UFC 3-280-04
17 DEC 2003
CHAPTER 6
AUXILIARY EQUIPMENT PROCESSES
6-1
DISPOSAL OF RESIDUALS. Filters are designed either to remove filtered
solids by backwashing or to dispose of them when the filter medium with the solids in-
corporated (disposable media) is discarded. Whether to use a backwashing filtration
system or disposable media is generally a question of economy, based on the concen-
tration of solids in the waste stream and the ability to treat the backwash water.
Backwash water is generally returned to the head end of the plant or back-
wash water recovery facilities. Because backwashing occurs at very high flows for short
durations, the design of treatment facilities where the backwash water is returned to the
process stream or backwash water recovery facilities must take into account high inter-
mittent flows that could produce hydraulic upsets. Some manner of holding tank or flow
equalization is often necessary and these flows and loadings should be considered in
any pre-filtration settling system design.
For the particulate residuals, the quantities of sludge produced can be esti-
mated based on the suspended solids in the raw water and the anticipated treated ef-
fluent water quality. Calculations used to estimate sludge generation must also include
any coagulant used as a filtration aid. Particulate residuals generated during filtration of
hazardous or industrial wastes must also be characterized to determine if the sludges
are hazardous and require secure, off-site disposal. In some cases sludges from HTRW
treatment may continue to be regulated as hazardous if the waste stream was associ-
ated with a listed hazardous waste. The designer should always check the source of
contamination when identifying treatment and disposal options. RCRA must be consid-
ered when identifying sludge treatment and disposal options.
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PRETREATMENT REQUIREMENTS. Pretreatment of the influent wastewater
can significantly enhance the treatability and improve effluent quality. Pretreatment can
be particularly important for hazardous and toxic wastewater streams. For example,
gravity separation or dissolved air flotation, or both, may need to be used when two-
phase liquid wastes (e.g., petroleum hydrocarbons and water) are present; chemical
pretreatment may be required where emulsions are present; or sedimentation may be
necessary where total suspended solids concentrations are prohibitively high. Pretreat-
ment may include oxidation of soluble forms of reduced metals, reduction of suspended
solids by chemical flocculation and sedimentation, chemical precipitation of dissolved
metals or other dissolved ions such as phosphorous, or coagulation by addition of
chemical filtration aids, such as poly-electrolytes. Where pretreatment is used to en-
hance filtration, the designer should keep in mind that the pretreatment must not be de-
signed independently from the filtration system, as the pretreatment facilities will depend
on the type of filtration system selected. Also, where direct discharge to surface water is
anticipated, water quality standards must be considered (e.g., for aluminum or iron).
6-2.1
Laboratory studies are generally required to determine type and degree of
pretreatment. These studies may range from simple jar-tests to column studies to plant
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