Manganese Greensand Filters are used for the removal of soluble iron,
manganese and hydrogen sulfide from the well water supplies. Manganese
greensand is a purple-black filter medium processed from glauconite
greensand. Manganese greensand is a proven technology for iron,
manganese and hydrogen sulfide removal. The unique chemical and
physical characteristics allow for the utilization of two different
methods of operation: the continuous regeneration (CR) method and the
intermittent regeneration (IR) method.
CONTINUOUS REGENERATION (CR) METHOD OF OPERATION:
Continuous Regeneration (CR) operation is recommended for well waters
where the main objective is iron removal with or without the presence of
manganese. Potassium permanganate (KMnO4) and/or chlorine (Cl2) are
fed directly into the raw water ahead of the Greensand Filter. Chlorine
should be fed upstream of the potassium permanganate by at least 10-20
seconds. The chlorine residual should be measured in the filter
effluent. Potassium permanganate, if required, should be fed to produce
a “just pink†color in the filter inlet. This slight excess of
potassium permanganate or a chlorine residual carried through the filter
will maintain the greensand in a continuously regenerated state. The
manganese dioxide coating of the greensand has the ability to
catalytically oxidize iron and/or manganese under certain conditions.
Pre-chlorination alone without potassium permanganate, may be all that
is required to carry out the oxidation process. The Manganese Greensand
CR process has been successful in removing radium and arsenic from well
water supplies. The iron and/or manganese precipitates that are formed
adsorb the radium and arsenic. Radium removal requires that soluble
manganese be present in the raw water supply. Arsenic removal has been
achieved with either iron or manganese being present in the raw water
supply. Pilot testing is recommended for Radium or Arsenic removal
applications.
Estimating the Chemical Demand:
1. The dosage of Cl2 and KMnO4 to be fed may be estimated as follows:
mg/l (Cl2) = mg/l of Fe
mg/l (KMnO4) = (0.2 x mg/l of Fe) + (2 x mg/l of Mn) + 5 x mg/l of H2S)
2. KMnO4 demand (without Cl2) may be estimated as follows:
mg/l of KMnO4 = (1 x mg/l of Fe) + 2 x mg/l of Mn) + 5 x mg/l of H2S)
LENGTH OF SERVICE RUN FOR CR SYSTEMS:
The length of service run is the amount of time that the filter can
provide water before backwashing is required. The equation below
provides a way to calculate that length of time in minutes. To convert
to hours; divide by 60. The Chemical Demand must be converted from mg/l
to gpg by dividing by 17.1 (17.1 mg/l = 1 gpg).
Capacity (700 grains/ft2
- - - - - - - - - - - - - - - - - / Service Flow Rate (gpm/ft2) = Service Run (minutes) Chemical Demand (gpg)
CR SYSTEM OPERATING PARAMETERS:
Bed Type..................... Manganese Greensand is commonly used as a
single media filter bed. The minimum bed depth for a filter with
Manganese Greensand only, is 24â€. It is recommended that an
anthracite cap be placed on top of the greensand in larger filters where
practical. The bed depth on a dual media filter is 15†- 24†for
the Manganese Greensand and 12†- 18†for the Anthracite cap.
Capacity...................... The removal capacity of greensand is 500 -
700 grains of oxidized iron and manganese per square foot of bed area
based on potassium permanganate demand and a maximum pressure drop of 8 -
10 psi during the service run. In some cases, ground water contains
iron that filters in depth and the pressure drop may only be 4 - 6 psi
before iron appears in the filter effluent, dictating that backwashing
is required.
Backwash.................... Backwash is normally 10 - 12 gpm/ft2,
dependent on water temperature and should be performed with filtered
water. Bed expansion should be at 40%. Air Scour can be utilized on
large commercial or industrial filters to supplement backwash water if
desired. The air should flow at the rate of 0.8 - 2.0 cfm/ft2 with a
simultaneous backwash with treated water at the rate of 4 - 5 gpm/ft2.
Backwash should last 10 to 15 minutes or until water runs clear. Rinse
is recommended at service flow rate for 3 - 5 minutes.
