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Dewatering of urban residual sludges: Filterability and hydro-textural characteristics of conditioned sludge
Separation and Purification Technology 72 (2010) 275–281
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Dewatering of urban residual sludges: Filterability and hydro-textural characteristics of conditioned sludge Thierry Ruiz, Thaniya Kaosol, Christelle Wisniewski ∗ UMR CIRAD 016, Génie des Procédés, Eau et Bioproduits, Université Montpellier 2, CC 05, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
a r t i c l e
i n f o
Article history: Received 15 April 2008 Received in revised form 15 February 2010 Accepted 19 February 2010 Keywords: Urban residual sludge Sludge conditioning Mechanical dewatering Water content limit Hydro-textural characterization
a b s t r a c t The hydro-textural properties of different urban residual sludges, characterized by their organic matter content, were estimated and correlated with their limit water content, corresponding to their minimum water content obtained after constant high-pressure filtration. A conventional compression-permeability (C-P) cell was used to determine the limit water content (wlim ) whereas standardized soil mechanics tests were run to determine the liquid limit (wL ) and plastic limit (wP ) content, characterizing the hydro-textural properties. The results demonstrated that wlim , wL and wP parameters depend on sludge composition: high organic matter content ratio is unfavorable to a low wlim but favorable to a high plastic range and so a high deformation of the sludge. It appeared that the wlim value was close to the wP value, but lower than that obtained after mechanical dewatering in the wastewater treatment plants (WWTPs). It appears thus that the totality of the mechanically extractable water is not extracted by the WWTPs mechanical dewatering devices. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Treatment of domestic sewage, via physicochemical and/or biological processes, produces large quantities of by-products, known as residual sludge. Sludge treatments are based on stabilization processes to block or reduce biological activity and on dewatering processes to reduce sludge volume for storage, pumping, transportation and handling. Sorensen [1] reports that 30–50% of annual treatment operating costs relate to the sludge dewatering stage. A good knowledge of physical properties allows the prediction of sludge behavior during all treatments, which it is necessary to apply to sludge handling and disposal [2,3]. The dewatering techniques used basically consist of mechanical separation processes (centrifuge, filter press or belt filter) which ensure the partial drainage of the water mobilized by mechanical action [4]. Sludge filterability must be characterized in order to choose and design the filtration unit but also to optimize the cycles of filtration operation and filter regeneration and/or washing. Capillary suction time (CST) and specific resistance to filtration, quantified in a compression-permeability (C-P) cell, are the most currently used parameters to define sludge filterability [5,6]. Limit water content, corresponding to the minimum water content of the sludge cake obtained after a constant pressure filtration, is another important parameter that can be useful to describe and forecast the efficiency of the filtration process or a purely mechanical action.
Many authors have studied the role of the nature and composition of sludge in regard to the specific resistance value [7–9] but rarely in terms of the limit water content value, even though this mechanical dewatering limit undoubtedly depends on sludge composition. Physical consistency is an important parameter in sewage sludge characterization as it affects all treatment, utilization and disposal operations [10]. Basically, despite the lack of studies on this aspect [2,3,11], it is important to evaluate the physical state of sludge and the limits between liquid and paste behaviors (liquid limit) and between solid and paste (plasticity limit), for the purposes of drying [11], storage, transportation, handling and landfilling. The objectives of this study were first to estimate the water limit content and hydro-textural characteristics of different kinds of sludge characterized by their organic matter content. A C-P cell was used to determine the limit water content whereas standard soiltesting procedure soil mechanics trials were run to determine the hydro-textural characteristics based on Atterberg’s limits quantification [12]. According to the results, correlations were established between the obtained parameters and a discussion about dewatering intensification by sludge conditioning is proposed. 2. Materials and methods 2.1. Sludge sample and conditioning
∗ Corresponding author. Tel.: +33 467548558; fax: +33 467548649. E-mail address: [email protected] (C. Wisniewski). 1383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2010.02.017
The studied sludges, sludge A and sludge B, were sampled from two urban wastewater treatment plants, WWTPs, A (5000 Popu-
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Table 1 Sludge characteristics. Parameters
Sludge A
Sludge B
Unit
VS/DS
0.75
0.59
–
Before dewatering wbd Sbd
5000 ± 100 2.00 ± 0.02
3240 ± 65 3.00 ± 0.06
% %
After dewatering wad Sad
645 ± 13 13.0 ± 0.3
295 ± 6 26.0 ± 0.5
% %
lation Equivalent) and B (280,000 Population Equivalent), whose common characteristic was the use of a biological treatment process. The sludge was sampled: (i) after digestion and before the dewatering stage to determine the limit water content, (ii) at the end of the mechanical dewatering stage (belt filter press for both WWTPs) to determine Atterberg’s limits. The two studied sludges differed in their organic content, directly linked by the type of WWTP digestion process (aerobic digestion for WWTP A and anaerobic one for WWTP B). The general characteristics of the sludges are summarized in Table 1. Dry solids (DS), volatile solids (VS) (identified with organic solids), dryness (S) and water content (w) (dry base) were determined as reported by Standard Methods [13]. All experiments were completed a week after sampling in order to minimize changes in sludge characteristics due to microbial activity. The sludge was stored at room temperature. The raw sludges were subjected to the addition of mineral material, to obtain a variation of the VS/DS ratio. Sludge was conditioned with mineral material extracted from the sludge and with quick lime (CaO), the most widely used reagent in water treatment applications. The sludge mineral material corresponded to sludge mineral dry solids obtained after the sludge was brought to 550 ◦ C for 2 h in a muffle furnace. The mineral composition of sludge is reported in Table 2. A conventional jar test (100 rpm, 5 min) or manual mixing was used to mix the sludge, liquid and paste, respectively, with the mineral additive. Concerning the addition of quick lime, a specific methodology was chosen according to exothermic reaction time [14]. The lime and sludge mixture was maintained under agitation for 24 h to reach thermodynamic equilibrium (Fig. 1).
