17 DEC 2003
combination with dewatering devices for the backwash stream or cartridge or bag filtra-
tion are possible treatment alternatives.
Chemical Addition. Chemical addition means the use of flocculants, coagu-
lants, or other chemicals that will enhance separation. For some processes, adding
chemicals may not be desirable from an operational point of view. This is a concern
when polymers and sub-micron cartridge filters are used together. The organic poly-
mers may coat or blind the filter media, causing premature failure. Electrolytes are often
used to precipitate colloids or enhance filtration of particulates and colloids. To improve
particulate removal, electrolytes that have a charge opposite in sign to the particles are
used (see EM 1110-1-4012).
Expendable Media. Expendable media are mostly filter cartridges, filter bags,
or precoat filter aids. For low solids loadings (less than 15 mg/L total suspended solids
[TSS]), expendable media may be economical. As the volume of filtered solids in-
creases, so do disposal costs for expendable media. Expendable media and either dry
cake or hazardous slurries are frequently incompatible. Filter cartridges or precoat filter
aids usually retain significant amount of filtrate and solids. For hazardous wastes, this
can substantially increase disposal costs. Expendable media may also be an adsorbent,
such as an oil adsorbent clay (see DG 1110-1-2).
Solids Output. Solids are removed from the separation or filtration device as
either a slurry from backwash of media type filters or as a relatively dry cake, as with
cartridge filters. If the solids are to be incinerated, then the driest possible cake is de-
sired. Where slurries may be land farmed or land applied, mechanisms such as dewa-
tering should be considered to reduce overall disposal volume and costs. Other specific
elevating requirements apply before solids can be landfilled. (See also UFGS 11360A,
11350, 11393, 11365A, and other USACE Guidance on dewatering processes).
Continuous or Batch Operation. Continuous or batch operations usually de-
pend on upstream or downstream processes. When using batch filtration with continu-
ous upstream or downstream processes, the designer must consider the size of inter-
mediate holding tanks to be used as feed or effluent control of the waste stream flow.
When using continuous filtration with upstream or downstream treatment processes, the
same is true. Typically, continuous backwash filtration processes use much more back-
wash water (as much as 10%) than batch processes (typically from 1 to 3%). This flow
needs to be returned to the head of the treatment system, which must be sized to han-
dle both the design flow and the return flow.
Precoat Filtration. Unlike the other filtration systems discussed in this docu-
ment, precoat filtration relates to a process technique, not a specific filter type. Many
commercially available filtration and solids separation processes can be used as pre-
coat filters. Some of these include rotary drum vacuum filters, vertical tube filters, and
recessed plate and frame filters. The designer must contact filter representatives re-
garding available data on the use of their filters for precoat applications. Generally, pre-
coat filters are used when there are high inert solids loadings at levels typically greater