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Effect of exo-polysaccharide concentration in the rheological properties and settling ability of activated sludge a
a
S. Akkache , I. Seyssiecq & N. Roche
a
a
Aix-Marseille University – CNRS, M2P2 UMR 7340, BP 80, 13545 Aix en Provence Cedex 4, France Accepted author version posted online: 03 May 2013.Published online: 22 May 2013.
To cite this article: S. Akkache, I. Seyssiecq & N. Roche (2013) Effect of exo-polysaccharide concentration in the rheological properties and settling ability of activated sludge, Environmental Technology, 34:22, 2995-3003, DOI: 10.1080/09593330.2013.798001 To link to this article: http://dx.doi.org/10.1080/09593330.2013.798001
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, No. 22, 2995–3003, http://dx.doi.org/10.1080/09593330.2013.798001
Effect of exo-polysaccharide concentration in the rheological properties and settling ability of activated sludge S. Akkache, I. Seyssiecq∗ and N. Roche Aix-Marseille University – CNRS, M2P2 UMR 7340, BP 80, 13545 Aix en Provence Cedex 4, France
Downloaded by [Florida International University] at 09:01 07 January 2015
(Received 5 February 2013; final version received 15 April 2013 ) The non-Newtonian properties of activated sludge (AS) suspension lead to transfer limitations (oxygen, substrate…) and operation difficulties in Waste Water Treatment Plants (WWTP). The current approach involves assuming the sludge behaves like water on a rheological point of view, and then oversizing pumping and aeration devices, which represent over 60% of the operating cost in WWTP. The objective of this work is to understand the effect of bioflocculation on the rheological properties and the settling ability of AS suspensions, by means of variations in concentration of exo-cellular polysaccharides. Experiments have been conducted in a 20 L laboratory scale bioreactor at a constant retention time of 20 days and with a total suspended solid concentration between 15 and 20 g L−1 . The bioreactor was fed with a synthetic substrate at a constant mass −1 loading rate of 0.3 kgDCO kg−1 TSS d . Our results show that increasing the exo-polysaccharide (EPS) concentration from 10 −1 to 80 mg gTSS leads to an increase in shear-thinning properties of AS. An improvement of the settling ability is also obtained, −1 at least when the EPSs increase from 10 to 45 mg g−1 TSS . Above 45 mg gTSS of adsorbed polysaccharides, the settling ability seems to decrease again. Keywords: wastewater treatment; activated sludge; flocculation; total suspended solids; exo-cellular polymers; rheology
1. Introduction Activated sludge (AS) suspensions are known to be nonNewtonian fluids.[1–3] They are described in the literature as shear-thinning fluids exhibiting a decrease in apparent viscosity as the shear rate is increased.[4] They are also known to exhibit (above a certain solid concentration) viscoplasticity. This latter property implies that no flow will occur below a certain stress, i.e. the yield stress.[4] Other papers also describe AS suspensions as thixotropic [5] (with a time-dependent viscosity) and viscoelastic fluids.[6] All these rheological complexities are due to an internal complex structure [7]: • A liquid phase mostly composed of water, salts, and macromolecules (‘free’ polymers) • Solid structures named bioflocs mostly composed of water and microorganisms structured by a macromolecular network (adsorbed polymers called exopolymers). Moreover, it has been clearly demonstrated that it is necessary to study the relationship between the rheological properties and the floc structure.[8] These bioflocs constitute the structural units (SU) of AS suspensions.[9] The SU are very porous (water is entrapped in their porosity), fragile and deformable. Under the effect of
a sufficient shear stress, SU’s morphology evolves, leading to a release of water and causing the shear-thinning of such suspensions. Among the parameters known to influence the rheological behaviour of biological sludge suspensions, the most important are their total suspended solid (TSS) concentration but also their physico-chemical and biological compositions.[10] The effect of TSS concentration on the rheology of AS suspension has been widely studied. Most of these studies [11–15] show that the shear-thinning and viscoplastic parameters (consistency index k (Pa sn ); flow index n (−) and yield stress τ0 (Pa)) can be modelled by linear, exponential, or power-increasing functions of the TSS concentration. The biological composition of bioflocs in terms of proportions of the different types of microorganisms (amount of filamentous organisms in the flocs for instance) is another parameter that needs to be taken into account. It is indeed directly related to the internal structure of bioflocs and therefore to their flowing properties or settling ability.[13] The exo-polymers (microorganisms metabolites) are the key to the bioflocculation process.[16] These macromolecules adsorbed at the surface of microorganisms, inducing their flocculation through macromolecular bridging. An increase in concentration of exo-polymers, notably
∗ Corresponding author. Email: [email protected] This article was originally published with erroneous pagination. This version has been corrected. Please see Erratum (http://dx.doi.org/ 10.1080/09593330.2013.869394).
