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Engineering Properties of Wastewater Treatment Sludge
Modified by Hydrated Lime and Fly Ash
Lee, K(1), Salgado, R. (2) , Jeon, W. (3), Kim, N(4)
(1) Assistant Prof., Dept. of Civil Engrg, Kyungsung Univ., Pusan, Korea
(2) Associate Professor, Sch. Of Civil Eng., Purdue Univ., W. Lafayette, IN, USA
(3) Engineer, Dodam Engineering Co., Ltd., Seoul, Korea
(4) Assistant Professor, Dept. of Arch. Eng., Korea Univ. of Tech. & Education, Korea
----------------------------------------------------------------------------------------------------------
Abstract
The purpose of this paper is to present engineering properties of modified sludge from
wastewater treatment by hydrated lime and fly ash as modifiers. The proper mixing ratio was
determined, which was the ratio to hold the pH of modified sludge above 12.0 for 2 hours.
Extensive laboratory tests were carried out, including particle analysis, compaction and CBR,
SEM, unconfined compression test, permeability test, TCLP test, etc. The main role of lime was
to sterilize microorganisms in the sludge. The unconfined strength of modified sludge by fly ash
and lime satisfied the criteria for landfill cover soil, above 1.0 kg/cm2. The permeability of all
the mixtures was around 1.0*10-7 cm/sec. Extraction tests for hazardous components in
modified sludge revealed below the regulated criteria, especially for cadmium, copper, and lead.
Judging from the extensive testing in the lab, the use of fly ash and hydrated lime would be
another alternative to modify or stabilize wastewater treatment sludge as construction materials
in civil engineering.
Key words : sludge, hydrated lime, fly ash, strength, SEM, permeability, TCLP
1. INTRODUCTION
Lime has been used extensively to improve the properties of clay soils worldwide, the
first records of the process dating back to Roman times. The lime reacts with water contained
within the clay, thereby releasing calcium cation (Ca2+) and hydroxyl anions (OH-) into solution.
The calcium-saturated solution surrounding the clay mineral particles results in cation
substitution and particle flocculation and agglomeration, thereby modifying the clay. Under
conditions of a high pH, slower, longer-term stabilization reactions occur, resulting in gel
formation and subsequent crystallization. Modification of the clay changes its nature, from
essentially cohesive to cohesionless, and stabilizations results in a brittle cemented material
being progressively formed. The rate and extent of property change vary with many factors,
including temperature, water content, lime content, and, most important clay mineralogy
(Rogers & Lee, 1994). It is nature that engineers should adopt lime stabilization for ground
improvement in preference to direct and indirect environmental costs of importing crushed rock
or other granular fill. The most widespread application must be for road subgrade stabilization,
although increasing use is being made of lime stabilization for bulk fill operations,
embankments, and cutting slope repair and as a bearing stratum for highly loaded foundation.
This is in addition to the more novel techniques such as lime slurry pressure injection, lime
columns and lime plies for ground improvement (Rogers & Bruce, 1991).
The purpose of this paper is to evaluate the engineering properties of modified sludge by
hydrated lime and fly ash. Extensive laboratory tests,, including particle analysis, compaction
and CBR, SEM, unconfined compression test, permeability test, and TCLP test, were conducted
to verify the engineering properties of modified sludge mixture as construction materials.
2. LITERATURE REVIEW OF LIME STABILIZATION
The addition of lime, fly ash, and lime-fly ash mixtures causes two basic set of
reactions with the soil : short-term reaction and long-term reaction (Nicholson et al., 1994). The
details and theory of these reactions, along with discussion of the physical, chemical, and
mineralogical processes involved, have been topics of several earlier studies (Diamond &
Kinter, 1965, Usmen & Bowders, 1990, Glenn & Handy, 1963). The short-term effect of the
addition of lime or fly ash to the soil is to cause flocculation and agglomeration of the clay
particles caused by ion exchange at the surface of the soil particles. The result of these short-
term reactions is to enhance workability and provide an immediate reduction in swell, shrinkage,
and plasticity.
