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OP-2 BAD SMELLING PRODUCT WATER
The reverse osmosis membrane is permeable by many gases including hydrogen sulfide, the gas
that causes rotten eggs to smell the way they do. If there are bad odors in the feed water they
will go through the membrane and the product water will be affected. Usually the source of the
odor is from the decay of planktonic creatures trapped in the sea strainer and prefilters
These
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tiny oxygen loving creatures soon suffocate and die inside the prefilter housings when the unit is
shut down. Once all the available oxygen is consumed, anaerobic bacteria begin to grow,
causing the odor. If a unit being used frequently begins to make smelly water, usually the
prefilters are the source of the problem. This occurs in a week or two in cold climates, but in less
than one night in very warm waters like the Sea of Cortes or the Red Sea. These bacteria can
spread throughout the watermaker, and begin to grow on the membrane, causing poor water
quality and high feed pressures.
Filling the system with fresh water after every use greatly slows this process, allowing the
automated units to operate with less frequent prefilter changes, but units operated for only an
hour or so each day will probably need to have the filters changed due to odor before they are
dirty enough to restrict water flow. Prefilters can be cleaned. We recommend that you have
three sets in service: one in the unit, one set soaking overnight in a bucket of clean fresh or salt
water, and one set drying for the next use. After shutting down the unit, remove the used
prefilters and install the dry set. Leave the housings full of air until the next use. On non-
automated systems, open the pressure relief when starting if there is a lot of air in the system
until the air is cleared out through the brine overboard. The filters will get just as clean when
soaked in sea water, but dry much faster if soaked in fresh. Given gentle handling, prefilters can
be reused many times.
Bad smelling product water is usually caused by bad smelling feed water, but can also be caused
by a fouled membrane if the membrane has been left unpickled. If the unit makes smelly but not
salty water after a long idle period and the prefilters are new, the smell can be eliminated by
running the unit unpressurized for an hour or so to flush the membrane.
Odors in the product water can also be eliminated by adding a charcoal filter in the product water
line. W.S. Darley & Company offers a product water filter kit.
More on this subject is available on our website at
www.purifiresystems.com
8/17/04
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