A.2 The Reverse Osmosis Membrane:
This is the heart of the Machine, and is the great protector. The membrane is a replica-
tion of human or animal stomach tissue, which permits the water molecule to pass through,
and holds dissolved molecules back. This is done naturally by osmotic pressure developed
because of the content of dissolved solids. To make this happen with the RO membrane, we
apply pressure to the water (reversing the process of generating pressure) and this pushes the
water molecules through, keeping most of the dissolved solid molecules behind. “Most”
means about 70% of nitrates are kept behind, and 99% of Copper and Lead, and high mol-
ecular weight material. The reverse osmosis technology will, on average, reject 95% of the
total dissolved solids in the incoming water. Over time, the RO membrane will foul with a
very thin layer of materials, and the efficiency drops, so that the TDS content of the clean
water will rise. When it rises from 10% of the inlet TDS value, it is an indicator that the RO
membrane should be replaced. In large units, the RO can be cleaned periodically by back
wash, but for home units, the cost of replacement is low and usually only needs to be made
every 3 years. About 4 gallons of water is discarded for every gallon of pure water made. In
Figure A-1, the discard is shown with a flow control orifice at the outlet of the RO stage.
This is designed to hold back the discard water to the above ratio, and maintain pressure on
the water in the membrane. For a 75-gallon per day membrane, approximately 788 milliliters
per minute of water are discarded. Note there is a check valve on the clean water outlet from
the RO. The purpose is to prevent backflow of water to the membrane from the tank or
faucet or because of autovalve failure. In the average household, 3 gallons of drinking water
are used a day.
A.3 Auto Control:
As water is produced , the pressure in the storage tank increases. To stop production of
water when the tank is full, an auto control valve is used in the system, as shown in Figure
A-1. The pressure in the tank is set nominally at 2/3 of the incoming line pressure. Normal
U.S. city water pressure is 60-psi, therefore the tank pressure when full and when water flow
stops is at a tank pressure of 40-psi.
The purified water goes to the tank where it is stored at pressure as described above.
The tank has a bladder in it, and on one side is air at 6-psi, initially. The initial volume of the
storage tank is 4.0 gallons. As the water fills the tank, it pushes against the bladder, and rais-
es the pressure as it takes space in the tank. When the pressure increases to 40-psi, water flow
stops. The net amount of water in the tank when full, less the space taken by the air at 40-
psi is approximately 2.5 gallons. When water is drawn by opening the clean water faucet on
the sink, water flows through the final post filter, another activated carbon filter, which pol-
ishes, or take out staleness of the water which has set in the tank, and the last few molecules
of taste or odor. When water is supplied to the refrigerator, the line should be therefore taken
after the post filter.
When the raw water pressure available is 40-psi or less, the RO membrane will not
operate efficiently or produce water at a reasonable rate. This is overcome by using a boost-
er pump which raises the water pressure to 80-psi.
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