© 2013 Taylor & Francis
Downloaded by [Florida International University] at 09:01 07 January 2015
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exo-polysaccharides (EPSs) is thus expected to induce an increase in floc size. Such an increase in floc size will in turn cause an increase in apparent viscosity of the sludge and enhance its non-Newtonian properties in terms of both shearthinning and viscoplasticity. Forster [17] shows that the exo-proteins and EPSs concentration affects the rheological behaviour of sludge (increasing the apparent viscosity when increasing the exo-polymeric substances concentration). Rosenberger and Kraume [18] and Li and Yang [19] show that the apparent viscosity of AS decreases when decreasing the exo-polymeric substances concentration. Mori et al. [15] shows that increasing the EPS concentration increases the shear-thinning and the viscoplastic properties of AS. The settling ability of AS suspensions is also linked to their biological composition. Filamentous or non-filamentous (zoogleal) bulking caused by instabilities in composition between the different microorganisms’ communities is known to be related to a poor ability to settle.[19] The state of flocculation is another important parameter. The settling ability of AS suspensions, is often a limiting step in Waste Water Treatment Plants (WWTP) processing. A low ability to settle can lead to a direct discharge of biomass in the environment. The objective of this work is to correlate the rheological properties of AS suspensions (15–20 g L−1 in TSS) and their ability to settle to their state of flocculation by varying the EPSs concentration. This could lead to a better understanding of the influence of the bioflocculation process on the flow properties of these materials. The 15–20 g/L TSS concentration range is chosen in order to be sufficiently sensible for the rheological measurements as well as for the determination of the EPS concentrations.
2. Methods and material 2.1. Reactor operating conditions AS suspensions used in this work come from an urban WWTP (Aix en Provence, France 165,000 inh. eq.). The sludge is sampled in the recirculation loop between aeration basins and secondary settlers at about 3 g L−1 in TSS. In order to obtain a TSS concentration of 15–20 g L−1 , the sludge is concentrated by soft (gravimetric) filtration using simple paper coffee filters (average size of pores around 100 μm). The reactor is a 20 L Plexiglas vessel with a liquid height kept equal to its diameter (H = D = 0.3 m). It is equipped with a mechanical stirrer of diameter d composed of eight inclined blades (d/D = 1/3) placed at the distance h from the bottom (h/H = 1/3) and operated at 250 rpm. The reactor is aerated by injection of compressed air through soft porous diffusers at 300 Lair h−1 . The temperature is kept at 20 ± 0.1◦ C due to a circulation of water in an immersed stainless steel coil.
The substrate used to feed the sludge is composed of −1 sugar as a source of organic carbon (0.3 gDCO g−1 TSS day ) and Viandox® as a source of proteins and oligo-elements (0.03 gDCO g−1 TSS once a week). The reactor works in a semi-continuous way. The age of biosolids is fixed at 20 days by withdrawing 500 mL of homogenous suspension every day. The acclimation step is assumed to be completed based on both the stabilization of TSS concentration and a visual observation of the sludge that shows that most of the filamentous bacteria initially present in the sludge had disappeared. 2.2.
Characterization of sludge suspension
2.2.1. TSS determination A 30 mL sludge sample is taken for centrifugation at 13,500 G during 15 min. Then, the solid is dried at 105◦ C until stabilization of the mass. TSS is calculated as the ratio of dried mass to the volume sampled and expressed in g L−1 . 2.2.2. Extraction of EPS substances Thirty millilitres of sludge sample is first taken for centrifugation at 4000 G during 15 min. The supernatant which is discarded is assumed to contain only free EPS substances. The solid phase is then diluted again into 30 mL of distillated water and sonicated at 42 W for 2 min in order to desorb the polymeric substances from the surface of flocs and cells.[15] This suspension is taken for centrifugation at 4000 G for 15 min. The supernatant now containing the desorbed EPS substances is used to perform their concentration measurement. 2.2.3. Measurements of EPS substances concentration Among exo-polymeric substances produced by sludge microorganisms, we have chosen to follow the evolution of one family of macromolecules the EPSs. The EPS represent one of the most important type of exo-polymeric substances (together with exo-proteins and humic acids). Their concentration is easy to measure using a simple colorimetric method (see below); and we work here under the assumption that feeding our sludge with a substrate mostly composed of sugar will mostly induce an increase in EPS than in other polymeric substances. The EPS concentration is obtained by the Dubois et al. (1956) method. It consists of adding 5 mL of 95% sulphuric acid and 1 mL of 5% phenol solutions to 1 mL of sludge supernatant. The mixture is heated at 105◦ C during 10 min. After 30 min at rest in the dark, an AquaMate Thermo Spectronic colorimeter is used to read the absorbance at 486.0 nm. The EPS concentration is directly obtained using a calibration curve previously obtained with solutions of glucose from 0 to 300 mg L−1 .
