05 Water Purification: Physical Treatment: Reverse Osmosis.
Description
This article is from the Water Treatment FAQ, by Patton Turner with numerous contributions by others.
05 Water Purification: Physical Treatment: Reverse Osmosis.
Reverse osmosis forces water, under pressure, through a membrane that
is impermeable to most contaminants. The most common use is aboard
boats to produce fresh water from salt water. The membrane is
somewhat better at rejecting salts than it is at rejecting non-ionized
weak acids and bases and smaller organic molecules (molecular weight
below 200). In the latter category are undissociated weak organic
acids, amines, phenols, chlorinated hydrocarbons, some pesticides and
low molecular weight alcohols. Larger organic molecules, and all
pathogens are rejected. Of course it is possible to have a
imperfection in the membrane that could allow molecules or whole
pathogens to pass through.
Using reverse to desalinate sea water osmosis requires considerable
pressure (1000 psi) to operate, and for a long time only electric
models were available. Competing for a contract to build a hand
powered model for the Navy, Recovery Engineering designed a model that
could operate by hand, using the waste water (90 percent of the water
is waste water, only 10% passes through the filter) to pressurize the
back side of the piston. The design was later acquired by PUR. While
there is little question that the devices work well, the considerable
effort required to operate one has been questioned by some survival
experts such as Michael Greenwald, himself a survivor of a shipwreck.
On the other hand the people who have actually used them on a liferaft
credit the availability of water from their PUR watermaker for their
survival.
PUR manual watermakers are available in two models: The Survivor 06
($500) produces 2 pints per hour, and the Survivor 35 ($1350) produces
1.4 gal/hr. The latter model is also available as the Power Survivor
35 ($1700), which produces the same water volume from 4 Amps of 12
VDC, and can be disconnected and used as a hand held unit A number of
manufactures, including PUR, make DC powered models for shipboard
use.. PUR recommends replacing the O rings every 600 hours on its
handheld units, and a kit is available to do this. Estimates for
membrane life vary, but units designed for production use may last a
year or more. Every precaution should be taken to prevent petroleum
products from contacting the membrane as they will damage or destroy
the membrane. The prefilter must also be regularly changed, and the
membrane may need to be treated with a biocide occasionally
Reverse osmosis filter are also available that will use normal
municipal or private water pressure to remove contaminates from water,
as long as they aren't present in the levels found in sea water.
The water produced by reverse osmosis, like distilled water, will be
close to pure H2O. Therefore mineral intake may need to be increased
to compensate for the normal mineral content of water in much of the
world.
Continue to:
My Books
