The influence of ionic strength and osmotic pressure on the dewatering behaviour of sewage sludge

Chemical Engineering Science 64 (2009) 2448 -- 2454

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The influence of ionic strength and osmotic pressure on the dewatering behaviour of sewage sludge Daan Curvers a,∗ , Shane P. Usher b , Adam R. Kilcullen b , Peter J. Scales b , Hans Saveyn a , Paul Van der Meeren a a b

Particle and Interfacial Technology Group, Faculty of Bioscience Engineering, Ghent University, Belgium Particulate Fluids Processing Centre, University of Melbourne, Victoria, Australia

A R T I C L E

I N F O

Article history: Received 28 November 2008 Received in revised form 21 January 2009 Accepted 24 January 2009 Available online 7 February 2009 Keywords: Filtration Separations Centrifugation Slurries Osmotic pressure Compressibility

A B S T R A C T

In this work, we investigated the importance of osmotic pressure in the overall dewaterability behaviour of a biotic sludge. Biotic sludges, such as activated or digested sludge from waste water treatment, are known to be difficult to dewater, due to their high compressibility and their gel-like water retention capacity. These properties are partly attributed to the presence of surface charges, which are due to the biological nature and the presence of weakly charged extra-cellular polymeric substances. Both in filtration and centrifugation experiments, charge related effects were partly neutralised through a controlled increase in the bulk ionic strength by the addition of NaCl. It was observed that an increase in the bulk ionic strength brings about an increase in the final solid volume fraction upon constant pressure filtration or centrifugation. Increasing the ionic strength did not result in a more classical filtration behaviour, however. The results further suggested that with increasing total pressure, the relative importance of the osmotic pressure in the total resistance against compression diminishes, and that more structural effects dominate the solid stress at high pressures. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction The biological treatment of sewage water produces large amounts of excess activated and/or anaerobically digested sludge. Disposal of this sludge is an issue and in order to reduce transportation and further treatment costs, the sludge is generally dewatered at the treatment site, either via centrifugation in a decanter centrifuge or via filtration in a plate and frame filter press. Sewage sludge, however, is known to be difficult to dewater and often exhibits non-traditional filtration behaviour (Stickland et al., 2005). On laboratory scale, pressure dewatering is often characterised using piston driven filtration. In a piston driven press, a piston forces a suspension towards a filter medium. The filter medium is permeable to the liquid phase, but retains the solids. The total dewatering process can be divided into two subprocesses: the cake formation and the cake compression phase. The cake formation phase is commonly described as the first part of the dewatering process whereby some of the solids are still in suspension, whereas the compression phase starts once all the solids are comprised in a filter cake, and the piston is in direct contact with the cake. Another, more general definition of the cake formation phase is the stage during which the

∗ Corresponding author. E-mail address: [email protected] (D. Curvers). 0009-2509/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2009.01.043

material that is in contact with the piston, is at its initial solid volume fraction. Classical filtration theory predicts the square of the total filtrate volume to increase linearly with time during the cake formation phase. Very compressible materials, however, often only display this initial linear filtration phase for a small portion of the total dewatering process. Buscall and White (1987) developed a unified theory of compressional rheology for the dewatering of flocculated suspensions. Key factors in this theory are the compressive yield stress Py () and the hindered settling function R(). The hindered settling function R() relates the liquid velocity through the cake to the free settling velocity of the individual particles. It is a measure for the cake permeability as a function of the solid volume fraction . Py () is related to the compressibility of the material. In a particulate suspension, a continuous network forms once the average particle volume fraction exceeds a gel point, g . From that moment, the particle pressure pp is the result of the direct interaction between the particles. The network structure can resist a compressive load elastically until pp exceeds a compressive yield stress Py (). Thus, at pressures higher than Py (), the network structure will yield and consolidate. It has been shown within this compressional rheology framework (Stickland et al., 2005), that non-traditional behaviour can be expected for dewatering at high pressures and/or high initial solid concentrations. Therefore, Stickland et al. (2005) concluded that no additional forces (e.g. osmotic pressure) are required to describe non-traditional behaviour.

