North Dakota State University
NDSU Extension Service
Treatment Systems for Household Water Supplies
7. Iron and Manganese Removal
AE-1030, February 1992
Bruce Seelig, Water Quality Specialist, North Dakota
Extension Service
Russell Derickson, Extension Associate in Water and
Natural Resources, South Dakota Extension Service
Fred Bergsrud, Water Quality Coordinator, Minnesota
Extension Service
Neither iron nor manganese in water present
a health hazard. However, their presence in water may cause
taste, staining, and accumulation problems.
Because iron and managanese are chemically
similar, they cause similar problems. Iron will cause
reddish-brown staining of laundry, porcelain, dishes, utensils,
and even glassware. Manganese acts in a similar way but causes a
brownish-black stain. Soaps and detergents do not remove these
stains, and the use of chlorine bleach and alkaline builders
(such as sodium carbonate) can actually intensify the stains.
Iron and manganese deposits will build up
in pipelines, pressure tanks, water heaters, and water softeners.
This reduces the available quantity and pressure of the water
supply. Iron and manganese accumulations become an economic
problem when water supply or softening equipment must be
replaced. There are also associated increased energy costs, like
pumping water through constricted pipes or heating water with
heating rods coated with iron or manganese minerals.
Iron and manganese are concentrated in water
by contact with rocks and minerals, and occasionally man-made
materials like iron and steel pipes. It is usually groundwater
supplies that may require treatment for high levels of iron and
manganese. Generally speaking, few surface water supplies have
high enough levels of either to cause problems. Occasionally
discharge of acid industrial wastes or mine drainage may increase
iron or manganese to problem levels in surface water.
Iron and manganese exist in many different
chemical forms. The presence of a given form of iron or manganese
in geologic materials or water depends on many different
environmental factors. We can often anticipate iron and manganese
problems in water by observing a few general principles that
affect water chemistry.
An important principle to remember about
chemical reactions is that, if allowed enough time, they will
reach an equilibrium with the surrounding environment. When the
conditions of that environment are changed, such as pumping water
from an underground acquifer, the chemical equilibrium is upset.
This will lead to either solution of certain elements such as
iron and manganese or their precipitation.
A general rule of thumb is that oxygenated
water will have only low levels of iron and manganese. The reason
is that both iron and manganese react with oxygen to form
compounds that do not stay dissolved in water. Surface water and
shallow groundwater (Figure 1) usually have enough dissolved
oxygen to maintain iron and manganese in an undissolved state. In
surface water, iron and manganese are most likely to be trapped
within suspended organic matter particles.
Waters that do not have regular contact with
the atmosphere tend to be low in oxygen (oxygen poor). Iron and
manganese carbonates in an oxygen poor environment are relatively
soluble and can cause high levels of dissolved iron and
manganese. However, if iron is associated with sulfur as iron
sulfide rather than iron carbonate, dissolved iron remains low.
Dissolved oxygen generally decreases with depth, so these types
of conditions are more likely to occur in deep wells. Sometimes
oxygen poor conditions can also occur in relatively shallow wells
that have stagnant water with very slow turnover.
Iron and manganese problems are most likely
to develop in water from wells with high carbonate and low oxygen
as shown in the middle well in Figure 1. Problems occur when this
type of water is pumped to the surface. The chemical equilibrium
is changed upon exposure to the atmosphere. The end result is
precipitation of iron and manganese compounds in plumbing, on
fixtures, and on clothing, dishes, and utensils.
Figure 1. The amount of iron and manganese dissolved
in water often follows a trend of low to high back to low again as depth of
the well increases. (Snoeyink, V. L. and D. Jenkins, 1980)
Some types of bacteria derive their energy
by reacting with soluble forms of iron and manganese. These
organisms are usually found in waters that have high levels of
iron and manganese in solution. The reaction changes the iron and
manganese from a soluble form into a less soluble form, thus
causing precipitation and accumulation of black or reddish brown
gelatinous material (slime). Masses of mucous, iron, and/or
manganese can clog plumbing and water treatment equipment. They
also slough off in globs that become iron or manganese stains on
laundry. Bacterial reactions with iron and manganese do not cause
any additional precipitation compared to normal exposure to
oxygen. However, precipitation caused by bacteria occurs faster
and tends to concentrate staining, thus making it more apparent.