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Environmental Technology 2.2.4. The SVI In order to evaluate the settling ability of the AS, we have measured the sludge volume index (SVI). The SVI is a commonly used indicator describing the functioning of the WWTP in terms of ability to separate biosolids from purified waters. It represents the volume of 1 g of sludge after 30 min of settling in a 1 L test tube. It is the ratio of the volume of settled sludge to the initial TSS concentration and is expressed in mL.g−1 . High values of SVI indicate a low ability to settle while on the contrary, values smaller than 150 mL g−1 are associated with good abilities to settle. The validity of this method is based on SVI values varying from 100 to 300 mL g−1 . It is, therefore, necessary to dilute sludge suspensions sampled between 15 and 20 g L−1 in the reactor to 2–3 g L−1 before performing this test. 2.3.
Rheological measurement
Rheological measurements are performed using an AR 500 rheometer (T.A. instrument) operated in a controlled shear stress mode using a Couette geometry (1 mm gap). The protocol was developed for WWTP AS for a range of TSS concentration between 10 and 30 g L−1 .[10] The temperature of the system is maintained at 20 ± 0.1◦ C using a recirculation the water bath. Rheometer control and data collection are performed using TA instrument’s softwares provided with the instrument. A pre-shear at 12 Pa for 1 min followed by a continuous linear ramp are imposed from 0.1 to 12 Pa over a period of 8 min are successively applied to the sample. 2.3.1. Rheological data analysis Using Microsoft ExcelTM , rheograms are fitted with the least squares method to determine the Herschel–Bulkley model parameters. τ = τ0 + k γ˙ n ,
(1)
where τ is the shear stress (Pa), τ0 the Herschel–Bulkley yield stress (Pa), k the consistency index (Pa sn ), and n the flow index (dimensionless) 0 < n < 1. 3. Results and discussion 3.1. TSS evolution with time Two different cultures (cultures I and II) of sludge have been conducted under the same operating conditions. Figure 1(a) and 1(b) successively show the time evolution of TSS for these two cultures. It can be observed that the TSS concentration remains steady after a period of acclimation. The aim of this work is to correlate the rheological properties and the ability to settle of AS suspensions, to their concentration in EPS. As a consequence, we can only focus our interest on period where the TSS concentration remains constant since this latter parameter is known to have a great influence on
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flow properties of such suspensions. The initial unsteady phase, lasting, respectively, 12 and 14 days according to the culture, known as the acclimation step,[20] is linked to an evolution in biological composition, i.e. proportions of the different types of microorganisms. This phenomenon is due to the differences in operating conditions (notably in terms of aeration, agitation and substrate composition) prevailing in the laboratory reactor compared to those prevailing at the WWTP. In the case of culture I, we can see in Figure 1(a) that the acclimation is completed after 12 days. From days 12 to 16 then, the sludge is said to be acclimated (with constant biological composition) and the TSS remains constant. During this latter period, variations in EPS concentration are the only factor affecting the state of flocculation of microorganisms and thus their rheological properties. In the case of culture II, it can be observed in Figure 1(b), that a period of constant TSS is also reached after the acclimation has been completed, between days 14 and 28. This period will also allow studying the effect of EPS concentration on the flocculation state and on the rheological properties. Another period of constant TSS is also observed during the acclimation step, i.e. while the biological composition of the sludge is still evolving (days 7–11). During this particular period not only the degree of flocculation (correlated to the EPS concentration), but also the evolution in biological composition of the flocs will superimpose their effects on the evolution of sludge rheology. 3.2. Rheology 3.2.1. Culture I The rheological measurements (rheograms) performed from days 12 to 16 are plotted in Figure 2(a). This figure shows an upwards translation of the rheograms as we go from days 12 to 16. After fitting the rheograms with the Herschel–Bulkley model and characterizing the sludge in terms of TSS and EPS, we obtain the following results (Table 1). It can be observed in Table 1 that, for an approximately constant TSS concentration, there is an evolution of the rheological behaviour which is probably due to an evolution in the bioflocculation state. As a matter of fact, the EPS concentration being multiplied by four from days 12 to 16, we can assume that the sludge is getting more and more flocculated. This implies that sludge flocs are probably bigger in size at the end of this period when compared to its beginning. Bigger flocs (i.e. bigger SU) will then lead to an enhancement of the shear-thinning properties as previously explained. An increase of the consistency index k and a decrease of the flow index n corresponding to such an increase in shear-thinning properties is well observed in Table 1 as the sludge evolves from days 12 to 16.
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Figure 1.
S. Akkache et al.
Evolution of TSS versus time: (a) for culture I and (b) for culture II.
Concerning the yield stress τ0 no significant variations of viscoplasticity is observed despite the increase in EPSs concentration. It can be assumed that, either the increase in SU size (due to the EPS concentration increase) is not important enough to induce an increase of the 3D interparticles network cohesion at rest and therefore results in no increase in viscoplasticity of the sludge; either this increase is too small to be detected using our experimental device. 3.2.2. Culture II In Figure 2(b), it can be observed that the time evolution of rheograms during the period from day 7 to day 11 during which the sludge shows a constant TSS, but a biological composition still in evolution.
