Mechanical properties of waterwork sludges — Shear strength

e

Pergamon

Wat. Sci. T~clr. Vo1.36. No. II. pp. 43-50. 1997. C 1997 IAWQ. Published by Elsevier Science Lid

PH: S0273-1223(97)OO667-7

Printed in Great Britain. 0273-1223197 $17'00 + 0-00

MECHANICAL PROPERTIES OF WATERWORKSLUDGES-SHEAR STRENGTH Knut Wichmann and Andreas Riehl DVGW-Forschungsstelle TUHH, Arbeitsbereich Wasserwirtschaft und Wasserversorgung, Technische Universitiit Hamburg-Harburg, Dampfschiffsweg II, 21079 Hamburg, Germany

ABSTRACT In order to ensure a regular and environmentally safe utilization and disposal of waterwork sludges especially the mechanical properties must be controlled. Monitoring of the mechanical properties is of great importance to almost all treatment. utilization and disposal operations. Depending on the raw water source and the treatment processes, different types of waterwork sludge can be described. The first step of this research project included the collection of data from literature reports, information about measuring methods on rheological and soil mechanical parameters and preliminary measurements on selected waterwork sludge samples. Measurement of vane shear strength is a method used in the field of waste disposalllandfill in Germany. Therefore this method was the main topic of the first investigations. Aims were to examine: I. Influence of water contents and dewatering capacity on shear strength. 2. Correlation between laboratory and field measurement devices. Sludge samples from different waterworks were investigated with following devices: laboratory vane shear apparatus, pocket vane apparatus, pocket penetrometer and capillary suction time measuring. e 1997 IAWQ. Published by Elsevier Science Ltd

KEYWORDS Mechanical properties; rheology; shear strength; soil mechanics; waterwork sludges; utilization. INTRODUCTION An orderly utilization and disposal of waterwork sludges needs control of the mechanical properties in order to ensure the quality that is demanded for storage, transport and handling. The main purpose in investigating the mechanical behaviour of sludges is to develop or to apply standards for the measurements at the production site of a sludge. These mechanical measurements must be simple and easy to handle methods to be done by the workers at the sludge production site to secure the requirements for treatment, storage, handling, transport, utilization etc. that are necassary for proper operation. These measurements are necessary to ensure that the whole operation of sludge utilization and disposal can be run in a proper way without disturbance. It is understood that the mechanical measurements are to be seen and done in connection with other measurements that have been or will be standardized. These include methods for chemical and physical parameters (chemical elements, dry solids, loss on ignition, pH value) and operational methods (capillary suction time CST, specific filtration resistance). The different composition of waterwork 43

44

K. WICHMANN and A. RIEHL

sludges with inorganic (Fe, Ca, Al etc.) and organic (Algae, humic substances, powdered activated carbon etc.) substances, depending on the source of raw water and water treatment processes, must be considered. There are no regulations for mechanical properties of sludges in connection with utilization existing in Germany. But for the disposal of wastes the TA Siedlungsabfall (1993) gives minimum values for the solidity: vane shear strength min. 25 kN/m2 or unconfined compression strength min. 50 kN/m2 combined with axial deformation of max. 20%. An inquiry by Schneider (1996) showed, that waterwork sludges of different types (Table I) in Germany amounted in 1992 to ca. 93.000 t OS. Table 1. Quantity of the different waterwork sludge types (1992) Waterwork sludge types

Quantity

Quota

Iron sludge AI-Flocculation sludge Fe-Flocculation sludge Lime sludge

ca. ca. ca. ca.

13.600 t OS 18.200 t OS 24.100 t OS 37.100 t OS

15% 19% 26% -10%

Total

ca. 93.000 t OS

100%

The first step of this research programme included preliminary measurements of mechanical properties of waterwork sludges from the main types in Germany. METHODS The range of 0.2 to 80% dry solid contents of waterwork sludges to be utilized is quiet wide, so that several different mechanical properties have to be measured. These properties are listed in Table 2 in connection with possible measuring methods coming mainly from the soil mechanical or rheological working fields. Table 2. Measuring methods for mechanical characterisation of waterwork sludges Property

Possible Measuring Methods

I. Sedimentation characteristics 2. Viscosity 3. Plasticity. Solidity 4. Consolidation grade 5. Thixotropy 6. Abrasion resistance

Sedimentation tests. Stirring tests Viscometer Liquid limit apparatus (Casagrande method), Cone penetrometer. Laborato~ vane shear test Oedometer Generation: Vibration table Measurement: Penetration of balls of different density and diameter. Penetration of cone Abrasion tester for granular activated carbon

[Rheology] [Soil Mechanics J [Soil Mechanics] [Soil Mechanics J

STATE OF KNOWLEDGE There are only a few data on mechanical properties of waterwork sludges published. Meyer and Dammann (1995) made measurements on the solidity/plasticity of iron sludges from Hamburg Waterworks with a laboratory vane shear test. The sludges were dewatered with a small laboratory filtration press and the preperation of the test samples did not follow the normal procedures due to the small quantities.