An additional source for dissolved iron may
be the pipelines through which water flows. Water with high
salinity or acidity from dissolved carbon dioxide or other acids
will be corrosive to metal pipes. In order to establish
equilibrium, iron and other metals will be dissolved from the
pipelines. If household pipes are being attacked by corrosive
water and cause problem levels of metals such as iron, copper,
and lead, the water can be treated to reduce corrosivity and
level of dissolved metals.
Acidity can be reduced by either adding
alkaline materials such as sodium carbonate or passing water
through filters made of alkaline material. Salinity can be
treated by either distillation or reverse osmosis.
Because different metals are more or less
corodable, a solution to the problem may be to use a more
resistant metal. A plumber should be consulted regarding
materials that are best suited to local water conditions.
How much iron or manganese in the water is
needed to cause these sorts of problems? There is no pat answer
to this question, because it varies with each household
situation. Standards for iron and manganese are based on levels
that cause taste and staining problems and are set under EPA
Secondary Drinking Water Standards. For most individuals 0.3
parts per million (ppm) of iron and 0.05 ppm of manganese is
objectionable. Usually iron and manganese do not exceed 10 ppm
and 2 ppm, respectively, in natural waters. Iron and manganese
are found at higher concentrations; however, that condition is
rare.
The need to test for iron and manganese in
water is not as critical as it is for other types of contaminants
that can cause health problems. Iron and manganese are not a
problem in household water until they become detectable by the
senses. Consequently, elaborate laboratory analyses are not
required to determine if iron or manganese are a problem.
Laboratory analyses for iron and manganese are needed to quantify
the problem.
Exposure of the sample to air will cause
precipitation of iron and manganese. To get an estimate of the
amount of iron and manganese originally dissolved in the well
water, precipitation must be prevented or the precipitated
material must be redissolved. Before sampling for iron and
manganese, a certified laboratory should be consulted. They will
recommend a sampling procedure that will provide an accurate
estimate of dissolved iron and manganese in the source water.
Polyphosphates react with dissolved iron
and manganese by trapping them in a complex molecule that is
soluble in water (Figure 2). As a result the iron and manganese
are not available to react with oxygen and precipitate.
Polyphosphates can be fed into the water system with controlled
injection equipment. Polyphosphates are not stable at high
temperatures. If water is treated prior to heating in a water
heater, the polyphosphates will release iron and manganese in the
heater as they break down. The released iron and manganese will
then react with oxygen and precipitate.
Polyphosphate treatment is a relatively
cheap way to treat water for low levels of iron and manganese.
Depending on the type of polyphosphate used, water with 1 to 3
ppm of iron can be adequately treated.
Figure 2. Polyphosphates protect dissolved iron and
manganese from reacting with oxygen and precipitating on household appliances,
bath/plumbing fixtures, and laundry.
Soluble iron and manganese (iron and
manganese dissolved in water) can be exchanged for sodium on an
exchange resin or zeolite (Figure 3). This process of iron and
manganese removal is the very same ion exchange process that
removes hardness or calcium and magnesium (refer to the softening
circular in the Treatment Systems for Household Water Supplies
series). Iron and manganese are removed during normal operation
of the water softener. They are later removed from the exchange
medium along with calcium and magnesium during regeneration and
backwashing. Some water softeners are capable of adequately
treating water having iron up to 10 ppm. However, others are
limited to treating water with iron no greater than 1 ppm. If
iron and manganese removal is desired in addition to hardness,
the manufacturer's recommendations should be checked.
Figure 3. Household water softening units will remove
some iron and manganese from water. Most units, however, are not designed to
handle very large amounts of iron and manganese and may become plugged when
concentrations are high.
One of the disadvantages of depending on
ion exchange for iron and manganese removal is precipitation by
oxygen. Some of the precipitate becomes tightly bound to the
exchange resin and over time reduces the exchange capacity by
plugging pores and blocking exchange sites. If iron bacteria are
present, the problem is even worse. Also, if suspended particles
of insoluble forms of iron or manganese are present in the water
prior to softening, they will be filtered out on the resin and
cause plugging. Suspended iron and manganese should be filtered
out before water enters the softener.