An evolution of the rheological properties is observed during this period, the rheograms being shifted downwards as the sludge evolves from day 7 to day 11. After fitting the rheograms with the Herschel–Bulkley model and characterizing the sludge in terms of TSS and EPS, we obtain the following results (Table 2). It can be observed in Table 2 that for a constant TSS content, as the sludge evolves from days 7 to 11, an increase in EPS concentration is obtained. This should be linked to an increase in flocs size (increase in bioflocculation) and should, therefore, be followed by an increase in the non-Newtonian properties of the suspension. On the contrary, a decrease in both shear-thinning and viscoplasticity (decrease in consistency index k and yield stress τ0 and increase in flow index n) is clearly observed in Table 2 during this period.
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Environmental Technology
Figure 2. to 28.
Evolution of rheograms: (a) for culture I from days 12 to 16; (b) for culture I from days 7 to 11; (c) for culture II from days 15
It is possible then to conclude that, as this period is located during the acclimation step (Figure 1(b)), not only the EPS concentration but also the biological composition (still unsteady between days 7 and 11) of the sludge will superimpose their effects on the time evolution of rheograms. It seems therefore that, despite the increase in
EPS concentration, the sludge structure is mostly subjected to a deflocculation stage between days 7 and 11. This phenomenon is thought to be due to the important modifications of the sludge biological composition, still in progress during the acclimation phase, and results in a downwards shift of sludge rheograms.
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Table 1. Day 12 13 14 15 16
TSS (g L−1 )
EPS (mg g−1 TSS )
k (Pa sn )
n (−)
τ0 (Pa)
R2
14.99 14.81 15.03 14.55 14.00
11.59 9.05 12.58 21.63 42.27
0.29 0.28 0.4 0.4 0.5
0.5 0.52 0.47 0.48 0.46
0.20 0.20 0.20 0.15 0.25
.9982 .9993 .9996 .9992 .9997
Table 2.
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Day
Culture I, evolution between days 12 and 16.
Culture II, evolution between days 7 and 11.
TSS (g L−1 )
EPS (mg g−1 TSS )
k (Pa sn )
n (−)
τ0 (Pa)
R2
19.1 18.3 18.8 18.8 19.3
2.83 8.47 16.46 26.39 39.00
0.52 0.52 0.43 0.32 0.35
0.44 0.43 0.55 0.48 0.46
1.00 0.60 0.60 0.40 0.40
.9986 .9996 .9997 .9976 .9994
7 8 9 10 11
Between days 14 and 28, another period of constant TSS is observed in culture II after the acclimation step has been completed. It can then be assumed that, only the EPS concentration will affect the flocculation state of the sludge during this period. The time evolution of some of the rheograms (for clarity purposes) obtained during this period are presented in Figure 2(c). A shift of the rheograms upwards is now clearly observed in Figure 2(c) as the sludge evolves during this period. After fitting the rheograms with the Herschel–Bulkley model and characterizing the sludge in terms of TSS and EPS, the results obtained are shown in Table 3. It can be observed in Table 3 an increase in consistency index k and a decrease in flow index n (increase in shear-thinning) together with an increase of the EPS concentration. This clearly indicates an enhancement of the flocculation state of the sludge. On the contrary no sensible evolution of the yield stress is observed. Table 3. Day 15 16 17 18 21 22 23 24 25 28
Culture II, evolution between days 15 and 28.
TSS (g L−1 )
EPS (mg g−1 TSS )
k (Pa sn )
n (−)
τ0 (Pa)
R2
16.4 16.6 16.7 17.9 16.7 16.3 16.2 16.6 16.9 17.0
17.78 21.47 34.21 42.31 47.10 43.95 50.84 57.38 60.71 75.54
0.45 0.47 0.52 0.56 0.80 0.69 0.66 0.73 0.78 1.02
0.46 0.46 0.46 0.44 0.43 0.45 0.45 0.45 0.45 0.44
0.40 0.45 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.30
.9993 .9991 .9998 .9998 .9997 .9995 .9995 .9992 .9988 .9993
These results are qualitatively identical with those obtained in the case of culture I (Figure 2(a) and Table 1). It seems that, after the acclimation of the sludge has been completed, and once its biological composition is stabilized according to our laboratory operating conditions, only the EPS concentration affects the bioflocculation state of the material. As a consequence, an increase in EPS induces an increase of the shear-thinning ability of the suspension. As it has been previously observed in the case of culture I, in the range of TSS studied here (15–20 g L−1 ), sludge suspensions are viscoplastic fluids with rather low-yield stresses (
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Effect of exo-polysaccharide concentration in the rheological properties and settling ability of activated sludge a
a
S. Akkache , I. Seyssiecq & N. Roche
a
a
Aix-Marseille University – CNRS, M2P2 UMR 7340, BP 80, 13545 Aix en Provence Cedex 4, France Accepted author version posted online: 03 May 2013.Published online: 22 May 2013.
