Removal of
Organic Refractories by Reverse Osmosis
PAUL J. BENNETT, Junior Sanitary Chemist
SHYAMALA NARAYARIAN, Junior Sanitary Chemist
ERVIN HINDIN, Associate Sanitary Chemist
Sanitary Engineering Section
College of Engineering Research Division
Washington State University
Pullman, Washington
In recent years, increased attention has been focused on the reuse of municipal and industrial wastewaters. Often, the reuse of such wastewater requires some
type of treatment. Conventional biological secondary waste treatment produces a
water of limited reuse. Therefore, tertiary or advanced wastewater renovation
must be employed to obtain the fullest utilization of the effluent. Which one of
the advance treatment methods has the potential of economically producing a
high quality water from wastewater? The reverse osmosis procedure is one approach which shows merit.
The use of the reverse osmosis method for the desalination of brackish and sea
water is well documented (l, 2, 3, 4). Because of the great versatility this process
possesses, an investigation was undertaken in 1963 to determine its applicability
in producing a water low in organic substances from secondary treated wastewater
effluents.
REVERSE OSMOSIS
To gain a better understanding of the reverse osmosis process, the basic principles of the osmosis phenomena must be understood.
When an aqueous solution is separated from water by a suitable membrane
the water molecules will permeate through the membrane into the solution. The
permeation of the water molecules into the aqueous solution will result in a decrease in the original mass of the water with an increase in the original mass of
the aqueous solution. Figure 1 is an illustration of this phenomena. Initially, at
time zero, the level of water in the container and that of the treated wastewater
in the tube are the same. A film of an ideal semi-permeable membrane covers
the end of the tube immersed in the water. After a period of time, "t", the water will be at a lower level than at time zero, while the level of the treated
wastewater in the tube will have risen.
It appears that in the osmotic phenomena a permeation or diffusion exists
which forces water molecules to diffuse through the membrane from a region of a
higher water concentration to that of a lower water concentration. As the treated
wastewater solution rises in the tube, a hydrostatic pressure develops in the tube.
Development of a hydrostatic pressure increases the tendency of the water molecules in the treated wastewater solution to permeate back into the water. Eventually a state is reached where the hydrostatic pressure equals the diffusion or
permeation pressure. The pressure differential under the dynamic equilibrium
state is known as the osmotic pressure.   This is what occurs during osmosis.
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