Fig. 1. Temperature evolution of the sludge and lime mixture vs. time.
In such a diagram, the filtration phase is linear and corresponds to cake formation; the second part, where linearity is no longer observed, represents the expression phase. The first linear plot is classically used to calculate the sludge specific resistance to filtration, ˛, according to Ruth’s equation, based on Darcy law [15]: 1 × W∗ × ˛ t ×V + = V Q0 2 × P × A2
Ruth’s equation is established in the case of incompressible materials and supposes a constant porosity throughout the cake thickness. It also assumes that the pressure drop across the cake and the filtrate medium is constant, as well as ˛. In the case of sludge (considered as compressible material), the hypothesis that the filtration cake does not present structural heterogeneities throughout its thickness, and during the running test, is debatable. Recent studies have proposed ameliorated methodologies or complementary trials to describe the local structure of particle deposits and validate the theoretical model and its hypothesis [16–18]. However, recent other publications have presented and exploited the results of ˛, obtained with this global approach; this approach is designed
2.2. Sludge characterization 2.2.1. Limit water content determination A conventional compression-permeability (C-P) cell was used to quantify the limit water content, wlim . The C-P cell, described in Fig. 2, consisted of a 0.42 L pressurized cell. The plane membranes used for filtration experiments were cotton cellulose membranes of 70 mm in diameter and 10 m in pore size. The cell was pressurized using a pneumatic piston submitted to defined gas (N2 ) pressure. The conventional methodology consists in filtering a defined volume of sludge under a constant pressure P. Sludge dewatering is classically described using the ratio V/t versus V (where V is the filter collected after a time t from the start of the test).
Table 2 Composition of the sludge mineral fraction. Element
Mg
Ca
P
Al
Si
Fe
Unit
Sludge A Sludge B
0.95 1.33
9.92 8.74
5.47 6.30
2.75 3.37
4.42 5.23
11.67 8.63
Atomic % Atomic %
(1)
Fig. 2. Schematic of the compression-permeability cell.
T. Ruiz et al. / Separation and Purification Technology 72 (2010) 275–281
Fig. 4. log(˛) vs. log(P).
Fig. 3. t/V vs. V (sludge A).
to provide a good approximation of the average specific resistance of the filtration cake, and ˛ is always considered to be a valuable parameter to characterize sludge filterability [19–21]. By measuring specific resistances to filtration under several pressures P and drawing the curve log(˛) versus log(P) the sludge compressibility coefficient, s, slope of the linear plot can be determined. This methodology is an indirect way of evaluating the deposit structure, as it is obtained from global measurements and gives access only to global values. In this study, ˛ and s were quantified only for the raw sludge. At the end of each filtration test, and for different pressures (ranged from 0.05 to 1.5 MPa), the water content of the sludge cake was measured (wfin ). It is shown that this final water content decreased with pressure to a constant value corresponding to the limit water content, wlim . 2.2.2. Hydro-textural characteristics determination The textural characterization of the pasty sludge obtained after mechanical dewatering, consisted of quantifying the water contents corresponding to the transitions (i) between solid and plastic states (plastic limit, wP ) and (ii) between plastic and liquid states (liquid limit, wL ). These transitional water contents, or so-called Atterberg’s consistency limits, were determined by standardized soil mechanics trials [11]: (i) wL was obtained by the use of a cone penetrometer and corresponded to a cone depression in the sludge of 17 mm and (ii) wP corresponded to the fissure of sludge roller of 3.0 ± 0.5 mm in diameter and 10 cm in length. The plasticity index, IP , corresponding to (wL –wP ), was quantified. This index, representing the range between plastic and liquid limits, determines the ability of the sludge to change from semisolid to liquid state, resulting in a significant decrease of cohesion. The IP index is also used to situate the consistency state of the sludge on the plasticity chart (Casagrande diagram), commonly used in soil mechanics [12]. On this chart, representing IP versus wL , shows the different consistency domains and the location of some types of soils.