To cite this article: S. Akkache, I. Seyssiecq & N. Roche (2013) Effect of exo-polysaccharide concentration in the rheological properties and settling ability of activated sludge, Environmental Technology, 34:22, 2995-3003, DOI: 10.1080/09593330.2013.798001 To link to this article: http://dx.doi.org/10.1080/09593330.2013.798001
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, No. 22, 2995–3003, http://dx.doi.org/10.1080/09593330.2013.798001
Effect of exo-polysaccharide concentration in the rheological properties and settling ability of activated sludge S. Akkache, I. Seyssiecq∗ and N. Roche Aix-Marseille University – CNRS, M2P2 UMR 7340, BP 80, 13545 Aix en Provence Cedex 4, France
Downloaded by [Florida International University] at 09:01 07 January 2015
(Received 5 February 2013; final version received 15 April 2013 ) The non-Newtonian properties of activated sludge (AS) suspension lead to transfer limitations (oxygen, substrate…) and operation difficulties in Waste Water Treatment Plants (WWTP). The current approach involves assuming the sludge behaves like water on a rheological point of view, and then oversizing pumping and aeration devices, which represent over 60% of the operating cost in WWTP. The objective of this work is to understand the effect of bioflocculation on the rheological properties and the settling ability of AS suspensions, by means of variations in concentration of exo-cellular polysaccharides. Experiments have been conducted in a 20 L laboratory scale bioreactor at a constant retention time of 20 days and with a total suspended solid concentration between 15 and 20 g L−1 . The bioreactor was fed with a synthetic substrate at a constant mass −1 loading rate of 0.3 kgDCO kg−1 TSS d . Our results show that increasing the exo-polysaccharide (EPS) concentration from 10 −1 to 80 mg gTSS leads to an increase in shear-thinning properties of AS. An improvement of the settling ability is also obtained, −1 at least when the EPSs increase from 10 to 45 mg g−1 TSS . Above 45 mg gTSS of adsorbed polysaccharides, the settling ability seems to decrease again. Keywords: wastewater treatment; activated sludge; flocculation; total suspended solids; exo-cellular polymers; rheology
1. Introduction Activated sludge (AS) suspensions are known to be nonNewtonian fluids.[1–3] They are described in the literature as shear-thinning fluids exhibiting a decrease in apparent viscosity as the shear rate is increased.[4] They are also known to exhibit (above a certain solid concentration) viscoplasticity. This latter property implies that no flow will occur below a certain stress, i.e. the yield stress.[4] Other papers also describe AS suspensions as thixotropic [5] (with a time-dependent viscosity) and viscoelastic fluids.[6] All these rheological complexities are due to an internal complex structure [7]: • A liquid phase mostly composed of water, salts, and macromolecules (‘free’ polymers) • Solid structures named bioflocs mostly composed of water and microorganisms structured by a macromolecular network (adsorbed polymers called exopolymers). Moreover, it has been clearly demonstrated that it is necessary to study the relationship between the rheological properties and the floc structure.[8] These bioflocs constitute the structural units (SU) of AS suspensions.[9] The SU are very porous (water is entrapped in their porosity), fragile and deformable. Under the effect of
a sufficient shear stress, SU’s morphology evolves, leading to a release of water and causing the shear-thinning of such suspensions. Among the parameters known to influence the rheological behaviour of biological sludge suspensions, the most important are their total suspended solid (TSS) concentration but also their physico-chemical and biological compositions.[10] The effect of TSS concentration on the rheology of AS suspension has been widely studied. Most of these studies [11–15] show that the shear-thinning and viscoplastic parameters (consistency index k (Pa sn ); flow index n (−) and yield stress τ0 (Pa)) can be modelled by linear, exponential, or power-increasing functions of the TSS concentration. The biological composition of bioflocs in terms of proportions of the different types of microorganisms (amount of filamentous organisms in the flocs for instance) is another parameter that needs to be taken into account. It is indeed directly related to the internal structure of bioflocs and therefore to their flowing properties or settling ability.[13] The exo-polymers (microorganisms metabolites) are the key to the bioflocculation process.[16] These macromolecules adsorbed at the surface of microorganisms, inducing their flocculation through macromolecular bridging. An increase in concentration of exo-polymers, notably
∗ Corresponding author. Email: [email protected] This article was originally published with erroneous pagination. This version has been corrected. Please see Erratum (http://dx.doi.org/ 10.1080/09593330.2013.869394).
