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%