Long-term reactions are accomplished over a period of time depending on the rate of
chemical breakdown and hydration of the silicates and aluminates. This results in further
amelioration and binds the soil grains together by the formation of cementitious materials. For
cementation to occur, there must be sufficient source of pozzolans available. Pozzolans are a
source of silica or alumina with high surface area that are available for hydration by alkali or
alkali earth hydroxides to form cementitious products in the presence of moisture at ordinary
temperatures. Soils that do not contain a suitable amount of pozzolans will not react with lime
admixtures. Fly ash provides a source of pozzolans for those deficient soils. The extent and
reaction rate is affected by fineness of the soil, which gives grater surface area, chemical
composition of both the fly ash and the soils to be mixed with it, and the temperature, moisture
content, and amount of stabilizer used (Usmen & Handy, 1990)
3. TESTING MATERIALS
3.1 Materials
In this study, sludge from wastewater treatment in Pusan, Korea were used. The
characteristics of the wastewater treatment sludge are showed seasonal variations. It has the
high water content, up to 250% for wastewater treatment sludge. So, the reduction of the high
water content is a major problem to recycle the sludge as construction materials. Its index
properties are summarized in Table 1, Fig. 1 and Fig. 2.
Hydrated lime and fly ash were adopted the modifier of sludge. Hydrated lime was
use, especially for the purpose of sterilizing microorganisms in the sludge. The fly ash used in
this research was generated at Tae-An thermoelectric power plant in Tan-An peninsula that used
anthracite coal as fuel. Until 1997, all the fly ash not used as cement admixture was disposed of
in waste ponds at the plant. The fly ash was at the stage in the process just before refining for
use cement mixing. It is classified as class-F fly ash.
Table 1. Index Properties of Sludge and Each Modifier
Fig. 1 Gradation of the Testing Materials
Fig. 2 The Shape of Particles by SEM
3.2 Determination of Optimum Mixing Ratio of Lime
The existence of a high pH, especially greater than 12, severely stresses
microorganisms. The high pH in the modified sludge is due primarily to the alkaline content of
the stabilizer. The measurement of pH was done following methods described in Standard
Methods (APHA, 1985). The results of the pH tests are given in Table 2. The modified-waste
water sludge mixtures showed the increase of pH with the increase of lime amount. The
modified-wastewater sludge mixture by 10% and 15% of lime showed a high pH, but 8% of
lime did not meet the criteria. Based on the laboratory pH test, the optimum mixing ratio of
lime in sludge was determined 10% for wastewater sludge.
Table 2. pH Value with Curing Time
3.3 Index Properties of Modified Mixtures
The Atterburg Limits tests were conducted on the sludge and modified sludge
mixtures after a further air drying to a moisture content near or slightly below its expected
platstic limit (PL). Actually, the liquid limit and plastic limit are substantial time-dependent
changes. In this study, the tests were performed just after 2 hour of curing. Table 3 represents
the test results for sludge-mixtures. Judging from the Atterburg Limit tests, the modified-
wastewater sludge mixtures showed increased engineering. The addition of lime and fly ash
showed a positive effect on the plastic index (PI), except for using fly ash of 200%.
Table 3. Test Results for Atterburg Limits
4. ANALYSIS OF MICROSTRUCTURE AND INGRIDIENT
In order to compare the microstructure of original sludge and modified sludge mixture,
SEM (Scanning Electron Microscope), which is Hitach S2400 type, was employed.
Micrographs of selected materials were obtained. These micrographs were closely analyzed for
any change in the microstructure of the original and modified sludge as a result of stabilization
or modification.
Micrographs of waster treatment sludge are shown in Fig. 3.The black areas on the
micrographs represented the voids in the sludge. The microstructure of sludge is not dense.
Fig. 4 represents the modified sludge with Ca(OH)2 and Fly Ash after 28-curing days. The
microstructure of lime-modified sludge is relatively dense. There is a little evidence to make
calcium-compound, which is a thin and polygon shape, due to the inclusion of lime. In case of
fly ash modification, the amount of calcium compound is very popular in the micrograph. The
increase of calcium-compound induced the increase of strength of modified sludge.
Fig. 3 Micrograph of Original Wastewater Treatment Sludge
Fig. 4 Micrograph of Modified Sludge Mixtures by Lime and Fly Ash
5. TESTING METHOD AND RESULTS
5.1 Compaction and CBR Test
Achieving the desired degree of relative compaction necessary to meet
specified or desired properties of a modified sludge is of great importance. As it was
mentioned before, the main problem of recycle of sludge is its high water content. In
order to reduce the water content, hydrated lime and fly ash were used, which allows
the desired relative compaction to be more easily achieve. Compaction was conducted
according to ASTM D 698-91. The testing specimen prepared by remolding was first
mixed with lime and water if needed and then cured for a period of up to 2 hours before
compaction. The OMC(optimum moisture content) and maximum dry weight of
wastewater treatment sludge are 68.4% and 0.81t/m3. The gradual increase in maximum
dry density and decrease in OMC with the addition of greater amounts of lime and fly
ash are shown in Fig. 5. There is a significant change of OMC and dry density with the
addition of lime and fly ash.
