EPA TTCR12040 ENC
906R80102
, _ _ 1OGY
TRANSFER
COLOR
REMOVAL
FROM
KRAFT
PULPING
EFFLUENT
BY LIME
ADDITION
U.S.
ENVIRONMENTAL
PROTECTION
AGENCY
INDUSTRIAL
DEMONSTRATION
GRANT
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Kraft pulp today represents the principal
type of chemical pulp produced in the United
States, accounting for 30 million tons, or
83 percent of the 36 million tons of chemical
pulp produced in 1971. There are
approximately 120 mills with a median
capacity of 1000 tons per day manufacturing
Kraft Pulp, concentrated in a broad arc of
southern states ranging from Virginia to Texas,
and in the Pacific Northwest, with lesser
concentrations in the upper Great Lakes and
New England states.
Discharge of a reddish brown colored
effluent is invariably associated with its
manufacture. While no national color pollution
standards have as yet been developed to guide
the development and application of kraft
effluent decolorization technology, there are a
growing number of locations where
decolorization efforts have been requested by
state or local regulatory authorities.
It has been known for twenty years that lime
addition was capable of decolorizing kraft
effluent, but intensified laboratory study in the
last ten years has indicated that several alternate
lime addition procedures are adaptable to
colored effluents from either the pulping or
bleaching stages of the kraft pulp production
process. The use of lime is made more
attractive by the existing major role occupied
by lime in the kraft process pulping chemical
recovery system, since it is already used in the
recausticizing stage. This provides a dual
opportunity for (1) recovery of lime used for
decolorization and (2) destruction of removed
color by conventional thermal oxidation
processes.
The reaction mechanisms governing lime
decolorization have been identified as
precipitation of calcium salts of weak organic
acids produced by chemical separation and
fragmentation of wood-derived lignin during
both pulping and bleaching.
In this EPA Demonstration Grant to
Interstate Paper Corporation a full scale
process for lime decolorization of kraft pulping
total mill effluent was operated. Slaked lime
addition was followed by primary
sedimentation for removal of the mixed
fibrous and lime decolorization sludges. The
clarified effluent subsequently underwent
biochemical stabilization in a quiescent storage
basin. Neutralization of excessive lime
alkalinity was achieved in this basin by
absorption of atmospheric carbon dioxide. The
grant also supported a pilot-scale study of
recarbonation for enhanced lime separation
and more rapid effluent neutralization prior to
biochemical stabilization.
While 83 of the 120 kraft mills produce
bleached kraft pulp whose effluent contains
substantially more color per ton of product
than that from the pulping process alone, the
demonstration at the Interstate unbleached
kraft mill has the following significance: (1) It
demonstrated an effective decolorization
process applicable to the 37 mills producing
only unbleached pulp, (2) It provided a
calcium-rich decolorized effluent for
subsequent pilot-plant recarbonation
investigations applicable to all effluents from
lime decolorization processes, (3) it raised the
possibility of bleached kraft total mill
effluent decolorization by a similar lime
treatment process, and (4) it laid the basis for
additional studies of decolorization lime
recovery in conjunction with primary sludge
disposal.
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Lime Kiln and Slaker
A calcium hydroxide dosage of 1000 ppm
derived from a calcium hydrate slurry at
1.04 sp.gr. produced in a conventional quick
lime slaker (S-l) is applied to a total mill
effluent of 1200 APHA color. Following a
35 minute flocculation period (T-l) gravity
clarification (C-l) at a rise rate of .13 gpm per
square ft. clear rise area yields a decolorized
effluent of 125 APHA units, and 10 ppm
suspended solids. The clarifier underflow
sludge contains 2 percent solids with
diminished dewatering characteristics, but can
be readily thickened to 6 percent solids. This
sludge is currently disposed of by lagoon
storage (L-l).
With natural recarbonation, the resultant
highly alkaline effluent (pH = 12.2) containing
700 to 750 ppm lime (as Ca(OH)2) first
undergoes neutralization by surface absorption
of atmospheric CO2 to pH 10.2, accompanied
by precipitation of nearly all the remaining
calcium in the inlet section of the stabilization
basin (B-l). Biochemical oxidation results in
further neutralization and overall BOD and
COD reductions of 98 and 95 percent
respectively. Clarifier sludge load totals about
30 tons per day, of which thirty percent
represents added lime.
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Lime Mixing Tank and Primary Clarifier
The use of a recarbonation process instead
of relying upon absorption of atmospheric CO2
was investigated on pilot scale to obtain
preliminary design data. Figure 2 shows the
pilot flow sheet. Decolorization clarifier (C-l)
overflow enters a carbonator (R-l) providing
16 to 20 minutes residence time, receiving
scrubbed lime kiln stack gas at 180°F.
Following clarification (C-2) at a rise rate of
2.5 gal. per minute per sq.ft. residual calcium
level is reduced to 1 Ib CaCO;i per 1000
gallons. This can be further reduced by
50 percent if the lime kiln gases are pretreated
with mechanical cyclones alone and enter the
carbonator at 325°F instead. Recarbonation at
this point adds approximately 10 tons of
sludge solids which would otherwise be
precipitated in the stabilization basin to the
30 tons obtained during decolorization and
improves overall dewatering characteristics.
The additional sludge is of high purity
however, and it could be introduced to the
lime recovery cycle. Reduction in pH by a
recarbonation process would probably extend
the effectiveness of the quiescent biochemical
oxidation lagoon since a portion of the capacity
is now required for neutralization by absorption
of atmospheric CO2.
