Page 205 - Industrial Ventilation Design Guidebook

4,4 WATER PROPERTIES AND TREATMENT 1 67

Vacuum Filters. If, due to the nature of the liquid, the gravity filter be-
comes unsuitable, a vacuum filter is used to create a substantial pressure dif-
ference. Vacuum filters can be divided into the following types:
* Intermittent filters
* Leaf filters
* Continuous (e.g., rotary vacuum) filters
* Vacuum pressure filters, as used in desulfurization plant
* Pressure filters
In the true gravity case, pumps are not used. If, however, the liquid is
highly viscous, to achieve efficient operation, pumps are required to force the
fluid through the pressure filters. The pump can be considered essentially as a
press with a plate-and-frame filter. The plate-and-frame filter consists of a se-
ries of frames over which the filter medium is stretched. A centrifugal basket
of fine mesh is another method of particulate removal.

Dialysis. If a solution containing colloidal particle is placed on one side
of a dialysis membrane, the water on the other side will allow the solution to
be reduced in concentration as it passes through the membrane.

Electrostatic Properties of Solids in Suspension. Some solids in suspen-
sion will migrate from one pole to another when placed between direct current
electrodes. The phenomenon of solids moving toward an electrode is known
as cataphoresis.
To close this section on treatment, two more methods that depend on bac-
teriological action are considered:
« Aerobic treatment
* Anaerobic treatment

Aerobic Treatment. The activated sludge process depends on aerobic
biological action. In this case the microorganisms, in searching for food,
break down the complex organic substances into simple stable substances.
This process results in the removal of soluble and suspended organic matter
from wastewater.
The growth of microorganisms in the presence of dissolved oxygen re-
moves the majority of pollutant matter; in turn, protozoa grow and feed on
these organisms. The resulting balance is of a living culture in suspended form
in the activated sludge floe. This process is ideally suited for the removal of
carbonaceous matter and nitrification from wastewater.
The principal elements of the system include an aeration tank in which the
wastewater is thoroughly mixed with continuously activated sludge and oxy-
gen. From this part of the process, it passes into a clarifier tank, where the set-
tled sludge is removed from the purified water to be recycled by the return
activated sludge pumps.
For this system to work, two exacting requirements must be met. The aer-
ation device must be capable of both transferring oxygen from the atmosphere
to the liquid, and distributing this oxygen throughout the wastewater to the