Fig. 5 Relationship between OMC and Dry Density with Addition of Lime and Fly Ash
The CBR value is an indicator of soil strength and bearing capacity that is widely used
in the design of civil engineering. According to the state-of-the-art report on lime
stabilization by the Transportation Research Record, the CBR is “not approprite for
characterizing the strength of cured soil- lime mixtures”, can only be used as a
comparison, and little practical significance or meaning as a measure of strength of
stability other than as a relative indicator test. To simulate the compaction carried out in
the field, the modified sludge was compacted to 95% of the maximum dry density as
obtained from the standard compaction test. Specimens were then soaked in water for a
further 96hr under a surcharge weight of 5.72kg. After swell was measured, the CBR
values were then recorded according to ASTM standards. Table 4 shows the CBR,
swelling and absorbed water content. The increase of CBR with the addition of lime
and fly ash is not significant, which means the alternative method should be consider,
especially for the use of construction materials as landfill cover soil, because of its low
CBR.
Table 4. Test Results for CBR of Modified Wastewater Sludge Mixtures
5.2 Unconfined Compression Test
The unconfined compression apparatus made by Marui in Japan was used in this
testing. The objective of the testing program was to obtain the undrained elasticity modulus
versus curing time. The diameter of the test material was 5cm, and the length was 12.5cm.
According to Head (1982), the compression velocity should be 2mm/min for the test of which
the diameter was 5cm; this velocity was equivalent to 1.6% strain per minute for the samples
tested in this program. Cured specimens after 0, 7 and 28 days were tested.
Table 5 represents the whole testing results including type and amount of additive, and
unconfined compression strength with curing time. The gain in strength of modified sludge
mixtures is primarily caused by the formation of various calcium silicate hydrates and calcium
aluminate hydrates. The exact products formed very with the kind of sludge mineralogy and the
reaction conditions, including temperature, moisture, and curing conditions. The addition of
lime and fly ash into wastewater sludge increased the unconfined compressive strength. When
lime is to be added with fly ash into wastewater treatment sludge, it has been suggested that for
a given ratio of lime to fly ash, the compressive strength of the modified sludge mixtures
increased with the amount of lime and fly ash.
Table 5. Unconfined Compressive Strength for Each Modified Mixture
5.3 Direct Shear Test
One of the major parameters in soil mechanics is strength parameter, like internal
friction angle and cohesion. Table 6 showed the strength parameter of modified wastewater
sludge mixtures, using unconfined compression test and direct shear test. The addition of lime
and fly ash showed the increase of internal friction angle and cohesion of modified sludge
mixtures.
Table 6. Strength Parameters of Modified Wastewater Sludge Mixtures
6. PERMEABILITY AND TCLP TEST
6.1 Permeability Test
A control panel and a flexible-wall permeability, which were made by Trautwein Soil
Testing Equipment and shown in Fig. 6, were used the devices for the permeability tests. In the
control panel, it is possible to apply pressure to the test materials, to subject vacuum at the same
time, and to supply deaired water. Therefore, saturation of materials and permeability tests are
simultaneously carried out. Flexible wall testing using the falling head condition was done in a
triaxial cell. The detailed test method is based upon ASTM(D 5084).
Fig. 7 showed the relationship between permeability and voids ratio of modified
sludge mixtures. The measured permeability of each modified sludge obtained from the testing
ranged from 1× 10-8 to 1× 10-4cm/s.
Fig. 6 Testing Equipment for Permeability Test
Fig. 7 Test Results for Permeability Test
6.2 Leaching Test
A common leaching test was carried out modified sludge mixtures. The goal of the
test was to determine the concentration of a number of chemicals in the leaching liquid from the
mixtures. Test results are shown in Table 7. Considering the leaching test results, the presence
of heavy metal in the modified sludge mixture is very small.
Table 7. Specification & Test Results for Leaching Test (mg/l)
7. CONCLUSION
The research presented in this paper aimed to characterize shear strength parameters
and physical properties of wastewater sludge modified by modifiers (hydrated lime and fly ash)
to be used as construction materials. A testing program was carried out to evaluate the index
properties, microstructure by SEM, unconfined compressive strength, permeability and
leatching potential. Within the limited laboratory test, following conclusion can be drawn.
