Makers of OilSorb™ and Other State-of-the-Art Filtration Media
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Makers of OilSorb™ and Other State-of-the-Art Filtration Media
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Put the Breaks on Wastewater Emulsions FEATURE REPORT George R. Alther Emulsions in wastewater pose a vexing problem for facilities attempting to recycle water and stay in compliance with permissible discharge limits. But the challenges are no less formidable for routine maintenance. The removal of emulsions, a major constituent of which are fats, oils and greases (FOGs), is necessary to prevent them from depositing on pipes and fouling filtration media. Not even facilities that use crossflow membranes are exempt from the problems caused by emulsions. While fouling may be eliminated, the concentrate that accumulates during processing remains to be dealt with. The concentrate coats evaporator heating elements, requiring frequent cleanup and disposal. Cost factors weigh in, with charges for hauling water ranging from 15¢ to more than $1 per gallon. As permissible discharge limits are lowered, facilities may find it increasingly difficult to stay in compliance. Indeed, a facility that is permitted to discharge 50 ppm of organic waste may believe it is in a safe range if the content of its water is only 30 ppm of oil. To the contrary, that oil may be putting a severe strain on the facility's discharge permit for total chemical oxygen demand (COD), which includes any compound that can be oxidized, but may be more difficult to remove. And for facilities attempting to recycle water in a closed-loop process, FOGs simply cannot be ignored. Some of the havoc caused by emulsions can be avoided if emulsions are broken and removed from wastewater streams. Successful emulsion breaking requires a basic understanding of emulsions, their chemical composition, and the technologies required to remove them from water. Chemical engineers must ask what types of emulsions are involved and then decide how to get rid of them. Those who choose not to take this methodical approach, may be lucky and select the proper removal method. Otherwise, they will be stumped from the beginning. EMULSION BASICS Whenever two immiscible liquids, such as oil and water, contact each other, one liquid tends to disperse, but not dissolve, in the other. This dispersion of liquid, typically in an aqueous medium, is an emulsion. Few emulsion droplets are smaller than 0.25 pm in diameter, but larger ones can be more than 100, times greater in size (Table 1). The two main categories of emulsions are oil in water (OW) and water in oil (WO), with water including the most highly polar, hydrophilic (water-loving) liquids. Hydrophobic (water-hating) non-polar liquids are considered "oils." Of concern for wastewater engineers is the OW phase, which is divided into three subgroups: less than 30% oil, 30-74% oil and 74% or more oil [1]. Emulsions consist of three phases: The internal, or discontinuous, phase consists of finely divided droplets. The external, or continuous, phase is the matrix that keeps droplets, in suspension. The interphase consists of an emulsifier, or stabilizer, that keeps the emulsion stable, binding the internal and external phases together, and preventing droplets from approaching each other and coalescing [2]. Usually emulsifiers are surfactants and soaps, present either by themselves or as part of the makeup of a detergent formulation. An emulsifier consists of a molecule with hydrophilic and hydrophobic ends. In the presence of immiscible liquids, the emulsifier migrates to the interface of the internal and external phases, forming a protective sheath around droplets of the dispersed phase, as shown in Figure 1. While the hydrophobic end of the molecule migrates, or partitions, into droplets, the hydrophilic end stays in the water [1, 2]. In effect, the emulsifier acts as a coupling agent, lowering the interfacial tension of the internal and external phases. When the interfacial tension is reduced to zero, an emulsion forms spontaneously. This means the surface area of the internal phase has reached its maximum. The dispersion of fine droplets, generally less than 1 μm dia., gives the emulsion a milky appearance. This effect can be achieved mechanically with colloid mills, centrifugal pumps and Waring-type blenders [1]. At equilibrium, the particle size of an emulsion's internal phase depends on the amount of emulsifier available to maintain that equilibrium. Hence, the concentration of an emulsifier must be balanced by its droplet size to keep particles from coalescing. The