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Viscous behaviour of sludge centrate in response to polymer conditioning
PII: S0043-1354(99)00143-8
Wat. Res. Vol. 34, No. 1, pp. 354±358, 2000 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
www.elsevier.com/locate/watres
TECHNICAL NOTE VISCOUS BEHAVIOUR OF SLUDGE CENTRATE IN RESPONSE TO POLYMER CONDITIONING DAVID H. BACHE*M and ELEFTHERIOS N. PAPAVASILOPOULOSM Department of Civil Engineering, University of Strathclyde, 107 Rottenrow, Glasgow G4 ONG, UK (First received 1 September 1998; accepted in revised form 1 March 1999) AbstractÐThis note outlines an analysis of the centrate viscosity in response to polymer conditioning of an alum sludge. Recognising the complexity of the controls on viscosity, an analytical scheme is described in which the centrate viscosity (at low contaminant concentrations) can be regarded as the linear sum of component terms. As the polymer dose is increased from zero, the centrate viscosity decreases initially, passes through a minimum and then increases. The interaction of viscosity contributions arising from the turbidity and residual polymer in the centrate appear to control the polymer dosage at which the minimum occurs, whereas the solvent viscosity, together with an electrolytic component contribute towards the overall viscosity. Associated analysis indicates that the minimum in the viscosity-dose trend coincides with the onset of adsorption saturation of a nonionic polymer. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐalum sludge, centrate, conditioning, polymer, turbidity, viscosity
INTRODUCTION
water and lower viscosity. It was also suggested that the behaviour of the centrate viscosity in this domain followed the Einstein relationship for viscosity (e.g. Probstein 1994):
When conditioning a sludge prior to dewatering, the polymer costs alone can represent a signi®cant expense and may exceed 50% of the overall solidshandling costs (US EPA, 1987; Vesilind, 1979). Thus it is generally a goal to establish optimum use of polymers. Of the various methods for identifying an optimum dose, Dentel and Abu-Orf (1995) found that the centrate viscosity of a digested sludge passed through a minimum (such as displayed in Fig. 1), this being consistent with the minimal behaviour in the Capillary Suction Timepolymer dose trend, and also occurred at a near zero value for streaming current in association with a cationic polymer used for conditioning. Studies on a waterworks sludge reported in Papavasilopoulos (1997) showed that its centrate viscosity-polymer dose trend followed a similar pattern. The trend illustrated in Fig. 1 can be considered in two parts. For the limb ab, in which the viscosity decreases with increasing dose, Dentel and Abu-Orf (1995) suggested that this was promoted by the collection of the ®ner sludge particles into separable ¯ocs and the exclusion of water from the solids, thus leading to a clearer centrate with more
Z Zo 1 2:5j
*Author to whom all correspondence should be addressed. Tel.: +44-141-5524400 Ext. 3351; fax: +44-1415532066. 354
1
in which Zo is the solvent viscosity and j the solids volume concentration. Equation (1) applies to suspensions of non-interacting, rigid particles with volume fractions <0.02 and is independent of particle size. Papavasilopoulos (1997) showed that region ab illustrated in Fig. 1 was strongly correlated with turbidity changes. However, the solids volume concentration were so low that the Einstein relation could not explain the observed changes in viscosity. A similar behavioural trend was evident in the study of Abu-Orf and Dentel (1997; Fig. 11) which showed parallel behaviour between the ®ltrate viscosity and turbidity in region ab. With regard to the rising limb bc in Fig. 1, there is a common view (e.g. Christensen et al., 1993; Dentel and Abu-Orf, 1995; Papavasilopoulos, 1997) that the viscosity behaviour is tied to increasing amounts of excess polymer in the liquid stream. This view had not been properly tested owing to the lack of a suitable analytical method of measuring polymer residuals. However, through use of a technique reported in Keenan et al. (1998), Papavasilopoulos and Bache
