced, is subject to the same limitations as flushing with water. A major advantage of air flushing versus water flushing is the higher fluid flow rates possible with air—provided that the soil permeability allows sufficient volumetric flow rates. SVE appears very promising for enhancing contaminant removal from dewatered sections of the saturated zone, although the degree to which this technology can remove contaminant sources has yet to be thoroughly evaluated.
In situ bioremediation systems stimulate subsurface microorganisms, primarily bacteria, to biodegrade contaminants. In situ bioremediation near the land surface can be achieved by using infiltration galleries that allow water amended with nutrients and electron acceptors to percolate through the soil. The most common electron acceptor for the full-scale in situ bioremediation systems used today is oxygen, although in the future other electron acceptors (such as nitrate) may become more common. In situ bioremediation has been well established as a successful method for treating soil and ground water contaminated with certain types of hydrocarbons, primarily petroleum products and derivatives. Before an in situ bioremediation project is initiated, a specific microbial enhancement feasibility study and a general hydrogeologic site investigation are essential. Like conventional pump-and-treat systems, in situ bioremediation systems are limited by geologic heterogeneities such as low-permeability zones, except the problem is reversed. Mass transfer limitations that slow the dissolution of sorbed or NAPL contaminants and create problems for conventional pump-and-treat systems also interfere with in situ bioremediation. Another limitation of in situ bioremediation is the requirement for a minimum contaminant concentration to maintain the microbial population and to induce the enzymes necessary for degradation.
An additional limitation is the difficulty of delivering sufficient oxygen to the microorganisms because of oxygen's low water solubility. In situ bioremediation has four unique advantages over conventional pump-and-treat systems. As shown in Figure 4-3, the components of a bioventing system resemble those of a soil vapor extraction system, with the addition of a mechanism for nutrient delivery. Another limitation of bioventing systems is that they may cause air quality problems if large quantities of volatile contaminants are vented to the atmosphere.
As for SVE, an advantage of bioventing is the greater ease of circulating air compared to circulating water. The simplest enhancements to conventional pump-and-treat systems are pulsed and variable pumping. Like conventional pump-and-treat systems, pulsed pumping systems are effective only in zones with sufficient permeability to sustain pumping. Pulsed pumping may reduce total pumping requirements and costs, although its effectiveness is currently being debated and needs to be evaluated on a case-by-case basis.
The physical components of in situ bioremediation systems for chlorinated solvent removal are the same as those for hydrocarbon removal: these systems use pumps, wells, and injection galleries to circulate materials through the subsurface to stimulate bacterial growth (see Figure 4-2).
Laboratory- and pilot-scale studies, along with a limited number of field tests, have documented two metabolic pathways for chlorinated solvent destruction. Equipment used to deliver nutrients and electron acceptors in an in situ bioremediation system. Methanotrophs have been tested for controlling chlorinated solvents at a limited number of field research sites.
Sometimes, reductive dechlorination is incomplete, and compounds that are less chlorinated than the original contaminant but still hazardous accumulate in the system. All of the limitations that apply to in situ bioremediation of hydrocarbons also apply to in situ bioremediation of chlorinated solvents. The need for large chemical inputs to stimulate the organisms is a third limitation, applicable to both aerobic and anaerobic pathways. Construction of an infiltration trench for delivering fluids in an in situ bioremediation system. An additional limitation is that if appropriate microorganisms for carrying out the desired contaminant transformation are not present, then introduction of a specific population may be required.
As for bioremediation of hydrocarbons, the extent to which this process can lower concentrations of chlorinated solvents is limited by the requirement for a minimum concentration threshold to stimulate microbial activity. A final potential limitation of anaerobic reductive dechlorination is the time required for the metabolic reactions to proceed.
Like in situ bioremediation of hydrocarbons, in situ bioremediation of chlorinated solvents is advantageous because it has the potential to completely convert the contaminants to innocuous products, instead of simply extracting them for disposal elsewhere. The physical systems used to promote in situ bioremediation of metals are like those for bioremediation of hydrocarbons and chlorinated solvents (see Figure 4-2).
As early as the beginning of the century, work has been ongoing to develop a liquid phase, regenerative process for converting hydrogen sulfide (H2S) into pure, elemental sulfur. In the late 1960’s the CIP process, which employed an aqueous solution of chelated iron, was introduced in the United Kingdom. Throughout the last half of the century, liquid phase oxidation has played an important role in the recovery of sulfur from various sources of hydrogen sulfide.