D. Curvers et al. / Chemical Engineering Science 64 (2009) 2448 -- 2454

Due to their biological nature, biotic sludges possess a complex chemical composition. Activated sludges are made up of microbial organisms and colonies, embedded in a matrix of extracellular polymeric substances (EPS) (Jorand et al., 1995; Higgins and Novak, 1997). The EPS consist typically of polysaccharides, proteins, humic compounds and nucleic acids (Higgins and Novak, 1997; Laspidou and Rittmann, 2002). All these polymeric substances carry charged functional groups, and are highly hydrated (Flemming and Wingender, 2001). Legrand et al. (1998) showed that the flocculation and dewatering behaviour of activated sludge is, to some extent, comparable to the behaviour of an anionic polymer gel. Furthermore, it has been suggested that the dewatering behaviour of biotic sludges can be described as a function of the osmotic pressure within the sludge, due to the presence of charged surface groups and their counterions (Keiding et al., 2001; Keiding and Rasmussen, 2003). When a solute is added to a solvent, the resulting solution will exhibit an osmotic pressure with respect to the pure solvent (Flory, 1969; Tanford, 1961; Tombs and Peacocke, 1974; Atkins, 1998). This is generally depicted using a semi-permeable membrane, which is permeable to the solvent but impermeable to the solute. The addition of solute decreases the chemical potential of the solvent in the solution by decreasing its mole fraction. The chemical potential of the solvent in the solution will increase when a pressure is applied to the solution. Equilibrium is reached when the extra pressure equals the decrease in the chemical potential. This is the osmotic pressure. For ideal solutions, the chemical potential is given by

i = 0i + RT ln(xi )

(1)

with i the chemical potential of component i, 0i the standard chemical potential of the pure component i, R the universal gas constant, T the absolute temperature and xi the molality of component i in the solution. For very dilute, ideal solutions, the osmotic pressure is given by the Van't Hoff equation as

 = RTC B

(2)

with  the osmotic pressure and CB the molar concentration of solute B. As an example, according to the Van't Hoff equation, any solution with a total species concentration of 0.1 M (e.g. 0.05 M NaCl, dissolving into Na+ and Cl− ) at 20 ◦ C, will possess an osmotic pressure of 244 kPa. This is in the same order of magnitude as pressures applied during sludge dewatering. In a polymer gel system, there is no physical membrane separating the solution from the solvent. In this case, the solute is separated from the free solvent by the bonds that form the network. Free solvent will flow into the gel, thus expanding it and stretching its bonds. This leads to an elastic pressure resisting the swelling. In the case of a non-ionic polymer gel, the polymer chains alone are responsible for the decrease in the chemical potential of the solvent in the gel. In the case of a polyelectrolyte gel, counterions are closely associated with the gel for reasons of electro-neutrality. Hence, they add to the total osmotic pressure in the gel and dominate it when the charge density of the polymer chain is relatively high. In an electrolyte, the distribution of coions and counterions is governed by the Donnan equilibrium (Tanford, 1961). The Donnan equilibrium fulfills the condition that the chemical potential of the salt is the same in the inside and the outside of the gel. Theoretically, the osmotic pressure is a property of the solvent. Even the introduction of one single particle reduces the chemical potential of the solvent, and induces an osmotic pressure. From this point of view, it is irrefutable that an osmotic pressure exists in sludge, and counteracts compression of the sludge. It can, however, be very small in comparison to other governing forces. Some researchers define the osmotic pressure of a colloidal dispersion more