© 2013 Taylor & Francis
Downloaded by [Florida International University] at 09:01 07 January 2015
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S. Akkache et al.
exo-polysaccharides (EPSs) is thus expected to induce an increase in floc size. Such an increase in floc size will in turn cause an increase in apparent viscosity of the sludge and enhance its non-Newtonian properties in terms of both shearthinning and viscoplasticity. Forster [17] shows that the exo-proteins and EPSs concentration affects the rheological behaviour of sludge (increasing the apparent viscosity when increasing the exo-polymeric substances concentration). Rosenberger and Kraume [18] and Li and Yang [19] show that the apparent viscosity of AS decreases when decreasing the exo-polymeric substances concentration. Mori et al. [15] shows that increasing the EPS concentration increases the shear-thinning and the viscoplastic properties of AS. The settling ability of AS suspensions is also linked to their biological composition. Filamentous or non-filamentous (zoogleal) bulking caused by instabilities in composition between the different microorganisms’ communities is known to be related to a poor ability to settle.[19] The state of flocculation is another important parameter. The settling ability of AS suspensions, is often a limiting step in Waste Water Treatment Plants (WWTP) processing. A low ability to settle can lead to a direct discharge of biomass in the environment. The objective of this work is to correlate the rheological properties of AS suspensions (15–20 g L−1 in TSS) and their ability to settle to their state of flocculation by varying the EPSs concentration. This could lead to a better understanding of the influence of the bioflocculation process on the flow properties of these materials. The 15–20 g/L TSS concentration range is chosen in order to be sufficiently sensible for the rheological measurements as well as for the determination of the EPS concentrations.
2. Methods and material 2.1. Reactor operating conditions AS suspensions used in this work come from an urban WWTP (Aix en Provence, France 165,000 inh. eq.). The sludge is sampled in the recirculation loop between aeration basins and secondary settlers at about 3 g L−1 in TSS. In order to obtain a TSS concentration of 15–20 g L−1 , the sludge is concentrated by soft (gravimetric) filtration using simple paper coffee filters (average size of pores around 100 μm). The reactor is a 20 L Plexiglas vessel with a liquid height kept equal to its diameter (H = D = 0.3 m). It is equipped with a mechanical stirrer of diameter d composed of eight inclined blades (d/D = 1/3) placed at the distance h from the bottom (h/H = 1/3) and operated at 250 rpm. The reactor is aerated by injection of compressed air through soft porous diffusers at 300 Lair h−1 . The temperature is kept at 20 ± 0.1◦ C due to a circulation of water in an immersed stainless steel coil.
The substrate used to feed the sludge is composed of −1 sugar as a source of organic carbon (0.3 gDCO g−1 TSS day ) and Viandox® as a source of proteins and oligo-elements (0.03 gDCO g−1 TSS once a week). The reactor works in a semi-continuous way. The age of biosolids is fixed at 20 days by withdrawing 500 mL of homogenous suspension every day. The acclimation step is assumed to be completed based on both the stabilization of TSS concentration and a visual observation of the sludge that shows that most of the filamentous bacteria initially present in the sludge had disappeared. 2.2.
Characterization of sludge suspension
2.2.1. TSS determination A 30 mL sludge sample is taken for centrifugation at 13,500 G during 15 min. Then, the solid is dried at 105◦ C until stabilization of the mass. TSS is calculated as the ratio of dried mass to the volume sampled and expressed in g L−1 . 2.2.2. Extraction of EPS substances Thirty millilitres of sludge sample is first taken for centrifugation at 4000 G during 15 min. The supernatant which is discarded is assumed to contain only free EPS substances. The solid phase is then diluted again into 30 mL of distillated water and sonicated at 42 W for 2 min in order to desorb the polymeric substances from the surface of flocs and cells.[15] This suspension is taken for centrifugation at 4000 G for 15 min. The supernatant now containing the desorbed EPS substances is used to perform their concentration measurement. 2.2.3. Measurements of EPS substances concentration Among exo-polymeric substances produced by sludge microorganisms, we have chosen to follow the evolution of one family of macromolecules the EPSs. The EPS represent one of the most important type of exo-polymeric substances (together with exo-proteins and humic acids). Their concentration is easy to measure using a simple colorimetric method (see below); and we work here under the assumption that feeding our sludge with a substrate mostly composed of sugar will mostly induce an increase in EPS than in other polymeric substances. The EPS concentration is obtained by the Dubois et al. (1956) method. It consists of adding 5 mL of 95% sulphuric acid and 1 mL of 5% phenol solutions to 1 mL of sludge supernatant. The mixture is heated at 105◦ C during 10 min. After 30 min at rest in the dark, an AquaMate Thermo Spectronic colorimeter is used to read the absorbance at 486.0 nm. The EPS concentration is directly obtained using a calibration curve previously obtained with solutions of glucose from 0 to 300 mg L−1 .
