EPA
TECHNOLOGY
TRANSFER
MA3NESUM
CARBONAE-
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RKX1ED
PRBAREDBt'
US.
FCRV\NER
PORTION
A3ENCY
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Approximately 1 million tons per year of dry
solids are produced from an estimated 3,600 water
treatment plants practicing coagulation throughout
the country. Of these, less than 5 percent receive
treatment of any kind before return to the water
course. Wastes from water treatment plants are today
recognized as a significant pollution problem.
Lime-soda softening plants produce considerably
more solids per million gallons of water treated than
are produced in a typical chemical coagulation plant.
It has been estimated that from 0.45 to 0.66
acre-foot per year of 50 percent solids sludge are
produced for each 100 ppm of hardness removed
from 1 mgd of water. The sludge produced in water
softening plants is primarily calcium carbonate, which
can be concentrated and dewatered more readily than
predominately metal hydroxide sludges.
A number of devices are available for dewatering
water plant sludges, although costs have been found
to be high. Dewatering alone still leaves a solid waste
management problem. To date, lime recovery has
been found practical on only relatively pure calcium
carbonate sludges.
The new magnesium recycle coagulation system
is based on a combination of water softening and
conventional coagulation techniques which can be
applied to all types of water. This system offers an
alternative approach to chemical sludge handling as
well as providing for reuse of the chemicals. The
development of this new process has been supported
by grants from the U. S. Environmental Protection
Agency; the Montgomery, Alabama, Water and
Sanitary Sewer Board; the city of Melbourne,
Florida; and the American Water Works Association
Research Foundation. In addition, the city of
Dayton, Ohio, has provided additional support.
The photograph on the opposite page is an aerial
view of the Montgomery Water Treatment Plant
showing the parallel operation with two coagulants —
(foreground) and alum.
The major areas of significance offered by this
new technology are:
• Sludge discharges are reduced or completely
eliminated and the sludge water, recovered as treated
water, is an important saving for large plants.
• The water is treated primarily by three chemicals
(lime, carbon dioxide, and magnesium carbonate),
which are recovered or recycled.
• Significant cost savings are realized for most
applications, particularly when sludge disposal costs
are considered.
• Floe characteristics allow significant increases in
clarifier loading rates, increasing the clarifier capacity
of many plants.
• The treated water produced is of high chemical
and bacteriological quality. Soft waters are made
chemically stable, hard waters are softened, and the
high pH of the process provides added assurance of
disinfection. In addition, many heavy metals are
removed in the water treatment process.
• The process can be readily adapted to the current
potable water filtration plant using chemical
coagulation, and to conventional lime-soda softening
plants.
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Detail of Dayton Water Treatment Plant
In this new technology, developed by Dr. A. P.
Black, the primary treatment chemicals (lime, carbon
dioxide, and magnesium carbonate) are recovered and
recycled.
The process development began in 1957 at the
Dayton, Ohio water treatment plant (front cover),
where a hard, clear, high magnesium water is softened
by the lime-soda process, involving the first six
reactions shown in Table 1. Sludge components are
underlined. Approximately 300 tons of sludge per
day, dry basis, are produced, containing 12 percent
magnesium hydroxide with the remainder primarily
calcium carbonate.
For all types of water, magnesium carbonate is
precipitated as magnesium hydroxide, which then
becomes the active coagulant. The resulting sludge is
composed of CaCOg, Mg(OH)2 and, in the case of
turbid waters, the turbidity removed from the raw
water. Reactions 7, 8a, and 8b describe how the
sludge is carbonated by injecting CO^ gas which
selectively dissolves the Mg(OH)2. The carbonated
sludge is then filtered with the magnesium being
recovered as soluble magnesium bicarbonate in the
filtrate, which is recycled to the point of addition of
chemicals to the raw water, reprecipitated as
Mg(OH)2 (Eqs. 3 and 4, Table 1), and a new
treatment cycle begun. The filter cake now contains
CaCOg and the raw water turbidity. In large plants, it
is possible to reduce the waste solids even further by
slurrying the filter cake and separating the raw water
turbidity from the CaCO^ by flotation. The purified
CaCOg is dewatered and recalcined as high quality
quicklime as shown in reaction 9. The stack gases
from the recalcining operation then provide the CC>2
gas for carbonating the sludge for magnesium
recovery and to recarbonate the water in the
treatment plant. When this latter lime recovery step is
practiced, the waste solids are reduced to only those
which constitute raw water turbidity.
