United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-090 Sept. 1984
SERA Project Summary
Trihalomethane Precursor
Removal by the Magnesium
Carbonate Process
J.S. Taylor, B.R. Snyder, B. Ciliax, C. Ferraro, A. Fisher, J. Herr, P. Muller,
and D. Thompson
A project was conducted to determine
and improve the ability to the magnesium
carbonate process to remove trihalo-
methane (THM) precursors in treated
drinking water. The project was con-
ducted at a drinking water treatment
plant in Melbourne, Florida, where the
process had been installed in the early
1970's (before THM regulation) to
reduce greatly the sludge produced
from water treatment. The process
involved recovering the magnesium
from the sludge by carbonation, reusing
the Mg(HCO3)2 liquor for coagulation,
recalcining the remaining CaCOa solids,
and reusing the recovered CaO for pH
control during coagulation. The project
consisted of seven phases, including a
jar test, recycle recovery, oxidation,
THMFP model development, distribu-
tion system, alternative disinfectant,
and granular activated carbon.
The magnesium carbonate process
showed no advantage over alum coagu-
lation with regard to the color remaining
in the water after treatment. With both
processes, the use of large doses ol
chlorine to decolorize the finished
water and provide a disinfectant residual
in the distribution system resulted in the
formation of THM's in excess of the 0.1
mg/L regulation.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory, Cincinnati, OH,
to announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
In 1980, the U.S. Environmental
Protection Agency (EPA), the City of
Melbourne, Florida, and the University of
Central Florida entered into a cooperative
agreement to determine the capability of
the magnesium carbonate process to
remove trihalomethane (THM) precursors.
This process is unique to potable water
treatment in that the principal coagulant,
Mg(HCO3)2, was recovered and recycled.
Melbourne, Florida, was the only city that
had built a plant specifically for the
magnesium carbonate process. No in-
formation had been gathered on the
ability of this process to remove THM
precursors, as THM control was not a
requirement or a concern at the time of
plant design. A schematic of the magne-
sium carbonate process at Melbourne
appears in Figure 1.
Recovery of the Mg(HCO3)2 coagulant is
accomplished by carbonation of the
Mg(OH)2 - CaCOs sludge taken from the
clarifiers. The form of magnesium is
changed to Mg(HCO3)2, which theoretically
allows complete magnesium recovery if
the total magnesium concentration in the
carbonation recovery unit does not
exceed the solubility of MgCO3.H2O. The
carbonated slurry is allowed to settle,
separating recovered Mg(HC03>2 liquor
from the remaining CaC03 solids. CaCO:,
is not solubilized because of the low
solubility product relative to that of
Mg(HC03)2.3H2O, 1(T83 as opposed to
10~54. The CaC03 solids were eventually
calcined in a lime kiln and reused,
theoretically producing no sludge for
disposal. The principal reactions of the
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Alum
Carbon
Lime
Coag. Aid
CO2 (NaPOalt
L. Wash/
Pumps
CI2
1
1
Flash
Mixer
a
.a
•§•
o
a
o
I
Floccu-
lator
Clari-
fier
Recycle Sludge
Ftecarb.
Basin
Filters
Recarb.
Cells
Storage
I
Thickener
Waste
Vacuum
Dryer
CaO
Storage
Furnace
Backwash
Storage
Pond
figure 1. Flow diagram {or Melbourne Water Treatment Plant.
Clear
Well
Storage
DBT
magnesium carbonate process are shown
in Equations (1), (2), and (3). Equation (1)
represents the recycle of dissolved
Mg(HC03)2 and solid CaO which is
solubilized and reacts to produce solid
Mg(OH)2 and CaCOs floe. Equations (2)
and (3) represent recovery of magnesium
and calcium by converting solid Mg(OH)2
to dissolved Mg(HC03)2 and solid CaCOa
to solid CaO by recalcination. The solid
CaO is converted to dissolved Ca(OH)2 by
slaking (hydrolysis) before recycle.
