Key Words

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The following are key words you will run across in reading over the next several chapters.

air flotation - This treatment usually follows oil-water separation. Wastewater is pressurized to 3-5 times normal atmospheric pressure in the presence of air to produce a saturated air-water solution. When this solution is released to normal pressure in the flotation unit, tiny air bubbles form throughout the liquid; the same effect is observed when a bottle of pop is opened. As the air bubbles form they become attached to tiny oil droplets and to suspended particles; a froth of bubbles and attached wastes rises to the surface and is skimmed off.

biological treatment - Microorganisms in aquatic systems feed on dissolved/suspended organic matter; their digestion processes decompose organic wastes. Oxygen is consumed during these decomposition processes thus decreasing the supply of dissolved oxygen in the water. If the rate of decomposition is excessive, the resultant oxygen depletion produces stresses on aquatic organisms. Biological (secondary) treatment systems address problems associated with biochemical oxygen demand. Break-down of wastes is transferred from natural waterways to lagoons and/or vessels where conditions can be controlled and therefore decomposition occurs efficiently. Biological treatment is simply a concentrated, controlled, application of a natural process.

carbon adsorption-Carbon adsorption is a technology that has been used widely in the drinking water treatment industry, and that is being used with increasing frequency in the wastewater and hazardous waste industry. The process takes advantage of the highly adsorptive properties of specially prepared carbon known as activated carbon. The porous structure of the carbon provides a large internal surface area onto which organic molecules may become attached. Many organic substances, including chlorinated solvents, PCBs, PAHs, pesticides, and others, may be removed from solution using carbon adsorption. Carbon adsorption is achieved by passing water residues through one or more columns containing granular activated carbon operated in parallel or in series. Carbon columns may be operated in either an upflow (expanded bed) or a downflow (fixed bed) mode. In theory, spent carbon may be regenerated. In practice, however, spent carbon must frequently be discarded, especially if high concentrations of PCBs are present.

coagulating agents - Wastes that are removed by this process are classed as suspended or colloidal. Colloids consist of small particles that are constantly moving; gravity does not cause them to settle out. Coagulating agents, eg. alum and ferric chloride, reduce the effects of electrical charges which keep the particles of waste separate from each other. The particles then join together to form masses called floes. These floes then rise to the surface or settle to the bottom. Flocculating agents are frequently used to bond floes together - this speeds the rate at which they rise to the surface or settle to the bottom.

equalization - Equalization systems contain large reservoirs together with piping and treatment processes. These systems minimize fluctuations in wastewater flows and thus give stability, ensuring that wastewater treatment is carried out under the best possible conditions.

flocculating agents - Flocculating agents are routinely used in municipal and industrial wastewater treatment in conjunction with clarifiers. There are many proprietary surfactant-type polymers designed for this purpose, although inorganic chemicals such as ferric chloride may also be used.

ion exchange-Ion exchange is a process in which ions held by electrostatic forces of charged functional groups on the surface of a solid are exchanged for ions of similar charge in a solution in which the solids are immersed. The "solids" are specific resins (usually in the form of beads) that have an affinity for metallic ions. The most common configuration is the fixed bed system, in which the wastewater flows through resin contained in a column. Ion exchange resins are either highly selective for specific metal contaminants or non-specific for a wide variety of metals.

leachate - The term "leachate" refers specifically to water that has flowed through the sediment, such as pore water, or precipitation that has infiltrated sediments in a CDF or landfill. The volume of leachate is generally much smaller than that of effluent, but the concentration of dissolved contaminants is typically higher. The flow rate of effluents and leachates is highly dependent on their source. The effluent from a CDF during filling operations from a hydraulic dredge can be quite substantial-hundreds or even thousands of liters per minute. The duration of such discharges, however, is limited to the duration of dredging, which is typically on the order of weeks or months. Sidestreams from pretreatment or treatment operations are technology-dependent, but generally will produce smaller flows over a longer period of time (months to years). Once the remediation project is completed, the need for effluent treatment is limited to storm water (runoff), which could remain a long-term source if water comes into contact with contaminated sediments. Leachate is generated over very long time periods, and therefore a permanent leachate collection and treatment system is a common requirement at municipal and industrial landfills.

metals removal technologies - Metal contaminants are primarily associated with suspended particulates in most water residues from sediment remedial alternatives. Suspended solids removal technologies should therefore be sufficient to address metals removal needs for the majority of applications. Removal of dissolved metals from water residues can be conducted using ion exchange or precipitation. These technologies have been widely used for industrial wastewater treatment.

