Page 201 - Industrial Ventilation Design Guidebook

4.4 WATER PROPERTIES AND TREATMENT 1 63

• Unaffected by the treatment to which it is subjected
» Capable of being reused with a minimum of treatment
• Not causing any problems when discharged into the drains
• Discharging into drains less than 40 °C with pH in the 6-11 range

Removal by Gravitational Force. This method simply involves a settling
chamber in which the fact that the liquid-solid mixture-solid pore interface
has little holding power. Thus, given sufficient time, the solids will settle into
the base of the chamber due to gravitational forces.
The efficiency of this operation can be improved by the use of finely per-
forated vee troughs, which will contain the particulate matter and allow the
liquid to descend to the base of the container, where it then drains away either
for further treatment or for reuse.

Removal by Centrifugal Force. This method is more efficient in extract-
ing the particulate matter from a liquid due to the fact that the forces devel-
oped are many times greater than the force of gravity.
The machine used is normally the basket-type centrifuge, which is a
rotating perforated drum with a vertical axis. The solids remain in the
drum and the liquid passes out through perforated holes. The smaller the
holes, the greater the collection efficiency. However, there is the risk of
hole clogging, causing a rapid fall in operating efficiency. The fluid viscos-
ity and the particulate size are of prime importance. Plant arrangement in
series using different-size perforations tends to overcome the clogging
problems.

Vacuum Removal. This approach is used in the paper industry for de-
naturing the paper, in which a vacuum is applied under the paper stock.

Mechanical Force. Liquid can be readily expelled from a spongelike
particulate mass of solid by using various pressing techniques. With this
method, mechanical energy is used to force the liquid containing the partic-
ulate matter through a porous bed. The particulate matter is held in the
pores in the bed. When the pressure drop reaches a certain level, replace-
ment or backwashing takes place. This process may be either intermittent
or continuous.

Solvent Action. Materials that tend to respond well to extraction by
pressing will be more effective in solids removal when solvents are used.
The complication is that it becomes necessary to separate not only the
solids and the containing liquid from the finished process, but the solvent
as well.

Displacement. This approach, which displaces one liquid from a solid
mass by the introduction of another, is seldom used.