SPIRAL WOUND MEMBRANE
Reversing a natural occurring process requires solutions to many problems. The first problem is how to effi-
ciently 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 mem-
branes 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 membrane 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.
COMPONENTS OF THE ROWPU
Table 1 outlines the major functional components of the ROWPU system. The item number corresponds to the
callouts in figure 3. The components are outlined in a logical fashion that will allow the reader to understand the
basic operating principles of the ROWPU. The ROWPU contains two units. ROWPU 1 (the starboard unit) is
fitted with two reverse osmosis membranes, and ROWPU 2 (the port unit) has only one membrane.