Recycle: Mg(HC03)2 + CaO(s) + H20—
Mg(OH)2
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increased the THMFP, TOC, and color
following coagulation, settling, and filtra-
tion (CSF). However, increasing doses of
doses of conventionally prepared MgfHCOgfe
recycled coagulant increased removal of
THMFP, TOC, and color following CSF. Of
the three magnesium sources tested,
unused MgS04.7H20 was the best,
removing 5% to 10% more color, TOC,
and THMFP than either the saline or con-
ventionally prepared Mg(HC03)2 recycled
coagulant. Thus recycling the Mg(HCOa)2
coagulant decreased the efficiency of
CSF. Typically, magnesium coagulation
at pH 11.3 to 12.0 accompanied 80% to
98% color removal, 20% to 40% TOC
removal, and 40% to 65%THMFP removal.
Optimum THMFP reduction was always
accompanied by optimum TOC and color
reduction. Approximately a 2% increase
in color, TOC, and THMFP removal was
obtained for every 0.1 pH increase from
pH 11 to 12. Not all of this increased re-
moval was attributable to increased mag-
nesium precipitation, as that was essen-
tially complete at pH 11.5. No significant
differences occurred in THMFP, TOC, or
color removal among the three different
lime sources (commercial, recalcined,
and reagent) that were used for pH con-
trol in magnesium coagulayion.
The reaction of color, TOC, or THMFP
with floe that was removed by filtration
through a 0.45-/um filter was independent
of variations in slow and rapid mixing
times. But the removal of formed turbidity
during coagulation was maximized at less
than 0.75 min of rapid mixing and 19 min
of slow mixing. These results indicated an
instantaneous reaction of color, TOC, or
THMFP with floe and (as expected) a mix-
ing dependent relationship for turbidity
removal. Alum used as a coagulant aid at
pH 11.5 increased THMFP, TOC, and
color removal by approximately 10% and
enhanced floe settling. An increase in
dissolved aluminum in the settled water
was not found until more than 2 mg/L
Al+3 was dosed in the alum form.
Statistical correlation of color, TOC, and
THM indicated that not all TOC produced
THM's or color.
Recycle Recovery
The color, TOC, and THMFP of the
recovered MgJHCOafe liquor typically
would exceed 3,000 chloroplatinate units
(CPU), 500 mg/L, and 50,000 fjg/L,
respectively. The purpose of the recycle
recovery subtask was to determine
whether recovered Mg(HC03)2 could be
improved as a coagulant for removing
i THMFP, TOC, or color by ozone or CIO2
oxidation. These studies indicated that
neither Oa nor ClOa oxidation of the
recovered Mg(HCO3>2 liquor would improve
the color, TOC, or THMFP removal when
the Mg(HCOs)2 was recycled.
Oxidation
The purpose of the oxidation phase was
to determine whether oxidation with Os
or CIO3 could improve removal of color,
TOC, and THMFP in the magnesium
carbonate process before chlorination.
In this process, Oa or CIO2 oxidation could
be executed either in the raw water
before coagulation, after coagulation at
pH 11.5, or after recarbonation at pH 8.3
and was investigated on a bench scale
under these conditions.
Ozone applied before coagulation
reduced color but did not reduce TOC or
THMFP of the raw water. When the
ozonated raw water was coagulated, the
final color was slightly lower, but the TOC
and THMFP were unchanged when com-
pared with the unozonated water. Ozone
applied to the recarbonated water after
coagulation also reduced the color but did
not change the TOC or THMFP of the
unozonated treated water. The color in
either case could be reduced to less than
the 15 CPU standard, but Oa doses of
more than 10 mg/L were required. Ozone
applied after coagulation at pH 11.5
reduced color, TOC, and THMFP more
effectively than at other pH's and
demonstrated that ozonation is used
most effectively at pH 11.5 after coagula-
tion.
ClOz applied before coagulation did
reduce the TOC and THMFP but did not
reduce the color of the coagulated water.
The same effect was found when CI02
was applied after coagulation at pH 11.5.
In each case, a CIOz dose of 10 mg/L
reduced the TOC 7%and the THMFP 35%.
CIOz applied after recarbonation did not
significantly affect TOC or color, but it
reduced THMFP 23% at the same CIO2
dose.
THMFP Model
A log-variant model for THMFP forma-
tion of CSF waters was developed from
more than 600 measured THMFP's from
three different sources chlorinated with
5, 10, 20, and 40 mg/L Cl at pH 6.0,7.5,
and 9.0, and at temperatures of 8°, 16°,
and 32°C for times of 0.005,1,6,24,48,
and 144 hr. The r2 and a values for the
model were 0.98 and 10~4. The model is
given in Equation (4) and shows that the
effect of TOC > time > Cl >Temperature
> OH on THM formation. The average
molecular weight of the THM's formed in
this study was 127 - approximately 85%
HCCk 14% HCCIzBr, and 1% HCBr2CI.
