Environmental Engineering Reference
In-Depth Information
material (or
filter media
). In purifying a surface water source (as we are dis-
cussing in this case), filtration is a very important process, even though filtra-
tion is only one step in the overall treatment process.
Note:
Filtration is the process that occurs naturally as surface waters
migrate (percolate) through the porous layers of soil to recharge ground-
water. This natural filtration removes most suspended matter and micro-
organisms and is the reason why many wells produce water that does
not require any further treatment.
The Surface Water Treatment Rule (SWTR) specifies certain filtration tech-
nologies. The most common treatment filter systems include rapid gravity
filters (either built onsite or packaged plants) and pressure filters. Other
types include direct filtration, slow sand filters, and diatomaceous earth (DE)
filters. The SWTR also allows the use of alternative filtration technologies,
such as cartridge filters.
Filtration treatment unit processes most commonly used in water puri-
fication systems include slow or rapid sand filtration, diatomaceous earth
filtration, and package filtration systems. Slow
and rapid filter systems refer
to the rate of flow per unit of surface area. Filters are also classified by the
type of granular material used in them. Sand, anthracite coal, coal-sand,
multilayered, mixed bed, and diatomaceous earth are examples of differ-
ent filtering media. Filtration systems may also be classified by the direc-
tion the water flows through the medium: downflow, upflow, fine-to-coarse,
coarse-to-fine. Finally, filters are commonly distinguished by whether they
are gravity or pressure filters. Gravity filters rely only on the force of grav-
ity to move the water down through the grains and typically use upflow
for washing (backwashing) the filter media to remove the collected foreign
material. Gravity filters are free surface filters commonly used for municipal
applications. Pressure filters are completely enclosed in a shell so most of the
water pressure in the lines leading to the filter is not lost and can be used to
push the water through the filter.
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The deeper the filtration bed, the better the filtration.
rapid Filter Systems
Slow sand filtration has been used in the United States since 1872. It is still
used in many older plants but is not commonly used today in most mod-
ern water treatment plants because of various problems associated with this
technique. One of the problems is related to the tiny size of the pore spaces in
the fine sand, which slows down the water's progress through the filter bed.