Mechanical propenies of watcrwork sludges

45

McTigue et al. (1990) made measurements on the solidity/plasticity of dewatered sludges from 14 waterworks in The Netherlands and Germany. The 72 measurement data of different sludge types and dewatering processes are shown in Fig. I. A laboratory vane shear apparatus was used. The vertical line marks the dry solids concentration of 35% that was given by LAGA (1979) as a minimum value for disposal of wastes in landfills. Sludges with more than 35% OS were then considered to be suitable for landfilling. The horizontal line marks the minimum value of 25 kN/m 2 for the vane shear strength that is now demanded from new regulations in the TA Siedlungsabfall (1993). Approximately 90% of the waterwork sludges tested after mechanical dewatering could not fulfil the required minimum value. 40

r--------------:-:-----...,

35

¥

30

~

€ 25 g' ~ 20

+------------:---------.---

en

to

.! 15 en ~

c:

~

10

.... . .. . ..,.... . . . :. ... ..... 04--+--+--+--+---t--+--I---+--+---t--+--l 5

.~

o

5

10

15

20

25 30 35 40 Dry Solid [% wlwl

45

50

55

60

Figure I. Comparison dry solid contents vs. laboratory vane shear strength.

Moller et al. (1985) executed an extensive measuring progranune on dewatered sewage sludges in Germany. Different devices for solidity/plasticity measurements were tested. Although the aims and slUdges were different, some interesting conclusions could be drawn for our research programme on waterwork sludges. As described in the ATV-Arbeitsbericht (1989), the laboratory vane shear test is a suitable reference method for the solidity measurements of sludges to be disposed of in landfills. PRELIMINARY MEASUREMENTS

Taking into account that the laboratory vane shear test could be a reference method for measurements of sewage sludges, it has to be examined whether this method is likewise appropriate for waterwork sludges. Furthermore it is necessary to find or to develop simple field measuring devices that are suitable for the use at the sludge management sites (waterworks, utilization etc.) and determine the correlation of values from these devices and laboratory instruments. Also the influence of other physical/mechanical properties on the solidity/plasticity bas to be determined. Here the dry solids concentration is a parameter of major interest. In addition also the influence of the dewatering properties, e.g. measured as capillary suction time (CST), is of interest. Inyestiialed devices There are no standardized methods for the preparation of samples for the solidity measurements of wastes existing. In Germany the preparation method described in the ATV-Arbeitsbericht (1989) is normaly used together with the apparatus described in DIN 18127 (1993), that is the small Proctor hammer (2.5 kg weight)

46

K. WICHMANN and A. RIEHL

and the small Proctor vessel. The sludge is filled into the vessel in three equal portions and is compressed with ten knocks. For the measurements a motor driven laboratory vane shear apparatus. a pocket vane shear apparatus (Fig. 2) and a pocket penetrometer after Neuschiifer (Fig. 3) was used. pointer carrier .......::=t1~!!II!;;;:r;;J

I 82.5 mOl

spring -

_

I

U vane No.1

~ vane No. 2

~ vane No. 3

Figure 2. Pocket vane shear apparatus including vanes. •

scale from 0 to 700 psi

30

.15

'-20

635mm

tl _

mdicator sleeve

d!Wjjj~_rr:.(O_=_=_"Jm_P\i)

[)

1800101

CIO

Figure 3. Pocket penetrometer including test cones (with scaling factors). Slud~e samples

The examined sludge samples came from 17 German waterworks. There were nine iron sludges. four iron resp. aluminium flocculation sludges. one aluminium flocculation sludge with powdered activated carbon and one lime sludge. Three samples were so low in dry solids content. that no solidity measurements were possible. They were dried by a thermal process to achieve a higher dry solids content. From one waterwork came three iron sludge samples with different OS contents (24.8. 51.6. 57.5% OS). RESULTS ANO DISCUSSION Influences on shear stren~th The relations between dry solid content and vane shear strength is given in Fig. 4. Samples from one waterwork are connected with a line. Four measurements were stopped at the end of the measuring range when no shearing occurred. These values are marked with an arrow. The relation between capillary suction time (CST) and laboratory vane shear strength was examined with one iron sludge. The samples were in steps dried with a thermal process and the respective values measured. Figure 5 shows that the vane shear strength can only be measured when the CST values are so high that normal measuring procedures are far exceeded. A defined boundary area where both methods will obtain measurable values is not to be found.