3. Results 3.1. Mechanical dewatering aptitude and hydro-textural characteristics of the raw sludges 3.1.1. Mechanical dewatering aptitude Fig. 3 presents the filtration curve t/V versus V for sludge A and for two defined pressures (0.6 and 1.5 MPa). Whatever the pressure P, the curve presents two main sections:
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(i) A straight section, corresponding to the filtration stage. The linear plot is used to calculate sludge specific resistance, ˛, according to Eq. (1). (ii) An asymptotic section over which an increase in filtration time yields no further increase in the volume of filtrate. The final water content wfin , defined as the cake water content obtained when there is no further filtrate run-off, was quantified for different pressures in order to estimate wlim . It is general practiced to establish a comparison between sludges using ˛0.05 MPa as a base line [22]. For urban residual sludge, ˛0.05 MPa ranges from 5 × 1010 to 2 × 1011 m kg−1 [23,24]. In these experiments, ˛0.05 MPa was equal to 1.2 × 1011 and 1.7 × 1012 m kg−1 for sludges A and B, respectively. The compressibility coefficient, s, was defined graphically by the slope of the straight line log(˛) versus log(P) (Fig. 4). The compressibility coefficients of sludges A and B were 0.98 and 0.77, respectively (Table 3). The compressibility coefficient, s, measures the intensity of the decreasing cake permeability under increasing pressures and consequently estimates the benefit of high-pressure filtration operations. In this experiment, s values were lower than 1, which signified that applying high pressures can favor high filtration rate. It was noted that the compressibility coefficient of sludge B was clearly lower than that of sludge A. It is important to note the difficulty in comparing these compressibility values with those found in the literature; indeed, the ways of evaluating the effect of pressure on the deposit structure are varied and not directly comparable. Fig. 5 presents the evolution with the pressure of the final water content wfin . The figure shows that the final water content decreased with pressure down to a constant value corresponding to the limit water content, wlim . This value is attempted for a pressure close to 0.8 MPa, whatever the sludge. wlim of sludges A and B were equal to 133% and 92%, respectively. According to various authors, a mechanical dewatering process can only remove the free and interstitial water content from sludge and for that reason, the water remaining in the sludge after filtration could be considered Table 3 Raw sludge dewatering aptitude and hydro-textural characteristics. Parameters
Unit
Sludge A
Sludge B
VS/DS wad ˛0.05 MPa s wlim wL wP IP
– % m kg−1 – % % % %
0.75 645 ± 13 1.2 × 1011 0.98 133 ± 2 538 ± 5 137 ± 5 401 ± 5
0.59 295 ± 6 1.7 × 1012 0.77 92 ± 2 240 ± 5 100 ± 5 140 ± 5
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Fig. 5. wfin vs. P.
as bound water [25–27]. Note that these water content values corresponded to dryness values close to 43% and 52% for sludges A and B, respectively. These results showed that sludges A and B did not present the same mechanical dewatering aptitude. Organic matter content seemed to be an essential parameter in mechanical dewatering efficiency. If the higher organic sludge (sludge A) presented lower specific resistance to filtration, the sludge appeared to be more compressible and the dryness obtained after filtration was significantly lower than sludge B. A great difference is observed between the value of wlim and the value of the water content after mechanical dewatering, wad . It appears that the mechanical dewatering implemented in the WWTP does not extract the totality of the mechanically extractable water. 3.1.2. Hydro-textural characteristics Atterberg’s limits could provide information regarding wP , wL and IP . Table 3 presents the values of wP , wL and IP obtained for sludges A and B. It can be noted that the water contents of the two sludges after mechanical dewatering (wad ) were considerably close to the liquid limits wL . Moreover, the wP values corresponded to wlim values. This relation between water contents corresponding to performances of mechanical dehydration and consistency limits, suggests that it may be judicious to consider the rheological behavior of the filtration cake to estimate the aptitude for such dewatering. While several works [15,28] reveal the importance of taking into account the rheological behavior of filtration cake compressibility, the change of consistency with the water content of the porous medium thus formed, has not been dealt with. The measured plasticity index places the sludges on the plasticity chart at the level of highly organic plastic soils such as fibrous clays and peat [12]. In this sense, IP value was seen to be high for sludge B, which is more organic than sludge A. 3.2. Mechanical dewatering aptitude and hydro-textural characteristics of the conditioned sludges 3.2.1. Mechanical dewatering aptitude Only the limit water content was quantified in order to compare this value to transition water contents. Fig. 6 presents the evolution of wlim with the VS/DS ratio for conditioning with sludge mineral material (a) and lime (b). It appears that the more organic the sludge, the higher the limit water content wlim increase with VS/DS ratio, regardless of the nature of the additive. This evolution can be described by the
Fig. 6. wlim vs. VS/DS: (a) sludge conditioned with mineral material and (b) sludge conditioned with quick lime.
following linear equation (2): wlim = a
VS +b DS
(2)
For experiments adding sludge mineral, a difference in slope value is noticed around a VS/DS ratio of 0.15. No explanation can be given of this sudden change of behavior, and complementary experiments around this VS/DS value need to be done. However, residual sludge with such low VS/DS ratio is not conventionally encountered in water treatment plants and the knowledge of the sludge behavior is such a range of VS/DS values is of limited interest. Table 4 gives the values of constants a and b for each sludge and type of additive. Simple mineralization by adding rough mineral material leads to a significant reduction of wlim . For a given VS/DS ratio, we observed that the combination of the chemical and biological effects of lime on the sludge involve higher limit water content. The water content decrease with the VS/DS ratio can be explained by the following relation: w=
VS Mw Mw = × DS DS VS
(3)
For a fixed organic matter (VS), the addition of mineral material (reduction in VS/DS ratio) would decrease the limit water content linearly if the quantity of water retained by the filtration cake Table 4 Values of the a and b constants (Eq. (2)). Sludge A
Mineral material Lime
VS/DS <0.15 VS/DS >0.15
Sludge B
a
b
R2
a
b
R2
349 56
36 92
0.99 0.94
101 42
39 50
0.96 0.97
117
32
0.97
81
26
0.99
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Table 5 Parameters of the mixture law (Eqs. (4) and (5)). wP org Sludge A Sludge B
80 3
wP min
R2
wL org
wL min
R2
59 49
0.97 0.99
593 470
64 50
0.99 0.98
Table 6 Parameters of the mixture law (Eq. (6)).