This model, shown in Equation (4), could
be used to estimated THMFP during plant
design or operation:
THMFP = 2.70 (TOC)124 (CI)'
L0.24
(OH)010 (T)017 (t)026
(4)
where:
THMFP=/umol/LTHM as THM
TOC=mmol/L TOC as C
CI=mmol/LCl2asCI
OH=mmol/LOH"asOH"
T=°C
t=hr.
This model can provide useful informa-
tion for THM control in similar waters.
Assuming a water temperature of 20°C, a
TOC of 10 mg/L, a pH of 9, and a chlorine
dose of 20 mg/L, THM's would be 547
/ug/L after 96 hr in the distribution
system. The TOC precursors would have
to be reduced to 2.6 mg/L to have pro-
duced no more than 0.10 mg/LTHM'sfor
the same conditions. Such a level is
impractical for most highly colored
surface supplies. If disinfection is changed
to chloramination with a chlorine dose of
10 mg/L before ammonia feed under the
same conditions, free chlorine contact
time would have to be less than 9 min to
avoid THM formation greater than 0.10
mg/L.
Distribution System
The water quality in the Melbourne
distribution system was monitored to
determine the effects of two process
changes which were instigated at the
water treatment plant to control THM's.
These were: (1) changing from Mg(HCOa)2
coagulation to alum coagulation; and (2)
changing from chlorine disinfection to
chloramination. Using alum resulted in
the removal of approximately 10% more
color, 13% more TOC, and 10% more
THM precursors in the coagulation
process.
The change from chlorine to chlora-
mines in June 1983 resulted in: (1)
THM's at the extremities of the distribution
system of less than 50jug/L, (2) disinfec-
tant residuals in water with summer
temperatures exceeding 85°F at distant
points where they had seldom, if ever,
been documented, (3) lower standard
plate counts (SPC) by orders of magnitude,
and (4) a significant reduction in consumer
complaints. As a result of these changes,
Melbourne is now experiencing the
highest residuals, lowest SPC's, and
lowest THM's that have been recorded in
the distribution system.
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Alternative Disinfectants
The purpose of the alternative disinfec-
tant phase was to determine the capability
of CI02, NH2CI,O3, and CI02, singularly or
in pairs, to remove trace color, reduce
THMFP, and provide residuals asfunctions
of time and temperature. NH2CI was not
effective for color reduction, but CIO2, O3,
and C\z could effectively reduce 1 to 2
CPU from 1 L of CSF water for every mg/L
dosed. Temperatures did not significantly
affect color removal. Color reduction
became increasingly difficult as the final
color decreased.
The rate of disinfectant dissipation was
03 > CI2 > CIO2 > NH2CI. This rate
generally increased with temperature
and exhibited exponential decay. When
dosed at 3 to 5 mg/L, NHaCI could maintain
the required residual of 0.6 mg/L for 7
days at 32°C. CIO2 and O3 could not
maintain a residual at doses of 10 mg/L
and 20 mg/L, respectively, under these
conditions.
Neither CI02, O3, nor NH2CI produced
THM's. But if chlorine were used for trace
color removal, the dose and contact
periods would have to be closely controlled
-typically less than 10 mg/L and 9 minto
avoid violation of the 0.1 mg/L THM
standard. The combination of O3 with
NH2CI and CI2 with NH2CI were effective
for: (1) reducing Term THM, SPC, and
trace color, and (2) producing a residual in
the distribution system. O3 with CIO2 and
CIO2 with NH2CI would also be effective if
CI02 doses higher than 2 mg/L could be
used, but such doses are not permitted in
Florida.
Granular Activated Carbon
Four three-column GAC pilot plants were
continuously operated at the Melbourne
Water Treatment Plant. Each column was
glass, end plates and valves were
stainless steel, and all interconnections
were made with Teflon* tubing. Each 5-ft-
high, 4-in.-diameter column had an empty
bed contact time (EBCT) of 10 min. Ozone
and CI02 contactors, made from Plexiglas,
were baffled into six compartments and
had a total 30-min detention time. Flow
was constant at 190 gallons per day and
bed depth in all columns was 2 ft.