smaller the droplets, the more emulsifier required to cover the larger surface area. In other words, the concentration of emulsifier determines the amount of stabilizer absorbed at the interface [2]. An important factor in emulsion stability is the diameter of the dispersed droplets, which correlates to the volume of the dispersed phase and the interfacial area available. For example, if an oil is added to a container partially filled with water, the oil, upon impact, will coalesce and float as sheen on the water's surface. The oil forms a second layer or phase because its specific gravity is lower than that of water. If water is then blasted into the container, the physical impact of the water on the oil will cause some droplets to emulsify, or disperse, in the water. When the blast ceases, oil droplets will coalesce quickly. A surfactant or emulsifier added to the container will disperse the oil into the water. The system can be imagined as a collection of small spheres dispersed in the continuous water phase. Unless the droplets are small enough to be kept in suspension by thermal forces, they will eventually settle out or rise to the surface and form a layer of droplets in a process called creaming. Left idle, oil droplets will collide and coalesce, unless enough emulsifier is added to cover the entire interfacial area. Over a period of time, larger particles will rise to the surface and coalesce into a single layer [1]. The creaming of an emulsion, or how fast an oil droplet will rise to the water's surface, is governed by Stoke's Law, which states: u = Gr2(d1-d2)/9v where: u = the rate of sedimentation of a spherical particle G = acceleration of gravity r = radius of the particle d1= density of liquid in the internal phase d2 = density of liquid in the external phase v = viscosity of the emulsion Large droplets rise, or fall, faster than small ones, and drops move faster in a low-viscosity liquid. Therefore, if d1 is less than d2, the particle will rise, but if d2 is larger than d1, it will fall. The settling velocity varies as the square of the particle diameter. Since oil's density is usually less than that of water, upward sedimentation, or creaming, will occur [1]. Flocculation, or aggregation, is a second process that takes place in emulsions with low-internal-phase ratios, as typically found in wastewater. Particles slide together without coalescing to form clumps, or chains of clumps, of larger effective size. The settling rate then increases, even though the particles do not behave like spheres, as Stoke's Law demands [2]. Emulsifier solubility can be enhanced, if necessary, by adding a co-solvent or co-emulsifier, such as propylene glycol, that will act as a coupler to assure stability at all temperatures. A blend of several surfactants can be added to form a tightly packed film around the oil droplets. The presence of finely divided solids, such as clays, can also act as emulsifiers. The oil droplets then coat these solids, resulting in an emulsion or settling. Electrical charge Across the interface of all solids and liquids is an electrokinetic gradient, called the Zeta potential, that is largely responsible for colloidal stability. Discharge of the Zeta potential, accompanied by precipitation of colloid, occurs by addition of polyvalent ions of a sign opposite that of colloidal particles. Adding an ionized emulsifier that is attracted to the oil-water interface will impart a positive or negative charge to each droplet. The coalescence rate will slow down as electrically repulsive forces build up between droplets. Oil droplets in an oil-in-water emulsion are likely to have a negative charge, as described by the Helmholtz theory of the electrical double layer, which states that if the negative charges are aligned or closely bound to the interlayer, charges of the opposite type will line up parallel to them, forming an electrical double layer that causes oil droplets to repel each other. Surfactants are essential components of formulations for processing textiles, pulp and paper, and detergents. They aid in dispersion, suspension, foam suppression and particle wetting. But surfactants can also make FOG in wastewater extremely difficult to clean up due to the solubility of oil emulsified by them. The molecular structure of surfactants consists of two distinct sections. One is polar and water soluble. Acetic acid, for example, the smallest carboxylic acid having any hydrocarbon character, is very soluble in water. The non-polar portion of a surfactant is insoluble in water [3]. When a surfactant, or emulsifier, is added to an oil-in-water emulsion, it has