Technical Note
355
Papavasilopoulos and Bache (1998) for a sludge of similar type. Freshly conditioned sludge samples (100 ml) were centrifuged at 2000 rpm for 5 min (acceleration 600 g) to obtain the centrate. Accurate viscosity measurements were obtained using an Ostwald viscometer (PSL Ltd, Wickford, UK), the error being in the region of 0.01± 0.1% (Shaw, 1992). All readings were taken at 20.020.28C. It should be stressed that temperature control is important since changes in the viscosity of water with temperature can be of the same order as the eects of turbidity and residual polymer. Turbidity of the centrate was measured using an SP6±250 Visible Spectrophotometer with a 20 mm cell. Measurements of residual polymer were carried out using Size Exclusion Chromatography in accord with Keenan et al. (1998). Fig. 1. Centrate viscosity as a function of polymer dosage (after Dentel and Abu-Orf, 1995). RESULTS
(1998) were able to provide a detailed view of the residual polymer behaviour during the conditioning of a waterworks sludge. Beyond the controlling features noted above, it is well known that many other factors contribute to the overall viscosity behaviour. For example, the review by Buscall et al. (1984) highlighted the signi®cance of eective particle size. Viscous behaviour also depends on the physicochemical interactions between particles in the suspension and the energy expended during deformation of particle structures (van de Ven and Hunter, 1977). At low contaminant concentrations (expected in the centrate), an analytical scheme presented in Stokes and Mills (1965) assumes that component terms act independently, such that the combined eect on viscosity can be regarded as the sum of the controlling components i.e. Z SZi
i 1, . . . , n
2
In this note, it is intended to explore a model of the form: Z Zw Zt Zp
Figure 2 illustrates the relationship between Zt and turbidity gained from suspensions of raw sludge samples in nanopure water. The trend is fairly close to that reported in Papavasilopoulos (1997), although a slightly dierent analytical approach was adopted in the former study. Figure 3 shows the dependence of Zp on the concentration of Flocmiser 50 in pure water. Both trends were used for the determination of the component factors of viscosity i.e. Zt and Zp. Turbidity and residual polymer measurements were carried out following the conditioning of fresh alum sludge samples with an initial solids concentration of 1500 mg/l. Component factors are shown in Fig. 4, their combined eect being shown in the lower trend of Fig. 5. Comparing the model trend (equation 3) with the observed trend, it is seen that these are well matched, but are displaced. The viscosity dierence appears to be more or less independent of the polymer dose. It is suggested that the viscosity displacement is caused by the presence of
3
in which Zw, Zt and Zp refer to the viscosity of pure water, the viscosity contribution caused by turbidity and the viscosity contribution caused by residual polymer respectively.
MATERIALS AND METHODS
Alum sludge was obtained from the sludge outlet of the clari®ers at Burncrooks water treatment works (West of Scotland Water) during the period of this analysis. The particular sludge emanates from the treatment of a low turbidity/ highly coloured water, typical of an upland catchment. The suspended solids concentration of the sludge ranged from 1200 to 1500 mg/l and had pH in the range 5.5±6.0. Conditioning was carried out using Flocmiser 50 (Watermiser Ltd, Bathgate, UK), a basically non-ionic (2% anionic) organic polymer. Polymer preparation, dosing and mixing were in accord with the methodology descibed in Papavasilopoulos (1997) and
Fig. 2. Dependence of viscosity component Zt on turbidity.