neutralization - Blending acidic and basic wastes is a function of most equalization systems. This action (neutralization) is essential before wastes are directed to biological treatment processes where microorganisms feed on organic substances. Extreme changes in pH often kill microorganisms.

oil separation-Some sediments contain very high concentrations of oil and grease. In most cases, the oil and grease will remain attached to the sediment particulates and be captured by suspended solids removal technologies. In some cases, oil and grease is released from sediment particles, forming a slick, a suspension of discrete particles, or an emulsion in the water residue. In such cases, the oil and grease must be captured or removed prior to treatment processes such as ion exchange, carbon adsorption, and filtration, because oily compounds will foul the surfaces of exchange resins and filters. Oil booms and skimmers are routinely used in CDFs to capture oil and floating debris. Coalescing plate separators employ a medium that provides a surface for the aggregation of small, emulsified oil droplets, which can then be removed by gravity separation. Emulsified oils are much more difficult to separate from water. Chemical de-emulsifying agents, heat, and/or acids are generally effective for breaking emulsions. Once the emulsion is broken, the oil is amenable to treatment processes.

oxidation - Oxidation is used to partially or completely degrade organic compounds. Complete oxidation of organic compounds can theoretically reduce complex molecules to carbon dioxide and water. Halogenated organic compounds will produce minor amounts of mineral acids (e.g., hydrochloric acid). However, oxidation is often not complete, resulting in the formation of simpler "daughter" compounds that are usually much less toxic or persistent than the original contaminants. Two forms of oxidation that might be applicable to water residues from sediment remedial alternatives are chemical oxidation and UV-assisted oxidation. Chemical oxidants suitable for treating wastewater include oxygen, ozone (03), hydrogen peroxide (H202), potassium permanganate, chlorine (or hypochlorites), and chlorine dioxide. The oxidizing power of hydrogen peroxide and ozone can be significantly enhanced through the use of UV light. This technology is effective for treating a wide variety of organic compounds, including PCBs and PAHs. Common oxidizing agents, in addition to oxygen, are chlorine, ozone, hydrogen peroxide and potassium permanganate. These substances oxidize wastes to make them more biodegradable and/or more readily removed by adsorption. Oxidation can be enhanced through control of pH and also through using catalysts.

precipitation - Precipitation is a chemical process in which soluble chemicals are removed from solution by the addition of a reagent with which they react to form a (solid) precipitate. This precipitate can then be removed by standard flocculation, sedimentation, and/or filtration processes. Most heavy metals can be precipitated from water as hydroxides with the addition of a caustic (e.g., sodium hydroxide or lime). Alternatively, sodium sulfide or ferric sulfide may be added to precipitate metals as sulfides. The sulfide process is effective for certain metals, such as mercury, which do not precipitate as hydroxides. Precipitation processes produce a sludge that may have to be managed as hazardous waste due to the presence of concentrated heavy metals. Disposal costs for these sludges may therefore be significant. Lime and caustic soda are common sources of hydroxide (OH-) ions.

OH- ions combine with ions of some metals to form insoluble metal hydroxides (precipitation). Precipitated metals settle out and thus are removed from the water; adsorption, using activated carbon, improves this separation process. Iron is one of many metals which is commonly removed in this way.

residue management-Residues are materials, products, or waste streams generated by components of a sediment remedial alternative. Residues may be water, wastewater, solids, oil fractions, or air and gas emissions. The management of these residues may involve treatment, containment, or discharge to the environment. The types of residues anticipated from most sediment remedial alternatives and management options for them are discussed in various sections of this book. Some sediment treatment technologies may generate unique residues, requiring special management considerations. At a minimum, the inert solid particles that were present in the original, untreated sediment, will still be present following the application of any treatment technology.

roughing filters - Roughing filters are used primarily as pretreatment for filter systems that may not be able to tolerate high turbidity or suspended solids in the source water. Many designs are upflow gravel filters. The upflow feature allows for maximum removal efficiency coupled with simple maintenance. By opening the downwash valves, flow through the filter is reversed at a high rate flushing filtered particles out. This system allows for years of use of the filter without gravel replacement. Typical applications include Remove sand and silt (settleable solids); Remove 50-80% turbidity (cloudiness); Pre-treatment for slow sand filters.