a tendency toward orientation, whereby the emulsifier's hydrophilic end is dissolved into the water and its non-polar hydrocarbon end is oriented toward the oil, when the molecules become crowded around an oil droplet, and space becomes limited, the film of surfactant around the droplet compresses and the surfactant molecules are packed in an oriented position, as shown in Figure 1. If enough emulsifier is added to coat the entire surface area of the droplets, a stable emulsion will form that can last for years - such as those found in cosmetics and shampoos. The emulsifier thus forms a skin around each droplet, preventing it from colliding with other droplets. When oil is emulsified in wastewater, usually at a few ppm to 5%, it takes only a small amount of detergent to emulsify the oil into a stable emulsion [2]. EMULSION BREAKING Emulsion breaking, or demulsification is the separation of a dispersed liquid from the liquid in which it is suspended. All chemical and mechanical methods of emulsion breaking conform to Stoke's Law. The objective of demulsification is to destroy the interface and drive the surfactant to either the oil side or the water side, allowing the oil particles and sediments to coalesce and rise to the surface, as in creaming. Demulsification can be enhanced by decreasing water-phase viscosity or increasing oil viscosity. Increasing the diameter of oil droplets and lowering the density of oil to water also works. Demulsification is divided into several processes: • Gravity separation of free, non-emulsified oil • Chemical treatment and separation of emulsified oil • Electrolytic methods, which are not covered in this article To design a demulsification program, the engineer must first answer a number of questions: • what type of emulsion is involved? • what kind of oil is in the emulsion? • what is the percent oil in water? • what is the wastewater's pH? • What is the oil-particle-size distribution? For example, are the droplets large enough to be visible? • Are there solids present? If so, are they oil-wetted or water-wetted? • Are surfactants present in either phase? If so, are they cationic, anionic or nonionic? Are they oil-soluble or water-soluble? • What type of mechanical treatment has the wastewater experienced prior to collection? • Is there violent mixing or sharp bends in the effluent pipes [4,5,6]? As a class, oils are nonionized and non-conductive. Inorganics are not soluble in most oils. The objective then is to move the emulsion breaker out of the oil droplets into the water phase. In wastewater, oils are defined as substances that can be extracted from water by hexane, carbon, tetrachloride, chloroform, fluorocarbon and Freon 113 [6]. These tests are found in ASTM D 2910 and other literature [7]. In addition to oil, used process water may contain metal shavings, silt, surfactants, cleaners, soaps, solvents, metal particles and other residue that will accumulate in the rag layer unless it settles out by gravity. Depending on the process, the FOGs found in these emulsions may be fats, lubricants, cutting fluids, heavy hydrocarbons such as tar, grease, crude oils and diesel oils, and light hydrocarbons, including gasoline, kerosene and jet fuel. Their concentrations can range from a few ppm to 5-10% of the wastewater [6]. The stability of emulsified droplets is maintained by anionic and nonionic surfactants, and solids present on the interfacial film. An emulsion achieved with solids is most stable when the contact angle of the solids with the interface is close to 90 deg, while the Zeta potential of the solids may increase the strength and stability of the emulsion, surfactants are largely responsible for the film's high Zeta potential, high interfacial-shear viscosity, high interfacial elasticity, and relatively low interfacial tension [2]. Counteracting emulsions There are several strategies for counteracting emulsions: • Decompose the emulsion, using dissolved air flotation, ozonation or other oxidation process. This method, however, is expensive • Chemically react the emulsion, modifying the surfactant's charge so that it no longer acts as an emulsifier. For an ionic surfactant, neutralization is often the simplest method, using an acid, base or ionizer. If a calcium or magnesium salt, such as CaCI or MgS04 is added to an emulsion stabilized by a sodium soap, the soap will convert into a calcium or magnesium soap, which is less soluble in water because the interfacial film has changed. The emulsion may break • Increase the solubility of the surfactant in either bulk phase. Alcohol or other polar solvents, such as acetone, can be