356
Technical Note
estimate of the dissolved solids can be made from the following equation (Todd, 1959): TDS EC=1:56
4
where EC is the electrical conductivity (in mS) and TDS, the total dissolved solids (in mg/l). Thus, it appears that the conductivity value of 120 mS corresponds to about 77 mg/l TDS. Inspection of data listed in Appendix 2.1 of Stokes and Mills (1965), shows that 77 mg/l TDS is consistent with a potential viscosity of 0.02 10ÿ6 m2/s for a wide range of salt solutions; thus the electrolytic contribution provides a likely explanation of the viscosity displacement shown in Fig. 5. With the inclusion of an electrolytic contribution, equation (3) should be cast in the modi®ed form: Z Zw Zt Zp Ze Fig. 3. Dependence of viscosity component Zp on polymer dose.
electrolytes in solution since the concentration of an electrolyte is known to aect viscosity (Stokes and Mills, 1965). From Fig. 5, the dierence between the trends is in the order of 0.02 10ÿ6 m2/s. To investigate whether the viscosity shift can be attributed to the presence of electrolytes, centrate derived at the dose corresponding to the minimum viscosity was sampled and diluted with nanopure water. At the minimum viscosity, the centrate had a conductivity close to 120 mS. A plot describing the relationship between the conductivity and viscosity is shown in Fig. 6. From the conductivity values, an
5
where the additional term (Ze) refers to the viscosity induced by dissolved substances as an electrolyte.
DISCUSSION
The analysis indicates that equation (5) provides a useful framework for describing the centrate viscosity behaviour on the basis of component terms. Within this expression, it is evident that the terms Zt and Zp exert greatest in¯uence on the position of the minimum. In order to demonstrate how the combined term Zt+Zp relates to the pattern of polymer adsorption (such as described in Papavasilopoulos and Bache, 1998), these are coplotted in Fig. 7. It is seen that the minimum in the
Fig. 4. Trends in the component viscosity contributions Zp and Zt as a function of polymer dose (sludge suspended solids: 1500 mg/l).
Technical Note
357
Fig. 5. Comparison of estimated trend (using eqn 3) with measured trend in centrate viscosity.
Zt+Zp trend coincides with what is termed as a `critical dose', i.e. the dose which marks the onset of adsorption saturation. Below the critical dose (13 mg/l) the residual polymer is close to zero (as evident in the viscosity behaviour shown in Fig. 4). In this phase (corresponding to the limb ab in Fig. 1), the reduction in viscosity is largely controlled by the reduction in the turbidity of the centrate. Above the critical dose (limb bc in Fig. 1), the measurements (such as shown in Figs 4 and 7) reinforce the view that the behaviour in this domain is caused by excess polymer. The analysis has shown how the individual com-
ponent terms in¯uence the total viscosity. It is recognised that the trend in Fig. 2 does not represent a unique relationship between viscosity and turbidity due to the role of particle size and other factors such as particle charge. Similarly, the term Ze depends on the type of electrolyte. Although many of the aspects described in this note need to be explored more fully, it is suggested that equation (5) provides useful insight into the controls on the viscosity behaviour. Equation 5 appears to provide a more comprehensive description of the viscosity behaviour than the mechanisms suggested in Dentel and Abu-Orf (1995).
Fig. 6. Kinematic viscosity as a function of conductivity.
358
Technical Note
Fig. 7. Behaviour of the combination Zt+Zp as a function of polymer dose in relation to the adsorption of the polymer on a sludge of similar solids content; the adsorption trend is from Papavasilopoulos and Bache (1998).
CONCLUSIONS
. At low contaminant concentrations, the viscosity of the centrate appears to be controlled by a series of components represented by Z Zw Zt Zp Ze . Within this relationship, the terms Zt and Zp representing the turbidity and polymer contributions, appear to control the position of the viscosity minimum. The terms Zw and Ze representing the pure water and the electrolytic contribution respectively, behave as constants for a given temperature. . Evidence from the analysis presented here indicates that the minimum in centrate viscosity more or less coincides with the onset of adsorption saturation.