Liquid phase oxidation processes for the removal of hydrogen sulfide (H2S) from gas streams were initially developed to correct certain problems associated with dry oxidation processes such as iron sponge.
Most early development work was done for systems processing coal gas or town gas with the objective of removing both hydrogen sulfide and ammonia by the formation of ammonium sulfate and elemental sulfur. After the failure of the polythionate processes, development shifted towards utilizing suspensions of iron oxide in aqueous solutions, which in essence was an attempt at a continuous iron sponge process. During the 1920’s the Thylox and Giammarco-Vetrocoke processes met with some commercial success.
Another group of processes which showed technical promise but were limited by toxicity problems were those employing iron cyanide solutions.
A summary of the various liquid phase oxidation processes, which have been developed throughout the years, is contained in Table I. The first liquid phase, oxidation process, which gained widespread commercial acceptance, was the Stretford process. These problems were corrected to a certain extent by the addition of alkali vanadates to the solution, which, in essence, replaced dissolved oxygen as the oxidant in the conversion of hydrosulfide ions (HS-) to elemental sulfur. Although the addition of vanadium to the Stretford Process increases the reaction rate of hydrosulfide ions to sulfur sufficiently to make the process commercial, it still produces a significant amount of byproduct thiosulfate. Iron is an excellent oxidizing agent for the conversion of H2S to elemental sulfur; however, due to the very low solubility of iron in aqueous solutions, the iron had to be present in the dry state (iron sponge) or in suspensions (the Ferrox process) or compounded with toxic materials such as cyanides.
In this process, iron, in its’ ferric state (+3), is held in solution by a chelating agent, namely ethylenediaminetetraacetic acid (EDTA). Equations 1 and 2 represents the absorption of H2S into the aqueous, chelated iron solution and its subsequent ionization, while equation 3 represents the oxidation of sulfide ions to elemental sulfur and the accompanying reduction of the ferric iron to the ferrous state. It is interesting to note that the chelating agents do not appear in the process chemistry, and in the overall chemical reaction, the iron cancels out. Although it appears that chelated iron would solve many problems associated with previous liquid oxidation processes, the CIP Process failed miserably. In the early 1970’s, a small company in the Chicago area (ARI Technologies) started development work on a process, which employed multiple chelating agents. Research found that the chelating agents were being oxidized to useless byproducts by a free radical mechanism.
Iron-based, liquid oxidation has developed into a very versatile processing scheme for treating gas streams containing moderate amounts of H2S. The three most common processing schemes encountered in iron-based, liquid oxidation systems are illustrated in Fig.
Although references are continuously made that iron-based, liquid redox systems are plagued with plugging and foaming problems and that the process cannot be operated at high pressure due to pump and foaming problems, these problems for the most part have been solved for some time.
Foaming occurrences are either a start-up phenomenon or the result of large amounts of liquid hydrocarbons entering the unit.
Continuous incursions of small amounts of liquid hydrocarbons are frequently experienced with no adverse effect on the operation of a unit; however, the introduction of large amounts of liquid hydrocarbons can present foaming and plugging problems. Operation of aqueous-based liquid redox systems at high pressure has been a problem due to difficulties with keeping the liquid circulation pumps running. Although iron-based, liquid redox processes have gained acceptance as evidenced by over 150 units being licensed worldwide, there are still areas in the process, which need to be improved upon. Operating costs for aqueous, iron-based redox systems are composed of replacing chemicals which are either oxidized in the unit or which are physically lost from the unit and of electrical power required for circulating solution and injecting air.
Besides replacing iron, which is physically lost from the system with the sulfur and blowdown streams, chelating agents are chemically oxidized into useless, non-toxic byproducts within the system and must be replaced. A large portion of the electrical consumption in an iron-based, liquid oxidation system is associated with blowing air through the solution to satisfy the oxygen demand of equation 5.
A considerable amount of research is currently underway to develop new mass transfer devices which will improve the oxygen utilization in liquid oxidation systems with the aim of reducing the quantity of air required (operating cost) and reducing the size of the oxidizing vessels (capital cost).
Neither the low or high head oxidizers are very good mass transfer devices; however, they generally do not plug with sulfur and they do supply solution inventory required for proper operation of the system.
Sulfur produced from liquid redox systems has the same chemical assay as Claus sulfur, and it does have several commercial uses2 in its unmelted form.