used to increase solubility in the water phase and pull the emulsifier out of the oil phase. If the aqueous phase is a brine, dilution with water may be all that is needed to achieve separation • Disrupt the oriented structure of the emulsifier's interfacial phase with demulsifiers. Because these materials are not very soluble in either phase, they concentrate at the interface. Separation occurs when the agents insert themselves between the surfactant molecules, increasing the intermolecular distance and weakening the binding forces constructed by the emulsifier [5]. Chemical demulsifiers provide the opposite charge to the emulsion, allowing the accumulated electrical charge on the interface of the emulsified oil droplets to be neutralized. Normally, cationic demulsifiers, which exhibit a positive charge when dissociated in water, are used to destabilize oil-water emulsions. Coalescence occurs when the Zeta potential of the surfactant approaches zero, at which point coalescence is in progress [6]. Though organic emulsion breakers produce less sludge than inorganic coagulants, ferric and aluminum chloride, continue to be among the most widely used chemicals for emulsion breaking (Table 2). Ferric chloride and aluminum chloride break emulsions by lowering the wastewater's pH, which aids in the coalescence of oil droplets. The disadvantage of using these inorganic compounds, however, is that they build up sludge and are difficult to remove. Salts such as aluminum sulfate add ionic strength and a high charge, which help later removal of surfactants by activated carbon, although pH must be raised back to 8 to do that successfully. However, removal of as much of the solids as possible prior to emulsion breaking will reduce the demand for treatment chemicals and lower processing costs [4]. Testing procedures Before applying treatment to break an emulsion, the first step is to establish that an emulsion, with two separate phases, is present. If the wastewater is cloudy or opaque, with visible amounts of a second liquid at the top or bottom of the sample container, or sticking to its side, it is probably an emulsion. But it could also be a slurry of finely divided, colloidal solids [5]. To test, mix the sample thoroughly, place it into a test tube, place the test tube into a hot bath. Does separation occur? Do solids settle? Is there a change of color? Many emulsions are light brown or cream colored. As they begin to break, they first darken, then separate. If the liquid is a clear, one-phase liquid, but becomes cloudy upon being heating, a microemulsion or nonionic detergent may be present. If simple heating (to about 162°F) results in separation, and this approach is economically feasible, skim the oil off the surface. Run the water through an absorber vessel filled with organoclay to remove the remaining traces of oil, and follow with a treatment of activated carbon. The water is then available for reuse. Another procedure, the Babcock Test (ASTM D1497), can also be used to determine the amount of oil in the water and its state of emulsification. In this procedure, a sample of the emulsion is placed into a long, thin, graduated flask. Sulfuric acid is added to destroy the effect of the emulsifier, then the bottle is spun in a centrifuge until the oil separates. The amount of oil present can be read off the bottle. Oil-particle-size distribution is also of use, and can be done by sight. If the drop size is above 200 pm, it is visible. Particle sizes of 100-10 μm cause a milky white appearance, 10-1 μm, a bluish-white color, and 1-0.5 μm, a smoky gray color. Below 0.5 μm, the liquid is transparent. Light scattering devices and refractive index measurements are the primary instruments used to measure droplet size [8]. Particle charge measurements have been reported to aid in determining the treatment method. Ionic emulsifiers contribute a charge to the interface that can be determined by electrodes. If the external phase is nonconductive, such as in a water-in-oil emulsion, which can consist of 80% water, the drops acquire a static charge. Charges can be induced on the particles by an electric or magnetic field [1]. Physical separation methods Most oily waste is a combination of free, nonemulsified oil, stable emulsified oil and insoluble solids, including grit, metal fines, carbon, paint, pigments and corrosion products. There are several methods for physical separation of emulsions. Some are more effective than others. There is a myriad of equipment and configurations available for emulsion separation, including coalescers, centrifuges and skimmers; Coalescers. Suitable for