Mechanical properties of waterwork sludges

47

200,----------------------, 180

o Iron Sludge • Fe-FlocculatIOn Sludge o AI-Flocculation Sludge • AI-IPAC-Flocculatlon Sludge • lime Sludge Test end Without shearing

_160

~ 140 ~

:; 120 Cl

c:

~

iii

100

~ 80

&.

~ 60 c:

~ 40

20 O+---_-~f___'''''-__+--__+--_+_--_+_--_4

o

10

30 40 Dry Solid [% w/wl

20

50

60

70

Figure 4. Dry solids content vs. laboratory vane shear strength.

9 0 0 . , . . . . . - - - - - - - - - - - r - - - - - - . . . , . 16 800

14

_ 700 .!!!.

• Capillary SuetJon rime

~ 600

o

....

'lane Shear Strengtll

l5 500

13

~ 400 ~

~ 300

a. ClI

u 200 •

100

2

0

o +---.........- --'F""=;.......-+-------+ 0 20

25

30

35

Dry Solid [% wlwl

40

45

Figure S. Capillary suction time vo. laboratory vane shear strength.

Correlations between laboratory and pocket instruments The smallest vane, No. I from the pocket vane shear apparatus, which was designed for samples with higher solidity, is not suitable for waterwork sludges. With samples of high solidity fissures form at the surface at the moment of penetration, so that a regular measurement is not possible. The manufacturer of the apparatus has given scaling factors to transform the measured values to shear strength. These factors are developed for soil measurement and are not suitable for waterwork sludges. Figure 6 shows for the vane No. 2 a linear and for vane No.3 a logarithmic correlation between values from field and laboratory measuring methods. For the pocket penetrometer Neuschafer developed a calibration curve for the transformation of penetrometer bearing capacity values to laboratory vane shear strength values for sewge sludges (ATV• Arbeitsbericht. 1989). Figure 7 shows the evaluation of waterwork sludge measurements. It must be noted that a new calibration curve has to be defined where the transformation in comparison with sewage sludge

48

K. WICHMANN and A. RIEHL

values leads to higher shear strength values. It could be suggested that there is no systematic influence of different waterwork sludge types on the shear strength measurement. 10

9

-r---------------------., , Vane _ Vane , Vane • Vane

No 3 No 3 No 3 No 2

Iron Sludge Fe-Flocculation Sludge AI-FlocculatIon Sludge Iron Sludge

.. Vane No 2 Fe-Flocculation Sludge

, Vane No 3 coefficIent of correlation r" 0,96

• Vane No 2 AI·Flocculatlon Sludge

Vane No 2 coeff,clOnt of correlation r. 0,95

0+-::::::::::.------+----+--------1 20 40 60 80 100 o Laboratory Vane Shear Strength [kN/m']

Figure 6. Pocket vane shear strength vs. laboratory vane shear strength.

500

.g

~

z

~ .?;-

450 400

U

350

ell

300

ell

a.

U

Ol

S

ro

200

~Q)

150

E

~ ~

cQ)

0..

coeffiCIent of correlation r,: 0.97

250

CD

Q)

i AI-Flocculation Sludge • Fe-Flocculaloon Sludge " Iron SlUdge • Lime Sludge

100 50 20

40

60

80

100

Laboratory Vane Shear Strength [kN/m~J

Figure 1. Penetrometer bearing capacity vs. laboratory vane shear strength.

CONCLUSIONS Evaluation of measurjni: methods All three apparatus for mechanical properties are useful for monitoring the measurement of shear strength of waterwork sludges. The laboratory vane shear apparatus could. as with the sewage sludges, be taken as reference device also for waterwork sludges. A comparison between the two field tests shows that the pocket penetrometer - especially because of good handling - could be more useful. An overview of the characteristics of the three devices is given in Table 3.