Liquid phase oxidation systems have undergone considerable evolution during the 20th century, and this will continue into the 21st century. All of the Cross section profile graphs showing oxygen, CO2 and temperature, are from passively aerated windrows.
Visit B&N to buy and rent textbooks, and check out our award-winning tablets and ereaders, including Samsung Galaxy Tab 4 NOOK and NOOK GlowLight. Organic molecules, like glucose, can then be stored as food to be broken down by cellular respiration when energy is needed. This causes the electrons inside the chlorophyll molecules to become excited to higher energy states. Also, some eukaryotic cells can be in an environment where oxygen is absent or in short supply.
However, use of innovative technologies has not been as extensive as might be expected, considering the potential size of the market.
Included in addition to reviews of technologies that treat ground water below the water table are reviews of technologies that treat soils above the water table, because ground water cleanup cannot be achieved if contaminants from the overlying soil continue to migrate downward. However, the increasing use of newer technologies applies mainly to soil above the water table.
Enhanced pump-and-treat systems all involve, to some extent, the pumping of fluids such as water, water solutions, or air and thus will face some of the same difficulties as conventional pump-and-treat systems; the advantage of these enhanced systems is their potential to significantly increase the rate at which contaminant mass can be removed from the subsurface. Because of the lack of performance data for most of the technologies reviewed here, the uncertainty associated with these methods is proportionately greater than the uncertainty associated with conventional pump-and-treat systems. Because of geologic complexity and slow rates of contaminant desorption and dissolution, these systems must displace many pore volumes of aquifer water to flush out contaminants, as explained in Chapter 3. Some of these technologies reduce the ultimate burden on the pump-and-treat system by removing from the soil contaminants that would otherwise migrate to the ground water or by removing volatile contaminants from the soil and ground water. They have been tested in laboratory-scale batch and column studies, in controlled field experiments, and in large-scale site trials.
The technology extracts organic contaminants (primarily from the unsaturated zone) by flushing with air. Numerous Records of Decision at Superfund sites have specified SVE as the technology of choice for unsaturated zone cleanup.

When present as a NAPL, contaminants will transfer from the pure liquid phase to the air via evaporation.
Finally, if the contaminants are dissolved in water in the soil pores, mass transfer must occur through the water-air interface.
Contaminant properties include vapor pressure, Henry's Law constant, hydrophobicity (usually quantified with the octanol-water partition coefficient), and diffusion characteristics. Principal design variables include the number of extraction wells, the rate of air flow (level of vacuum applied or rate of air injection), and the depth and length of the screened zone. Most of these systems addressed contaminant removal from the unsaturated zone rather than from dewatered portions of the saturated zone. The air stream is unlikely to flush zones of low permeability, which can contain significant quantities of contamination. Large numbers of pore volumes of air can be flushed through the subsurface in a short time, which permits recovery of a significant mass of released contaminants. It is probable that SVE will be more successful at sites with light NAPLs (LNAPLs) than at sites with dense NAPLs (DNAPLs) because LNAPLs tend to remain above the water table, where they are more accessible, whereas DNAPLs tend to sink.
When given the proper stimuli, microorganisms can transform the contaminants to innocuous mineral end products, such as carbon dioxide and water.
As shown in Figure 4-2, some in situ bioremediation systems use extraction and injection wells in combination to control the flow of electron acceptors and nutrients and to hydraulically isolate the contaminated area.
In situ bioremediation systems typically supply oxygen by bubbling air or pure oxygen into the injection water or by dosing the water with hydrogen peroxide. In situ bioremediation was first successfully demonstrated for cleaning up subsurface petroleum hydrocarbons at a Sun Oil pipeline leak in Ambler, Pennsylvania, in 1972 (Lee and Ward, 1985). The microbial study will help determine the types and amounts of substances required to stimulate optimum contaminant degradation. For pump-and-treat systems, geologic heterogeneities limit the ability to extract contaminants, whereas for in situ bioremediation, geologic heterogeneities interfere with the ability to inject the necessary electron acceptors and nutrients. Microorganisms with the metabolic capability to degrade a contaminant will not do so if the contaminant is unavailable to the cell because it is contained in a NAPL or sorbed to subsurface particles.
The existence of such a concentration threshold means that, theoretically, there is a minimum concentration below which no further bioremediation will occur.
Injecting air directly into the ground water, rather than applying it in dissolved form in the nutrient-amended water, is one approach used to improve oxygen delivery.