emulsions that have been induced mechanically, process separators can remove free oil droplets by gravity if they are larger than 0.015 cm dia. Proper sizing and piping design of the separator, which can be circular or parallel, is essential. Settling requires a tank and a period of rest time. Corrugated plate separators consist of a fiberglass or steel separation tank, with inlet diffuser, oil and water baffles, an adjustable oil skimmer with a v-shaped hopper for settling out solids. Fiberglass units feature an integral oil reservoir, effluent chamber, a vapor-sealing cover, fiberglass fittings that are bonded to the tank. Inside, there is a coalescing media pack made of corrugated polyvinyl chloride, fiberglass or steel plates. A typical design consists of 50 plates, arranged in parallel on 3/4-in, centers. The plates are supported by a series of gutters, which isolate the collected contaminants from the wastewater stream. The plates are encased in a polyvinyl chloride or fiberglass pack, and placed in a separation tank at a 45-deg angle. As wastewater flows through the pack, oil droplets coalesce on the plate and rise to the surface. Sediments settle into the sludge hopper. Centrifugal separators for liquid-liquid separation use centrifugal force to separate oil from water. This is based on the density differences between the two liquids. One or two inlet streams enter the annular mixing zone. Emulsion breakers or other chemicals can be added through a petcock, or inlet. The input liquids flow into the rotor by way of bottom vanes. The liquid is directed by the self pumping separator upward through the rotor. Centrifugal force causes separation in the rotor, and the separated liquids exit through the outlet ports. Air flotation Oil droplets and light solids can be removed from water by introducing small bubbles of air or gas into the water. The air bubbles act as scavengers, attaching themselves to oil droplets and solids, and floating them up to the liquid's surface. The two systems in use are dissolved air flotation (DAF) and induced air flotation (IAF). DAF uses pressurized water, supersaturated with air, to release bubbles 30-120 μm in size. In this system, fine solid particles and oil droplets attach to small air bubbles, which are derived from compressed air that is injected into the bottom of the flotation tank. As the air bubbles rise to the surface of the flotation tank, solids and oils can be skimmed off the surface. Agglomeration of oil droplets into large droplets and into large flocs is achieved by introducing coagulants into the wastewater. Dissolved air is added at 100-300 kPa. The wastewater flow is pressurized with air, held for about 3 min. in the retention tank, upon which pressure is released to ambient pressure in the flotation chamber. DAF variables are solids and hydraulic loading rates, air-solids ratios, recycled ratios and feeds-solids concentration. Systems consist of a retention tank, pressurizing pump, pressure-reducing valve, air-injection equipment and a flotation tank. IAF uses a pump to induce air into the system, either on the suction side or with a venturi. The entrained air in water produces bubbles up to 1,000 pm dia. With IAF, which yields larger air bubbles than those provided by DAF, the percent removal is lower. However, IAF can remove larger amounts of oil-solids flocs, which are brought to the surface as froth. Evaporation Evaporation works well for small volumes of water. For energy, it is more economical to use gas, which costs about $0.07/gal. Electricity costs about $0.12/gal. Pre-polishing or post-polishing with organoclay is recommended to boost evaporation and reduce energy consumption. Evaporators work on the principle that by raising the temperature of the wastewater, the emulsion will be firmly broken. Water will evaporate, while oil is left behind. This method is not effective, however, if solids are too high, or if the emulsion is chemical and cannot be broken by heat. An evaporator system consists of a tank where the wastewater is stored, a heat exchanger fueled by natural gas, liquid propane, steam or electricity. In the system, water is boiled to 212°F. A blower draws ambient air through the burner and an opening in the tank. As air is drawn across the surface of the heated liquid, water vapor is swept away as it breaks the surface and escapes into the atmosphere. Moisture-saturated air and flue gases leave the tank via separate passageways, and are joined at the blower entrance. The two air streams are mixed in the blower and released through the stack. Free oils and oils from emulsions that are firmly broken