REFERENCES
Abu-Orf M. M. and Dentel S. K. (1997) Polymer dose assessment using the streaming current detector. Wat. Environ. Res. 69(6), 1075±1085. Buscall R., Stewart R. F. and Sutton D. (1984) Structure in colloid suspensions and eects on solid±liquid separation. Filtration and Separation (May/June), 183±188. Christensen J. R., Sorensen P. B., Christensen G. L. and Hansen J. A. (1993) Mechanisms for overdosing in
sludge conditioning. J. Environ. Engng, ASCE 119(1), 159±171. Dentel S. K. and Abu-Orf M. M. (1995) Laboratory and full scale studies of liquid stream viscosity and streaming current for characterization and monitoring of dewaterability. Wat. Res. 29(12), 2663±2672. Keenan H. E., Papavasilopoulos E. N. and Bache D. H. (1998) Measurement of polymer residuals in an alum sludge. Wat. Res. 32(10), 3173±3176. Papavasilopoulos E. N. (1997) Viscosity as a criterion for the optimum dosing of polymers in waterworks sludges. J. Chart. Inst. Wat. Environ. Mgmnt 11(3), 217±224. Papavasilopoulos E. N. and Bache D. H. (1998) On the role of aluminium hydroxide in the conditioning of an alum sludge. Wat. Sci. Tech. 38(2), 33±40. Probstein R. F. (1994) Physicochemical Hydrodynamics. An Introduction, 2nd ed. John Wiley & Sons Inc. Shaw D. J. (1992) Introduction to Colloid and Surface Chemistry, 4th ed. Butterworth Heinemann Ltd. Stokes R. H. and Mills R. (1965) Viscosity of electrolytes and related properties. In The International Encyclopedia of Physical Chemistry and Chemical Physics, vol. 3. Pergamon, Oxford. Todd D. K. (1959) Groundwater Hydrology. John Wiley & Sons Inc. US Environmental Protection Agency (1987) Design Manual. Dewatering Municipal Wastewater Sludges. EPA-652/1-87-014, Cincinnati, Ohio. van de Ven T. G. M. and Hunter R. J. (1977) The energy dissipation in sheared coagulated sols. Rheol. Acta. 16, 534±543. Vesilind P. A. (1979) Treatment and Disposal of Wastewater Sludges, 2nd ed. Ann Arbor Science Publishers Inc, Michigan.
Wat. Res. Vol. 34, No. 1, pp. 354±358, 2000 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
www.elsevier.com/locate/watres
TECHNICAL NOTE VISCOUS BEHAVIOUR OF SLUDGE CENTRATE IN RESPONSE TO POLYMER CONDITIONING DAVID H. BACHE*M and ELEFTHERIOS N. PAPAVASILOPOULOSM Department of Civil Engineering, University of Strathclyde, 107 Rottenrow, Glasgow G4 ONG, UK (First received 1 September 1998; accepted in revised form 1 March 1999) AbstractÐThis note outlines an analysis of the centrate viscosity in response to polymer conditioning of an alum sludge. Recognising the complexity of the controls on viscosity, an analytical scheme is described in which the centrate viscosity (at low contaminant concentrations) can be regarded as the linear sum of component terms. As the polymer dose is increased from zero, the centrate viscosity decreases initially, passes through a minimum and then increases. The interaction of viscosity contributions arising from the turbidity and residual polymer in the centrate appear to control the polymer dosage at which the minimum occurs, whereas the solvent viscosity, together with an electrolytic component contribute towards the overall viscosity. Associated analysis indicates that the minimum in the viscosity-dose trend coincides with the onset of adsorption saturation of a nonionic polymer. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐalum sludge, centrate, conditioning, polymer, turbidity, viscosity
INTRODUCTION
water and lower viscosity. It was also suggested that the behaviour of the centrate viscosity in this domain followed the Einstein relationship for viscosity (e.g. Probstein 1994):
When conditioning a sludge prior to dewatering, the polymer costs alone can represent a signi®cant expense and may exceed 50% of the overall solidshandling costs (US EPA, 1987; Vesilind, 1979). Thus it is generally a goal to establish optimum use of polymers. Of the various methods for identifying an optimum dose, Dentel and Abu-Orf (1995) found that the centrate viscosity of a digested sludge passed through a minimum (such as displayed in Fig. 1), this being consistent with the minimal behaviour in the Capillary Suction Timepolymer dose trend, and also occurred at a near zero value for streaming current in association with a cationic polymer used for conditioning. Studies on a waterworks sludge reported in Papavasilopoulos (1997) showed that its centrate viscosity-polymer dose trend followed a similar pattern. The trend illustrated in Fig. 1 can be considered in two parts. For the limb ab, in which the viscosity decreases with increasing dose, Dentel and Abu-Orf (1995) suggested that this was promoted by the collection of the ®ner sludge particles into separable ¯ocs and the exclusion of water from the solids, thus leading to a clearer centrate with more
