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requirements and recommendations (see DG 1110-1-2, PWTB 200-01-05, or API
Publication 421 for additional information).
Space Limitations. Where space is limited, bag and cartridge filters are good
selections, depending on waste stream characteristics. In some instances, the designer
may not be able to consider space saving filter options because of high solids loadings,
the high cost of disposal of expendable cartridge and bag filter elements, or high oper-
ating costs if frequent filter changes are required. Again, the designer needs to consider
whether these filters will provide the ease of operation and effluent quality for a particu-
Settling Velocity. Sedimentation velocity can help the designer determine if
settling or filtration may be required for a given waste stream. Waste streams with high
settling velocities (more than 0.5 cm/s) typically depend on gravimetric or centrifugal
type treatment processes prior to filtration to reduce the quantity of solids. Filtration in
this case would be used to remove the solids remaining in suspension. Settling veloci-
ties can be determined in the laboratory. Test method "2701 E. Zone Settling Rate" of
the 20th Edition of Standard Methods for the Examination of Water and Wastewater
(APHA 1999) should be considered for determining settling velocities.
Solids Concentration. The feed solids concentration is the weight percent of
dry solids in the waste stream to be treated. This can be obtained by filtering a known
quantity of waste through a tarred and previously weighed dry filter paper (see Standard
Methods, 20th Edition, Method 2540 D, "Total Suspended Solids" [APHA 1999]). Where
solids concentration is high, the designer must consider using gravimetric or centrifugal
solids separation processes prior to filtration.
Treatment Flow Rate. Treatment flow rate is important in determining the de-
gree of automation that can be economically allowed. In general, low flow rates require
less operator attention, thereby reducing O&M costs, versus higher flow rates that may
require frequent operator input. The tradeoff is that the lower flows typically have higher
capital investments, where the economy of scale decreases as the volume of waste
treated increases. For HTRW sites, the treatment flow rate is often determined during
the feasibility study (FS) phase of work.
3-2.9 Environmental Hazards. If the waste stream to be treated is toxic, flammable, or
explosive, it is best treated in an isolated environment. For this parameter, filtration de-
vices that are closed vessels (i.e., pressure filters) are more suitable than open, gravity-
type systems. In the case of cartridge filters, operator contact during filter changes may
affect selection of this type of filtration device.
Objective of Solids Separation. As the concentration of the solids increases
(e.g., greater than 100 mg/L) two treatment processes are typically required. The first
should remove the bulk of the solids, usually via some sedimentation type device (see
EM1110-1-4012), followed by filtration to clarify or remove those solids not removed
earlier. At lower solids concentrations, only filtration may be required. Solids separation
may also be used to capture and concentrate the solids. In this case media filtration in