SITE INFORMATION AND CORRESPONDENCE

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• suspended solids,Oper filtering of process water can Ineffective Filtration Media often eliminate the need for a metals removal system. Metals concentrations in extracted water may also Filters are often included in treatment trains to remove decrease over time because the mobility of metals is solids and metal precipitates. Filters help the treatment sensitive to their oxidation states. Metals such as iron, plant meet discharge criteria for solids and metals. manganese, and arsenic become relatively immobile They also protect the other treatment components, such when oxidized and relatively mobile when reduced. as GAC units,that might otherwise become fouled. Ground water with elevated levels of organic For instance, if filters are ineffective, the GAC units contaminants may initially have highly reducing may require more frequent replacement due to fouling conditions, making these metals more mobile. Once than would otherwise be required due to chemical pumping begins,however, the reducing conditions loading alone. Because costs of GAC replacement may diminish due to mixing and/or contaminant substantially exceed the cost of properly maintaining a removal. Therefore,as remediation progresses,the filtration unit, ineffective filtration may result in extracted water may have significantly lower metals increased O&M costs. concentrations than anticipated from remedial investigation data. In some cases, metals Premature fouling of carbon or decreased reduction in concentrations may fall below the discharge criteria, total suspended solids are indications of ineffective rendering metals treatment unnecessary. filtration units. Metals treatment via precipitation involves chemicals Inefficient Air Strippers for pH adjustment, significant labor(i.e., one or more operators full time), and generation and disposal of Removal of volatile organic compounds(VOCs) is filter cake. As a result, metals removal systems are often most effectively achieved with air stripping. extremely costly and should be eliminated, shutdown, Packed towers and tray aerators are two types of or bypassed if they are unnecessary. At some sites systems that,when properly designed, effectively strip where metals, such as iron and manganese, are not VOCs. COCs but frequently cause fouling of other system components, it may be cost-effective to frequently In some cases, typically in systems that at one point clean the P&T system than it is to operate a metals utilized biotreatment, air stripping is achieved by removal system. diffused air strippers(i.e., large storage tanks that use large blowers to diffuse air through process water). GAC Polishing Steps Such an approach typically uses a 20-horsepower blower and results in 80%removal of VOCs whereas a Although not always the case, air strippers can often well-designed tray aerator may use a 5-horseblower reduce VOC concentrations in extracted ground water blower and achieve 99%removal of VOCs. Thus, without a GAC polishing step. If a GAC polishing switching to a well-designed air stripper from a step is planned for or is part of a P&T system, efforts diffused air stripper might reduce power costs should be made to optimize the air stripper because substantially and allow for removal of a GAC polishing the polishing step may not be required. Often, step due to improved removal efficiency. strippers can be made more effective by increasing the air/water ratio, changing the packing material (for Remove Redundant or Unnecessary Components packed towers), or adding another tray (for tray aerators). A second air stripper can also be considered Eliminating unnecessary components that stem from as a polishing step. A cost benefit analysis should be over-design or changing site conditions may result in conducted to determine which approach is most substantial savings. Three common examples of appropriate for a specific site. redundant or unnecessary components are provided. Parallel Systems or Components Metals Removal Systems Providing redundancy (e.g., a spare component, Metals removal is a common treatment component that perhaps installed in parallel to the operational may be unnecessary shortly after a system becomes component) for filters or mechanical equipment such operational and functional, at some other point before as pumps and blowers is often warranted. However, site closure, or with proper filtering. Because elevated splitting the flow of process water into parallel metals concentrations in extracted water may be due to treatment trains or providing an additional treatment 14