Civil Engineering Reference
In-Depth Information
The processes of coagulation and filtration are inseparable, and their interrelationships
must be considered for best treatment results. One important advantage of filtration
over coagulation, relative to the removal of very dilute concentrations of colloidal
particles, is the much greater opportunity for contact afforded by the granular bed as
compared to the number afforded by mixing the water. The removal efficiency of a
filter bed is independent of the applied colloidal particle concentration, whereas the
time of mixing depends upon colloidal particle concentration.
There are two basic approaches to achieving optimum filterability of water. One is
to establish the dosage of primary coagulant needed for maximum filterability rather
than for production of the most rapid-settling floc. Another approach is to add a second
coagulant as a filter aid to improve the filterability of the settled water as it enters the
filter. Filter aid must be added continuously (to coat the filter media rather than pre-
coat). Excessive dosages of filter aids cause filters to clog too rapidly.
For effective filtration, the objective of pretreatment should be to produce small,
dense floc, so that the particles are small enough to penetrate the bed surface and
migrate partially down into the filter bed. Removal of floc within a bed is accomplished
primarily by contact of the floc particles with the surface of the grains or previously
deposited floc, and adherence thereto. Contact is brought about principally by the
convergence of flow at contractions in the pore channels between the grains. Of minor
importance are the flocculation, sedimentation, and entrapment of particles that occur
within the pores of the bed.
Adsorption of suspended particles on the surface of the filter grains is also an
important factor in filter performance. Physical factors affecting adsorption are the
nature of the filter and the suspension. Adsorption is a function of the filter grain size,
floc particle size, and the adhesive characteristics and shearing strength of the floc.
Chemical factors affecting adsorption include the chemical characteristics of the sus-
pended particles, the aqueous suspension medium, and the filter medium. Two of the
most important chemical characteristics are the electrochemical forces and van der
Waals forces (molecular cohesive forces between particles).
Filtration Efficiency
Filters are highly efficient in removing suspended and colloidal materials from water.
Many impurities can be removed by the combination of proper coagulation and filtra-
tion. Among the many are: turbidity, bacteria, algae, viruses, protozoa color, oxidized
iron and manganese, radioactive particles, chemicals added in pretreatment, and heavy
metals. Because filtration is both a physical and a chemical process, there are a large
number of variables that influence filter efficiency. These variables exist both in the
water applied to the filter and in the filter itself. Knowledge of the factors affecting
filter efficiency has increased quite rapidly since the 1960s. Use of this information in
the design and control of filters makes possible the water quality needed to meet the
continually more stringent filter performance requirements of federal and state regu-
lations.
Filter efficiency is affected by several properties of the applied water: temperature,
filterability, and the size, nature, concentration, and adhesive qualities of suspended
and colloidal particles. Cold water is notably more difficult to filter than warm water,
but for most applications there is no control over water temperature. Filterability, which
is related to the size and surface chemistry of the suspended and colloidal impurities
in the water, is the most important property. One practical way to measure filterability
