b. Design Considerations. Surface area of tank and retention time are the two
primary considerations in designing and operating a coagulation-sedimentation tank
system. Surface area is the area of the tank liquid surface exposed to the atmosphere.
This parameter allows for adequate chemical-solids interaction. Detention time is the
amount of time a solid particle or unit volume of water will spend in the tank. This
parameter also allows for the chemical-solids interaction to occur.
c. Residues. The settled residues collect on the bottom of the sedimentation
tank because their density is greater than that of water. Depending upon the type of
tank, mechanical collectors scrape the settled residues or pumps suck the sludge swells
from the bottom into hoppers, which are then transported for ultimate disposal via land
application, centrifuging, or ponding.
2-12. MEMBRANE PROCESSES
a. Electrodialysis. In this procedure, stacks of alternating positively and
negatively charged membranes in single and multi-stage configurations constitute the
semipermeable membrane. Different impurities are removed in different degrees
according to the selectivity of the membrane.
(1) Pretreatment. Plugging of the membrane can be prevented by
pretreatment with carbon adsorption or filtration. The water to be treated is held in large
tanks, with the membranes serving as obstacles to flow. All correctly charged particles
in the water are attracted to the membrane obstacle, but only certain ones are able to
pass through. This allows for selective distribution of the particles and a cleaner water.
Chlorine oxidation disinfects the matter.
(2) Efficiency factors. Factors such as air temperature, water temperature,
water pH, and electrical power applied to the equipment determine the efficiency of this
b. Reverse Osmosis.
(1) Use of reverse osmosis. This process is used primarily to remove salt
from sea water (desalination) for drinking or manufacturing purposes. Reverse osmosis
operates at pressures of 500 to 1,500 pounds per square inch with hydraulic application
rates of 3 to 50 gallons per day per square foot. The reverse osmosis membrane will
reject from 90 to 99 percent of the total dissolved solids presented to it. Either process
will remove 100 percent of the suspended solids. High temperature conditions allow for
more water to be treated with this process.
(2) Eguipment. Reverse osmosis equipment is usually designed as hollow-
fiber, spiral-wound membrane, or as tubular units. Pressure can be applied to the high
salt-strength water in the tubes. This forces the water present through the barrier
membrane while trapping the salt and solids in the tubes.