float to the surface, and are skimmed off and removed via an overflow trough. Removal capacity is 85-99%. If higher removal is required, the exiting water can be polished in a tank of organoclay. Heating This method may break poorly emulsified oils. The mixture must be heated to at least 160°F and allowed to settle for eight hours. Removal strategies More than 99% of the oil in water industrial streams contains anionic and nonionic surfactants. When the Zeta potential of these oil droplets is neutralized, the following can be done: Add 1-4lb/100 gal of wastewater of a metal salt, such as aluminum, iron or calcium chloride, or sulfate to adjust the pH. If the pH is 5-9, no adjustment is needed. If the pH is high, above 10, an application of sulfuric acid may be needed. At low pH of about 3-4, the surfactant splits. Care must be taken that it does not recombine at a higher pH. This requires the addition of 0.5lb/100 gal of sulfuric acid. The solution is then allowed to react for five minutes. Slowly, add sodium hydroxide (50% concentration) to bring the pH back to 7.5. The exact amount depends on the amount of acid and aluminum salt in the wastewater. Allow the mixture to stand for 30-90 min, to allow the floc to rise or fall. Cationic polymer may still be added to form a tighter floc and improve settling [6]. This method costs from 2-4¢/gal. Capitalization of a 10-gal/min facility is about $50,000. Alternatives to sulfuric acid are tannin, an organic chemical, and polyaluminum chloride, which requires no pH adjustment and produces less sludge than do other metal salts. To determine if the charge is neutralized, a bench test can be performed with a cationic polymer - or several different types of high- and low-charge cationic polymers - to cause coalescence of the oil droplets, and thereby formation of aggregates, until the lower and upper addition range is defined. In these aggregates, the droplets have not completely lost their identity, although, from the point of view of creaming, they behave as a single drop. During this stage of neutralization, to overcome the repulsive forces of the double layer, aggregation is still reversible. In a third step, a small amount of anionic polymer can be added to cause flocculation of solids and oil droplets, resulting in creaming. This step is irreversible. Total coalescence results and the oil becomes a single drop, totally demulsified. Surfactants and solids accumulate in the rag layer. Polymer cost is 0.1-1¢/gal. However, there is still the cost of sludge disposal and elimination that has to be factored in [4]. To test this method, take a 1-gal sample of wastewater with 1% (10,000 ppm) emulsified oil. The pH is 9.1. Add 10-2,000 ppm of polyaluminum chloride (or tannin) to neutralize the charge. Observe separation. If separation does not occur, gradually add more. A high surfactant presence, such as 8-10%, requires a high tannin level of 100 ppm plus. Add cationic polymer, at 1-1,000 ppm addition. Observe coagulation and further separation, and water clarity. Shake the sample jar during this process, or stir at low speed. Enough time should be allowed between Steps 1 and 2 to insure complete reaction, from several seconds to 5 min. Add 5-10 ppm anionic polymer. There should be two layers, one being clear water. If the anionic does not work, try a cationic or nonionic one. The flocculant, which can also be cationic, should be of high molecular weight, greater than 1 million. Residence time between Steps 2 and 3 should be at least 30 s [4]. Not all of the oil will be removed in this manner because polymers are cost effective only to about 30 ppm. For water suitable for recycle and reuse, pass the waterwater through a column of organically modified clay and activated carbon. The organoclay removes the remaining oil; the carbon removes the surfactant. Since the charges are neutralized, the oil should not reemulsify (Figure 3). Bentonite-based powders For small facilities that don't have a wastewater engineer onsite to dose organic polymers precisely, the use of bentonite powder, pre-mixed with aluminum salts, polymers and other agents, is a safe alternative for demulsification. This treatment is also recommended for facilities that prefer discarding filter cake that passes the U.S. Environmental Protection Agency's Toxicity Characteristic Leachability Procedure (TCLP), to hauling away concentrated sludge. Often the job can be done in one step, in a batch or continuous process, using either flow-through or batch treatment equipment. Excellent removal of organics and metals yields reusable water at a cost of 25¢/gal. Capital costs for