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The rate of oxidation of ferrous
iron by aeration is slow under conditions of low pH and is
fast under high pH conditions. Rate of precipitation and filtration
are accelerated in practice by contact and catalysis. Water
is allowed to trickle over coke or crushed stone. The deposition
of hydrated oxides of iron and bacteria on the contact media
is believed to act as catalysis which accelerate the oxidation
of Iron. |
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| Simple Technique for Iron removal |
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A simple and inexpensive treatment
unit for the removal of iron is suggested so that the difficulties
of operation and maintenance can also be minimized. |
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When the source is a well or a
sump and the water consumption rate is in the order of 40 lpcd
and when hand pump is used, a tray type aerator with two trays
operated at an aeration rate of 1.26 m 3/m 2/hr are employed
and the water aerated. Then the water is settled in a sedimentation
basin having a detention period of 3 hours and the clarified
water passed through a sand filter having a depth of 0.3 m
supported by gravel 3-6 mm in size 0.1 m deep. The effective
size of the sand is 0.30-0.45 mm and its uniformity coefficient
2-3.Sand is cleaned by manual scraping. |
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| Iron Removal: A World Without Rules |
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| Iron can often be detected visibly
in water or by staining on plumbing fixtures. |
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There
is one rule to keep in mind when selecting a method for iron
removal — and that is
there is no rule. You will find — as with all problem
water applications — the solution is 50
percent science and 50 percent experience. |
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The
following information describing the different types of iron
removal process applications are the basics. Before using
any of these applications, it’s
good to have an understanding of the type of iron present;
the equipment and its limitations; and the product and processes
involved with method. |
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Care must be taken when considering
iron removal advice from different regions of the country as
water temperature, pH, alkalinity, dissolved oxygen content
and other factors will affect the actual results. |
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Most application
failures are caused simply by not selecting the right equipment
for the water conditions present. It is important to follow
manufacturer’s guidelines
regarding flow rates, backwash rates, pH levels, maximum iron
input levels, water temperatures and any other application
limitations that the manufacturer has noted in order for the
equipment and media to deliver their best result as designed |
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Most iron filtration systems operate
on the principal of oxidizing the iron (oxidation) to convert
it from a ferrous (dissolved or soluble) to a ferric or undissolved
state. Once in the ferric state, iron can be filtered. |
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Water filters are the most widely
used equipment in removing iron. Its popularity comes from
its versatility due to the various media products available
and the process involved with each media. |
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The most common reasons for filter
failure are a lack of flow in backwash or a lack of frequency
of regenerations. Low pH levels when using filters are another
reason for unsatisfactory results. |
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Water softeners exchange ions by
design. When used in iron removal, the softener uses a cation
resin to exchange iron for sodium, in addition to the calcium
and magnesium exchanged for sodium in the softening process. |
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Softeners are commonly used in removing
low levels of ferrous iron (1-3 ppm), though it is not uncommon
to remove 10 or more ppm depending on water conditions and
control settings. |
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The last thing a water softener
needs is for the ferrous iron to oxidize and convert to a ferric
state. Since pH plays a big part in how quickly this conversion
takes place, it is important to note that softeners
perform better on low pH, which will also prolong
bed life. |
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In the ferric state, iron will coat
the resin, plugging the exchange sites and fouling the resin.