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pragmatically as a characterisation of the strength of the interactions between particles, including electrostatic interactions and Van der Waals forces (Bowen and Williams, 1996). From this point of view, the osmotic pressure is equal to the resistance to compression of a colloidal suspension consisting of hard spheres. It can be shown that in a stable (dispersed) suspension, the total particle pressure is thermodynamically equivalent to the osmotic pressure (Landman and White, 1994). Note that the electrostatic interaction pressure between charged surfaces in an electrolyte can be expressed as the excess osmotic pressure of the ions in the midplane over the bulk pressure (Israealachvili, 1985). In this work, we will regard the osmotic pressure in the sludge matrix as the resistance against compression that arises from the presence of charged groups within the sludge. All other forms of resistance against compression or deformation will be grouped together as a structural resistance against compression (e.g. purely elastic deformation or restructuring of material within the floc and even osmotic pressure due to non-ionic components). Increasing the ionic strength of the bulk solution should decrease the osmotic pressure. From the viewpoint of electrostatic interactions, this is caused by a compression of the electrical double layer, while from the osmotic viewpoint, this is caused by a decrease in the difference between the chemical potential of the solvent on the inside and the outside of the floc due to a shift in the Donnan equilibrium. While the existence of an osmotic contribution to the resistance against compression cannot be denied, the goal of this work is to empirically evaluate the importance of the contribution to the overall dewatering behaviour. The EPS component of biotic sludges makes them ideal candidates to examine this balance between the structural and osmotic contributions to the total compressive force. The aim of this work is thus to examine the relative contribution of the osmotic force to the compressive force for a biotic sludge material in the pressure regime of 0–4000 kPa. 2. Materials and methods 2.1. Sludge The sludge used in this work is a mesophilic, anaerobically digested sludge from a waste water treatment plant in Carrum, Australia. The sludge was refrigerated (4 ◦ C) for 14 days before use to reduce the temporal variability in the sludge properties. Although anaerobic sludges contain less organic material than aerobic sludges, they still possess a high compressibility and a low permeability and exhibit typical sludge dewatering behaviour (Ayol et al., 2006). The sludge as received had a total dry solid content of 1.7% (w/w), a pH of 7.33 and a conductivity of 6.25 mS/cm. The dry solid density was calculated to be 1590 kg/m3 , based on the dry solid content, corrected for dissolved solids, and the densities of the sludge and the bulk liquid. The dissolved solid were determined by centrifuging the sludge, filtering the supernatant over a 0.65 m filter membrane and drying the filtrate, resulting in a value of 0.13%. 2.2. Flocculation Prior to filtration, the sludge was flocculated using a cationic polyelectrolyte, Zetag 7587 (Ciba). The polymer was prepared at a concentration of 2 g/L, and stirred overnight. After preparation, the polymer was stored at 4 ◦ C and used within 5 days of preparation. Flocculation was performed in a way that allowed for maximum control over the experimental conditions, in order to maximise repeatability. An amount of 469 g of sludge was poured into a round, flat bottomed beaker with a four baffle insert. A Rushton turbine was used to induce mixing and shearing. The sludge was stirred at 1000 rpm for 1 minute. Subsequently, 49 mL of polymer, resulting in

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a total dose of 12 g polymer per kg dry solids, was added through a tube in a fixed position in the vortex. After 40 s, the Rushton turbine was switched off. The average velocity gradient in the beaker during the mixing was calculated to be 455 s−1 , according to the method described in Hulston (2005). After flocculation, the sludge sample was left to settle, and after 45 minutes, 100 mL of supernatant was removed, leaving little to no free supernatant. The flocculated sample was gently homogenised by stirring, and 2 × 100 mL sub-samples were taken. To one of the subsamples, an amount of 0.582 g NaCl was added, resulting in a bulk concentration increase of approximately 0.1 M NaCl. Subsequently, both samples were placed on a roller at low speed for 45 minutes to allow for the salt to dissolve and homogenise. 2.3. Filtration For each data set or sub-sample, two filtration tests were performed simultaneously on similar piston driven filtration rigs. The pistons were fitted with pressure transducers, and the pressure applied on the sample was kept constant using a software PIDcontroller. This method ensures that friction between the piston and the filtration cylinder does not affect the determination of the actual pressure transferred to the sample. A linear encoder was used to record the piston height as a function of time. The cylinder diameter was 0.040 m. Millipore PVDF membranes with a mean pore diameter of 0.65 m were used as filter medium. Preliminary experiments showed that the rigs did not influence the filtration results (i.e., for a given sample, the filtration behaviour was the same on both rigs). The devices have been described in detail elsewhere (De Kretser et al., 2001). After gently homogenising the samples, 75 mL of each sample was transferred to one of the filtration rigs and the filtration was started. For high pressure runs, a set-up with a smaller piston in a smaller cylinder (diameter of 0.020 m) without a pressure transducer was used. Using the smaller diameter enabled a four-fold increase in the applied pressure. Because of the lower precision in the measurement of the piston height, and a reduced control over the applied pressure, the results from these tests are only used as indicative values. 2.4. Filtration analysis Landman and White (1997) have shown that for constant pressure filtration, the kinetics of the cake compression are described by a Taylor's series. The first term of this series describes the asymptotic compressional behaviour, and can be rearranged to yield (Usher, 2002) t = E1 − E2 ln[h(t) − h∞ ]