First, while pump-and-treat systems extract contaminants to the surface for disposal or treatment elsewhere, in situ bioremediation treats contaminants in place and can convert them to innocuous products (such as carbon dioxide and water).
Like soil vapor extraction, bioventing involves inducing air movement through the unsaturated soil. The technology is particularly useful in cases where excavation of the site is impractical, such as under buildings, where underground utilities are present, or where the contaminated soils are deep. Soil permeability to air is two to three orders of magnitude greater than its permeability to water (Wilson and Ward, 1987). They have been tested in the laboratory, in controlled field experiments, and in some cases at a limited number of sites.
These methods intermittently slow or stop pumping to allow the contaminant concentration to build up, with the goal of increasing the mass of contaminant removed per unit volume of water pumped. In conventional systems, the subsurface flow rates induced by pumping may be too rapid to allow enough time for sorbed or trapped chemicals to enter the bulk ground water for extraction. The key design criteria are the duration of the cycle of maximum pumping and the duration of the cycle during which the pumping rate will be slowed or stopped.
However, the metabolic processes for chlorinated solvent degradation are more complex than those for hydrocarbon degradation.
Transformation by methanotrophs does not appear effective for compounds such as carbon tetrachloride and perchloroethylene that are fully substituted with chlorine atoms. In the process of consuming the methane, the bacteria produce an enzyme, methane-monooxygenase, that incidentally transforms the chlorinated compound.
Commonly observed intermediates include vinyl chloride, chloroform, and various isomers of dichloroethene and dichloroethane. In situ bioremediation of chlorinated solvents also has limitations that are not factors in hydrocarbon bioremediation. However, introducing new microorganisms to the contaminated zone is difficult because the subsurface is an efficient filter medium that generally restricts microbial transport. Aerobic cometabolism of chlorinated solvents by methanotrophs is rapid, with half-lives ranging from hours to days (Semprini et al., 1990). In addition, the pumping rates for delivering growth-stimulating materials to promote bioremediation are lower than pumping rates for contaminant extraction. However, while bacteria can destroy hydrocarbons and chlorinated solvents, they can change the form of metals but cannot destroy them. Anaerobic microorganisms can affect metal dissolution and precipitation by one or more of the following mechanisms: (1) direct enzymatic reduction of the metal, (2) biochemical alteration of conditions that influence the oxidation state of the metal, (3) excretion of microbial metabolites or decomposition products that can chelate or sequester the metal, and (4) bioaccumulation and release of metals elsewhere in the subsurface. This work has lead to the introduction of over 25 different processes, most of which with very little commercial success. The process failed miserably; however, its failure did lead to the successful introduction of the LO-CAT® Process in the late 1970’s, which solved many of the problems encountered with the CIP Process and the Stretford Process. Of all the processes available for converting H2S to sulfur, current liquid phase oxidation systems are the most versatile.
The problems being mainly large plot requirements, replacement of the oxidation media on a frequent basis, and safety problems. The hydrogen sulfide reacts with an alkaline compound to form hydrosulfide, which reacts with iron oxide to form iron sulfide, which in turn reacts with oxygen to form iron oxide and sulfur. However, both of these processes employed thioarsenate solutions, which resulted in toxicity problems caused by the arsenic. The process was developed by the North Western Gas Board and the Clayton Aniline Company in England to remove H2S from town gas.
The reaction is still slow enough that air streams cannot be treated due to the high rate of thiosulfate formation. In the 1960’s development work was begun in England to increase the solubility of elemental iron in aqueous solutions.
The intent of the process was to oxidize sulfide (S=) and hydrosulfide (HS-) ions to elemental sulfur by the reduction of the ferric (Fe+3) iron to ferrous (Fe+2) iron, and the subsequent reoxidation of the ferrous ions to ferric ions by contact with air.
Equations 4 and 5 represents the absorption of oxygen into the aqueous solution followed by oxidation of the ferrous iron back to the ferric state.
