Reverse Osmosis Water Treatment
Reverse osmosis (RO) is becoming a common home
treatment method for contaminated drinking water. RO is
probably best known for its use in desalination projects
(turning seawater into drinking water). However, it is
also effective for treating water quality problems in the
home. RO can reduce the amounts of organics, in-organics,
bacteria and particulates that can be found in
contaminated drinking water. Because the efficiency of
removal of various contaminants can vary, homeowners
should evaluate this when considering using RO for home
treatment Other home treatment methods may be better for
specific contaminant.
RO is based on the process of osmosis. Osmosis
involves the selective movement of water from one side of
a membrane (a plastic film that looks similar to
cellophane) to the other. To make the process work,
pressure is applied to the contaminated water, forcing
water through the membrane. Since contaminants do not
move with the water as it moves across the membrane,
purer water collects on the other side of the membrane.
The purified water that accumulates on one side of the
membrane can then be used or stored. A specific amount of
pressure is necessary to separate purified water and
contaminants. This required pressure is based on the type
and concentration of contaminants in the water. Supplying
even more pressure to the contaminated water than is
required provides better separation and a higher
production rate.
The levels of most dissolved compounds and suspended
matter present in water can be reduced by RO treatment.
However, not all compounds can be efficiently removed by
this process. The efficiency with which membranes reject
the contaminant molecules depends on the pollutant
concentration and chemical properties of the pollutant.
Membrane type and operating conditions will also affect
the degree of pollutant removal.
Efficiency of removal is often 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. For example, the
rejection percentage for nitrate can be as high as 90
percent with some systems, indicating the membrane is
highly efficient in rejecting nitrate. However, for an
incoming nitrate concentration of 110 milligrams per liter
(mg/l) --- an unrealistically high level --- 90 percent
removal would still leave 10 percent of the nitrate in the
purified water, or 11 mg/l. This is greater than the 10
mg/l maximum contaminant level for nitrate allowed in
drinking water supplies. It is important to know not only
rejection percentages, but also incoming pollutant
concentrations to effectively reduce contaminant
concentrations in the drinking water to safe levels. 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 flow when
the storage tank is full. The system must also provide for
waste flow to drains. Pre-filters containing activated
carbon are commonly used to protect chlorine-sensitive
membranes. All of these components can be purchased from
the dealer.
To continually perform well, RO systems, like all
other home water treatment devices, require regular
maintenance and replacement of various components.
Pre-filters and post-filters need to be re-placed 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 removal determine when
post-filters should be replaced. RO membranes should
typically last for one to three years, depending on
operating conditions, membrane type and pre-filter
performance.
A particularly major disadvantage of RO is the large
amount of contaminated wastewater generated. This can be
as much as 50 to 90 percent of the incoming water. This
amount depends largely on the pressure difference across
the membrane. The larger the pressure difference, the
smaller the wastage rate.