Flow Rate.................... Recommended service flow rates with CR
operation are 2 - 5 gpm/ft2. Intermittent flow rates of 8 - 10 gpm/ft2
are possible. The higher the concentrations if iron and manganese, the
lower the flow rate should be to achieve equivalent lengths of service
runs. Higher flow rates are possible with very low concentrations of
iron and manganese, but test units should be run to prove that the
system will perform as required.
INTERMITTENT REGENERATION (IR) METHOD OF OPERATION:
Intermittent Regeneration (IR) operation is the process by which the
Manganese Greensand Filter is backwashed, then regenerated down flow
with a solution of potassium permanganate (KMnO4) in a batch process,
after a predetermined volume of water has been treated. The manganese
and soluble iron are removed when it comes in contact with the surface
of the greensand granules. This process is referred to as contact
oxidation. Hydrogen Sulfide utilizes the oxidizing capacity of the
greensand with the resultant precipitates removed by filtration within
the bed. Anthracite can be used in conjunction with the greensand, if
iron is present in the raw water to be treated. The batch regeneration
process is done at the end of the service cycle. The unit is backwashed
and then regenerated with a solution of potassium permanganate to
restore the oxidative capacity of the manganese greensand. The
regeneration level should be 2 - 4 ounces of potassium permanganate per
cubic foot of media.
Removal Capacity:
Total KMnO4 capacity* = 10,000 mg/l KMnO4 per ft3
Iron (Fe2+) only = 10,000 mg/l Fe per ft3
Manganese (Mn2+) only = 5,000 mg/l Mn per ft3
Hydrogen Sulfide (H2S) only = 2,000 - 3,000 mg/l H2S per ft3
- - - - - - - - - - - - - - - - - - - - - - - - -
1. KMnO4 demand = (1 x mg/l of Fe) + (2 x mg/l of Mn) + (5 x mg/l of H2S)
10,000 mg/l KMnO4 per ft3
2. Capacity = - - - - - - - - - - - - - - - - - - = gallons/regeneration/ft3
KMnO4 demand
* Total KMnO4 capacity is the total capacity of the Manganese Greensand filter media per cubic foot.
IR SYSTEM OPERATING PARAMETERS:
Bed Type..................... The majority of Intermittent Regeneration
Manganese Greensand filters are used for domestic applications rather
than commercial or industrial applications and are single media depth
filtering units. A minimum bed depth of 30†has been established for
single media manganese greensand filters. The bed should be regenerated
with potassium permanganate prior to placing it in service. An
anthracite cap is not required but can be used to improve the removal of
iron by acting as a filtration media for oxidized iron.
Capacity...................... Manganese greensand has the capability of
removing 550 grains per cubic foot of iron alone from a water supply.
If iron and manganese exist together, then the removal capacity is 400
grains per cubic foot. If manganese exists without iron then the
capacity is only 300 grains per cubic foot. Manganese greensand can
only remove up to 175 grains of hydrogen sulfide.
Backwash.................... Backwash is normally 10 - 12 gpm/ft2,
dependent on water temperature and should be performed with filtered
water. Bed expansion should be at 40%. Backwash should last 10 to 15
minutes or until water runs clear. Rinse is recommended at service flow
rate for 3 - 5 minutes.
Flow Rate.................... Recommended service flow rates with IR
operation are 2 - 5 gpm/ft2 or 1 - 2 gpm/ft3. The higher the
concentrations of iron and manganese, the lower the flow rate should be
to achieve equivalent lengths of service runs. Higher flow rates are
possible with very low concentrations of iron and manganese, but test
units should be run to prove that the system will perform as required..
Regeneration................A batch regeneration must be performed on a
regular basis. The frequency of regeneration is dictated by the
capacity of the manganese greensand in relation to the amount of
contaminants (iron, manganese and hydrogen sulfide) present in the water
to be treated. The regeneration level is 1 - 4 oz. of KMnO4 per cubic
foot of greensand. A gallon of water will dissolve 2 – 4 ounces of
potassium permanganate dependent on the temperature of the water.