Z Zo 1 2:5j
*Author to whom all correspondence should be addressed. Tel.: +44-141-5524400 Ext. 3351; fax: +44-1415532066. 354
1
in which Zo is the solvent viscosity and j the solids volume concentration. Equation (1) applies to suspensions of non-interacting, rigid particles with volume fractions <0.02 and is independent of particle size. Papavasilopoulos (1997) showed that region ab illustrated in Fig. 1 was strongly correlated with turbidity changes. However, the solids volume concentration were so low that the Einstein relation could not explain the observed changes in viscosity. A similar behavioural trend was evident in the study of Abu-Orf and Dentel (1997; Fig. 11) which showed parallel behaviour between the ®ltrate viscosity and turbidity in region ab. With regard to the rising limb bc in Fig. 1, there is a common view (e.g. Christensen et al., 1993; Dentel and Abu-Orf, 1995; Papavasilopoulos, 1997) that the viscosity behaviour is tied to increasing amounts of excess polymer in the liquid stream. This view had not been properly tested owing to the lack of a suitable analytical method of measuring polymer residuals. However, through use of a technique reported in Keenan et al. (1998), Papavasilopoulos and Bache
Technical Note
355
Papavasilopoulos and Bache (1998) for a sludge of similar type. Freshly conditioned sludge samples (100 ml) were centrifuged at 2000 rpm for 5 min (acceleration 600 g) to obtain the centrate. Accurate viscosity measurements were obtained using an Ostwald viscometer (PSL Ltd, Wickford, UK), the error being in the region of 0.01± 0.1% (Shaw, 1992). All readings were taken at 20.020.28C. It should be stressed that temperature control is important since changes in the viscosity of water with temperature can be of the same order as the eects of turbidity and residual polymer. Turbidity of the centrate was measured using an SP6±250 Visible Spectrophotometer with a 20 mm cell. Measurements of residual polymer were carried out using Size Exclusion Chromatography in accord with Keenan et al. (1998). Fig. 1. Centrate viscosity as a function of polymer dosage (after Dentel and Abu-Orf, 1995). RESULTS
(1998) were able to provide a detailed view of the residual polymer behaviour during the conditioning of a waterworks sludge. Beyond the controlling features noted above, it is well known that many other factors contribute to the overall viscosity behaviour. For example, the review by Buscall et al. (1984) highlighted the signi®cance of eective particle size. Viscous behaviour also depends on the physicochemical interactions between particles in the suspension and the energy expended during deformation of particle structures (van de Ven and Hunter, 1977). At low contaminant concentrations (expected in the centrate), an analytical scheme presented in Stokes and Mills (1965) assumes that component terms act independently, such that the combined eect on viscosity can be regarded as the sum of the controlling components i.e. Z SZi
i 1, . . . , n
2
In this note, it is intended to explore a model of the form: Z Zw Zt Zp
Figure 2 illustrates the relationship between Zt and turbidity gained from suspensions of raw sludge samples in nanopure water. The trend is fairly close to that reported in Papavasilopoulos (1997), although a slightly dierent analytical approach was adopted in the former study. Figure 3 shows the dependence of Zp on the concentration of Flocmiser 50 in pure water. Both trends were used for the determination of the component factors of viscosity i.e. Zt and Zp. Turbidity and residual polymer measurements were carried out following the conditioning of fresh alum sludge samples with an initial solids concentration of 1500 mg/l. Component factors are shown in Fig. 4, their combined eect being shown in the lower trend of Fig. 5. Comparing the model trend (equation 3) with the observed trend, it is seen that these are well matched, but are displaced. The viscosity dierence appears to be more or less independent of the polymer dose. It is suggested that the viscosity displacement is caused by the presence of
3
in which Zw, Zt and Zp refer to the viscosity of pure water, the viscosity contribution caused by turbidity and the viscosity contribution caused by residual polymer respectively.