Sludge A Sludge B
IP org
IP min
R2
513 449
5 9
0.99 0.99
following equations:
wP =
VS VS wP org + 1 − DS DS
wL =
VS org VS + 1− wL DS DS
wP min
(4)
wL min
(5)
The slope and origin ordinate of the obtained curves allow the quantification of worg and wmin , given in Table 5. In our VS/DS range, the mixture laws were validated. The plastic and liquid limits of the mineral matter are in the range of those usually measured for clays [12]. The total data obtained from the different sludges show that the higher the organic fraction, the more water retained, with a consistency that remains plastic. The plasticity index represents the plastic range and increases with VS/DS ratio (Fig. 8), by a same mixture law: Fig. 7. win , wL and wP vs. VS/DS: (a) sludge conditioned with mineral material and (b) sludge conditioned with quick lime.
remained constant. The difference between the water contents given by Eq. (3) and obtained in experiments (Fig. 6) shows that the cake retains additional water mass (Mw ). This quantity depends on the nature and proportions of the conditioning, which plays an important role on the limit dryness obtained. 3.2.2. Hydro-textural characteristics Consistency limits are highly dependent on sludge composition. For a mixture of sludge and mineral matter, Figs. 7 and 8 show the evolution of wP , wL and IP with VS/DS ratio >0.15. We note that wP and wL evolutions versus VS/DS ratio are linear for the tested VS/DS range. If we suppose that the relations of plastic and liquid limits to VS/DS ratio can be represented by mixture laws, we obtain the
Fig. 8. Consistency vs. VS/DS.
IP =
VS org VS IP + 1− DS DS
IP min
(6)
where IP org represents the plasticity index of the organic fraction and IP min represents the plasticity index of the mineral fraction (Table 6). Fig. 9 shows that the plasticity index is slightly dependent on the nature of the sludge. This result, already obtained for various raw sludges [29], has been extended here to the case of conditioned sludges. 4. Discussion The experimental result reveals that the more organic the sludge, the higher the limit water content (Fig. 6). Aptitude for mechanical dewatering is thus lower. However, its plastic range is wider, meaning a greater capacity of deformation (Fig. 9). Fig. 10 shows the Casagrande diagram, plotting the different mixtures and sludges studied. As can be seen, the values for most of these mate-
Fig. 9. Plasticity index vs. VS/DS.
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ical behavior with water and its intrinsic consistency limits. The proportions of the mixture can be deducted from the various presented relations according to expected performances. 5. Conclusion
Fig. 10. Casagrande diagram.
rials plot above the A-line, in the zone typically characteristic of highly compressible materials, with high plasticity at the limit of inorganic and organic clays. It can be noticed that the mixture law, used in Eqs. (4)–(6), is verified. Thus a high organic sludge is less mechanically dewatered than a mineral conditioned sludge. This point shows that the plastic range is not a sufficient criterion to characterize compressibility. The liquid limit marking the transition between a liquid state and a plastic state is very close to the water content obtained after mechanical dewatering from WWTP (Fig. 11). This result shows the influence of the consistency state for this kind of dewatering in industrial conditions. Indeed, during centrifugation or press filtration, sludge is flowing in the equipment used. The sludge undergoes stresses that are transmitted to interstitial water and ensure its drainage. When consistency becomes plastic, the flow is not comparable with that of a suspension. This consistency probably does not allow any further water drainage. This point seems to corroborate the results obtained with the C-P cell. In this case, sludge does not flow and all the transmitted stresses are used for the drainage of interstitial water and compression of the cake. The limit water content obtained under these conditions is very close to the plastic limit (Fig. 11). The process efficiency is related to the transition in consistency from a plastic state to a solid state. This analysis confers on conditioning a role that is strongly related to its capacity to modify consistency. The additional amount of water retained by the physicochemical interactions with the added matter, can be controlled by the choice of a product that is more or less chemically reactive or hydrophilic. The type of mineral material to be added can be chosen according to its physicochem-
From this study, it can be concluded that the hydro-textural characteristics of sludge are significantly correlated with mechanical dewatering aptitude. The influence of mineral material conditioning on the mechanical dewatering of two waste sludges is shown. The hydro-textural approach confirms the validity and interest of standardized consistency tests (Atterberg’s tests). These tests taken from soil science constitute complementary characterizations to determine the water contents characteristic of this kind of dewatering. The plasticity index is close to the water content obtained after traditional mechanical dewatering. The plasticity index correlates very closely with the limit water content. By measuring these water contents, associated to the correlation established between the plasticity index and VS/DS ratio, it is possible to examine the influence of conditioning on the performances of mechanical dewatering. Nomenclature
A DS IP M P Q s S t V VD w W*
membrane surface area [L2 ] dry solids [M] plasticity index [%] mass [M] transmembrane pressure [M L−1 T−2 ] flow rate [L3 T−1 ] compressibility coefficient dryness time [T] volume of filtrate [L3 ] volatile solids [M] water content [M M−1 ms ] dry solids deposited per [M L−3 ]
Greek letters ˛ specific resistance to filtration [L M−1 ] dynamic viscosity of filtrate [M L−1 T−1 ] Subscripts/superscripts bd before WWTP dewatering ad after WWTP dewatering fin final in after conditioning L liquid lim limit min mineral 0 initial time of filtration org organic p plastic w water References
Fig. 11. wP and wlim vs. VS/DS.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Dewatering of urban residual sludges: Filterability and hydro-textural characteristics of conditioned sludge Thierry Ruiz, Thaniya Kaosol, Christelle Wisniewski ∗ UMR CIRAD 016, Génie des Procédés, Eau et Bioproduits, Université Montpellier 2, CC 05, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
a r t i c l e
i n f o