This is in contrast to the oxidation reactions in the Stretford process when using vanadium. So the obvious question is why is chelated iron required at all, if it doesn’t part take in the overall reaction. In very short order after starting up, all of the iron precipitated out of solution as iron sulfide, FeS. The idea being that by employing chelating agents with overlapping pH ranges where the chelation strengths were high, the iron would stay in solution at all times. After a few years of experimentation, the chelate oxidation rate was reduced to an acceptable level by the introduction of free radical scavengers and by switching to chelating agents, which were much more resistant to oxidation. Advantages of these systems include the ability to treat both aerobic and non-aerobic gas streams, removal efficiencies in excess of 99.9%, essentially 100% turndown on H2S concentration and quantity, and the production of innocuous products and byproducts. 4) is the aerobic unit (air contaminated with H2S) in which equations 1 through 5 all occur within the same vessel, at the same time and without separation of the absorber and the oxidizer. During the initial start-up of a unit, the surface tension properties of the fresh solution are such that the foaming may occur during the first few days of operation. This would also be true of Claus units, selective oxidation processes and hydrocarbon-based, redox systems. Packing plugged, static mixers plugged, pipes plugged, heat exchangers plugged and distributors plugged. Current areas of R&D efforts are reduction in operating costs, reduction in equipment size and improvement in molten sulfur color.
For any iron-based system there is an economic tradeoff between iron concentration and the solution circulation rate.
As stated previously, different chelating agents have different resistances to chemical oxidation.
Due to the low solubility of oxygen in water, a large excess of air is generally employed depending on oxidizer design. A main goal of current research is to develop a mass transfer device, which will reduce the amount of air required to approximately stoichiometric quantities, will reduce the oxidizer volume and will not plug with sulfur. In fact Lubrizol in France has been recycling its produced sulfur with no adverse effect for quite some time.
Foreseeable developments for the near future will be smaller equipment sizes and lower operating costs which will be achieved by the development of better oxygen mass transfer devices reducing the amount of air required and the size of oxidizers and by the addition of free radical scavengers into the systems. Then, the electrons move from one molecule to another inside the thylakoids, and energy is released with each move.
Glycolysis (in cytosol)The first stage of aerobic respiration is glycolysis, which takes place in the cytosol of cells. For example, while conventional pump-and-treat systems were selected for use at 73 percent of Superfund sites with ground water contamination through fiscal year 1992, at the remaining 27 percent of sites the most common "remedies" were not innovative technologies but nontreatment measures such as providing alternative water supplies, aquifer use restrictions, and wellhead treatment (Kelly, 1994; K. The most striking example of this desired technical evolution is the increased use of soil vapor extraction systems, which have now become a leading cleanup technology for soil (Kovalick, 1993).
Thorough characterization of the site's geologic and chemical characteristics, field tests of the remediation method, and continual monitoring of the full-scale system are all essential steps for minimizing uncertainties. Conventional pump-and-treat systems thus are inherently inefficient for removing contaminants from the subsurface.
Other innovative technologies improve the efficiency of contaminant extraction by increasing the amount of contaminant removed with each volume of pumped water. The technique has also been extensively used for cleanups at gas stations and other sites where large quantities of volatile organic compounds have leaked from underground storage tanks. The rate of extraction thus depends in part on the efficiency of each of these molecular-scale mass transfer processes. Soil properties include stratigraphy (for example, size distribution, permeability, and porosity), organic carbon content, mineralogy, and moisture. Especially important to consider is the vapor flow path relative to the contaminant location. Typically, if the soil's permeability to air is less than 1 darcy (10-16 m2), flow rates may be too low to achieve successful removal in reasonable time frames. In addition, SVE must overcome mass transfer limitations that inhibit the desorption of strongly adsorbed contaminants or contaminants that have penetrated the microstructure of the aquifer materials. Whether this increased flushing is sufficient to remove contaminants to acceptable levels is highly site specific. As explained in Chapter 2, the necessary stimuli for microbial growth in aquifers are oxygen or other electron acceptors (such as nitrate or sulfate) and nutrients (such as nitrogen and phosphorus). Alternately, they may supply oxygen by injecting air directly into the ground water, with nutrients added through injection wells or infiltration galleries. Since then, the technique has been used to clean up subsurface spills of refinery wastes, crude oil, and fuels. Slow dissolution from NAPLs and slow desorption from soils decrease the biodegradation rate, thereby increasing the cleanup time and the amount of chemicals that must be added to sustain microbial activity. For example, concentrations within a NAPL pool are likely to be toxic and restrict bioremediation to the periphery of the NAPL zone. As a result, in situ bioremediation reduces the requirement for surface treatment and disposal of the recovered water and minimizes the contaminant exposure hazard. However, the main purpose of bioventing is not to extract volatile contaminants but to enhance aerobic biodegradation of contaminants by supplying oxygen to soil microbes. Since they are designed to promote biodegradation rather than physical removal of vapors, air recovery wells are located at the periphery of the contaminated area, and air flow rates are kept at the minimum rate required to deliver oxygen.