MATERIALS AND METHODS
Alum sludge was obtained from the sludge outlet of the clari®ers at Burncrooks water treatment works (West of Scotland Water) during the period of this analysis. The particular sludge emanates from the treatment of a low turbidity/ highly coloured water, typical of an upland catchment. The suspended solids concentration of the sludge ranged from 1200 to 1500 mg/l and had pH in the range 5.5±6.0. Conditioning was carried out using Flocmiser 50 (Watermiser Ltd, Bathgate, UK), a basically non-ionic (2% anionic) organic polymer. Polymer preparation, dosing and mixing were in accord with the methodology descibed in Papavasilopoulos (1997) and
Fig. 2. Dependence of viscosity component Zt on turbidity.
356
Technical Note
estimate of the dissolved solids can be made from the following equation (Todd, 1959): TDS EC=1:56
4
where EC is the electrical conductivity (in mS) and TDS, the total dissolved solids (in mg/l). Thus, it appears that the conductivity value of 120 mS corresponds to about 77 mg/l TDS. Inspection of data listed in Appendix 2.1 of Stokes and Mills (1965), shows that 77 mg/l TDS is consistent with a potential viscosity of 0.02 10ÿ6 m2/s for a wide range of salt solutions; thus the electrolytic contribution provides a likely explanation of the viscosity displacement shown in Fig. 5. With the inclusion of an electrolytic contribution, equation (3) should be cast in the modi®ed form: Z Zw Zt Zp Ze Fig. 3. Dependence of viscosity component Zp on polymer dose.
electrolytes in solution since the concentration of an electrolyte is known to aect viscosity (Stokes and Mills, 1965). From Fig. 5, the dierence between the trends is in the order of 0.02 10ÿ6 m2/s. To investigate whether the viscosity shift can be attributed to the presence of electrolytes, centrate derived at the dose corresponding to the minimum viscosity was sampled and diluted with nanopure water. At the minimum viscosity, the centrate had a conductivity close to 120 mS. A plot describing the relationship between the conductivity and viscosity is shown in Fig. 6. From the conductivity values, an
5
where the additional term (Ze) refers to the viscosity induced by dissolved substances as an electrolyte.
DISCUSSION
The analysis indicates that equation (5) provides a useful framework for describing the centrate viscosity behaviour on the basis of component terms. Within this expression, it is evident that the terms Zt and Zp exert greatest in¯uence on the position of the minimum. In order to demonstrate how the combined term Zt+Zp relates to the pattern of polymer adsorption (such as described in Papavasilopoulos and Bache, 1998), these are coplotted in Fig. 7. It is seen that the minimum in the
Fig. 4. Trends in the component viscosity contributions Zp and Zt as a function of polymer dose (sludge suspended solids: 1500 mg/l).
Technical Note
357
Fig. 5. Comparison of estimated trend (using eqn 3) with measured trend in centrate viscosity.