Article history: Received 15 April 2008 Received in revised form 15 February 2010 Accepted 19 February 2010 Keywords: Urban residual sludge Sludge conditioning Mechanical dewatering Water content limit Hydro-textural characterization
a b s t r a c t The hydro-textural properties of different urban residual sludges, characterized by their organic matter content, were estimated and correlated with their limit water content, corresponding to their minimum water content obtained after constant high-pressure filtration. A conventional compression-permeability (C-P) cell was used to determine the limit water content (wlim ) whereas standardized soil mechanics tests were run to determine the liquid limit (wL ) and plastic limit (wP ) content, characterizing the hydro-textural properties. The results demonstrated that wlim , wL and wP parameters depend on sludge composition: high organic matter content ratio is unfavorable to a low wlim but favorable to a high plastic range and so a high deformation of the sludge. It appeared that the wlim value was close to the wP value, but lower than that obtained after mechanical dewatering in the wastewater treatment plants (WWTPs). It appears thus that the totality of the mechanically extractable water is not extracted by the WWTPs mechanical dewatering devices. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Treatment of domestic sewage, via physicochemical and/or biological processes, produces large quantities of by-products, known as residual sludge. Sludge treatments are based on stabilization processes to block or reduce biological activity and on dewatering processes to reduce sludge volume for storage, pumping, transportation and handling. Sorensen [1] reports that 30–50% of annual treatment operating costs relate to the sludge dewatering stage. A good knowledge of physical properties allows the prediction of sludge behavior during all treatments, which it is necessary to apply to sludge handling and disposal [2,3]. The dewatering techniques used basically consist of mechanical separation processes (centrifuge, filter press or belt filter) which ensure the partial drainage of the water mobilized by mechanical action [4]. Sludge filterability must be characterized in order to choose and design the filtration unit but also to optimize the cycles of filtration operation and filter regeneration and/or washing. Capillary suction time (CST) and specific resistance to filtration, quantified in a compression-permeability (C-P) cell, are the most currently used parameters to define sludge filterability [5,6]. Limit water content, corresponding to the minimum water content of the sludge cake obtained after a constant pressure filtration, is another important parameter that can be useful to describe and forecast the efficiency of the filtration process or a purely mechanical action.
Many authors have studied the role of the nature and composition of sludge in regard to the specific resistance value [7–9] but rarely in terms of the limit water content value, even though this mechanical dewatering limit undoubtedly depends on sludge composition. Physical consistency is an important parameter in sewage sludge characterization as it affects all treatment, utilization and disposal operations [10]. Basically, despite the lack of studies on this aspect [2,3,11], it is important to evaluate the physical state of sludge and the limits between liquid and paste behaviors (liquid limit) and between solid and paste (plasticity limit), for the purposes of drying [11], storage, transportation, handling and landfilling. The objectives of this study were first to estimate the water limit content and hydro-textural characteristics of different kinds of sludge characterized by their organic matter content. A C-P cell was used to determine the limit water content whereas standard soiltesting procedure soil mechanics trials were run to determine the hydro-textural characteristics based on Atterberg’s limits quantification [12]. According to the results, correlations were established between the obtained parameters and a discussion about dewatering intensification by sludge conditioning is proposed. 2. Materials and methods 2.1. Sludge sample and conditioning
∗ Corresponding author. Tel.: +33 467548558; fax: +33 467548649. E-mail address: [email protected] (C. Wisniewski). 1383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2010.02.017
The studied sludges, sludge A and sludge B, were sampled from two urban wastewater treatment plants, WWTPs, A (5000 Popu-
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Table 1 Sludge characteristics. Parameters
Sludge A
Sludge B
Unit
VS/DS
0.75
0.59
–
Before dewatering wbd Sbd
5000 ± 100 2.00 ± 0.02
3240 ± 65 3.00 ± 0.06
% %
After dewatering wad Sad
645 ± 13 13.0 ± 0.3
295 ± 6 26.0 ± 0.5
% %
lation Equivalent) and B (280,000 Population Equivalent), whose common characteristic was the use of a biological treatment process. The sludge was sampled: (i) after digestion and before the dewatering stage to determine the limit water content, (ii) at the end of the mechanical dewatering stage (belt filter press for both WWTPs) to determine Atterberg’s limits. The two studied sludges differed in their organic content, directly linked by the type of WWTP digestion process (aerobic digestion for WWTP A and anaerobic one for WWTP B). The general characteristics of the sludges are summarized in Table 1. Dry solids (DS), volatile solids (VS) (identified with organic solids), dryness (S) and water content (w) (dry base) were determined as reported by Standard Methods [13]. All experiments were completed a week after sampling in order to minimize changes in sludge characteristics due to microbial activity. The sludge was stored at room temperature. The raw sludges were subjected to the addition of mineral material, to obtain a variation of the VS/DS ratio. Sludge was conditioned with mineral material extracted from the sludge and with quick lime (CaO), the most widely used reagent in water treatment applications. The sludge mineral material corresponded to sludge mineral dry solids obtained after the sludge was brought to 550 ◦ C for 2 h in a muffle furnace. The mineral composition of sludge is reported in Table 2. A conventional jar test (100 rpm, 5 min) or manual mixing was used to mix the sludge, liquid and paste, respectively, with the mineral additive. Concerning the addition of quick lime, a specific methodology was chosen according to exothermic reaction time [14]. The lime and sludge mixture was maintained under agitation for 24 h to reach thermodynamic equilibrium (Fig. 1).