The addition of lime and fly ash into wastewater sludge decreased the waster content
of original sludge, which is one of the major problems in recycling the sludge. Especially, the
use of fly ash showed a significant effect, but the large amount of fly ash, more than 200%, did
a negative effect. As the addition of modifiers increased, the optimum moisture content
decreased and the dry density increased. Also, there is a little increase of CBR value, but this is
not enough to use as subgrade soil. The unconfined strength of modified sludge by fly ash and
loess was above 1.0 kg/cm2. The main reason is pozzolan effect. Form the SEM, the
microstructure of lime-modified sludge is relatively dense. There is a little evidence to make
calcium-compound, which is a thin and polygon shape, due to the inclusion of lime. In case of
fly ash modification, the amount of calcium compound is very popular in the micrograph. The
increase of calcium-compound induced the increase of strength of modified sludge. The range
of permeability of all the mixtures was between 1.0*10-4 cm/sec to 8.0*10-8 cm/sec. Extraction
tests for hazardous components in modified sludge revealed below the regulated criteria,
especially for cadmium, copper, and lead. Judging from the extensive testing in the lab, the use
of fly ash, and lime would be another alternative to modify or stabilize wastewater treatment
sludge as construction materials in civil engineering
ACKNOWLEDGEMENT
This research has been performed as a part of Advanced Highway Research Center
Project funded by Korea Ministry of Science and Technology, Korea Science and Engineering
Foundation.
REFERENCES
ASTM (1991), Annual Book of ASTM Standards
American Public Health Association (1980), Standard Methods for the Examination of
Waste and Wastewater, 15th Edition, Washington, D.C.
Diamond, S. and Kinter, E.B. (1965), “Mechanisms of Soil-Lime Stabilization : An
Interpretive Review”, Highway Research Record 92, HRB, National Research Countil,
Washington, D.C., pp. 83-96
Glenn, G. R. and Handy, R. L. (1963), “Lime-Clay Mineral Reaction Products”,
Highway Research Record 29, HRB, National Research Countil, Washington, D.C., pp.
70-82
Head, K. H (1982), Manual of soil laboratory testing, Pentech Press
Nicholson, P. G., Kashyap, V. and Fujii, C. F. (1994), “Lime and Fly Ash Admixture
Improvement of Tropical Hawaiian Soils”, TRR 1288, TRB, National Research Countil,
Washington, D.C., pp. 71-78
Park, J. S., Yoon, K. K., Kim, T. K, Lee, J. H., Paek, M. K., and Kim, N. Y.(1996),
"Optimum Mix Design of Concrete Incorporating Waste Foundry Sand", Institute of
Industrial Technology, Research Report Vol. 2, Kangwon Univ.
Rogers, C.D.F. & Lee, S. J. (1994), “Drained Shear Strength of Lime-Clay Mixs”, TRR
1440, Washington, D.C., pp. 53-62
Rogers, C.D.F. & Bruce, C. J. (1990), “The Strength of Lime-Stabilized British Clays”,
In Proc., British Aggregates Construction Materials Industries Lime Stabilization ’90
Conference, Sutton Coldfield, UK, pp. 57-72
Usmen, M. A. and Bowders, Jr. (1990), “Stabilization Characteristics of Class F Fly
Ash”, TRR 1288, TRB, National Research Countil, Washington, D.C., pp. 59-60
Xu, A., and Sarkar, S. L.(1994), "Microstructural development in high-volume fly-ash
cement system", Journal of Materials in Civil Engineering, Vol. 6, No. 1, February, pp.
117-136.