Optimum regeneration time is 30 minutes. Rinse volume is 40 -50
gal./cu-ft. or until all traces of potassium are gone.
Initial Conditioning..... Manganese greensand is NOT shipped in a
regenerated form; therefore, it is necessary to regenerate the manganese
greensand with a solution containing 1 gallon of water and 2 ounces to 4
ounces of potassium permanganate for every cubic foot of media. The
filter media should be soaked in this solution for a minimum of 1 hour,
then rinsed of all traces of potassium permanganate prior to placing the
unit in service.
Removing Fines.......... Prior to placing the filter into service,
Manganese Greensand should be thoroughly backwashed and the top layer
(approximately 1â€) of fine material removed. This is especially
important if anthracite is placed on top of the greensand bed.
Article source : http://softwateradvice.com/archive/index.php/t-966.html
March
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Ground water, such as that found in Wha Ti, Fort Liard and Nahanni Butte,
often contains iron and manganese. Although usually referenced together, they
can be found separately. Typical concentrations for iron and manganese are 10
ppm and 2 ppm, respectively.
Neither element has any direct adverse health effects for humans. Both are
found in multivitamins; however, iron and manganese in normal drinking water
have no nutrient value. For the water to contain beneficial amounts, the taste
of the water would be rather unpleasant. Iron and manganese in drinking water
are normally not considered to be a health concern. However, there are a number
of problems which can occur if too much iron and manganese are present in the
water. These include:
·
staining of laundry and plumbing fixtures;
·
discolouration of the water;
·
taste to the water;
·
growth of iron bacteria is encouraged;
·
formation of deposits in distribution systems and plumbing;
·
interference with treatment processes such as
disinfection; and
The manganese greensand process has been used effectively for removing
iron, manganese, and hydrogen sulfide since the 1950s in the U. S. Manganese
greensand is processed from what is commonly known as “New Jersey greensand,”
but is more correctly identified as glauconite.
Glauconite Powder Glauconies Stones
For iron and manganese removal, the naturally
occurring singular grains of glauconite are washed and classified to produce a
filtration media having a sieve analysis of 18 x 60 mesh with a resulting
effective size of 0.3–0.35 millimeters (mm) and a uniformity coefficient of
1.60 or less, giving the media excellent filtration characteristics.
The glauconite is first stabilized
then coated with manganese oxide. This coating provides the glauconite with its
special chemical oxidation-reduction properties for the removal of iron and
manganese, as well as small quantities of hydrogen sulfide.
Manganese greensand has qualities that, in some cases, offer advantages over
other iron and manganese removal media:
- It has an
optimum grain size and shape to retain oxidation precipitation products of iron
and manganese.
- All grains have the same finite uniform coating, which is firmly attached.
- All manganese greensand is processed to exact specifications and tested prior to shipment.
- It has unequalled oxidation-reduction buffer capacity, and can tolerate a slight over- or underfeed of continuously fed oxidants.
- It provides high effluent water quality.
- Manganese oxide coating is not removed during backwashing or during the water-saving, but more physically demanding, air/water washing.
- Manganese greensand is not a proprietary medium of any equipment manufacturer.
- No detention time is normally required.
WATER TREATMENT PROCESS
Pretreatment Processes
To assist in iron and manganese removal, there are a
number of oxidation or other approaches that can be used. These include
the following:
1)
Aeration: This is frequently
useful for the conversion or oxidation of iron. Aeration is normally too
slow to be used with successful manganese oxidation unless the water’s pH is
very high. Aeration is generally used before other chemical treatment,
since it is relatively inexpensive to operate and it can reduce the amount of
chemicals that might subsequently be used.
The common types of aeration equipment are either pressure aerators or forced
draft aerators:
- in the pressure type, compressed air is diffused into the water in very fine bubbles and the oxygen from the air converts the ferrous iron to ferric iron. This type of aeration treatment is used to avoid double pumping; and
- in the forced draft aerator, the raw water is introduced into the top of a rectangular or circular tank and the water falls over trays breaking up into droplets as it goes to the bottom. Air is added from the bottom by a blower and is blown out through the top. This way there is a good mixing effect between the water droplets and the air so that the iron is easily converted to the ferric type.