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Environmental Technology 2.2.4. The SVI In order to evaluate the settling ability of the AS, we have measured the sludge volume index (SVI). The SVI is a commonly used indicator describing the functioning of the WWTP in terms of ability to separate biosolids from purified waters. It represents the volume of 1 g of sludge after 30 min of settling in a 1 L test tube. It is the ratio of the volume of settled sludge to the initial TSS concentration and is expressed in mL.g−1 . High values of SVI indicate a low ability to settle while on the contrary, values smaller than 150 mL g−1 are associated with good abilities to settle. The validity of this method is based on SVI values varying from 100 to 300 mL g−1 . It is, therefore, necessary to dilute sludge suspensions sampled between 15 and 20 g L−1 in the reactor to 2–3 g L−1 before performing this test. 2.3.
Rheological measurement
Rheological measurements are performed using an AR 500 rheometer (T.A. instrument) operated in a controlled shear stress mode using a Couette geometry (1 mm gap). The protocol was developed for WWTP AS for a range of TSS concentration between 10 and 30 g L−1 .[10] The temperature of the system is maintained at 20 ± 0.1◦ C using a recirculation the water bath. Rheometer control and data collection are performed using TA instrument’s softwares provided with the instrument. A pre-shear at 12 Pa for 1 min followed by a continuous linear ramp are imposed from 0.1 to 12 Pa over a period of 8 min are successively applied to the sample. 2.3.1. Rheological data analysis Using Microsoft ExcelTM , rheograms are fitted with the least squares method to determine the Herschel–Bulkley model parameters. τ = τ0 + k γ˙ n ,
(1)
where τ is the shear stress (Pa), τ0 the Herschel–Bulkley yield stress (Pa), k the consistency index (Pa sn ), and n the flow index (dimensionless) 0 < n < 1. 3. Results and discussion 3.1. TSS evolution with time Two different cultures (cultures I and II) of sludge have been conducted under the same operating conditions. Figure 1(a) and 1(b) successively show the time evolution of TSS for these two cultures. It can be observed that the TSS concentration remains steady after a period of acclimation. The aim of this work is to correlate the rheological properties and the ability to settle of AS suspensions, to their concentration in EPS. As a consequence, we can only focus our interest on period where the TSS concentration remains constant since this latter parameter is known to have a great influence on
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flow properties of such suspensions. The initial unsteady phase, lasting, respectively, 12 and 14 days according to the culture, known as the acclimation step,[20] is linked to an evolution in biological composition, i.e. proportions of the different types of microorganisms. This phenomenon is due to the differences in operating conditions (notably in terms of aeration, agitation and substrate composition) prevailing in the laboratory reactor compared to those prevailing at the WWTP. In the case of culture I, we can see in Figure 1(a) that the acclimation is completed after 12 days. From days 12 to 16 then, the sludge is said to be acclimated (with constant biological composition) and the TSS remains constant. During this latter period, variations in EPS concentration are the only factor affecting the state of flocculation of microorganisms and thus their rheological properties. In the case of culture II, it can be observed in Figure 1(b), that a period of constant TSS is also reached after the acclimation has been completed, between days 14 and 28. This period will also allow studying the effect of EPS concentration on the flocculation state and on the rheological properties. Another period of constant TSS is also observed during the acclimation step, i.e. while the biological composition of the sludge is still evolving (days 7–11). During this particular period not only the degree of flocculation (correlated to the EPS concentration), but also the evolution in biological composition of the flocs will superimpose their effects on the evolution of sludge rheology. 3.2. Rheology 3.2.1. Culture I The rheological measurements (rheograms) performed from days 12 to 16 are plotted in Figure 2(a). This figure shows an upwards translation of the rheograms as we go from days 12 to 16. After fitting the rheograms with the Herschel–Bulkley model and characterizing the sludge in terms of TSS and EPS, we obtain the following results (Table 1). It can be observed in Table 1 that, for an approximately constant TSS concentration, there is an evolution of the rheological behaviour which is probably due to an evolution in the bioflocculation state. As a matter of fact, the EPS concentration being multiplied by four from days 12 to 16, we can assume that the sludge is getting more and more flocculated. This implies that sludge flocs are probably bigger in size at the end of this period when compared to its beginning. Bigger flocs (i.e. bigger SU) will then lead to an enhancement of the shear-thinning properties as previously explained. An increase of the consistency index k and a decrease of the flow index n corresponding to such an increase in shear-thinning properties is well observed in Table 1 as the sludge evolves from days 12 to 16.
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Figure 1.
S. Akkache et al.
Evolution of TSS versus time: (a) for culture I and (b) for culture II.
Concerning the yield stress τ0 no significant variations of viscoplasticity is observed despite the increase in EPSs concentration. It can be assumed that, either the increase in SU size (due to the EPS concentration increase) is not important enough to induce an increase of the 3D interparticles network cohesion at rest and therefore results in no increase in viscoplasticity of the sludge; either this increase is too small to be detected using our experimental device. 3.2.2. Culture II In Figure 2(b), it can be observed that the time evolution of rheograms during the period from day 7 to day 11 during which the sludge shows a constant TSS, but a biological composition still in evolution.
