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|Homeowners are increasingly concerned about contaminants in their water supply that may affect
health or cause taste and odor problems. The reverse osmosis (RO) water treatment method has
become popular for household drinking water treatment to resolve these concerns. This article
discusses the principles and process of RO treatment for household drinking water.
Contaminants removed from water by reverse osmosis
Reverse osmosis (RO) systems frequently are used to reduce the levels of total dissolved solids and
suspended particles within water. These systems remove a variety of ions and metals as well as
certain organic, inorganic and bacterial contaminants. Some contaminants treated effectively by RO
are listed in Table I. This table is not an exhaustive list of contaminants that RO may remove, but
rather lists those for which RO can be a practical treatment method for treating household drinking
water. Most RO systems also include activated carbon (AC) filters and the carbon provides the
treatment for some contaminants, as noted in the table. The RO membrane alone may not be an
effective method for total removal of these contaminants, but a properly designed system may be
effective in reducing these contaminants to safe levels. Contaminant removal by the system may vary
depending on operating conditions and equipment. Refer to the equipment section of this guide for
further explanation of activated carbon filters combined with RO.
Reverse osmosis can remove microorganisms. However, it is not recommended for that use (i.e., only
coliform-free water should be fed to the system) because membrane deterioration can occur due to
the bacteria, and contamination may occur through pinhole leaks.
Contaminants not removed from water by reverse osmosis
There are some contaminants not removed from water by RO systems. These include dissolved
gases such as hydrogen sulfide, a common nuisance contaminant with characteristic rotten egg odor,
which passes through the RO membrane. Some pesticides, solvents and other volatile organic
chemicals (VOCs) are not completely removed by RO.
The RO membrane's efficiency in reducing the amount of contaminant in the water depends on the
contaminant concentration, chemical properties of the contaminant, the membrane type and
condition, and operating conditions. Refer to the section in this guide on the RO process for
explanation of these factors.
No one piece of treatment equipment manages all contaminants. All treatment methods have
limitations and often situations require a combination of treatment processes to effectively treat the
water. AC filtration and/or sediment filtration is commonly used in conjunction with RO to help remove
silt particles or chlorine that may foul the RO membrane and also remove certain pesticides and
organic solvents that the RO membrane does not remove. The section in this guide on equipment
discusses this concept.
Regardless of the water treatment system being considered, the water should first be tested to
determine which contaminants are present. Public water systems are routinely tested for
contaminants. Water utilities are required to publish Consumer Confidence Reports (CCRs), which
inform consumers on the source of the water, contaminants present, potential health effects of those
contaminants, and methods of treatment used by the utility. Depending on the population served by
the utility, CCRs may be mailed, posted in newspapers or posted on the Internet. Copies of the CCR
can be obtained from the local water utility. Public supplies must conform to federal standards
established by the Safe Drinking Water Act. If contaminants exceed the Maximum Contaminant Level
(MCL), the water must be treated to correct the problem and/or another source of water suitable for
drinking must be provided.
In contrast, monitoring private water systems is the responsibility of the homeowner. Therefore
contamination is more likely to go undetected in a private water supply. Knowledge of what
contaminants may be present in the water should guide the testing, since it is not economically
feasible to test for all possible contaminants. It is essential to know what contaminants are present,
their quantities, and reasons for removal (i.e., to reduce contaminants posing health risks, to remove
tastes or odors, etc.) prior to selecting treatment methods or equipment.
RO is based on the principle of osmosis. In osmosis, a membrane separates two solutions containing
different amounts of dissolved chemicals. The membrane allows some compounds like water to pass
through it, but does not allow larger compounds through (i.e., a semipermeable membrane). Pressure
differences cause pure water to pass through the membrane from the dilute to the more concentrated
solution. The pressure is called osmotic pressure and this process is osmosis. The natural tendency
is for water to move through the membrane from the dilute to the concentrated solution until chemicals
reach equal concentrations on both sides of the membrane. Figure 1 shows the natural osmotic
In reverse osmosis, pressure is applied to the concentrated side of the membrane (the contaminated
side). This forces the osmotic process into reverse so that, with adequate applied pressure, pure
water is forced from the concentrated (contaminated) side to the dilute (treated) side. Treated water is
collected in a storage container. The rejected contaminants on the concentrated side of the
membrane are washed away as wastewater. Figure 1 shows the reverse osmosis process.