2)
Chlorination: Chlorine is a good
oxidizing agent particularly for a lot of the ferrous ions. Chlorine is
normally not that effective for manganese removal or conversion. Chlorine
can be applied as a hypochlorite solution – either calcium hypochlorite (65%
available chlorine) or sodium hypochlorite (12% available chlorine) – or
as chlorine gas. The use of chlorine also provides disinfection of the
water. If chlorine or chlorine solution is used for the pretreatment, it
is important that it be used in the right dosages and constantly to avoid
problems with the filter. Obviously, if the right amount of chlorine is
not added, the iron will not be converted and it will pass right through the
sand filters.
Remember: Keep Chemical Feeders Working
Have Backup Equipment or Parts on Hand
3)
Potassium Permanganate: This
purple chemical has to be used when manganese greensand filters are used.
Potassium permanganate in dilute solution is a pink colour. When it
reacts with iron or manganese, the colour is yellow to orange unless extra
permanganate is available. The normal operation is to feed potassium
permanganate in solution form ahead of a manganese greensand filter in such
dosages that the water is slightly pink as it goes onto the filter. The
presence of the pink colour tells you there is enough of the chemical being
added. Potassium permanganate is normally fed in a pretreatment process
following the application of other oxidizing agents such as air or
chlorine. It is important that enough mixing time or reaction time be
provided to oxidize as much as possible the manganese in solution. The
time required depends on the pH – the higher the pH, the shorter the time that
is necessary.
4)
Filtration
The filtration step involves the final removal of iron and manganese from
the water. It therefore is a critical link in the process. There
are two basic types of filters that are used; gravity high rate filters and
pressure filters. Basically, they include a means of introducing the
water, the filter media and a collection system for the filtered water.
The collection system also serves as a distribution system for the backwash
water used to clean the filters. Since pressure filters are used in the
vast majority of iron and manganese removal plants in Saskatchewan, the
operation of pressure filters will be stressed. Note that the selection
of filtration media and operational cycle of a gravity filter is somewhat
similar to that of a pressure filter.
Treatment
Methods
A manganese greensand filter is capable of removing
both iron and manganese from the water. A greensand filter is very similar to a
regular sand filter except that the granular material has been treated with
potassium permanganate. The filter can be operated in three modes :
1.
Continuous Regeneration,
2.
Intermittent Regeneration, and
3.
Catalytic Regeneration.
1.
Continuous Regeneration
In the continuous regeneration process, chlorine is
first added, oxidizing most of the metallic iron and manganese present in the
raw water. Then a slight excess of potassium permanganate is added to remove
the rest of iron and manganese. Finally, the water is passed through the
Greensand filter where two things occur: (1) the insoluble iron and manganese
oxides are filtered and (2) the excess permanganate is reduced to manganese oxides,
regenerating the greensand. Once the head loss is too high through the filter,
the filter is then backwashed.
ADVANTAGES
ü Can
remove moderate concentrations of manganese and iron in the water.
DISADVANTAGES
o
Requires the addition of chlorine and if
required, a dechlorination step;
o
Manganese oxidization efficiency is very
low and hence, this treatment method is not valid where manganese
concentrations are high.
2.
Intermittent Regeneration (IR)
The intermittent regeneration process is suitable
where mostly manganese is present, having very little iron in the raw water.
Oxidation occurs directly on the greensand as raw water flows over it. In this
process small amounts of iron are also removed. Lastly, the filter is
backwashed when the head loss becomes too large.
ADVANTAGES
ü Suitable
for situations where manganese removal is the main treatment requirement;
ü Does
not require chlorine or dechlorination.
DISADVANTAGES
o
Cannot effectively treat water with
significant iron concentrations;
o
Filter must be backwashed frequently.
3.