Because bioventing requires air flow, it is more easily applied to permeable soils such as sand than to clays.
The added liquid affects soil moisture content and, consequently, may inhibit air movement. Variable pumping differs from pulsed pump-Lug in that the pumping rate cannot be diminished to zero because hydraulic control of the plume is required.
Pulsed and variable pumping allow more contact time between the moving water and residual contaminants that are sorbed, contained in NAPLs, or trapped in low-permeability zones. Therefore, in situ bioremediation of chlorinated solvents often requires circulation not only of electron acceptors and elemental nutrients, but also of other growth-stimulating materials specific to the metabolic process by which the contaminants are degraded. Therefore, stimulating methanotrophic bacteria to transform chlorinated solvents requires adding methane to the site, in addition to oxygen and nutrients. In reductive dechlorination, the chlorinated compound becomes an electron acceptor, and microbially catalyzed reactions replace a chlorine atom on the compound with a hydrogen atom. Probably the major obstacle to using anaerobic processes is the possibility that hazardous intermediates will accumulate.
Delivering these large quantities of chemicals to the proper locations is difficult at geologically complex sites and sites where the locations of contaminants are unknown. However, anaerobic reductive dechlorination rates appear to be slow, with half-lives on the order of weeks to months in acclimated laboratory systems (Bouwer and Wright, 1988). Finally, while in situ bioremediation using reductive dechlorination may be slow, in situ bioremediation using methanotrophs is relatively rapid. At metal-contaminated sites, it may be possible to stimulate anaerobic microbial activity and control these mechanisms to influence the state of the metals. However, in the late 1940’s, the North Western Gas Board and the Clayton Aniline Company developed the Stretford Process, which utilized an aqueous solution of vanadium and anthraquinone (ADA).

In the late 1980’s, another chelated iron process, the Sulferox Process, was introduced; however, the developers of the LO-CAT and Sulferox processes have recently combined efforts to improve liquid redox processing even further. They are able to treat any type of gas stream containing H2S, at a wide variety of operating conditions and all at removal efficiencies exceeding 99%. This development work led to the utilization of various oxygen carriers dissolved or suspended in a liquid phase, which could be regenerated continuously at ambient temperatures. Development work in this area lead to the introduction of the Burkheiser, Ferrox and Manchester processes. To reoxidize the Va+4 ions back to the +5 State, ADA is added as an oxygen carrier, and the ADA is subsequently regenerated with air. This work led to the introduction of the CIP process, CIP being an acronym for "Chelated Iron Process".
Consequently, iron-based systems generally produce relatively small amounts of byproduct thiosulfate ions, and in properly designed units, air streams can actually be processed.
The problem was that the chelation strength of many chelating agents varies considerably with solution pH, and unfortunately, the pH’s experienced in the CIP process were outside the range of EDTA. These are generally less expensive units than the other two schemes; however, because there is always oxygen in the presence of sulfide ions, consequently, these units produce the most byproducts.
However, for aqueous-based redox systems, "Designer" surfactants1 have been developed, which in essence totally alleviates the problems caused by the introduction of large amounts of liquid hydrocarbons.
The logic being that closed-impeller pumps would plug with sulfur particles or possibly erode.
Since 2 moles of iron are required for every mole of hydrogen sulfide (equation 3), the amount of circulating solution required is dependent on the iron concentration in the solution and the amount of H2S in the sour gas stream — the higher the iron concentration, the lower the circulation rate and hence, the lower the power consumption. In addition, chemicals may be added to the system or made in the unit, which act as free radical scavengers, thus retarding chelate oxidation. These oxidizers provide mass transfer coefficients which are approximately 4 times better than the low head oxidizers; however, this is at the expense of higher discharge heads on the air blowers. However, there is always a desire to improve the appearance (color) of redox sulfur, which is degraded due to the presence of iron polysulfides. Glycolysis involves the conversion of glucose (C6H12O6) into two 3-carbon molecules of pyruvate. For cleaning up petroleum hydrocarbons, in situ bioremediation is also becoming increasingly common. Another group of innovative technologies pumps minimal amounts of fluids to stimulate treatment of contaminants in place, either biologically or chemically, rather than requiring contaminant extraction and surface treatment.
Although SVE also can remove contaminants from dewatered portions of the saturated zone, in which the water table has been purposely lowered through pumping, its use for this purpose has not been as extensive as for unsaturated zone treatment. If the air stream bypasses zones of low permeability, the slow process of diffusion will dominate, making contaminant removal extremely slow.