equipment up to 30 gal/min are less than $50,000. Post-polishing with granular organoclay provides water that is recyclable. Post-polishing Organophilic clay is bentonite or other clay material modified with a quaternary amine. Bentonite, whose main clay constituent is montmorillonite, has an ion exchange capacity of 70-90 milliequivalent per 100 grams. The quaternary amine has a hydrophilic nitrogen end that is positively charged. This nitrogen ion exchanges unto the clay, replacing sodium, potassium, calcium and other metals. When this now organically modified clay is placed into water, the amine chain stands perpendicular to the clay surface (montmorillonite clays have the shape of a rectangular platelet). The result is a hydrophobic, oil-loving end of the quaternary amine with a halide attached to it, extending into the water in an undissolved state. As an oil droplet passes by this end of the quat chain, it enters, dissolves, and partitions into the oil droplet, assuming that the oil is either mechanically emulsified or poorly emulsified by a surfactant. Formed is a solid-liquid emulsion, whereby the hydrophilic end, instead of being dissolved into water, is "dissolved" into the clay. The quat is wetted by the oil, or vice versa. An organically modified or organophilic clay can be used to remove small amounts of oil from water - typically the amounts that oil-water separators allow to pass through, because the droplets are too small to coalesce (in the absence of solids), or they are poorly emulsified. Partitioning denotes an uptake, in which the sorbed organic chemical permeates into the network of an organic medium, by forces common to solutions, such as Van der waals forces. When the organic phase is a solid, partition is distinguished from adsorption by the homogeneous distribution of the sorbed material through the entire volume of the solid phase. The organoclay is the sorbent phase. Anytime something is put between two phases and that dissolves into both of them, it partitions. This describes the activity of the emulsifier. How much partitions into oil and water, is called the partition coefficient, Kd, which is "the amount of organic absorbed on the organoclay, the amount of organic left in solution x volume of solution/mass of organoclay. For example, benzene and water do not mix, but the addition of anhydrous acetic acid partitions into both sides. The amount depends on the concentration and temperature. The process is pH and temperature sensitive. The solubility of oil increases with increased temperature, lowering the removal capacity of the organoclay. The percent removal capacity of the organoclay varies with the size of the oil droplets. The smaller they are the more quat is needed on the clay surface. Organoclay is used either as a powder in batch treatment systems to remove oil and other organics, or it is used as a granule for flow-through systems, at a particle size of 8 x 30 mesh (smaller sizes cause pressure-drop problems). Organoclay is blended with anthracite to prevent early plugging of interstitial pores in the adsorber vessel. Because of this partitioning capacity, the organoclay removes mechanically emulsified oil at seven times the rate of activated carbon. Activated carbon While the organoclay removes the oil, activated carbon is generally used as a post-polisher to remove traces of oil (1 ppm or less), to achieve ultrapure water, or to remove the surfactants in the wastewater. As a rule, the less soluble, more hydrophobic and more chlorinated an organic compound, the more effectively organoclay will remove it from water. For a substance to adsorb onto activated carbon, it has to diffuse into the carbon's pores. Nonionic surfactants adsorb well onto carbon, especially in alkaline environments. When they are emulsified, the surfactants are difficult to remove, however, because, while they can be broken apart at low pH, the fractions join back together as the pH is raised [4]. An exception is a new family of nonionic surfactants, the Triton SP series, developed by Union Carbide (CE, Dec. 1996, p.17). Their chemistry combines a novel hydrophobe and an ethoxylate chain hydrophile. When the pH of the wastewater is lowered, the bond between the surfactants hydrophobe and liopophile is permanently broken. For maximum efficiency, a pH of 3-5 is required to break it apart. The plant engineer who is sensitive to wastewater cleanup may consider using such surfactants. In any case, engineers should consult surfactant manufacturers before making any changes to a system that is installed and online. |
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