If magnesium is to be recovered, the magnesium
bicarbonate solution, clarified by either settling or
filtration, passes to a heat exchange unit where it is
warmed to 35° C - 45° C after which it is aerated
by compressed air in a mechanically mixed basin. The
MgCOj • 3H20 (Eq. 10, Table 1) precipitation is
complete in about 90 minutes and the resulting
product is vacuum filtered, dried, and sold.
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CHEMICA L SUM MA RY OF THE NEW PROCESS
CHEMICAL REACTIONS OF LIME-SODA SOFTENING
^,,,: \ ..*'
,
CHEMICAL REACTIONS FOR SLUDGE CARBONATION
±: MgC03
-9.- '
CHEMICAL REACTION FOR CALCINATION
' ' -'-:•;'••. - »^ CaO
10.
CHEMICAL REACTION FOR PRODUCT RECOVERY
° -°
• 3 H?0 + C02
"™~
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COMPA RISON OF CON VENT/ON A L COAGULANTS WITH MA GNESIUM CA RBONA TE
CHARACTERISTICS
pH of optimum coagulation
Flocculation
Settling
Sludge
Sludge water
CONVENTIONAL
Usually in the range pH 5.2
through 5.7 for organic color,
somewhat higher for turbidity.
MAGNESIUM
CARBONATE
pH 11.3 when magnesium
recovery is to be made. For
magnesium recycle only pH
10.9 through 11.1, but may be
pH 11.3.
Hydrolysis product. Forms Large, dense, the Mg(OH)2
slowly, more so at low coagulant weighed down by
temperatures. Sensitive to high the coprecipitated CaCO3-
velocity gradients. Less sensitive to temperature
changes.
Settles slowly. Clarifier flow Settles rapidly. Clarity of
rate less than 0.75 gal/ft^/min. settled water at Montgomery
dramatically better than alum.
Upflow rate in Melbourne's
clarifiers double that of alum.
Sensitive to convection
currents.
Gelatinous sludge normally 1
to 2 percent solids. Thickens
to only about 6 percent solids.
Rarely, if ever, recovered.
Primary sludge 3 to 5 percent
solids. Carbonated sludge
thickens to 40 to 50 percent
solids.
About 90 percent recovered,
recycled and added to plant
effluent as treated water. This
represents a significant saving
for large plants.
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CHARACTERISTICS
CONVENTIONAL
MAGNESIUM
CARBONATE
Finished water
Filtration
Low total hardness. Low
alkalinity, usually in range 10
to 25 ppm. Not possible to
stabilize by pH adjustment.
"Red water" problems
common.
Fine gelatinous floe may
reduce length of filter runs.
Somewhat higher total
hardness. Alkalinity 35 to 50
ppm. May be stabilized by pH
adjustment to produce
non-corrosive effluent. Total
hardness depends upon
noncarbonate hardness
present.
turbidity of stablized
water does not reduce filter
runs. Filter runs at
Montgomery consistently
longer than with alum.
Filtered turbidities lower.
Adaptability to existing
plants
Usually designed for the use of
alum or a ferric salt.
Readily adaptable without
plant changes in most areas.
When coagulation is at pH
11.3, two-stage carbonation
used; stage 1 to pH 10.3; stage
2 to final pH of water.
Bacteriological quality of
treated water
Dependent upon quality of
raw water and type of residual
carried.
High pH of coagulation
provides complete disinfection
and should eliminate
pre-chlorination.
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There are three general applications of this new
technology in water treatment dependent upon
the quantity and character of the raw water treated.
These applications are:
(1) Soft, Turbid and/or Colored Waters
Figure 1 shows magnesium being used as the
coagulant, with the recycle of magnesium bicarbonate
and sludge dewatering as an integral part of the
process. The Montgomery study demonstrated this
application to be successful.
Figure 1. Soft, turbid and/or colored waters
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Figure 2. Moderately hard, turbid and/or colored waters
(2) Moderately Hard, Turbid and/or Colored Waters
Figure 2 demonstrates magnesium being used as a
recycled coagulant, but lime recovery is also included.
In some cases, the flotation step will not be required
due to low suspended solids in the raw water. Carbon
dioxide produced in lime recovery is used for softened
water stabilization and carbonation of the sludge.
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(3) Hard, High Magnesium, Clear or Turbid Waters
In this application, both calcium and magnesium
are recovered. Part of the lime is recycled and the
remainder is sold. In addition, the magnesium
carbonate is sold. Figure 3 illustrates the unit
operations necessary for this application when
treating a hard (125 to 300 mg/l), turbid, high
magnesium (10 to 30 mg/l) water.
There are a number of major cities which fall
into this category. Dayton, Ohio, where a clear,
high magnesium, hard water is softened, is a good
illustration of an application in this category. Lime
recovery is now practiced, with magnesium recovery
soon to be added.
