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.
8 comments:
Oliver,Interesting!
Do you know any North American greensand manufacturers ?
Regards
Omar Mauricio
hey Man, very cool blog with lots of details in it, Water Treatment Process and Filteration Media stuffs, thanks for posting..!!!
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