Mechanical propenies of walerwork sludges

49

Table 3. The characteristics of the three apparatuses for monitoring mechanical properties .-\pparatus

Characteristics

Laboratory ,'ane shear apparatus

• length oftime: ca. 60 minutes I for 3 measurements) • good handling (motorized test procedure) - reproducibility oftest "alues (a"erage +/- IS %) - measuring range: 1- 180 kN/m~ • I~nl!th of time: ca. 5 minutes (for 3 measurements) - me~suring range: 0 - ca. I00 k~ m~ -low costs (ca. 8 % of the Labor:llory Vane Shear Apparatus) - bad handling (specially by high shear strength) - bad reproducibility of test values (average _1- 19 %) - length oftime: ca. 5 minutes (tor 3 measuuments) - good handling - reproducibility oftest "alues (a,'erage +/- 10%) - measuring range: 0 - ca. 90 kN m~ (Vane Shear Strength) - low costs (ca. 8 % of the Laboratory Vane Shear Apparatus)

Pocket vane shear apparatus

Pocket penetrometer

The costs for the complete measuring apparatuses are given in Table 4. Table 4. Net investment costs of the apparatus (March 1997) Appar:ttus I. Proctortest apparatus (DIN I ~ 1:7) - f-Iand operated rammer. ~.5 kg .., Motorized laboralo~ \ ane sh.:ar npparalus - Incl." ditTerent c;llibrated springs and .. different vanes 3. Pock~t "ane shear ;Ipparatus - Incl.; different H\I1l:S ... Pocket penetrometer - Incl." different COlll:S (NEt'SCIIAFERl 5. CST measuring instrument

Trade prices [ECU]

~.600 ~oo

760

Prgposal for a pro~ramme of inyesti~ations A proposal for a programme of investigations for mechanical characterization of waterwork sludges is shown in Table S. Therein are listed all mechanical properties which should be able to be measured with the operation of now known utilization possibilities. To measure sedimentation characteristics simple sedimentation tests are used. Measuring methods for the mechanical properties of sludges (viscosity, plasticity, solidity) should give infonnation about the behaviour of sludges in the boundary areas between: - liquid and plastic - solid and plastic. Also the thixotropic behaviour from solid to liquid and vice versa must be measured together with the piling behaviour referring both to compaction and stability. Abrasion resistance could be measured in special cases e.g. utilization with metallurgical processes.

50

K. WICHMANN and A. RIEHL

Table 5. Proposal for a programme of investigations for mechanical characterization ofwaterwork sludges Property I. Sedimentation characteristics 2. Viscosity 3. Solidity 4. Consolidation grade 5. Thixotropy

6. Abrasion resistance

Investigation energy requirement for stirrer, ma.ximal stop period between pumping t1owability. pumpability maximal stacking height. handling te.g. conveying belt) control of mechanical dewatering tresidual pore water) no liquefaction during transport and handling: time depending re-solidification abrasiveness of thermal dried and granulated iron sludges

REFERENCES ATV-Arbeitsbericht (1989). Die Bestimmung der Deponierfahigkeit von Schllimmen mit der Referenzmethode .Laborfliigelscherfestigkeit". ATV-Fachausschu6 3.2 und 3.6, Korrespondenz Abwasser, 36(8). 903-909. DIN 18127 (1993). Proctorversuch. Beuth-Verlag. Berlin. LAGA (1979). Die geordnete Ablagerung von Abflillen. Merkblau. McTigue. N. E.. Koppers. H. M. M. and Cornwell, D. A. (1990). Regulations, Characteristics. and Analytical Considerations. in: Slib. Schlamm, Sludge. Cornwall, D. A. (ed.). Koppers, RM.M. (ed.) American Water Works Association Research Foundation (AWWARF) und Keuringsinstituut voor Waterleidingartikelen (KIWA). Denver. Meyer. E. and Dammann, E. (1995). Verwertung von Eisenhydroxidschllimmen aus der Trinkwasseraufbereitung in der Baustoffinduslrie. Sch1uBbericht. Forschungsgemeinschaft fUr Wasserwirtschaft und Wasserversorgung an der Technischen Universillit Hamburg-Harburg. Hamburg. Moller. U.• Gay, G. C. W., Kassner, W., Kohlhoff. D., Loll, U. and Oue-Witte, R. (1984). Neudefinition der Deponierfahigkeit von Abwasserschllimmen, Teil 1. Schriftenreihe Siedlungswasserwirtschaft Bochum, Band 6, Bochum. Schneider. S. (1995). RiJckstlinde aus der Trinkwasseraufbereitung in Deutschland: Mengen. Zusammensetzung und Entsorgungswege. Abschlu6bericht. ESWE-Institut, Wiesbaden. TA Siedlungsabfall (1993). Driue allgemeine Verwaltungsvorschrift zum Abfallgesetz. Bundesanzeiger.