2.5. Centrifugation analysis Centrifugation experiments were performed using a Lumifuge LF110 (Lum GmbH, Germany). This centrifuge allows for the determination of the sediment bed height during the centrifugation. The samples were subjected to four different rotational speeds, and the equilibrium bed heights at the different speeds were used to estimate Py (). The method used for extracting Py () from the centrifugation data is discussed more extensively in Curvers et al. (2008). Because of the limited dimensions of the centrifuge tubes, the sludge was not flocculated prior to the centrifugation experiments. Preliminary experiments had shown that the sludge possessed a significant amount of very fine particles. These fine particles only settled after an extended period of time at high rotational speeds. As such, these particles caused an increase in the sediment bed height, halfway through the experiment. To circumvent these problems, the sludge was centrifuged for 1800 s in a Jouan refrigerated centrifuge. The supernatant was decanted, and the sediment with a smaller amount of fines was resuspended in tap water (conductivity: 177.5 S/cm). Prior to the centrifugation tests, 2 × 100 mL samples were taken from this reconstituted sludge. To one of these samples, the appropriate amount of NaCl was added, and subsequently both samples were placed on a lab roller for 45 minutes. 3. Results 3.1. Effect of salt addition It could be argued that the addition of salt could have an effect on the physical properties of the sludge, other than the change in swelling properties. The major concern would be a salt-induced coagulation of the sludge. To rule out this effect, the particle size distribution was measured for both the original sludge and a sample with an increased (+0.1 M NaCl) salt concentration, using laser diffraction (Malvern Mastersizer, 2000). The data indicated comparable particle size distributions with a volume weighted mean diameter of 56.2 m for the original sludge and 58.8 m for the sample with increased salt concentration. The d0.9 values were 124.1 and 120.7 m, respectively. Besides determining the particle size distribution, the initial centrifugal settling behaviour of both samples was compared. Fig. 1 shows the settling velocity as a function of time for a sample of the original sludge and a sample with increased salt concentration. Clearly, the addition of salt does not affect the settling velocity, which confirms that the sludge particles have not been coagulated. Finally, coagulation of the particles would result in a higher sediment bed volume, whereas the addition of salt results in a decrease in the sediment bed volume. These three observations lead us to believe that coagulation effects may be ruled out.

(3) 3.2. Filtration behaviour

with E1 and E2 constants, h(t) the piston height during the compression and h∞ the equilibrium piston height. For highly compressible materials, such as activated sludge, the time to reach the equilibrium solid volume fraction at a given pressure can be very long. Eq. (3) can be used to estimate the equilibrium solid volume fraction from a filtration experiment that is close to, but not at equilibrium. This is done by searching for the equilibrium height h∞ which yields the best fit of Eq. (3) to the final part of the compression kinetics. Eq. (3) only represents the first order term of Taylor's series. As such, it only describes the final part of the compression and it does not require any knowledge about the precise transition point between cake formation and cake compression. From the equilibrium height, the equilibrium solid volume fraction can be calculated using a mass balance.

Fig. 2 shows the filtration time as a function of the square of the specific filtrate volume (filtrate volume divided by the total filtration area). Fig. 3 shows the same curves for the runs at 50, 100 and 200 kPa, but with both variables scaled by their final value, i.e., at the end of the run. The 20 kPa run has not been included because it had been stopped too far from the final equilibrium. Fig. 4 shows the slope of the filtration plot (Fig. 2) as a function of the extent of filtration. All of the filtration runs show a region of more or less constant slope—the filtration phase—followed by a region of increasing slope—the expression phase—as predicted by the classical filtration theory. However, the slope during the filtration phase is less constant as what can be observed with mainly inorganic slurries (e.g. alum sludge in Scales et al., 2004). Interestingly, the increase in the

D. Curvers et al. / Chemical Engineering Science 64 (2009) 2448 -- 2454

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1.2 Original + 0.1M NaCl

Settling velocity [mm/s]

1

0.8

0.6

0.4

0.2

0 1.00

10.00

100.00 Time [s]

1000.00

10000.00

Fig. 1. The settling velocity as a function of time for the original sludge and the same sludge after addition of 0.1M NaCl.