List of Tables
Table 1. Index Properties of Sludge and Each Modifier
Table 2. pH Value with Curing Time
Table 3. Test Results for Atterburg Limits
Table 4. Test Results for CBR of Modified Wastewater Sludge Mixtures
Table 5. Unconfined Compressive Strength for Each Modified Mixture
Table 6. Strength Parameters of Modified Wastewater Sludge Mixtures
Table 7. Specification & Test Results for Leaching Test (mg/l)
List of Figures
Fig. 1 Gradation of the Testing Materials
Fig. 2 The Shape of Particles by SEM
Fig. 3 Micrograph of Original Wastewater Sludge
Fig. 4 Micrograph of Modified Sludge Mixtures by Lime and Fly Ash
Fig. 5 Relationship between OMC and Dry Density with Addition of Lime and Fly Ash
Fig. 6 Testing Equipment for Permeability Test
Fig. 7 Test Results for Permeability Test
Table 1. Index Properties of Sludge and Each Modifiers
Wastewater Sludge
Hydrated Lime
Fly Ash
Specific Gravity (g/am3) 2.059 2.199 2.173 Water Content (%) 217.0 - -
Classification (UIUC) OH/Peat - - Cu 9.34 48.26 16.44 Cg 1.18 2.75 0.63
Mean Size (um) 123.7 232.4 111.0 Standard Deviation 171.0 244.0 135.0
Coefficient of Variation (%) 139.0 105.0 122.0 Table 2. pH with Curing Time
Sludge Lime (%) 0 30 60 120 180 240 8 12.1 12.1 12.0 11.8 11.7 11.6 10 12.4 12.4 12.3 12.2 12.1 12.0
Wastewater
15 12.5 12.5 12.4 12.2 12.1 12.0 Table 3. Atterburg Limits
Sludge Lime(%) Fly Ash(%) LL PL PI SL 0 0 233.3 182.2 51.1 39.6
0 193.5 180.0 13.5 43.3 50 140.5 129.8 10.7 46.5 100 120.0 115.0 5.0 46.5
10
200 88.0 NP NP NP 0 182.0 175.0 7.0 52.3
50 136.8 128.9 7.9 54.1 100 126.5 120.0 6.5 54.6
Wastewater
15
200 88.4 NP NP NP Table 4. Test Results for CBR of Modified Wastewater Sludge Mixture
Lime (%) Fly Ash (%) Absorbed Water(%) Expansion (%) CBR (%) 0 0 3.70 1.37 2.74 10 50 6.45 2.39 3.49 10 100 9.33 2.31 4.52 10 200 10.0 2.30 5.13
Table 5. Unconfined Compressive Strength (kg/cm2) for Each Modified Mixture
Sludge Lime (%) Ash (%) 0 day 7 day 28 day 0 0 0.084 0.093 0.180 Waste
Water 10 50 0.059 0.983 1.610 Table 6. Strength Parameters of Modified Sludge Mixtures
28 day Sludge Lime(%) Ash(%) C angle
0 0 9.74 26.8 10 50 15.81 34.06
Waste Water
10 100 18.31 43.4 Table 7. Specification & Test Results for Leaching Test (mg/l)
Korean Standard TCLP by EPA Lime (%)
Curing Time (day) Cd Cu Pb Cd Cu Pb
0 N.D 0.05 0.10 N.D 0.15 0.15 3 N.D 0.04 0.12 N.D 0.12 0.15 7 N.D 0.05 N.D N.D 0.16 0.14
0
28 N.D 0.05 N.D N.D 0.16 0.15 0 N.D N.D N.D N.D 0.19 0.14 3 N.D 0.04 N.D N.D 0.17 0.14 7 N.D 0.05 N.D N.D 0.19 0.14
5
28 N.D 0.07 0.12 N.D 0.18 0.14 0 N.D 0.05 N.D N.D 0.20 0.15 3 N.D 0.07 N.D N.D 0.17 0.16 7 N.D 0.05 N.D N.D 0.18 0.18
10
28 N.D 0.04 0.14 N.D 0.20 0.15 0 N.D 0.05 N.D N.D 0.20 0.15 3 N.D 0.06 N.D N.D 0.20 0.15 7 N.D 0.05 N.D N.D 0.19 0.17
15
28 N.D 0.04 N.D N.D 0.19 0.15 ND : Not Detected
0
20
40
60
80
100
0.1 1 10 100 1000
wastewater-sludge
l ime
fly ash
Particle Size (um)
Pas
sing
Per
cent
(%
)
F i g 1 . G r a d a t i o n o f t h e T e s t i n g M a t e r i a l s
(a) hydrated lime (b) fly ash
F i g . 2 The P a r t i c l e S h ape s b y SEM
Fig. 3 Micrograph of Original Water/Wastewater Sludge
(a) 10% lime for (waste) (b) 10% lime + 50% fly ash (waste) Fig. 4 Micrograph of Modified Sludge Mixture by Lime and Fly Ash
0.6
0.7
0.8
0.9
1
1.1
1.2
0 20 40 60 80 100 120 140
Water content(%)
Dry
Den
sity
(t/m
3 )
S
S+L10%
S+L15%
S+L10%+F50%
S+L10%+F100%
S+L10%+F200%
s ludge100% f i xed
Fig. 5 Relationship between OMC and Dry Density by Modification
Fig. 6 Testing Equipment for Permeability Test
Fig. 7 Testing Results for Permeability Test
Void Ratio (e).6 .8 1.0 1.2 1.4 1.6
Kv (
cm/s
ec)
1e-7
1e-6
1e-5Sludge Sludge + Lime 10%Sludge + Lime 10% + Fly ash 50%Sludge + Lime 10% + Fly ash 100%Sludge + Lime 10% + Fly ash 200%