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LIME STORAGE TANK
VARIABLE SPEED
SCREW CONVEYOR
FUTURE LIME RECOVERY
iSLUDGE DISPOSAL;
MILL EFFLUENT
EFFLUENT LIFT PUMPS
HOLDING LAGOON
FIGURE 1
FULL SCALE DECOLORIZATION SYSTEM
BIOCHEMICAL
TREATMENT LAKE
650 ACRES-900 MG
(180 DAYS RETENTION)
RICEBORO CREEK
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EXCESS GAS
REACTOR
THICKENER
CLARIFIER
SIMULATED
LIME KILN
EXHAUST GAS
FUTURE
LIME
RECOVERY
SLUDGE BLOW DOWN
FIGURE 2
RECARBONATION PILOT PLANT FLOW SCHEME
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TYPICAL EFFLUENT COMPOSITIONS AT INTERSTATE PAPER CORPORATION
RICEBORO, GEORGIA MILL
(All Data in Ibs. per ton Pulp/Board unless otherwise noted.)
Flow (Gal/Ton)
pH
Suspended Solids
BOD
COD
Color, APHA Units
Calcium as CaCO,
Estimated Overflow
from Primary Clarifier
without Lime Treatment
15000
8.5-9.5
5.0 (40 ppm)
41(330ppm)
121 (970 ppm)
1200
<6(<50ppm)
Overflow from Primary
Clarifier with 1000 ppm
CaCO3 Lime Dosage
15000
12.2
1.25 (10 ppm)
28 (220 ppm)
66 (525 ppm)
125
125 (1000 ppm)
Final Effluent at Interstate
Paper after Lime Treatment
plus Natural Recarbonation
and Biological Treatment
in Stabilization Basin
15000
7.5-8.5
1.25 (10 ppm)
.6 (5 ppm)
6 (50 ppm)
125
4 (35 ppm)
Sludge Solids 105 150 255!
Includes estimated Sludge deposited in Stabilization Basin during Natural Recarbonation
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Construction costs for the decolorization
portion of the treatment facilities totalled
$481,600 including land. These costs covered
site development, lime slaking, storage and
feed control facilities, flocculation, a portion of
the clarification unit and sludge storage
facilities costs, project engineering, and
construction supervision. First year operating
costs for chemicals, power, labor, and plant
overhead totalled $289,000 with lime costs
estimated at $15.35 per ton lime (as 90 percent
CaO). First year special study costs including
administration, contingencies, system revisions
and post-construction studies and reports
totalled $133,100. These special study costs
are not included in the economics presented.
(2) A major portion of the operating costs is
accounted for by the non-recovered lime,
including that separable by treated effluent
recarbonation, as well as the lime present in
the settled sludge.
The grant at Interstate did not include
testing these modifications. The potential for
raw material savings by recovery of the lime
used is valued at approximately $78,000
annually as lime rock or $0.56 per ton of
pulp/board. Lime recovery would increase
fixed maintenance, labor, and power costs and
decrease chemical and long term sludge
disposal costs.
Aenal View of Sludge Lagoons and Effluent entry
into Oxidation Pond
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140,000 NOMINAL TONS/YR (197,000 TONS/YR ACTUAL)
OF
UNBLEACHED KRAFT PULP
TOTAL INVESTMENT
(including engineering, excluding land) $455,000
operating costs (all cost in $/Yr)
I Wages (operator/shift plus allotted supervision and technical assistance) 71,000
II Electric Power (288 hp, including lift station, at 8 mils) 14,000
III Maintenance 30,000
IV Laboratory (included in Technical Assistance)
V General Plant Overhead 45,000
VI Raw Materials Lime ($15/Ton) 129,000
TOTAL OPERATING COSTS 289,000
FIXED CHARGES
Depreciation @ 10 years 45,000
Interest on Loan @ 9% 41,000
Taxes, Insurance 9,000
TOTAL FIXED CHARGES 95,000
NET YEARLY COST TOTAL 384,000
OPERATING COST=$1.47/ton pulp
TOTAL COSTS=$1.95/ton pulp
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The process as described, and with the
potential for adding features for lime recovery,
believed directly applicable to unbleached
kraft mills. This process with lime recovery is
currently being applied to the Continental Can
unbleached kraft mill at Hodge, Louisiana.
The process might also find application at
bleached kraft mills where use of lime
precipitation techniques on the heavily colored
caustic extraction stage effluent prove
inadequate to accomplish the desired degree
of decolorization. It might also find application
in treating the mixed color-containing mill
effluents from a bleached kraft pulp mill,
particularly where acidic effluent neutralization
is required prior to biological treatment.
Aenal View of Aeration Basin at Outfall into
Receiving Stream
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FOR FURTHER INFORMATION:
DETAILED INFORMATION ON THIS PROJECT,
INCLUDING EQUIPMENT LIST, IS AVAILABLE
FROM THE SUPERINTENDENT OF
DOCUMENTS AS EPA REPORT 12040 ENC
"COLOR REMOVAL FROM KRAFT PULPING
EFFLUENT BY LIME ADDITION"
TECHNOLOGY TRANSFER
U.S. ENVIRONMENTAL
OR WRITE PROTECTION AGENCY
WASHINGTON, D.C.
20460
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TECHNOI.OGY
TRANSFER
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