Fig. 3. The filtration time as a function of the square of the specific filtrate volume, both scaled by their final value.

1E+08

Slope [s/m²]

8E+07

6E+07

4E+07

2E+07 Original + 0.1M NaCl 0E+00 0

0.01

0.02 0.03 0.04 0.05 Specific filtrate volume [m]

0.06

Fig. 4. The slope of the filtration curve shown in Fig. 2 as a function of the specific filtrate volume V. Fig. 2. The filtration time as a function of the square of the specific filtrate volume.

bulk salt concentration does not induce a significant change in the kinetics during the filtration phase. Apart from some starting effects, due to the different set-ups, the initial sections of the filtration curves overlap for the runs with and without an increased salt level, also illustrated by similar slopes. This implies that the increased salt levels did not influence the permeability. They did influence the final filtrate volume, however, implying that the compressibility is affected. These results conform with the theory that an osmotic term is responsible for at least a part of the resistance of the matrix to the applied pressure. Fig. 3 shows that this osmotic component, or the partial neutralisation thereof, did not influence the final shape of the scaled filtration curve. Fig. 5 shows the compressive yield stress curves for the sludge with and without additional salt, i.e., the compressive yield stress as a function of the equilibrium solid volume fraction. The equilibrium solid volume fractions were calculated according to the method ex-

plained in Section 2.4. Figs. 2 and 4 show that the filtration runs at 20 kPa did not reach equilibrium. In many biotic sludge systems, the trade off between achieving equilibrium while maintaining material integrity is an issue at long filtration times. The extrapolation technique for determination of the equilibrium height will probably result in an over-estimation of the final solid volume fraction for these runs. The lines in Fig. 5 represent the best fitting power law functions, with the general form (Tiller and Leu, 1980): 

 = g 1 +

 Py  . pa

(4)

For the full lines, g , pa and  were used as variable optimisation parameters, which yielded 0.0516, 87.06 Pa and 1.369, respectively, for the original sludge, as compared to 0.0574, 91.97 Pa and 1.370 upon salt addition. The broken lines represent the best fit of Eq. (4) with the gel point fixed at g = 0.015, a value estimated from gravity settling of the sludge. This yielded pa and  values of 6.433 Pa and

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D. Curvers et al. / Chemical Engineering Science 64 (2009) 2448 -- 2454

Compressive yield stress [kPa]

1000 Original + 0.1 M NaCl

100

10 0

0.1

0.2

0.3

0.4

Solids volume fraction [-]

0.8258, respectively, for the original sludge and 5.105 Pa and 0.7907, respectively, upon addition of salt. Both fits are probably extremes, and the real compressive yield stress curve will be somewhere in between. The high pressure values (at 1000 kPa) were determined using a different and less precise set-up, as described in Section 2.3. Hence, the values are only indicative, and were not included in further analysis. However, they do display the same trend, where an increase in the bulk ionic strength increases the final solid content after correction for the additional salt. One should note that Eq. (4) can only be used for a limited pressure range, as it will always predict volume fractions higher than 1 at sufficiently high pressures, which is not physically plausible. The fits can be used to calculate the difference in pressure that is required to reach the same solid concentration with both sludges. Fig. 6 shows the relative difference in pressure that is required to reach the same volume fraction in a sludge with an increased bulk ionic strength as compared to the original sludge. This relative pressure difference has been plotted as a function of the pressure that needs to be applied to the original sludge to reach the given solid volume fraction. The broken and solid lines correspond to the fits in Fig. 5. Results are shown for pressures ranging between 50 and 200 kPa. On the second vertical axis in Fig. 6, the corresponding equilibrium solid volume fractions for the original sludge are shown. Fig. 6 indicates that by increasing the bulk ionic strength of the sludge with 0.1 M NaCl, the pressure required to reach a given solid volume fraction decreases by about 6–15%. The calculations reveal that the relative pressure difference becomes less important at higher pressures, i.e., at higher solid volume concentrations. One must not, however, extrapolate these results naively to pressures above the range considered in Fig. 6. In order not to reach solid volume fractions higher than 1, which are physically impossible, the compressive yield stress will eventually have to curve up. Even though the measurements at 1000 kPa were performed using a different set-up, and cannot be related directly to the lower pressures, they are in accordance with this trend. Furthermore, it is clear that for solid volume fractions higher than the maximum attainable value for the original sludge, the pressure difference will be infinitely high.