A clogged water softener can be cleaned by
acid regeneration if the unit is made to withstand acid
corrosion. The manufacturer should be consulted before this is
attempted. The problem with iron bacteria can be eliminated by
chlorinating (Refer to the Chlorination circular in the Treatment
Systems for Household Water Supplies series) and filtering the
water at some point before it reaches the softener. As long as
levels of iron and manganese in the water do not exceed the
manufacturer's recommendations, iron and manganese clogging
should not be a significant problem. When iron and manganese
levels are higher than recommended by the manufacturer, iron and
manganese removal will be necessary prior to softening.
One of the first types of filters to be
used to treat water was the "greensand" filter. The
active material in "greensand" is glauconite.
Glauconite is a green clay mineral that contains iron and has ion
exchange properties. Glauconite often occurs mixed with other
material as small pellets, thus the name "greensand."
The glauconite is mined, washed, screened, and treated with
various chemicals to produce a durable greenish-black product
that has properties that allow it to adsorb soluble iron and
manganese.
As water is passed through the filter,
soluble iron and manganese are pulled from solution and later
react to form insoluble iron and manganese. Insoluble iron and
manganese will build up in the greensand filter and must be
removed by backwashing. Backwashing should be done regularly
twice a week or as recommended by the manufacturer.
Eventually the greensand must also be
regenerated by washing with a permanganate solution. Regeneration
will leave the greensand grains coated once again with a
manganese material that adsorbs soluble iron and manganese.
Frequency of regeneration will depend on the level of iron,
manganese, and oxygen in the water and the size of the filter.
The manufacturer's recommendations should be followed.
Most greensand filters are rated to be
effective treating water with iron concentrations up to 10 ppm.
Because some greensand filters are not rated this high, the
manufacturer's recommendations should always be checked. The
acidity or pH of the water will influence the ability of the
filter to remove both iron and manganese. If the pH of the water
is lower than 6.8, the greensand will probably not adequately
filter out the iron and manganese. The pH can be raised above 7.0
by running the water through a calcite filter.
Regular backwashing is essential for
effective filter performance and require flow rates that are
often three to four times the normal household useage rate. A
backwash rate of about eight gpm/square foot of filter bed is
recommended. If the household system cannot support the needed
flowrate for adequate backwashing, poor filter performance and
failure are likely.
Chemical oxidation followed by filtration
is the accepted method of iron and manganese removal when
concentrations are greater than 10 ppm. There are a number of
strong oxidants that have been used in this procedure; however,
chlorine is generally used in household systems (Refer to the
Chlorination circular in the Treatment Systems for Household
Water Supplies series).
A chlorine solution is injected with a
chemical feed pump ahead of a sand filter. Soluble iron and
managanese begin to precipitate almost immediately after contact
with the chlorine solution. However, approximately 20 minutes of
contact time is needed for the precipitate to form particles that
can be filtered. Often the standard 42 gallon pressure tank used
on many household systems will provide the needed contact time if
water is forced through the tank. A simple T-connection from the
pipeline to the pressure tank will not work, since much of
the water bypasses the tank. Additional contact time can be
provided by connecting another tank in series or using a plastic
pipe coil.
This type of system will remove both
soluble and suspended particles of insoluble iron and manganese
from the source water. Backwashing the sand filter to remove
precipitated iron and manganese is an important part of continued
filtration. As with the "greensand" filter, the system
flow rate should be checked to make sure it can provide the
needed rates for backwashing.
An additional advantage of using the
chlorination system is its bacteriacidal effect. Iron and
manganese bacteria, along with other bacteria, are destroyed.
Potential clogging problems in the sand filter are eliminated.
Chlorination does produce trihalomethanes (THM) when organic
matter is present in the water. THMs are considered to be
carcinogenic (maximum contaminant level permissible in public
water systems is 0.1 parts per million) and if necessary can be
filtered out with an activated charcoal filter (refer to the
activated charcoal filtration circular in the Treatment Systems
for Household Water Supplies series).
The optimum rate of oxidation of iron and
manganese by chlorination is at a pH of about 8.0 and 8.5,
respectively. Soda ash injected with the chlorine will increase
the pH to optimum levels. Adjusting the pH to alkaline levels
also reduces the corrosivity of the water to pipes and plumbing.