Catalytic Regeneration
Catalytic Regeneration is suitable where iron and manganese
concentrations are small, less than 1.0mg/L and where the pH is greater than
7.0. Sufficient chlorine is added to the raw water before the filter to
maintain a chlorine residual of 0.5 to 1.0 mg/L. As the water passes though a
special grade of greensand, the chlorine regenerates the greensand and the
manganese is oxidized right on the filter.
ADVANTAGES
ü Suitable
for situations where iron and manganese concentrations are relatively low and
the pH is above 7.0;
ü Longer
filter run lengths are observed in comparison with the previous two methods;
ü Low
chemical operating costs;
ü Low
suspended solids in backwash wastes.
DISADVANTAGES
o
Cannot treat water with high iron and
manganese concentration;
o
Requires a specially refined greensand,
often Pyrolox.
Equipment
The filter usually consists of 3 different types of layers. One layer on
top consists of 45 cm (18 inches) of anthracite coal, followed by 45 cm of
manganese greensand, with 30
cm (12
inches) of graded gravel on the bottom. The greensand filter is different from
a conventional sand filter as the greensand contains much finer sand, having a
slower filtering and backwash rate.
The greensand filters can remove 95% of the iron and manganese in the water
if iron concentrations are below 10 mg/L, manganese concentrations are below
5 mg/L and also a little hydrogen sulfide concentrations. Reactions for oxidation of iron, manganese and hydrogen sulfide with potassium
permanganate are listed below.
3Fe(HCO3)2 + KMnO4 + 2H2O -> 3Fe(OH)3
+ MnO2 + KHCO3 + 5CO2
3Mn(HCO3)2 + 2KMnO4 -> 5MnO2
+2KHCO3 +2H2O +4O2
2KMnO4 + 3H2S à
2MnO2 + 2K+ + 2OH- + 2H2O + 3S
However, when these concentrations are exceeded, the efficiency of
filtration is reduced and the frequency of backwashing is increased resulting
in an overall decrease in plant efficiency. In these situations, pre-treatment
is often required.
Physical Properties of
Manganese Greensand Filter :
• Color: Purple-black
• Bulk Density: 85 lbs./cu.
ft.
• Specific Gravity: 2.4-2.9
• Effective Size: 0.30-0.35 mm
• Uniform Coefficient: 1.6
• Mesh Size: 16-60
Conditions For Operation
• Water pH range: 6.2-8.5
• Maximum water temperature:
80°F/26.7°C
• Bed depth: 30 in.
• Regeneration: 1.5-2 oz of
KMnO4 by weight per cu. ft.
• Service flow rate: 3-5
gpm/sq. ft., 8-10 gpm/sq. ft. intermittent flow possible
• Backwash flow rate: 10-12
gpm/sq. ft.
CAPACITY PER CU. FT.
·
Iron alone
600 grains (10,000 gal. of water containing 1 ppm iron/cu. ft.)
·
Iron and
manganese 400 grains (7,000 gal. of water containing ½ ppm iron and ½ ppm
manganese/cu. ft.)
·
Hydrogen
sulfide 175 grains (3,000 gal. of water containing 1 ppm hydrogen sulfide/cu.
ft.)
Figure 2-13 Greensand gravity-feed
filter diagram
Process
Control
The amount of chlorine required can be determined by:
Cl2 Required, mg/L = 1 x [Fe] Conc., mg/L
potassium permanganate (KMnO4) is added to complete the oxidation of any
remaining iron and soluble manganese.
KMnO4
Required, mg/L = (0.2 x [Fe] Conc., mg/L) + (2 x [Mn] Conc., mg/L)
If the influent flow to the greensand filter was properly treated, the
influent should have a slight pink colour. As the pre-treated water flows
through the greensand filter, the permanganate will be reduced to manganese
oxide and regenerate the filter while removing most of the remaining iron and
manganese in the water.
pH Control for Manganese
Manganese is often more difficult to remove when compared to iron and H2S.
During commissioning of the Nahanni plant a bench study showed that raw water
pH had to be increased from about pH 8 to over pH 9 to see some reduction of
manganese.
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