Fig. 1. Temperature evolution of the sludge and lime mixture vs. time.
In such a diagram, the filtration phase is linear and corresponds to cake formation; the second part, where linearity is no longer observed, represents the expression phase. The first linear plot is classically used to calculate the sludge specific resistance to filtration, ˛, according to Ruth’s equation, based on Darcy law [15]: 1 × W∗ × ˛ t ×V + = V Q0 2 × P × A2
Ruth’s equation is established in the case of incompressible materials and supposes a constant porosity throughout the cake thickness. It also assumes that the pressure drop across the cake and the filtrate medium is constant, as well as ˛. In the case of sludge (considered as compressible material), the hypothesis that the filtration cake does not present structural heterogeneities throughout its thickness, and during the running test, is debatable. Recent studies have proposed ameliorated methodologies or complementary trials to describe the local structure of particle deposits and validate the theoretical model and its hypothesis [16–18]. However, recent other publications have presented and exploited the results of ˛, obtained with this global approach; this approach is designed
2.2. Sludge characterization 2.2.1. Limit water content determination A conventional compression-permeability (C-P) cell was used to quantify the limit water content, wlim . The C-P cell, described in Fig. 2, consisted of a 0.42 L pressurized cell. The plane membranes used for filtration experiments were cotton cellulose membranes of 70 mm in diameter and 10 m in pore size. The cell was pressurized using a pneumatic piston submitted to defined gas (N2 ) pressure. The conventional methodology consists in filtering a defined volume of sludge under a constant pressure P. Sludge dewatering is classically described using the ratio V/t versus V (where V is the filter collected after a time t from the start of the test).
Table 2 Composition of the sludge mineral fraction. Element
Mg
Ca
P
Al
Si
Fe
Unit
Sludge A Sludge B
0.95 1.33
9.92 8.74
5.47 6.30
2.75 3.37
4.42 5.23
11.67 8.63
Atomic % Atomic %
(1)
Fig. 2. Schematic of the compression-permeability cell.
T. Ruiz et al. / Separation and Purification Technology 72 (2010) 275–281
Fig. 4. log(˛) vs. log(P).
Fig. 3. t/V vs. V (sludge A).
to provide a good approximation of the average specific resistance of the filtration cake, and ˛ is always considered to be a valuable parameter to characterize sludge filterability [19–21]. By measuring specific resistances to filtration under several pressures P and drawing the curve log(˛) versus log(P) the sludge compressibility coefficient, s, slope of the linear plot can be determined. This methodology is an indirect way of evaluating the deposit structure, as it is obtained from global measurements and gives access only to global values. In this study, ˛ and s were quantified only for the raw sludge. At the end of each filtration test, and for different pressures (ranged from 0.05 to 1.5 MPa), the water content of the sludge cake was measured (wfin ). It is shown that this final water content decreased with pressure to a constant value corresponding to the limit water content, wlim . 2.2.2. Hydro-textural characteristics determination The textural characterization of the pasty sludge obtained after mechanical dewatering, consisted of quantifying the water contents corresponding to the transitions (i) between solid and plastic states (plastic limit, wP ) and (ii) between plastic and liquid states (liquid limit, wL ). These transitional water contents, or so-called Atterberg’s consistency limits, were determined by standardized soil mechanics trials [11]: (i) wL was obtained by the use of a cone penetrometer and corresponded to a cone depression in the sludge of 17 mm and (ii) wP corresponded to the fissure of sludge roller of 3.0 ± 0.5 mm in diameter and 10 cm in length. The plasticity index, IP , corresponding to (wL –wP ), was quantified. This index, representing the range between plastic and liquid limits, determines the ability of the sludge to change from semisolid to liquid state, resulting in a significant decrease of cohesion. The IP index is also used to situate the consistency state of the sludge on the plasticity chart (Casagrande diagram), commonly used in soil mechanics [12]. On this chart, representing IP versus wL , shows the different consistency domains and the location of some types of soils.