-------------------------------------------
Range of Soluble
Iron Removed
Treatment Method (parts per million)
-------------------------------------------
Polyphosphate 0-3
Ion Exchange (softener) 0-10*
Greensand Filter 0-10**
Chlorination and Filtration 0->10***
-------------------------------------------
*Most softeners are rated for use at the
lower end of the range. Check with the
manufacturer.
**Most greensand filters are rated for use at
the upper end of the range. Check with the
manufacturer. If water pH is less than 6.8,
greensand filters will not perform as rated.
***Chlorination and filtration will work at
all levels of soluble iron; however, it is
recommended only for levels above 10 ppm
of soluble iron.
The Water Quality Association (WQA) has
set voluntary performance standards for oxidative filtration
methods. "An oxidizing filter shall reduce 10.0 ppm plus or
minus 1.0 ppm soluble iron to not more than 0.2 ppm and 2.0 ppm
plus or minus 0.2 ppm soluble manganese to not more than 0.05
ppm." A directory of validated equipment that meet these
standards is available from WQA, National Headquarters and
Laboratory, 4151 Naperville Road, Lisle, IL 60532 (708/505-0160).
WQA also recognizes that the following
water treatment methods can be used to meet EPA's Secondary
Drinking Water Standards for both soluble iron and manganese: 1)
oxidizing filters; 2) cation exchange; and 3) chlorination -
precipitation/filtration. Polyphosphate treatment does not meet
the drinking water standards, because it ties up iron and
manganese but does not remove it. Reverse osmosis, distillation,
(Refer to the Reverse Osmosis and Distillation circulars in the
Treatment Systems for Household Water Supplies series) and
pressure aeration/filtration are also recognized by WQA as water
treatment methods that can be used to meet the iron and manganese
drinking water standards.
The consumer should be cautioned to note
that different water treatment systems vary considerably in their
ability to treat a specific contaminant. Water treatment
equipment should be selected only after careful consideration of
the water problem and type of equipment to be used for its
removal. As a part of the installation procedure, the performance
of the equipment should be tested by water analysis. Periodic
water analysis is recommended to check for continued equipment
performance.
For further information contact your local county extension
Office or state health department. Additional information can be found in other
publications in this series: Treatment Systems for Household Water Supplies
- AE1029---Activated Carbon Filtration (1992)
- AE1031---Softening (1992)
- AE1032---Distillation (1992)
- AE1045---Identification and Correction (1992)
- AE1046---Chlorination (1992)
- AE1047---Reverse Osmosis (1992)
References
Hem, J. D. 1967. Equilibrium chemistry of iron in groundwater.
In S. D. Faust and J. V. Hunter (ed.) pp. 625-643, Principles and
applications of water chemistry. John Wiley and Sons, Inc., New
York.
Machmeier, R. E. Reviewed 1990. Iron in drinking water,
AG-FO-1318. Minnesota Extension Service, University of Minnesota,
Agriculture.
O'Connor, J. T. 1971. Iron and Manganese. In M. E. Flentje and
R. J. Faust (ed.) pp. 380-396, Water quality and treatment - a
handbook of public water supplies, 3rd Edition. Prepared by The
American Water Works Association, Inc. McGraw-Hill Book Co., New
York.
Snoeyink, V. L. and D. Jenkins. 1980. Water Chemistry. John
Wiley and Sons, Inc., New York.
Water Quality Association. 1988. Recommended industry
standards for household and commercial water filters - a
voluntary industry standard, S-200. National Headquarters and
Laboratory, Lisle, Illinois.
Water Quality Association. 1989. Recognized treatment
techniques for meeting the national secondary drinking water
regulations with the application of point-of-use systems, R28.
National Headquarters and Laboratory, Lisle, Illinois.
AE-1030, February 1992
NDSU Extension Service, North Dakota State University of
Agriculture and Applied Science, and U.S. Department of
Agriculture cooperating. Sharon D. Anderson, Director, Fargo,
North Dakota. Distributed in furtherance of the Acts of Congress
of May 8 and June 30, 1914. We offer our programs and facilities
to all persons regardless of race, color, national origin,
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This publication will be made available in alternative format
upon request to people with disabilities (701) 231-7881.
North Dakota State University
NDSU Extension Service
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