The amount of treated water that an RO membrane typically used in the home can produce per day is
in the range of 10 to 35 gallons per day. The amount of treated water produced depends on several
factors, including membrane type and condition, operating conditions (such as flow control and
pressure) and feed water quality (i.e., contaminant concentration, temperature and pH). Two
measures of performance of an RO membrane are recovery rate and rejection rate. Recovery rate
refers to the fact that only part of the water that flows into an RO system comes out as treated water.
Part of the water fed into the system is used as waste water to wash away the rejected contaminants.
The recovery rate is therefore a measure of efficiency calculated as:
% Recovery = (Volume of treated water produced / Total volume of feed water) x 100
The use of large quantities of water to produce little treated water may be avoided by properly
designed RO systems. Most household RO systems are designed with a 20% - 30% recovery rate.
This means that a system with 100 gallons/day of untreated water fed to it and a 20% recovery rate
would yield 20 gallons/day of treated water and dispose of 80 gallons/day in the waste stream. Proper
adjustment of the flow regulator on the side of the waste stream is important. If the flow of waste water
is slow, more time is available for water to pass through the membrane so the recovery rate is higher.
However, RO membranes are readily fouled if concentrated contaminants are not washed away soon
enough. Conversely, if the waste flow rate is too fast, the recovery rate is low and excessive water
flows down the drain.
Closely related to flow rate, water pressure is another key factor in RO systems. The incoming feed
line pressure must be adequate to overcome the osmotic pressure and any backpressure generated
from the storage tank "down-line" from the membrane. Auxiliary pumps can be added to increase
incoming water pressure as necessary. Generally, the higher the pressure difference across the
membrane the better the rejection of contaminants and recovery rate. Also, some RO systems have
shut off valves to stop flow whenever storage tank pressure is too high for efficient recovery or if the
storage tank is full.
Temperature and pH of the feed water are also factors in performance. There is a 1 to 2% decrease
in treated water produced for every degree below the standard 77° F. Well water at 45° F (a typical
temperature for groundwater) would produce about half the amount of treated water that would be
produced at 77° F. Also, slightly acidic feed water may prolong the life of the membrane and help
decrease scale buildup in the system.
The rejection rate is the percentage of contaminant that is not allowed to move through the
membrane. A rejection rate is calculated for each contaminant separately, as well as for Total
Dissolved Solids (TDS). For contaminants that cause health concerns, the rejection rate needs to be
high enough to reduce the contaminant to a safe level. The quality of the incoming water, or feed
water, is crucial here. For example, if the water supply contains nitrate at 40 mg/L, an RO membrane
with 85% rejection would reject 40 x 0.85 = 34 mg/L nitrate, leaving 6 mg/L in the treated water.
However, if the water supply contains 80 mg/L nitrates, an 85% rejection rate would reduce the nitrate
concentration to 12 mg/L in the treated water. This nitrate level, even after RO treatment, is above the
maximum contaminant level (MCL) of 10 mg/L nitrate set by the EPA.
Treatment systems can be classified as either Point-of-Use (POU) or Point-of-Entry (POE). POU
devices treat water at the point it is used, such as the faucet. Most RO systems are POU systems
placed under the sink or on the countertop. A separate faucet is generally installed at the sink to allow
the option of using treated water only for drinking and cooking. Water treated by RO can be more
corrosive than untreated water so special plumbing, in addition to the faucet, is installed with RO
systems. POE devices treat water as it enters the household so all water used within the house is
treated. POE reverse osmosis units are more costly to purchase, install and operate than POU
Although the RO process is simple, the complete system is often complex. Typical RO systems consist
of one or more pretreatment filters, the RO membrane, flow regulator, post-treatment filter, storage
tank and dispensing faucet. AC or sediment filters before the RO membrane and AC filters after the
RO membrane are commonly used. Pre-filters help extend the life of the system by removing silt and
other large particles and/or chlorine that may be harmful to the RO membrane. If the feed water is not
chlorinated, AC filters should not be used for pre-filtration because they can encourage microbial
growth on the membrane surface. In this case only a sediment pre-filter is recommended. AC
post-filters can also remove certain pesticides and organic solvents that the RO membrane does not
remove. The AC treatment process is also improved since the RO membrane removes compounds
that may hinder adsorption by the carbon.