Because of the complex interrelationships among all the factors that influence SVE, the effectiveness of SVE should be evaluated carefully on a site-by-site basis. Unfortunately, the efficiency of the systems in terms of contaminant recovery was not reported. Thus, all of the factors that inhibit release of contaminants during traditional pumping and treating also limit the performance of air flushing systems. Typical in situ bioremediation systems therefore perfuse electron acceptors and nutrients through the contaminated region, as shown in Figure 4-2.
It has also been used to treat other easily biodegraded organic contaminants such as phenols, cresols, acetone, and cellulosic wastes. These parameters affect nutrient and electron acceptor availability, which may be hindered by sorption to the soils or reactions with naturally occurring subsurface chemicals. Since there is currently no scientific consensus on what factors affect bioavailability or how bioavailability ultimately affects bioremediation, contaminant bioavailability must be considered on both a site- and a compound-specific basis.
Fortunately, the soluble concentrations of hydrocarbons normally observed at field sites are well below the toxic range. Second, pumping requirements are likely to be lower for in situ bioremediation than for conventional pump-and-treat systems. Air flow requirements are therefore much lower for bioventing systems than for soil vapor extraction systems.
The change in soil moisture can also affect the load-bearing capacity of the soil—an important consideration when treating soil under or near a building.
It is therefore possible to move relatively large amounts of oxygen with a bioventing system, even through soils with moderate to low permeabilities. More controlled field studies and large-scale site trials are necessary to generate reliable performance data. The increased contact time permits the system to approach chemical equilibrium, increasing desorption, dissolution, and diffusion. Pulsed pumping can work in heterogeneous, less permeable rocks, but with far longer projected cleanup times. For reductive dechlorination to proceed, an electron donor, such as a low-molecular-weight organic compound (lactate, acetate, methanol, glucose, or toluene) or hydrogen (H2), must be available to provide reducing equivalents.
In addition, injecting large quantities of electron donors results in the buildup of large amounts of end products such as carbon dioxide, methane, and biomass—much more than in bioremediation of hydrocarbons under aerobic conditions. Consequently, it may be possible for optimized biotransformation systems to meet relevant regulatory end points. Extrapolation of optimal rates presently observed in the laboratory suggests cleanup times of years in the field. Although the Stretford process had some serious process, operational and environmental problems, the process filled a much-needed niche and became fairly popular throughout the 50’s, 60’s and 70’s. This paper will discuss the major liquid oxidation processes, describing in detail their advantages and disadvantages. However, as in the Stretford process, equations 1 and 4 are relatively slow and are the rate controlling steps in all chelated iron processes.
The process worked well for units processing small quantities of H2S; however, in the first unit processing tons per day of sulfur, the process, in essence, failed.
2 shows a "Conventional" unit, which is employed for processing gas streams, which are either combustible or cannot be contaminated with air such as carbon dioxide, which is being treated for beverage purposes. In addition, this foaming tendency is only experienced when the entire unit is filled with fresh solution, which only happens during the initial start-up of the unit. Vessels with random packing are no longer used, on-line cleaning procedures have been developed for static mixers1, which require very little operator attention, proper pipe design has eliminated pipe plugging, proprietary heat exchanger designs and proper operating procedures have minimized heat exchanger plugging, newly designed absorber spargers are being installed, which have greatly extended the life of sour gas spargers and improved quality control of oxidizer sparger materials and proper operation of the process has minimized oxidizer sparger plugging.
Consequently, for high head applications in which open-impeller pumps would not apply, plunger type pumps were chosen. Conversely, the higher the iron concentration, the higher the catalyst makeup rate required to replace iron from physical losses such as solution lost with sulfur withdrawal.
Research continues on the development of oxidation resistant chelates and on economical, free radical scavengers. Initial results indicate that the first two objectives of the research — stoichiometric air and high mass transfer coefficients — can be obtained.
Because only 2 ATPs are made for each initial glucose molecule, fermentation is much less efficient than aerobic respiration. ATP made from aerobic breakdown of glycogen.The last two sources of ATP have different characteristics. Furthermore, technologies that treat ground water in place rather than extracting it were specified as remedies at fewer than 2 percent of Superfund sites (Kelly, 1994). Despite the increasing use of these two technologies, application of innovative technologies for cleaning up ground water remains rare.