Fig. 6. The relative difference in pressure required to reach the same volume fraction for a sludge with a 0.1M increased bulk ionic strength as opposed to the original sludge, as a function of the required pressure to reach that volume fraction with the original sludge. The broken and solid lines correspond to the broken and solid lines in Fig. 5, respectively. On the second vertical axis, the corresponding equilibrium solid volume fractions for the original sludge are shown.

0.01 Original + 0.1M NaCl

0.009 Sediment bed height [m]

Fig. 5. The compressive yield stress Py as a function of the solid volume fraction , determined by pressure filtration. The symbols represent measured values, the lines represent power law function fits.

0.008 0.007 0.006 0.005 0.004 0.003 0.002 0

1000 2000 3000 Rotational speed [rpm]

4000

Fig. 7. The equilibrium height of the sediment bed as a function of the rotational speed for the original sludge and sludge with an extra 0.1 M NaCl. Symbols represent measured data, while the full lines show the result of the model fit.

3.3. Equilibrium centrifugation The pressure range that can be investigated with pressure filtration is limited, especially on the low pressure side. Centrifugation can be used to assess low pressure compressibility behaviour (Curvers et al., 2008; Green et al., 1996; Buscall and White, 1987). Figs. 7 and 8 show the equilibrium bed height as a function of the rotational speed for the original sludge and sludge with an additional +0.1 or +0.5 M NaCl, respectively. The symbols represent measured data, while the full lines represent the model fits, fitted to the data using a least squared error method, as described by Curvers et al. (2008). In agreement with the filtration experiments, the addition of

D. Curvers et al. / Chemical Engineering Science 64 (2009) 2448 -- 2454

0.01

0.32 Original + 0.5M NaCl

0.28 Relative pressure difference [-]

Sediment bed height [m]

0.009 0.008 0.007 0.006 0.005 0.004 0.003

0.24 0.2 0.16 0.12 0.08 + 0.1M NaCl + 0.5M NaCl

0.04

0.002 0

1000

2000

3000

4000

Rotational speed [rpm] Fig. 8. The equilibrium height of the sediment bed as a function of the rotational speed for the original sludge and sludge with an extra 0.5 M NaCl. Symbols represent measured data, while the full lines show the result of the model fit.

Table 1 Parameter values yielding the best fit of Eq. (4) to the equilibrium bed height centrifugation data. Parameters

g (dimensionless)  (dimensionless) pa (Pa)

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0.1 M NaCl

0 0

500

1000 1500 Applied pressure [Pa]

2000

Fig. 9. The relative difference in pressure required to reach the same volume fraction for a sludge with an increased bulk ionic strength as compared to normal sludge, as a function of the required pressure to reach that volume fraction with the normal sludge.

4. Discussion and conclusion

0.5 M NaCl

Original

+NaCl

Original

+NaCl

0.0114 0.2562 1.461

0.0114 0.2659 1.4579

0.0118 0.2550 1.4966

0.0118 0.2651 1.3734

salt leads to a lower sediment bed height, or by inference, a higher compression of the bed. The model fits yielded an estimation of the relationship between local solid pressure and local porosity of the form of Eq. (4). The parameter values resulting from the fitting process are listed in Table 1. The resulting compressive yield stress curves can again be used to calculate the difference in pressure that is required to reach the same solid concentration with both sludges. Fig. 9 shows the relative difference in the pressure that is required to reach the same volume fraction in a sludge with an increased bulk ionic strength as compared to the original sludge. Again, increasing the bulk salt concentration decreases the pressure required to reach a certain equilibrium solid concentration significantly. At a relatively high salt concentration with a + 0.5 M NaCl increase, the required pressure decreased by 25–30%. A + 0.1 M NaCl increase resulted in a pressure decrease of 15–20%. This is higher than the results from the filtration experiment, and suggests that, indeed, the importance of the osmotic component lessens with increasing pressure, i.e., structural factors become more and more limiting at higher pressures. It can be expected, however, that the higher value is partly due to the physical differences between the sludges used for the experiments: the sludge used for the centrifugation test had been concentrated and resuspended in water with a lower ionic strength than the original bulk ionic strength. Furthermore, the sludge was flocculated prior to filtration, using a cationic polyelectrolyte. As the polyelectrolyte neutralises part of the surface charge of the sludge, it can be expected that this would decrease the importance of the osmotic component. For these reasons, the centrifugation and filtration experiments are not to be compared quantitatively. This being said, they both provide an indication of the influence of the osmotic pressure on the total compressibility.

