UFC 3-280-04
17 DEC 2003
CHAPTER 3
PRINCIPLES OF OPERATION AND THEORY
3-1
DESCRIPTION OF THE FILTRATION PROCESS
3-1.1
Purpose of Filtration. This filtration document describes technologies for the
separation of solids from a liquid through a permeable medium, generally a porous, fi-
brous, or granular substance, which retains the particles. This chapter will discuss the
theory of solids removal, the application of filtration within the liquid treatment process,
important wastewater parameters, and the application of pilot studies.
3-1.2
Mechanics of Filtration. Solids removal within the filter is affected by five
major factors: the size of the filter medium, the rate of filtration or surface loading
[(L/s)/m2 or gpm/ft2], the influent particle size and size distribution, the flow rate, and the
amount of solids that has already been removed within the filter. The size of the filter
medium determines the total available surface area for removal and the flow channels.
The rate of filtration determines the contact time. Influent particle size and size distribu-
tion affect the mechanism of removal, available surface area, and porosity, which will
change with run time. The flow rate determines shear forces. As solids are removed,
available removal sites are decreased and flow channels are altered.
3-1.2.1 The efficiency of particulate collection in a filter is defined as the number of
successful collisions for all particulates in the cross-sectional area of the collector di-
vided by the total possible number of collisions between the particulates and the col-
lector. The overall efficiency can be described by the summation of the different mecha-
nisms by which particulates are removed from the aqueous stream. This relationship, as
developed by Yao (1971), includes the following three mechanisms.
3-1.2.1.1 Removal by Interception. Particles moving along the streamline are re-
moved as they come in contact with the surface of the filtering media.
3-1.2.1.2 Removal by Impaction, or Settling. When particles are heavier than water,
they do not follow the flow streamlines and, instead, settle out.
3-1.2.1.3 Removal by Diffusion. Small particles can diffuse to the collector through
Brownian motion.
3-1.2.2 The overall removal can be closely estimated as the sum of these three re-
moval mechanisms. Diffusion will predominate at smaller particle diameters, whereas
settling will predominate at larger particle diameters.
3-1.2.3 In addition to these removal mechanisms, straining and adsorption play a part
in particulate removal. Straining occurs when the particle is larger than the pore size,
resulting in the particle being strained out mechanically. In the case of granular media
filtration, excessive straining is undesirable because head loss will increase rapidly be-
cause a surface mat forms. Chemical or physical adsorption will occur where bonding,
3-1
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