All of these technologies have in common the requirement to pump fluids through the subsurface, meaning that to varying degrees the geologic and chemical conditions that impose limitations on conventional pump-and-treat systems also present problems for these innovations. Typically, concentrations of volatile compounds in the extracted air stream decreased rapidly with time and approached asymptotic values similar to those seen in ground water pump-and-treat systems. Although in situ bioremediation of other types of or-garlic contaminants, such as chlorinated solvents, is possible, the technology has not yet been demonstrated for these other applications. Although biodegradation rates improve with high moisture levels, high soil moisture inhibits air movement. In addition, flushing nutrients through the soil may transport contaminants from the unsaturated into the saturated zone. Consequently, in situ bioremediation using reductive dechlorination requires the addition of an electron donor, in addition to nutrients. Consequently, complete detoxification of chlorinated solvents appears possible under certain anaerobic conditions.
In addition, the current R&D efforts in the field of liquid phase oxidation will be discussed with a glance of what future developments may occur. The solution had a very low capacity for dissolved sulfides resulting in large liquid circulation rates and hence, high power consumption.
It was found that the chelating agents were disappearing very rapidly requiring extremely high chemical makeup rates.
In this scheme, equations 1 through 3 are performed in the Absorber while equations 4 and 5 are performed in the oxidizer. The plunger pumps had no difficulty supplying the required head; however, seal rings had extremely short lives.
There is an optimum iron concentration based on the incremental cost of power and the amount of solution, which is normally lost from the system.
For example, many compounds from the polyamine family have proven to be excellent free radical scavengers reducing chelate degradation to essentially zero. These oxidizers are relatively poor mass transfer devices; however, they do provide much need solution inventory for proper operation of the system. Long term testing is currently being carried out to determine the plugging tendencies of the membranes. However, due to the extremely low price of sulfur and due to the relatively low quantities of sulfur produced in liquid redox plants, it is difficult justifying much work in this area. Anaerobic breakdown of glycogen is very fast (maximal power output in about 5 seconds) because few steps are involved and it doesn't need oxygen from blood.
Researchers have investigated the possibility of using gaseous ammonia as a nitrogen source to eliminate these problems, but this method has not been very successful. These metals cause taste and odor problems and stain pipes, bathroom and kitchen fixtures, and clothes. Biomass growth is likely to fill up the pore space, markedly reducing the formation's permeability. In addition, the sulfur formation reaction was very slow requiring large liquid inventories and resulting in high byproduct formation (thiosulfates). To solve this problem, a multi-staged, closed-impeller, centrifugal pump was installed in one high pressure application with excellent results. Unfortunately, the oxidation rates of the polyamines are extremely high and consequently, uneconomical. If this last phase of testing is successful, liquid oxidation systems will become much smaller and less expensive to operate. However, it is inefficient in terms of ATP yield, it creates lactic acid, which accumulates and eventually causes muscle fatigue.
Due to the apparent lack of detailed field investigations, a detailed assessment of SVE performance under controlled field conditions is needed.
In addition, anaerobic organisms excrete metabolites that increase the concentration of organic matter. 3 illustrates an "Autocirculation" unit, which is used for processing acid gas (CO2 and H2S) streams or for other non-combustible streams, which can be contaminated with air. The pump was in continuos operation for approximately1_ years without any signs of plugging or erosion. Also new chelating agents have been developed which have very high resistances to oxidation; however, they currently are uneconomical to manufacture.
The organic matter may react with disinfectants used for drinking water purification to form hazardous byproducts such as trihalomethanes. All the chelating agents do is to increase the solubility of iron in water, thus reducing the circulation rates required to furnish the two moles of iron required in equation 3.
In this scheme, equations 1 through 3 are performed in the "Centerwell" which is nothing more than a piece of pipe open on each end.
For all future high-pressure applications, closed-impeller single or multi-stage centrifugal pumps will be specified. The microbial metabolites may also dissolve cadmium, copper, lead, and zinc oxides, facilitating their passage into the drinking water distribution systems (Francis and Dodge, 1988). As electrons move down the chain, they give up energy, and thirty-two molecules of ATP are formed. The volume within the centerwell is essentially the same as the absorber in a conventional unit.
The other unique feature of the Autocirculation scheme is that no pumps are required to circulate solution between the centerwell (absorber) and the oxidizer. In these units there is a larger volume of air than acid gas; consequently, the aerated density on the outside of the centerwell is less than on the inside resulting in a natural circulation from the oxidizer into the centerwell.

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Author: admin | 09.08.2015

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