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Reverse Osmosis
Reverse Osmosis (RO) units are usually installed as point-of-use (POU) systems. They
are generally placed at the kitchen faucet and used to purify water intended for
drinking and cooking purposes only.
RO IS BASED ON the process of osmosis. Osmosis
is the selective
movement of water from one side of a membrane to the other. Pressure is applied to
the contaminated water, forcing water through the membrane. Because contaminants
can not move with the water across the membrane, only purer water passes through
the membrane. The process splits the liquid flow into a purified
stream of liquid which has passed through the membrane (permeate) and a
concentrated stream of liquid which has passed over the surface of the membrane
(concentrate).
BASIC COMPONENTS of an RO system should include a
pre-filter to remove fouling
agents such as rust and lime; an RO module containing the membrane; an activated
carbon post-filter to remove residual taste, odor and some compounds from the
purified water; a storage tank; and various valves, including a shut-off valve
that stops the water. There are enhanced RO systems available with
pressure pumps and up to 10 filter stages.
RO is effective at filtering or
reducing:
- arsenic, chloride, fluoride,
nitrate, sulfate
- lead, mercury, other heavy
metals
- some organics
- bacteria and viruses
Performance
Membrane selection affects
performance and is based on various water characteristics such as acidity,
hardness, total dissolved solids and chlorine content. Another characteristic influencing an RO system's performance is
water pressure. The higher the water pressure, the better the rejection of
pollutants and the more purified the water. Water pressures vary
significantly changing the efficiency of the unit. Water
production and rejection rates in most units decrease as the storage tank fills,
since this increases pressure on the purified water side of the membrane. For maximum
efficiency the storage tank should be emptied daily.
The amount of water passing
through the membrane is referred to as flux and is generally expressed in
gallons per square foot of membrane per day (GFD). Membranes have a
maximum recommended flux, the higher the flux the more water is produced.
Contaminant rejection by the membrane remains constant and therefore the more
water that passes through the membrane the higher the quality of permeate water.
Typical water pressure in homes
may not be adequate for efficient RO treatment, however, booster pumps can be added. Doubling the pressure across a membrane more than doubles the output
flow rate of purified water.
The production rate of purified
water is also influenced by temperature.
The higher the water temperature, the better the production rate. A drop in
temperature from 75 degrees to 45 degrees Fahrenheit
virtually cuts the production of purified water in half.
Considerations
RO systems require regular maintenance and replacement of various components.
Pre-filters and post-filters need to be replaced on a regular basis. The length of
time between changing pre-filters will depend on the water quality, especially
the concentration of solids, The contaminant concentration, membrane rejection
percentages, and efficiency of activated carbon. RO membranes typically last for one to
three years, depending on operating conditions, membrane type and pre-filter
performance. Consider
maintenance.
It is difficult to know when to replace the various components
of an RO system. RO membranes may become clogged by rust particles, Fe/Mn bacteria, silt,
etc., if pre-filters are insufficient water conditions, if this occurs the
membrane cannot be regenerated. Verification of a system's performance can only be determined by
chemical
testing. However, if we have access to your water analysis and
have installed your RO system we can usually accurately determine when filters and
membranes need to be replaced, presuming that contaminant concentrations have not
increased significantly over time.
A major disadvantage of RO is the large amount contaminated
wastewater generated. This can be from as little as 50 percent to higher than 90 percent of the incoming
water. This amount depends largely on water temperature and the pressure difference across the
membrane. The warmer the temperature and larger the pressure difference, the smaller the wastage rate.
Consider water
consumption and disposal.
Efficiency is described using the term "rejection
percentage," which is the percent of a particular contaminant that doesn't
cross the membrane, i.e., is rejected by the membrane. However, rejection
percentages do not tell the whole story. If an incoming concentration of a
chemical is high, a rejection rate of 80 or 90 percent may not be sufficient
to reduce contaminant concentration to safe levels. Consider
testing your water before purchasing a solution.
Be sure to size your system to your
needs. RO water generation is relatively slow process, RO units are
rated at optimal conditions (70 degree water at 60 PSI, into an empty storage
tank) in US gallons (US) per day. Expect half of the rated capacity if
you have city or well water at 45 degrees and water pressure of 45 PSI. On the
other side of the equation in optimal conditions you do not want to store RO
water longer than a day. RO like distilled water is relatively
pure. Pure water is frequently termed as the universal solvent because
it "wants" to pick up or dissolve many of the things it comes in
contact with. Consider
your daily requirements.
Both Distilled Water RO water
have been implicated in the leaching of nutrients from the body. Our
investigation of this while definitely not as thorough as the paper
done by the WQA on this topic has not found scientific proof of this.
Like Distilled Water RO can create rather "flat" tasting
water, much of this can be alleviated by aeration and chilling the water.
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