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 TM 55-1925-273-10-1
0028 00
PRINCIPLES OF REVERSE OSMOSIS
Osmosis is defined as the spontaneous passage of a liquid from a diluted solution to a more concentrated solution across a
semi-permeable membrane. This allows the passage of the solvent (water), but halts the dissolved solids (solutes). In
naturally occurring osmosis, this transfer of water from one side of the membrane to the other will continue until the pressure
is equalized on both sides of the membrane. When at equilibrium, the quantities of liquid passing in either direction is equal.
The force created that causes osmosis to occur is called `osmotic pressure'. In natural occurring functions, fluids will flow
from the purer side to the saline side of the membrane until the osmotic pressures are equalized. If the osmotic pressure on
the saline side of the membrane is higher, fluid is forced to flow through the membrane in reverse. This is how the solution
containing the higher salt concentration flows into the solution with the lower salt concentration. This process is called
Reverse Osmosis (RO).
SPIRAL WOUND MEMBRANE
Reversing a natural occurring process requires solutions to many problems. The first problem is how to efficiently recreate
the effect so that a sufficient quantity of fresh water can be produced. Natural osmosis occurs at the cellular level. This
means that since there are billions of cells in the body using osmosis, there must be billions of reactions occurring at the same
time. All these reactions occur in a relatively small space. We can't create billions of cells, but we can create one large one
that does the same thing. So, the solution is to increase the surface area using one giant cell. The giant cell theory led to the
development of the spiral wound membranes that are used in the ROWPU.
If each side of the membrane were unrolled and laid flat, they would cover an area of fifteen square acres. The membrane
sheets are only two molecules thick, and they are rolled into spirals and placed into tubes that are about four feet long and
three inches in diameter. Reverse osmosis requires the membranes to be very thin, and they are delicate and sensitive to
damage when mishandled. The membranes are also very sensitive to clogging. In natural osmosis, the fluids do their job by
going back and forth. When water is made for potable use, the fluid only flows one way. This causes clogs due to the build
up of solids, so the solids must be removed.
The solids are washed away by allowing more water to flow past the saline side of the membranes than is allowed to pass
through the membrane. The ratio for the ROWPU is five gallons of raw water used for every two gallons of fresh water
produced. Mechanically, the membranes act as filters to keep the salt and other impurities from passing to the fresh water
side. If the solids were not removed, the membranes would quickly clog. The raw water is also passed through various
filters that remove impurities down to 5 microns in size. This results in greatly increasing the life of the membranes and a
cost effective and low maintenance unit.
WATER TEMPERATURE EFFECT
Water flow through the membrane is significantly affected by the water temperature. The higher the water temperature, the
more active the water molecules. The more active the water molecules, the easier it is to push them through the membranes.
This means that at any given pressure, the flow rate will increase as water temperature increases. When the temperature is
reduced, the flow rate for a given pressure is reduced. The operating pressure also has a direct effect on product water
quantity. It takes less pump pressure to achieve the unit's rated capacity in warmer water. In order to reach the rated service
life of the membranes, the system must be operated at the lowest pressure necessary to achieve the designed product water
flow rate. In warm water, the system will easily make more water than rated capacity. However, exceeding rated capacity
will shorten the service life of the membranes.
REVERSE OSMOSIS WATER PURIFICATION UNIT (ROWPU) OPERATING PRINCIPLES
The LT's General Service (GS) system provides raw water to the ROWPU. ROWPU 1 can make 1500 gallons (5678 liters)
a day, and ROWPU 2 can make 800 gallons (3028 liters) a day. These values are easily achievable if the high pressure
pumps do not exceed 900 PSI (62 bar) and if overall differential pressure never exceeds 25 PSI (1.7 bar). The raw water
entering the system passes through three filtering cycles so that solids no larger than 5 microns will enter the membranes.
After being filtered, the raw water is pressurized and directed to a semi-permeable membrane. The semi-permeable mem-
brane permits the passage of water and rejects the other contaminants. After passing through the membrane, waste water is
expelled via the general service system overboard discharge, and potable water is sent to the brominator for chemical
treatment before being discharged to the potable water tanks.
0028 00-6
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