3. Results 3.1. Mechanical dewatering aptitude and hydro-textural characteristics of the raw sludges 3.1.1. Mechanical dewatering aptitude Fig. 3 presents the filtration curve t/V versus V for sludge A and for two defined pressures (0.6 and 1.5 MPa). Whatever the pressure P, the curve presents two main sections:
277
(i) A straight section, corresponding to the filtration stage. The linear plot is used to calculate sludge specific resistance, ˛, according to Eq. (1). (ii) An asymptotic section over which an increase in filtration time yields no further increase in the volume of filtrate. The final water content wfin , defined as the cake water content obtained when there is no further filtrate run-off, was quantified for different pressures in order to estimate wlim . It is general practiced to establish a comparison between sludges using ˛0.05 MPa as a base line [22]. For urban residual sludge, ˛0.05 MPa ranges from 5 × 1010 to 2 × 1011 m kg−1 [23,24]. In these experiments, ˛0.05 MPa was equal to 1.2 × 1011 and 1.7 × 1012 m kg−1 for sludges A and B, respectively. The compressibility coefficient, s, was defined graphically by the slope of the straight line log(˛) versus log(P) (Fig. 4). The compressibility coefficients of sludges A and B were 0.98 and 0.77, respectively (Table 3). The compressibility coefficient, s, measures the intensity of the decreasing cake permeability under increasing pressures and consequently estimates the benefit of high-pressure filtration operations. In this experiment, s values were lower than 1, which signified that applying high pressures can favor high filtration rate. It was noted that the compressibility coefficient of sludge B was clearly lower than that of sludge A. It is important to note the difficulty in comparing these compressibility values with those found in the literature; indeed, the ways of evaluating the effect of pressure on the deposit structure are varied and not directly comparable. Fig. 5 presents the evolution with the pressure of the final water content wfin . The figure shows that the final water content decreased with pressure down to a constant value corresponding to the limit water content, wlim . This value is attempted for a pressure close to 0.8 MPa, whatever the sludge. wlim of sludges A and B were equal to 133% and 92%, respectively. According to various authors, a mechanical dewatering process can only remove the free and interstitial water content from sludge and for that reason, the water remaining in the sludge after filtration could be considered Table 3 Raw sludge dewatering aptitude and hydro-textural characteristics. Parameters
Unit
Sludge A
Sludge B
VS/DS wad ˛0.05 MPa s wlim wL wP IP
– % m kg−1 – % % % %
0.75 645 ± 13 1.2 × 1011 0.98 133 ± 2 538 ± 5 137 ± 5 401 ± 5
0.59 295 ± 6 1.7 × 1012 0.77 92 ± 2 240 ± 5 100 ± 5 140 ± 5
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Fig. 5. wfin vs. P.
as bound water [25–27]. Note that these water content values corresponded to dryness values close to 43% and 52% for sludges A and B, respectively. These results showed that sludges A and B did not present the same mechanical dewatering aptitude. Organic matter content seemed to be an essential parameter in mechanical dewatering efficiency. If the higher organic sludge (sludge A) presented lower specific resistance to filtration, the sludge appeared to be more compressible and the dryness obtained after filtration was significantly lower than sludge B. A great difference is observed between the value of wlim and the value of the water content after mechanical dewatering, wad . It appears that the mechanical dewatering implemented in the WWTP does not extract the totality of the mechanically extractable water. 3.1.2. Hydro-textural characteristics Atterberg’s limits could provide information regarding wP , wL and IP . Table 3 presents the values of wP , wL and IP obtained for sludges A and B. It can be noted that the water contents of the two sludges after mechanical dewatering (wad ) were considerably close to the liquid limits wL . Moreover, the wP values corresponded to wlim values. This relation between water contents corresponding to performances of mechanical dehydration and consistency limits, suggests that it may be judicious to consider the rheological behavior of the filtration cake to estimate the aptitude for such dewatering. While several works [15,28] reveal the importance of taking into account the rheological behavior of filtration cake compressibility, the change of consistency with the water content of the porous medium thus formed, has not been dealt with. The measured plasticity index places the sludges on the plasticity chart at the level of highly organic plastic soils such as fibrous clays and peat [12]. In this sense, IP value was seen to be high for sludge B, which is more organic than sludge A. 3.2. Mechanical dewatering aptitude and hydro-textural characteristics of the conditioned sludges 3.2.1. Mechanical dewatering aptitude Only the limit water content was quantified in order to compare this value to transition water contents. Fig. 6 presents the evolution of wlim with the VS/DS ratio for conditioning with sludge mineral material (a) and lime (b). It appears that the more organic the sludge, the higher the limit water content wlim increase with VS/DS ratio, regardless of the nature of the additive. This evolution can be described by the
Fig. 6. wlim vs. VS/DS: (a) sludge conditioned with mineral material and (b) sludge conditioned with quick lime.
following linear equation (2): wlim = a
VS +b DS
(2)
For experiments adding sludge mineral, a difference in slope value is noticed around a VS/DS ratio of 0.15. No explanation can be given of this sudden change of behavior, and complementary experiments around this VS/DS value need to be done. However, residual sludge with such low VS/DS ratio is not conventionally encountered in water treatment plants and the knowledge of the sludge behavior is such a range of VS/DS values is of limited interest. Table 4 gives the values of constants a and b for each sludge and type of additive. Simple mineralization by adding rough mineral material leads to a significant reduction of wlim . For a given VS/DS ratio, we observed that the combination of the chemical and biological effects of lime on the sludge involve higher limit water content. The water content decrease with the VS/DS ratio can be explained by the following relation: w=
VS Mw Mw = × DS DS VS
(3)
For a fixed organic matter (VS), the addition of mineral material (reduction in VS/DS ratio) would decrease the limit water content linearly if the quantity of water retained by the filtration cake Table 4 Values of the a and b constants (Eq. (2)). Sludge A
Mineral material Lime
VS/DS <0.15 VS/DS >0.15
Sludge B
a
b
R2
a
b
R2
349 56
36 92
0.99 0.94
101 42
39 50
0.96 0.97
117
32
0.97
81
26
0.99
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Table 5 Parameters of the mixture law (Eqs. (4) and (5)). wP org Sludge A Sludge B
80 3
wP min
R2
wL org
wL min
R2
59 49
0.97 0.99
593 470
64 50
0.99 0.98
Table 6 Parameters of the mixture law (Eq. (6)).
