Filtration

Fig. 1 gives a general summary and classification of filtra-

tion equipment. Each technique has its advantages and

limitations.

Gravity Filtration

In this mode of operation, the liquid-solid mixture flows

under ambient atmospheric pressure through the filter

medium with no pumping. The difference in water level

between the influent and effluent streams provides the

driving force for the flow of water through the filter med-

ium. This method of filtration is used to remove unwanted

particulate material from solutions. The filter media could

be sand, anthracite, anthracite-sand, multilayered mixed-

media, or diatomaceous earth. In addition to physical inter-

ception of the particulate material, the filter medium may

also adsorb or attach to impurities. This method is used in

water treatment facilities to remove impurities such as iron,

manganese, hydrogen sulfide, color, turbidity, bacteria,

and viruses from the water. Clogging of the filter medium

will occur and it will need frequent clearing by backwash-

ing. The concentration, size distribution, and properties of

the suspended solids will influence design considerations,

such as the selection of the type and coarseness of the

filtration medium, the configuration, size, and depth of

the filtration bed. The design will also depend on the

modes of operation of these filters, namely, under constant

pressure, under constant rate with control of either the

effluent or the influent rates, and variable declining-rate

filtration.