Both the filtration and the centrifugation results show that increasing the bulk ionic concentration does not result in a more classical filtration behaviour of the sludge, but it has an effect on the compressibility of a biotic sludge. The effect is observed in both flocculated and non-flocculated sludges. The change in filtration behaviour could not be explained as flocculation or coagulation effects. Such effects would, on the contrary, result in a lower solid volume fraction due to the more open structure of the flocs. Furthermore, no increase in floc size or sedimentation velocity could be observed. The observed effect can be explained by a shift in the Donnan equilibrium. The difference in ionic concentration between the sludge flocs and the bulk is reduced, which in turn decreases the osmotic swelling pressure of the sludge flocs. This can be observed as a relative increase in solid volume fraction at a certain pressure, and confirms earlier research suggesting that biotic sludges show behaviour similar to gel systems (Legrand et al., 1998; Keiding and Rasmussen, 2003). Alternatively, a decrease in the resistance against compression can be explained as a collapse of the diffuse double layer of the particles within the sludge upon an increase in the bulk ionic concentration. The increased equilibrium solid volume fraction is in accordance with the results from Jean and Lee (1999), who noticed an improvement in consolidation behaviour of wastewater sludge upon the addition of salt. Because only scaled consolidation data were shown in their work, final solid volume fraction figures are not available, and direct comparison is not possible. At relatively high salt concentrations and low pressures (+0.5 M NaCl and centrifugation), the pressure required to reach a given equilibrium solid volume fraction during centrifugation was reduced by approximately 30%. Furthermore, it was seen that with increasing pressure, the relative importance of the osmotic component decreased. This indicates that, although the osmotic pressure is an important factor in the total resistance against compression, other structural components are not to be neglected. These structural components can be due to the presence of a particular material that does not show gel-like behaviour. Non-charge-related osmotic pressure components can be present as well. Flory (1969) describes the swelling pressure in a gel using three different components:

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 = m + el + i . m is related to the free energy of mixing of the polymer chain with the solvent, el is related to the elastic free energy (in case of swelling of the gel, this is due to the stretching of the polymer chains, and this force counteracts the swelling tendency) and i is the osmotic pressure component due to the mixing of the solvent with the ionic constituents. In case of compression, el can be regarded as an elastic modulus of the polymer backbone resisting further compression. Acknowledgements This work was conducted with the sponsorship of the Particulate Fluids Processing Centre, a Special Research Centre of the Australian Research Council. Furthermore, Daan Curvers acknowledges the support he receives from Research Foundation—Flanders as an aspirant of the foundation. References Atkins, P., 1998. Physical Chemistry. sixth ed. Oxford University Press, Oxford. Ayol, A., Filibeli, A., Dentel, S., 2006. Evaluation of conditioning responses of thermophilic–mesophilic anaerobically and mesophilic aerobically digested biosolids using rheological properties. Water Science and Technology 54, 23–31. Bowen, W., Williams, P., 1996. The osmotic pressure of electrostatically stabilized colloidal dispersions. Journal of Colloid and Interface Science 184, 214–250. Buscall, R., White, L.R., 1987. The consolidation of concentrated suspensions. 1. The theory of sedimentation. Journal of the Chemical Society—Faraday Transactions I 83, 873–891. Curvers, D., Saveyn, H., Scales, P.J., Van der Meeren, P., 2008. A centrifugation method for the assessment of low pressure compressibility of particulate suspensions. Chemical Engineering Journal, DOI: 10.1016/j.cej.2008.09.030. De Kretser, R.G., Usher, S.P., Scales, P.J., Boger, D.V., 2001. Rapid filtration measurement of dewatering design and optimization parameters. A.I.Ch.E. Journal 47, 1758–1769. Flemming, H.C., Wingender, J., 2001. Relevance of microbial extracellular polymeric substances (EPSs)—Part I: structural and ecological aspects. Water Science and Technology 43 (6), 1–8.

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