Iron fouling will eventually happen in any iron application
and requires replacement of the media. |
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High saltings, longer backwashes,
frequent regenerations and the use of iron cleaners are keys
to longer bed life. However, even after taking these steps
to prevent the bed from fouling, the resin will eventually
succumb to the iron and require replacement. |
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Each
type of treatment has its own strengths and weaknesses. As
in the selection of equipment, it is important to follow
manufacturers’ recommendations
and note any application limitations such as water temperature,
pH alkalinity and dissolved oxygen content to get the best
result. |
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To do this, water treatment professionals
need a clear understanding of all limitations of the product
and equipment selected. |
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Filtration using various means of
oxidation is the most common method of iron removal. Depending
on the media selected, other common processes such as ozone,
aeration, chlorine or peroxide injection may be used to boost
the oxidizing properties of the water being treated. |
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Greensand is one of the oldest but
proven oxidation technologies. Potassium permanganate, itself
an oxidizer, is used to regenerate the greensand. |
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In
this application, potassium permanganate produces manganese
dioxide on the surface of the mineral and — once
the water comes in contact with it — any iron is immediately
oxidized. The iron can be filtered and then cleaned away in
the backwash cycle. Greensand is also effective with low levels
of H 2S (hydrogen sulfide) and manganese. |
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Synthetic greensand is a granular
mineral with a manganese dioxide coating having the same ability
as regular greensand. It is much lighter and requires less
of a backwash rate than standard greensand. |
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Manganese dioxide is a naturally
mined ore with the ability to remove iron, manganese and hydrogen
sulfide. The hydrogen sulfide capability exceeds that of either
greensand or synthetic greensand and requires no chemicals
to regenerate. |
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It does, however, require adequate
amounts of dissolved oxygen in the water as a catalyst and
may require some type of pre-oxidation to achieve its maximum
ability. |
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Birm has the ability to remove iron
and manganese and has no effect on hydrogen sulfide. Like manganese
dioxide, birm also uses dissolved oxygen as a catalyst and
may require some type of pre-oxidation in cases where the dissolved
oxygen content is too low to affect a maximum iron removal
result. |
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Redox media, which requires adequate
dissolved oxygen to be effective, consists of two metals -
85 percent copper and 15 percent zinc. These two dissimilar
metals create a small electrical field in the bed that will
not allow bacterial growth in the media. |
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This property earns redox the unique
distinction of being effective on bacterial iron without the
use of chlorine injection and being rated as bacterial static. |
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| Effective on removal of iron and
hydrogen sulfide, able to reduce chlorine and heavy metals
such as lead and mercury, redox is not effective with
manganese. |
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| The biggest
drawback for this media is its weight. Being almost twice
as heavy as other minerals, it requires more than twice the
backwash rate of other minerals. Sizing
mineral tanks is crucial. |
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| Catalysts & Considerations |
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Once you have identified the enemy
and selected the equipment with compatible backwash and flow
rates for the media selected, the water itself must be scrutinized. |
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Check for dissolved oxygen and pH
levels and determine what, if any, pre-treatment is necessary
for the selected application to deliver maximum iron removal
efficiency. |
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The pH of a given water source plays
an important role in how quickly ferrous (dissolved) iron converts
to a ferric (solid) state. The higher the pH, the faster
iron will convert to the ferric state that can then be filtered. |
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This is good in all equipment selections
with the exception of a water softener where the ferric iron
plugs the exchange sites and fouls the resin. |
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When using an iron filter a
pH above 6.5 is necessary for iron to properly convert
and is the recommendation of most manufacturers. However,
most experienced water treatment professionals agree that
a pH above 7.0 is a must and an 8.0 to 8.5 pH greatly enhances
the chance of a successful application. |
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If it is necessary to increase the
pH level, chemical feed of either sodium carbonate (soda ash)
or sodium hydroxide (caustic soda) is preferred over a filter
filled with calcium carbonate or magnesium oxide, as the filter
method may foul quickly |
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Most chemical-free iron filters
and several chemical filter media require some dissolved oxygen
in the water to act as a catalyst. Pre-oxidation is required
in cases where the dissolved oxygen content is too low. |
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| Pre-oxidation can come from aeration,
chlorine or peroxide injection, ozone and other
methods. |
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There are several types of chemical
feed applications. Using sodium carbonate or sodium hydroxide
to raise pH is common. Using 5 percent to 10 percent chlorine
or 7 percent hydrogen peroxide as oxidizers to the water before
a filter is also widely used. |
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Different
rules apply to each of these methods, from retention or contact
tanks to using static mixers. When using different chemicals
together, it’s
important to understand the compatibility of the chemicals
and the safety considerations. |
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For
greater success, follow the manufacturers’ recommendations
closely regarding proper feed rates and installation when
injecting chemicals. |
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| When aeration is used as a pre-oxidizer
it is generally done with either an air inductor or an air
pump. |
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An air inductor is a venturi installed
inline. The water flowing through the inductor creates a vacuum
and sucks air into the water line. The faster the water flows,
the more air induced into the water. |
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Watch
for pressure drop and perform routine maintenance of the
inductor, as they will clog with iron over time. The air
pump method allows more air induced into the water, as a
mechanical pump is used to force air into the water. A contact
tank is often used. |
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This method has proven effective
with the only cautions being maintenance to the pump and injection
fittings. |
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