The primary emphasis of each of these
applications is the elimination of sludge disposal
problems by the recycle or recovery of the three
water treatment chemicals used (lime, carbon
dioxide, and magnesium). This technology also has
potential for the treatment of municipal and
industrial wastewater.
Finished Water
FILTERS 4
• -. IJ^^J!
Cooling
Exchanger
Raw Water
Carbonation
0
.. Heating Aeration Product
Storage Exchanger Cells Dewatering
n
Cells
Filtrate
CO.
7T^
I J Thickener MgCO3. 3 H2O
Repulp
D
•^«^BI^^^d
Vacuum Filter
\
Sludge Carbonation and Storage
. Magnesium Recovery
Lime Recovery
CaO
CaC03
Flotation Cells
Kiln w I 1
Vacuum Filter
Turbidity
^s^
Figure 3. Hard, high magnesium, clear or turbid waters
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Equalizing sludge lagoon at Dayton
The economics of the new process are most
favorable in the case of hard, high magnesium waters
where recalcination is practiced. This is because all of
the three essential chemicals are recovered,
recycled or sold.
The economic benefits of the new technology
can be substantial. During the 16-year period 1958
through 1973, Dayton produced 505,462 tons of
high quality lime valued at $9 million. Approximately
100,000 tons of lime valued at more than $1 million
has been produced and sold to other during this
period.
Chemical treatment costs are substantially
reduced in several ways. The coagulant used by all
water treatment plants, alum or an iron salt, is
replaced by the magnesium present in the raw water.
The recovery of high quality quicklime by
recalcining reduces lime costs by about 50 percent
and provides, at no cost, sufficient carbon dioxide
from the kiln stack gas to carbonate the treated
water. Carbon dioxide is also used to recover or
recycle the magnesium in the sludge. Sludge water is
recycled to the plant and recovered.
The following additional savings will be realized:
a. Sale of excess lime produced.
b. Sale of magnesium produced.
c. Cost of an alternate method of sludge
disposal.
Other potential savings include:
a. The possibility of reducing the amount of
chlorine used for pre-chlorination or perhaps
eliminating it entirely.
b. Production of a soft, stable treated water.
c. Potential significant increase in treatment
capacity with minor plant modification.
In the case of soft, turbid and/or colored waters,
plant size and daily lime consumption are the key
factors determining the cost of treatment. If daily
lime usage is sufficient to justify recalcination of
sludge, treatment costs should be as low or lower
than present treatment costs. However, without
recalcination, treatment costs will probably be higher
because the kiln provides both lime and carbon
dioxide. In such cases, the cost of sludge disposal
becomes the most important determining factor.
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&
Since recalcination is an important unit process
of the new technology, the initiation of a new
procedure requiring approximately 9 million
BTU/ton of quick-lime requires careful consideration.
The following facts would appear to apply to all
plants where recalcination would be used:
1. Using commercial quicklime, in larger tonnage
than any other water treatment chemical,
requires the following energy-consuming
operations for its manufacture:
a. Mining of limestone or marble in quarry.
b. Transporting to kiln.
c. Crushing and grinding before burning.
d. Burning in vertical shaft or rotary kilns.
e. Transporting in bulk shipment, frequently
for hundreds of miles.
f. Unloading and transporting to the water
plant.
Interior of kiln
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Lime kiln at the Dayton Water Treatment Plant
2. Recalcining carbonated sludge would:
a. Eliminate a, b, c, e, and f above.
b. Evaporate the moisture from the filter or
centrifuge cake. The amount of heat
required to do this is more than
compensated for by the fact that
recalcination produces more lime than is fed
to the water.
c. Produce more than sufficient CC^ in the
stack gas to carbonate the plant's finished
water, for stabilization.
d. Produce more than sufficient CC>2 to
carbonate the plant sludge and make
possible the recycling or recovery of the
magnesium.
e. Provide for recovery, in a heat exchanger, of
the latent heat of evaporation of the sludge
cake water. This is later used for recovering
the MgC03 « 3 H20 and drying it.
f. Decrease the cost of hauling sludge to
disposal point.
The new process actually conserves energy and is
a major factor in eliminating the sludge disposal
problem.
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TECHNOLOGY
TRANSFER
For Further
Information:
Detailed information on this
project is available from the
Superintendent of Documents as
EPA Report 12120 HMZ "Plant
Scale Studies of the Magnesium
Carbonate Water Treatment
Process."
Or Write:
U.S. Environmental Protection Agency
Environmental Research Information Center
Cincinnati, Ohio 45268
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