Nebraska Cooperative Extension
G03-1490-A
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 guide discusses the
principles and process of RO treatment for household drinking water.
Jodi Kocher, Extension Engineer;
Bruce Dvorak, Extension Environmental Engineering Specialist;
Sharon Skipton, Extension Educator
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.
| Table I.
Contaminants removed by household reverse osmosis units. |
| Ions and Metals |
Arsenic, Aluminum,
Barium, Cadmium, Calcium, Chloride, Chlorine1,
Chromium, Copper, Fluoride, Iron, Lead, Magnesium,
Manganese, Mercury, Nitrate, Potassium, Radium, Radon1,
Selenium, Silver, Sodium, Sulfate, Zinc |
| Organic Chemicals |
Benzene1, Carbon
tetrachloride1, Dichlorobenzene1,
Toluene1, Trichloroethylene1, Total
Trihalomethanes (THM's)1 |
| Particles |
Asbestos, Protozoan cysts,
Cryptosporidium |
| Pesticides |
1,2,4-trichlorobenzene1,
2,4-D1, Atrazine1, Endrin, Heptachlor,
Lindane, Pentachlorophenol |
| 1Activated
carbon filters, commonly included in RO systems, can provide
treatment for these contaminants. |
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) not listed in the table above are not completely
removed by RO. Refer to Extension Circular EC03-703, Drinking
Water Treatment: An Overview for a discussion of possible water
quality problems and appropriate treatments for these contaminants.
Further information can be obtained from the appropriate treatment
guide in the Drinking Water Treatment series (listed at the end of
this publication). NebGuides are available online at http://ianrpubs.unl.edu/water/
or can be obtained from your local or state Extension office.
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.
Water testing
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.
Refer to NebGuide G89-907 Testing for Drinking Water Quality for
testing information.
Treatment principles
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.
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.
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 Nebraska
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.
Equipment
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 systems.
Although the RO process is simple,
the complete system is often complex. Typical RO systems consist of
a pretreatment filter, the RO membrane, flow regulator,
post-treatment filter, storage tank and dispensing faucet as shown
in Figure 3. 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 the membrane.
Selection Requirements
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.
Other important guidelines for
consumers purchasing drinking water treatment equipment are
discussed in NebGuide G03-1488 Drinking Water Treatment: What
You Need to Know When Selecting Water Treatment Equipment.
Drinking water treatment NebGuides and guides on specific
contaminants are listed at the end of this publication. The NebGuide
series on drinking water treatment focuses on contaminants most
likely to be encountered in Nebraska drinking water supplies. 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.
Summary
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.
Related Drinking Water
Treatment Publications
EC03-703 Drinking Water Treatment: An
Overview
G03-1488 Drinking Water Treatment: What You Need to Know When
Selecting Water Treatment Equipment
G03-1489 Drinking Water Treatment: Activated Carbon Filtration
G03-1491 Drinking Water Treatment: Water Softening (Ion Exchange)
G03-1492 Drinking Water Treatment: Sediment Filtration
G03-1493 Drinking Water Treatment: Distillation
G03-1494 Drinking Water Treatment: Emergency Procedures
G03-1496 Drinking Water Treatment: Continuous Chlorination
G95-1255 Shock Chlorination of Domestic Water Supplies
Related Drinking Water
Contaminant Publications
G89-907 Testing for Drinking Water
Quality
G90-989 Drinking Water: Bacteria
G96-1274 Drinking Water: Hard Water
G96-1275 Drinking Water: Sulfates and Hydrogen Sulfide
G96-1279 Drinking Water: Nitrate-Nitrogen
G96-1280 Drinking Water: Iron and Manganese
G96-1282 Drinking Water: Man-made Chemicals
G97-1333 Drinking Water: Lead
G98-1360 Drinking Water: Copper
G98-1376 Drinking Water: Fluoride
G98-1369 Drinking Water: Nitrate and Methemoglobinemia
G02-1448 Drinking Water: Bottled or Tap?
G03-1489 Drinking Water: Storing an Emergency Supply
NF02-505 Drinking Water: Chloramines Water Disinfection in Omaha
Metropolitan Utilities District
Technical Review
provided by: Joe Harrison, Technical Director, Water Quality
Association; Tom Schuerman, Nebraska Department of HHS Regulation
and Licensure; Mike Wentink, Nebraska Department of HHS Regulation
and Licensure; Glenn Hoffman, Cooperative Extension; Darrel Siekman,
Cooperative Extension; John Wilson, Cooperative Extension.
Views expressed in this publication are
those of the authors and do not necessarily reflect the views of
either the technical reviewers or the agencies they represent.
References
"Treatment Systems for
Household Water Supplies: Reverse Osmosis,"
North Dakota State University Extension Service
"Water Treatment Notes:
Reverse Osmosis Treatment of Drinking Water," Cornell
Cooperative Extension, New York State College of Human Ecology
"Reverse Osmosis for Home
Treatment of Drinking Water,"
Michigan State University Extension Service
"Reverse Osmosis for Home
Treatment of Drinking Water,"
Purdue University Cooperative Extension Service "Drinking
Water Standards," www.epa.gov/safewater/mcl.html
"Understanding the New
Consumer Confidence Report,"
www.awwa.org/Advocacy/bluethumb98/consumer.cfm
"Quality Water on Tap
Through Reverse Osmosis," Water Quality Association,
Lisle, Illinois, 1999.
File G1490 under: WATER RESOURCE
MANAGEMENT
A-31, Water Quality
Issued October 2003 2,000
Electronic version issued November
2003
pubs@unl.edu
Issued in
furtherance of Cooperative Extension work, Acts of May 8 and June
30, 1914, in cooperation with the U.S. Department of Agriculture.
Elbert C. Dickey, Director of Cooperative Extension, University of
Nebraska, Institute of Agriculture and Natural Resources.
University of Nebraska
Cooperative Extension educational programs abide with the
non-discrimination policies of the University of Nebraska-Lincoln
and the United States Department of Agriculture.
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