Science of Water : Concepts and Applications

Water Treatment 313
effi ciency with highly turbid waters. On the downside, operation and maintenance costs of rapid
sand fi lters are much higher because of the increased complexity of the fi lter controls and backwashing
system.
In backwashing a rapid sand fi lter, cleaning the fi lter is accomplished by passing treated water
backward (upward) through the fi lter media and agitating the top of the media. The need for backwashing
is determined by a combination of fi lter run time (i.e., the length of time since the last
backwashing), effl uent turbidity, and head loss through the fi lter. Depending on the raw water quality,
the run time varies from one fi ltration plant to another (and may even vary from one fi lter to
another in the same plant).
√ Note: Backwashing usually requires 3–7% of the water produced by the plant.
Pressure Filter Systems
When raw water is pumped or piped from the source to a gravity fi lter, the head (pressure) is lost
as the water enters the fl oc basin. When this occurs, pumping the water from the plant clear well
to the reservoir is usually necessary. One way to reduce pumping is to place the plant components
into pressure vessels, thus maintaining the head. This type of arrangement is called a pressure fi lter
system. Pressure fi lters are also quite popular for iron and manganese removal and for fi ltration of
water from wells. They may be placed directly in the pipeline from the well or pump with little head
loss. Most pressure fi lters operate at a rate of about 3 gpm/ft2 .
Operationally the same, and consisting of components similar to those of a rapid sand fi lter, the
main difference between a rapid sand fi ltration system and a pressure fi ltration system is that the
entire pressure fi lter is contained within a pressure vessel. These units are often highly automated
and are usually purchased as self-contained units with all necessary piping, controls, and equipment
contained in a single unit. They are backwashed in much the same manner as the rapid sand fi lter.
The major advantage of the pressure fi lter is its low initial cost. They are usually prefabricated,
with standardized designs. A major disadvantage is that the operator is unable to observe the fi lter
in the pressure fi lter and so is unable to determine the condition of the media. Unless the unit has
an automatic shutdown feature on high effl uent turbidity, driving fi ltered material through the fi lter
is possible.
Diatomaceous Earth Filters
Diatomaceous earth is a white material made from the skeletal remains of diatoms. The skeletons
are microscopic, and in most cases, porous. There are different grades of diatomaceous earth, and
the grade is selected based on fi ltration requirements.
These diatoms are mixed in water slurry and fed onto a fi ne screen called a septum, usually of
stainless steel, nylon, or plastic. The slurry is fed at a rate of 0.2 lb/ft 2 of fi lter area. The diatoms
collect in a precoat over the septum, forming an extremely fi ne screen. Diatoms are fed continuously
with the raw water, causing the buildup of a fi lter cake approximately 1/8–1/5 in. thick. The
openings are so small that the fi ne particles that cause turbidity are trapped on the screen. Coating
the septum with diatoms gives it the ability to fi lter out very small microscopic material. The fi ne
screen and the buildup of fi ltered particles cause a high head loss through the fi lter. When the head
loss reaches a maximum level (30 psi on a pressure-type fi lter or 15 in. or mercury on a vacuum-type
fi lter), the fi lter cake must be removed by backwashing.
Slurry of diatoms is fed with raw water during fi ltration in a process called body feed. The body
feed prevents premature clogging of the septum cake. These diatoms are caught on the septum,
increasing the head loss and preventing the cake from clogging too rapidly by fi ltering of the particles.
While the body feed increases head loss, head loss increases are more gradual than if body
feed were not used.