Sludge A Sludge B
IP org
IP min
R2
513 449
5 9
0.99 0.99
following equations:
wP =
VS VS wP org + 1 − DS DS
wL =
VS org VS + 1− wL DS DS
wP min
(4)
wL min
(5)
The slope and origin ordinate of the obtained curves allow the quantification of worg and wmin , given in Table 5. In our VS/DS range, the mixture laws were validated. The plastic and liquid limits of the mineral matter are in the range of those usually measured for clays [12]. The total data obtained from the different sludges show that the higher the organic fraction, the more water retained, with a consistency that remains plastic. The plasticity index represents the plastic range and increases with VS/DS ratio (Fig. 8), by a same mixture law: Fig. 7. win , wL and wP vs. VS/DS: (a) sludge conditioned with mineral material and (b) sludge conditioned with quick lime.
remained constant. The difference between the water contents given by Eq. (3) and obtained in experiments (Fig. 6) shows that the cake retains additional water mass (Mw ). This quantity depends on the nature and proportions of the conditioning, which plays an important role on the limit dryness obtained. 3.2.2. Hydro-textural characteristics Consistency limits are highly dependent on sludge composition. For a mixture of sludge and mineral matter, Figs. 7 and 8 show the evolution of wP , wL and IP with VS/DS ratio >0.15. We note that wP and wL evolutions versus VS/DS ratio are linear for the tested VS/DS range. If we suppose that the relations of plastic and liquid limits to VS/DS ratio can be represented by mixture laws, we obtain the
Fig. 8. Consistency vs. VS/DS.
IP =
VS org VS IP + 1− DS DS
IP min
(6)
where IP org represents the plasticity index of the organic fraction and IP min represents the plasticity index of the mineral fraction (Table 6). Fig. 9 shows that the plasticity index is slightly dependent on the nature of the sludge. This result, already obtained for various raw sludges [29], has been extended here to the case of conditioned sludges. 4. Discussion The experimental result reveals that the more organic the sludge, the higher the limit water content (Fig. 6). Aptitude for mechanical dewatering is thus lower. However, its plastic range is wider, meaning a greater capacity of deformation (Fig. 9). Fig. 10 shows the Casagrande diagram, plotting the different mixtures and sludges studied. As can be seen, the values for most of these mate-
Fig. 9. Plasticity index vs. VS/DS.
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ical behavior with water and its intrinsic consistency limits. The proportions of the mixture can be deducted from the various presented relations according to expected performances. 5. Conclusion
Fig. 10. Casagrande diagram.
rials plot above the A-line, in the zone typically characteristic of highly compressible materials, with high plasticity at the limit of inorganic and organic clays. It can be noticed that the mixture law, used in Eqs. (4)–(6), is verified. Thus a high organic sludge is less mechanically dewatered than a mineral conditioned sludge. This point shows that the plastic range is not a sufficient criterion to characterize compressibility. The liquid limit marking the transition between a liquid state and a plastic state is very close to the water content obtained after mechanical dewatering from WWTP (Fig. 11). This result shows the influence of the consistency state for this kind of dewatering in industrial conditions. Indeed, during centrifugation or press filtration, sludge is flowing in the equipment used. The sludge undergoes stresses that are transmitted to interstitial water and ensure its drainage. When consistency becomes plastic, the flow is not comparable with that of a suspension. This consistency probably does not allow any further water drainage. This point seems to corroborate the results obtained with the C-P cell. In this case, sludge does not flow and all the transmitted stresses are used for the drainage of interstitial water and compression of the cake. The limit water content obtained under these conditions is very close to the plastic limit (Fig. 11). The process efficiency is related to the transition in consistency from a plastic state to a solid state. This analysis confers on conditioning a role that is strongly related to its capacity to modify consistency. The additional amount of water retained by the physicochemical interactions with the added matter, can be controlled by the choice of a product that is more or less chemically reactive or hydrophilic. The type of mineral material to be added can be chosen according to its physicochem-
From this study, it can be concluded that the hydro-textural characteristics of sludge are significantly correlated with mechanical dewatering aptitude. The influence of mineral material conditioning on the mechanical dewatering of two waste sludges is shown. The hydro-textural approach confirms the validity and interest of standardized consistency tests (Atterberg’s tests). These tests taken from soil science constitute complementary characterizations to determine the water contents characteristic of this kind of dewatering. The plasticity index is close to the water content obtained after traditional mechanical dewatering. The plasticity index correlates very closely with the limit water content. By measuring these water contents, associated to the correlation established between the plasticity index and VS/DS ratio, it is possible to examine the influence of conditioning on the performances of mechanical dewatering. Nomenclature
A DS IP M P Q s S t V VD w W*
membrane surface area [L2 ] dry solids [M] plasticity index [%] mass [M] transmembrane pressure [M L−1 T−2 ] flow rate [L3 T−1 ] compressibility coefficient dryness time [T] volume of filtrate [L3 ] volatile solids [M] water content [M M−1 ms ] dry solids deposited per [M L−3 ]
Greek letters ˛ specific resistance to filtration [L M−1 ] dynamic viscosity of filtrate [M L−1 T−1 ] Subscripts/superscripts bd before WWTP dewatering ad after WWTP dewatering fin final in after conditioning L liquid lim limit min mineral 0 initial time of filtration org organic p plastic w water References
Fig. 11. wP and wlim vs. VS/DS.
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