Zt+Zp trend coincides with what is termed as a `critical dose', i.e. the dose which marks the onset of adsorption saturation. Below the critical dose (13 mg/l) the residual polymer is close to zero (as evident in the viscosity behaviour shown in Fig. 4). In this phase (corresponding to the limb ab in Fig. 1), the reduction in viscosity is largely controlled by the reduction in the turbidity of the centrate. Above the critical dose (limb bc in Fig. 1), the measurements (such as shown in Figs 4 and 7) reinforce the view that the behaviour in this domain is caused by excess polymer. The analysis has shown how the individual com-
ponent terms in¯uence the total viscosity. It is recognised that the trend in Fig. 2 does not represent a unique relationship between viscosity and turbidity due to the role of particle size and other factors such as particle charge. Similarly, the term Ze depends on the type of electrolyte. Although many of the aspects described in this note need to be explored more fully, it is suggested that equation (5) provides useful insight into the controls on the viscosity behaviour. Equation 5 appears to provide a more comprehensive description of the viscosity behaviour than the mechanisms suggested in Dentel and Abu-Orf (1995).
Fig. 6. Kinematic viscosity as a function of conductivity.
358
Technical Note
Fig. 7. Behaviour of the combination Zt+Zp as a function of polymer dose in relation to the adsorption of the polymer on a sludge of similar solids content; the adsorption trend is from Papavasilopoulos and Bache (1998).
CONCLUSIONS
. At low contaminant concentrations, the viscosity of the centrate appears to be controlled by a series of components represented by Z Zw Zt Zp Ze . Within this relationship, the terms Zt and Zp representing the turbidity and polymer contributions, appear to control the position of the viscosity minimum. The terms Zw and Ze representing the pure water and the electrolytic contribution respectively, behave as constants for a given temperature. . Evidence from the analysis presented here indicates that the minimum in centrate viscosity more or less coincides with the onset of adsorption saturation.
REFERENCES
Abu-Orf M. M. and Dentel S. K. (1997) Polymer dose assessment using the streaming current detector. Wat. Environ. Res. 69(6), 1075±1085. Buscall R., Stewart R. F. and Sutton D. (1984) Structure in colloid suspensions and eects on solid±liquid separation. Filtration and Separation (May/June), 183±188. Christensen J. R., Sorensen P. B., Christensen G. L. and Hansen J. A. (1993) Mechanisms for overdosing in
sludge conditioning. J. Environ. Engng, ASCE 119(1), 159±171. Dentel S. K. and Abu-Orf M. M. (1995) Laboratory and full scale studies of liquid stream viscosity and streaming current for characterization and monitoring of dewaterability. Wat. Res. 29(12), 2663±2672. Keenan H. E., Papavasilopoulos E. N. and Bache D. H. (1998) Measurement of polymer residuals in an alum sludge. Wat. Res. 32(10), 3173±3176. Papavasilopoulos E. N. (1997) Viscosity as a criterion for the optimum dosing of polymers in waterworks sludges. J. Chart. Inst. Wat. Environ. Mgmnt 11(3), 217±224. Papavasilopoulos E. N. and Bache D. H. (1998) On the role of aluminium hydroxide in the conditioning of an alum sludge. Wat. Sci. Tech. 38(2), 33±40. Probstein R. F. (1994) Physicochemical Hydrodynamics. An Introduction, 2nd ed. John Wiley & Sons Inc. Shaw D. J. (1992) Introduction to Colloid and Surface Chemistry, 4th ed. Butterworth Heinemann Ltd. Stokes R. H. and Mills R. (1965) Viscosity of electrolytes and related properties. In The International Encyclopedia of Physical Chemistry and Chemical Physics, vol. 3. Pergamon, Oxford. Todd D. K. (1959) Groundwater Hydrology. John Wiley & Sons Inc. US Environmental Protection Agency (1987) Design Manual. Dewatering Municipal Wastewater Sludges. EPA-652/1-87-014, Cincinnati, Ohio. van de Ven T. G. M. and Hunter R. J. (1977) The energy dissipation in sheared coagulated sols. Rheol. Acta. 16, 534±543. Vesilind P. A. (1979) Treatment and Disposal of Wastewater Sludges, 2nd ed. Ann Arbor Science Publishers Inc, Michigan.