sedimento/jo«—Sedimentation is the basic form of primary treatment employed at most municipal and industrial wastewater treatment facilities. There are a number of process options available to enhance gravity settling of suspended particles, including chemical flocculants, CDFs, sedimentation basins, and clarifiers. Of these, gravity settling in CDFs has been used most extensively with contaminated sediments. CDFs have long served the dual role of a settling basin and storage or disposal facility for dredged sediments. Gravity settling in CDFs, with proper design and operation, can take a hydraulically dredged slurry (typically having 1015 percent solids by weight) and produce an effluent with 1-2 g/L suspended solids. Many CDFs on the Great Lakes produce effluents with suspended solids less than 1 g/L (e.g., 100 mg/L) by gravity settling alone. At most CDFs, a hydraulically dredged slurry is discharged into the CDF at one end and effluent is released over a fixed or adjustable overflow weir at the opposite end, as shown in Settling times of several days are commonly achieved at larger CDFs. Improved settling efficiencies can be achieved by dividing the CDF into two or more cells or through operational controls to increase the detention time and prevent short-circuiting. As the CDF becomes filled, and detention times shorten, dredging production rates may have to be reduced or mechanical dredging used instead of hydraulic dredging to provide suitable settling efficiencies. Sedimentation basins or clarifiers are typically open, concrete or steel tanks with some type of solids collection system that operates on the bottom. Inclined plates may be incorporated into the tanks to improve solids capture for a given flow rate and reduce the size of the clarifier. Rectangular and circular clarifiers are commonly used in municipal and industrial wastewater treatment, but have only been used on a limited basis in applications with contaminated sediments.

solid residues - Solid residues include the bulk of sediment solids following treatment as well as smaller fractions of solids separated from the sediments or produced by the treatment processes. For most remedial alternatives involving a properly designed and thorough treatment system, the treated solids will not require additional treatment and can be disposed using the technologies discussed in Chapter 8. Exceptions to this may include solid residues with special physical properties or concentrations of contaminants requiring special handling. Some treatment technologies produce small volumes of sludges. Other solid residues include debris and oversized materials separated during dredging or pretreatment, sludges from water or wastewater treatment systems, spent media from granular filters or carbon adsorption systems, and particulates collected from air pollution control systems.

stripping - As steam rises through the column it removes (strips) contaminants from wastewater that is moving in the opposite direction. Hydrogen sulphide and ammonia are two contaminants that are stripped from refinery wastes. Methods that are used to recover contaminants from the flow of steam (as it exits from the stripper) include condensation/vacuum recovery systems, biox and incineration.

suspended solids removal - The removal of suspended matter is generally the most important process in the treatment of effluents and leachates from sediment remedial alternatives because most of the contaminants in water residues are associated with the solid particles. An effective solids removal system can significantly reduce contaminant concentrations, leaving behind only those contaminants that are dissolved or associated with colloidal material. Solids removal is a frequently required pretreatment for processes that remove dissolved contaminants (e.g., ion exchange, carbon adsorption). The primary technology types for suspended solids removal are sedimentation and filtration.

wastewater reduction - Reductions in wastewater production almost always result in decreased amounts of wastes that enter the river. Typical reductions include: replacing once-through cooling water (OTCW) with water that has been recirculated through cooling towers; keeping clean stormwater separate from wastewater that requires treatment; reusing treated wastewater eg. in process units; in cooling towers; as feed water for boilers.

water residues - Water is likely to be the most important residue for consideration at most sediment remediation projects simply because of the volumes generated. The removal and transport technologies selected will have a profound effect on how much water residue is generated through the treatment process. For example, if the sediments are dredged hydraulically and transported by pipeline, a large area will probably be needed for gravity settling. In contrast, if the sediments were removed with a mechanical dredge and transported by truck, there would be much less "free water" to handle. Some pretreatment and treatment processes may require the addition of even more water. For final disposal of sediments and solids residues, most of this water must be removed. Depending on how the sediments are handled, treated, and disposed, the volume of water that must ultimately be managed can be less than one-half of the volume of sediments (in place) dredged, or greater than five times this volume. Water residues from a sediment remedial alternative are commonly referred to as effluent or leachate. The term "effluent" may be applied to a wide variety of water residues, including: Discharges from an active CDF; Surface runoff from a landfill or CDF; Sidestreams from a dewatering process (e.g., filtrate from a filter press or centrate from a centrifuge); Wastewater or condensate from a pretreatment or treatment process.

water residue treatment - Technologies for treating wastewater from municipal and industrial sources are well established and well documented (Weber 1972; Metcalf & Eddy, Inc. 1979; Corbitt 1990). Averett et al. (in prep.) evaluated the applicability of these technologies to effluent and leachate from sediment remedial alternatives on the basis of cost, effectiveness, implementability, and availability. Effluent/leachate treatment technologies may be categorized according to the type(s) of contaminants that are removed. This chapter discusses technologies that remove the following contaminant categories: Suspended solids; Metals; Organic compounds. While there is some degree of overlap between the processes, these categories reflect the primary areas of treatment.

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