TM 55-1925-282-14&P
OPERATOR, UNIT, DIRECT SUPPORT, AND GENERAL SUPPORT MAINTENANCE
REVERSE OSMOSIS WATER PURIFICATION UNIT FOR
INLAND AND COASTAL LARGE TUG (LT)
THEORY OF OPERATION
INTRODUCTION
The purpose of this work package is to introduce the reader to the Reverse Osmosis Water Purification Unit
(ROWPU) used aboard the LT. This work package explains what osmosis is and how the process is manipulated
mechanically to produce fresh water. The work package also explains how reverse osmosis water purification is
accomplished onboard the LT.
PRINCIPLES OF REVERSE OSMOSIS
The specific process through which osmosis occurs is called ion exclusion. A concentration of ions at the mem-
brane surface forms a barrier that allows pure water molecules to pass while excluding molecules of foreign
materials.
OSMOSIS
Osmosis is the diffusion of two mixable solutions through a semi-permeable membrane in such a manner as to
equalize their concentrations. Natural osmosis occurs when a substance of lesser salinity naturally diffuses
through a membrane into a higher concentration salinity. Cellular functions in the human body function in this
manner. The process is very energy efficient in that it requires very little heat or pressure. However, the warmer
the liquids, the more efficient the process due to the activity of the molecules. It is a natural function for liquids of
unequal density to try to equalize themselves with one another when they are in molecular contact. In natural
osmosis, liquids that are less saline attempt to equalize pressure by becoming more saline. The membrane of a
living cell (figure 1, item 1) passes lower density saline solution (figure 1, item 2) into higher density saline
solutions (figure 1, item 3). This happens because the lower density solution (figure 2, item 1) inside the cell
naturally moves to equilibrium with the higher density solution (figure 2, item 2) the cell is located in. The cell
membrane (figure 2, item 3) allows liquid to pass, but it prevents particles (figure 2, item 4) from passing.
REVERSE OSMOSIS
Osmosis is an important process to life, but it does not produce water that is palatable to humans. In order to
produce water, the osmosis process must be reversed. The same principles apply, but some form of energy is
necessary to force osmosis reversal. Thus, the process called `reverse osmosis' was engineered to overcome
the natural phenomenon of osmosis. Reverse osmosis works by forcing water of a higher salinity content through
some type of semi-permeable membrane in an effort to remove the impurities. Salt is one of the impurities that
is removed during the reverse osmosis process, and this makes it possible to create potable drinking water from
undrinkable water sources.
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 Osmo-
sis (RO).
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