Membrane selection is an important aspect of RO treatment that can significantly affect performance.
The most common membrane materials are polyamide thin film composites (TFC) or cellulose-type
membranes. Both are synthetic fibers. The membrane can be spiral wound (like a rolled up
newspaper), or individual hollow fibers can be bundled together. This provides a very large surface
area for water treatment within a compact tube element.
TFC membranes are more costly, but have greater strength and durability than cellulose-types. They
have higher total dissolved solids (TDS) rejection rates, are more resistant to microbial attack and are
more tolerant of high pH. Cellulose type membranes are less costly and can tolerate chlorine which is
commonly used for disinfection of drinking water. TFC membranes deteriorate in chlorinated water. If
the feed water is chlorinated and a TFC membrane is used, an AC prefilter is needed to remove
chlorine from the water. Another type of membrane is a sulfonated polysulfone (SPS) membrane. SPS
membranes are tolerant of chlorine and can withstand higher pH levels, but are more costly than
cellulose-types and less effective than TFC membranes. SPS membranes can be used in RO systems
when the water is soft and pH is high.
The storage tank generally has a capacity of 2 to 5 gallons. It is pressurized to provide adequate flow
when the tap is open. Post filters can be used for removing any taste and odor compounds or residual
organics not removed by the RO process. If an AC filter is used for pre-filtration, post-filtration can be
eliminated. Monitoring gauges and lights are also becoming increasingly common. Shut-off valves are
important to stop water flow when the storage tank is full so excess water is not wasted. Since RO
treatment uses significant amounts of water, consideration must be given to the adequacy of the
household septic system. The wastewater, carrying rejected contaminants, typically is connected to a
household drain and this wastewater increases the load on the septic system.
As with any drinking water treatment system, regular maintenance is important to extend the life of the
system and to help ensure peak performance. Pre-filters and post filters require regular replacement.
The length of time before prefilter replacement depends upon water volume, quality and contaminant
concentration. Post filter replacement also depends on contaminant concentration, as well as
membrane rejection percentages and AC removal efficiency. Manufacturers and dealers can assist in
determining replacement intervals.
Microorganisms (alive or dead) can clog RO membranes. This is called bio-fouling and RO systems
must be disinfected regularly with products provided by the manufacturer. Clogged RO membranes
can decrease water flow in the system and cause poor performance. If membrane fouling is detected
early it is possible to clean and regenerate the membrane; the method depends on the type of
membrane and fouling. Completely clogged or torn membranes require replacement. However,
damaged RO membranes are not easily detected. It is important to periodically test water to determine
if the membrane is intact and functioning properly. Many systems are equipped with a monitor that
indicates high total dissolved solids content or inadequate TDS rejection, one indicator of improper
functioning. For relatively hard water, pretreatment of the water by a softener can increase the life of
Federal, state or local laws do not regulate home RO drinking water treatment systems. The industry
is self-regulated. NSF (formerly known as the National Sanitation Foundation) and the Water Quality
Association (WQA) evaluate performance, construction, advertising, and operation manual
information. The NSF program establishes performance standards that must be met for endorsement
and certification. The WQA program uses the same NSF standards and provides equivalent American
National Standards Institute (ANSI) accredited product certifications. WQA-certified products carry the
Water Quality Association Gold Seal. Though these certifications and validations should not be the
only criteria for choosing an RO system, they are helpful to ensure effectiveness of the system.
It is possible that some water supplies may contain contaminants not addressed here, such as
cryptosporidium, giardia, hexavalent chromium and others. Reverse osmosis systems may remove
some of these contaminants as well.
Drinking water treatment using RO is one option for the homeowner to treat drinking water problems.
RO is an effective method to reduce certain ions and metals, such as nitrate and arsenic. It also can
remove certain pesticides, organic and inorganic compounds, though it is not effective for others. It is
often used in combination with AC filtration. Selecting an RO system should be based on water
analysis and assessment of the individual homeowner's needs and situation. Regular maintenance of
the membrane and replacement of any filters/cartridges are critical factors in maintaining
effectiveness and reducing bacterial contamination of the system. NSF and the WQA test and certify
products and this certification and validation can help guide selection.