Four different influents were compared
for color, TOC, and THMFP removal by
GAC adsorption. The influents were raw
water, CSF water, CSF and ozonated water,
and CSF and CIO2 treated water. These
systems are denoted as RW/GAC,
CSF/GAC, CSF/03/GAC, and CSF/CICV
GAC. The CSF/03/GAC system was
more effective than the other three
systems (see Table 1), removing 11%
more THMFP, 10% more TOC, and 16%
more color during the 473 days of
operation than the next most effective
system (CSF/GAC). The additional re-
movals occurred entirely in the O3 con-
tactor and were due solely to ozonation.
The effluent SPC's and dissolved oxygen
(DO) decreases from the columns indi-
cated a higher biological activity in the
CSF/Os/GAC columns compared with
that in the CSF/GAC columns. Thus
biological activity in the GAC columns did
not appear to be a significant means of
color, TOC, or THMFP removal. 03 and
CI02 pretreatment alone did increase
removals of THMFP, TOC, and color, but
they also reduced the absorbability of the
organics and therefore the overall
efficiency of the GAC systems. Ozone
was 2 to 3 times as effective as CIO2 for
reducing THMFP, TOC, and color. Chang-
ing to alum coagulation removed more
THMFP such that the GAC efficiency was
increased fivefold. This and the relative-
ly poor performance of the RW/GAC
system compared with the CSF/GAC
system indicate that optimum treatment
by GAC could be achieved by positioning
GAC columns following coagulation and
before any oxidation processes, and by
maximizing coagulation for color, TOC,
and THM precursor removal. The percen-
tage of color removal was generally twice
the TOC or THMFP removal in the GAC
columns.
DO and SPC measurements taken
during the GAC pilot plant experiments
indicate that significant biological activity
existed in all the GAC columns but that
the removal of color, TOC, or THM
precursors generally decreased with
time. Complete exhaustion of the RW/GAC
columns for TOC removal occurred even
though significant biological activity was
always present in the columns. These
observations suggest that a removal
mechanism other than biological activity.
Table 1. GAC Capacities to Meet Color Standard and THM Regulation Under Melbourne Test
Conditions
principally physical adsorption, is respon-
sible for color, TOC, and THM precursor
removal, and that eventual exhaustion of
these columns would be reached at
Melbourne.
Conclusions
Because chlorination must be used to
remove color remaining after coagulation,
the magnesium carbonate process will
produce excessive THM's. Thus it will be
difficult to use this system as a potable
water process and still meet the THM
regulation. Oxidation of the recycled
Mg(HCO3)2, the raw water, or the CSF
water could not reduce the InstTHM to
the 0.10 mg/L regulation. The THMFP
model did accurately predict THM forma-
tion and showed that, as a primary
disinfectant, chlorine could only be used
for a short contact period to form less
than 0.1 mg/L THM's. Chlorine followed
by NH2CI produced a water with the
lowest SPC, THM's belowO.10 mg/L, and
the highest residuals ever recorded at the
extremities of the Melbourne distribution
system. Biological activity on GAC was
not indicated to be a major mechanism of
color, TOC, or THMFP removal.
The full report was submitted in
fulfillment of Cooperative Agreement No.
807704 by the Unrversity of Central
Florida under the sponsorship of the U.S.
Environmental Protection Agency.
Capacities to Meet 15 CPU
Color Standard and 0.10 mg/L THM Regulation
System
Color (CPU/gm GAC)
THMFP (mg/gm GAC)
'Mention of trade names or commercial products
does not constitute endorsement or recommendation
for use.
CSF/GAC
CSF/Os/GAC
CSF/CIOZ/GAC
RW/GAC
CSF/GAC*
240
265
264
140
252
0.56
1 04
0.54
0.48
2.49
^Received only alum coagulated influent.
*USGPO: 1984-759-102-10666
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J. S. Taylor, B. R. Snyder, B. Ciliax, C. Ferraro. A. Fisher, J. Hen, P. Muller, and D.
Thompson are with the University of Central Florida. Orlando, FL 32816.
J. Keith Carswell is the EPA Project Officer (see below).
The complete report, entitled "Trihalomethane Precursor Removal by the
Magnesium Carbonate Process," (Order No. PB 84-191 147; Cost 25.00,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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