An evolution of the rheological properties is observed during this period, the rheograms being shifted downwards as the sludge evolves from day 7 to day 11. After fitting the rheograms with the Herschel–Bulkley model and characterizing the sludge in terms of TSS and EPS, we obtain the following results (Table 2). It can be observed in Table 2 that for a constant TSS content, as the sludge evolves from days 7 to 11, an increase in EPS concentration is obtained. This should be linked to an increase in flocs size (increase in bioflocculation) and should, therefore, be followed by an increase in the non-Newtonian properties of the suspension. On the contrary, a decrease in both shear-thinning and viscoplasticity (decrease in consistency index k and yield stress τ0 and increase in flow index n) is clearly observed in Table 2 during this period.
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Environmental Technology
Figure 2. to 28.
Evolution of rheograms: (a) for culture I from days 12 to 16; (b) for culture I from days 7 to 11; (c) for culture II from days 15
It is possible then to conclude that, as this period is located during the acclimation step (Figure 1(b)), not only the EPS concentration but also the biological composition (still unsteady between days 7 and 11) of the sludge will superimpose their effects on the time evolution of rheograms. It seems therefore that, despite the increase in
EPS concentration, the sludge structure is mostly subjected to a deflocculation stage between days 7 and 11. This phenomenon is thought to be due to the important modifications of the sludge biological composition, still in progress during the acclimation phase, and results in a downwards shift of sludge rheograms.
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S. Akkache et al.
Table 1. Day 12 13 14 15 16
TSS (g L−1 )
EPS (mg g−1 TSS )
k (Pa sn )
n (−)
τ0 (Pa)
R2
14.99 14.81 15.03 14.55 14.00
11.59 9.05 12.58 21.63 42.27
0.29 0.28 0.4 0.4 0.5
0.5 0.52 0.47 0.48 0.46
0.20 0.20 0.20 0.15 0.25
.9982 .9993 .9996 .9992 .9997
Table 2.
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Day
Culture I, evolution between days 12 and 16.
Culture II, evolution between days 7 and 11.
TSS (g L−1 )
EPS (mg g−1 TSS )
k (Pa sn )
n (−)
τ0 (Pa)
R2
19.1 18.3 18.8 18.8 19.3
2.83 8.47 16.46 26.39 39.00
0.52 0.52 0.43 0.32 0.35
0.44 0.43 0.55 0.48 0.46
1.00 0.60 0.60 0.40 0.40
.9986 .9996 .9997 .9976 .9994
7 8 9 10 11
Between days 14 and 28, another period of constant TSS is observed in culture II after the acclimation step has been completed. It can then be assumed that, only the EPS concentration will affect the flocculation state of the sludge during this period. The time evolution of some of the rheograms (for clarity purposes) obtained during this period are presented in Figure 2(c). A shift of the rheograms upwards is now clearly observed in Figure 2(c) as the sludge evolves during this period. After fitting the rheograms with the Herschel–Bulkley model and characterizing the sludge in terms of TSS and EPS, the results obtained are shown in Table 3. It can be observed in Table 3 an increase in consistency index k and a decrease in flow index n (increase in shear-thinning) together with an increase of the EPS concentration. This clearly indicates an enhancement of the flocculation state of the sludge. On the contrary no sensible evolution of the yield stress is observed. Table 3. Day 15 16 17 18 21 22 23 24 25 28
Culture II, evolution between days 15 and 28.
TSS (g L−1 )
EPS (mg g−1 TSS )
k (Pa sn )
n (−)
τ0 (Pa)
R2
16.4 16.6 16.7 17.9 16.7 16.3 16.2 16.6 16.9 17.0
17.78 21.47 34.21 42.31 47.10 43.95 50.84 57.38 60.71 75.54
0.45 0.47 0.52 0.56 0.80 0.69 0.66 0.73 0.78 1.02
0.46 0.46 0.46 0.44 0.43 0.45 0.45 0.45 0.45 0.44
0.40 0.45 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.30
.9993 .9991 .9998 .9998 .9997 .9995 .9995 .9992 .9988 .9993
These results are qualitatively identical with those obtained in the case of culture I (Figure 2(a) and Table 1). It seems that, after the acclimation of the sludge has been completed, and once its biological composition is stabilized according to our laboratory operating conditions, only the EPS concentration affects the bioflocculation state of the material. As a consequence, an increase in EPS induces an increase of the shear-thinning ability of the suspension. As it has been previously observed in the case of culture I, in the range of TSS studied here (15–20 g L−1 ), sludge suspensions are viscoplastic fluids with rather low-yield stresses (
