Effect of Thermal Hydrolysis on Rheological Behavior of Municipal

Article pubs.acs.org/IECR

Effect of Thermal Hydrolysis on Rheological Behavior of Municipal Sludge Guohong Feng,†,‡ Liyan Liu,‡ and Wei Tan*,‡ †

School of Environment & Safety, Taiyuan University of Science & Technology, Taiyuan, China School of Chemical Engineering & Technology, Tianjin University, Tianjin, China

‡

S Supporting Information *

ABSTRACT: Properly understanding of sludge rheological properties is important for designing of pumping and translating. Effect of thermal hydrolysis on rheological properties of municipal sludge was studied using a rheometer (DHR-2) with concentric cylinder geometry. Test results indicated both raw and thermal treated sludge displayed non-Newtonian rheological behavior with shear thinning, exhibiting thixotropic properties and viscoelasticity. The Herschel−Bulkley model could represent their flow behavior more accurately than other models. However, for the raw sludge, as solid content reached 120 g/L, fluidity disappeared, and all the rheological models could not describe it accurately. After thermal hydrolysis, the consistency index (k) decreased significantly, while the flow index (n) increased, suggesting that thermal treated sludge was much closer to the Newtonian fluid compared to the raw sludge. Both raw and treated sludge exhibited strong dependence on solid content and temperature. Correlations between solid content, temperature, and viscosity were expressed by an exponential equation and an Arrhenius type equation, respectively. Analysis of thixotropic properties illustrated that evolution of viscosity over time could be expressed by a first-order (solid content lower than 100 g/L) and a second-order thixotropic kinetic equation (solid content higher than 100 g/L), respectively, for raw sludge. For treated sludge, it could be simulated by a first-order thixotropic kinetic equation. Furthermore, the dynamic test indicated viscoelasticity of treated sludge decreased remarkably. For treated sludge, as the solid content was larger than 120 g/L, viscoelastic parameters were linearly correlated with logarithm frequency.

1. INTRODUCTION Rheology plays an important role in characterizing sludge hydrodynamic properties and strongly affects almost all sludge treatment, utilization, and disposal operations, including mixing, pumping, translating, filtration, drying, and land filling.1,2 Correctly predicting flow behaviors of these engineering hydrodynamic processes requires accurate knowledge on sludge rheology. However, sludge rheology is difficult to describe due to its complex composition, which is made of intricate hyphae of microorganisms, colloids, or large flocs. Also its rheological behavior is highly dependent on its intrinsic properties and treatment processes.3,4 Rheological measurements could provide interesting information concerning sludge characterization and floc structure. In rheology, there are two types of measurements: steady and dynamic tests. Both tests could provide complete information about internal structure of suspension complementarily. Steady tests are used to obtain information on viscous behavior, such as flow model, thixotropy, while dynamic tests are usually carried out to measure viscoelastic properties.5,6 Generally, wastewater sludge is known as a non-Newtonian fluid and usually exhibits thixotropy, solid behavior within shorter time frames and liquid behavior over longer durations.7−10 The general models used to describe rheological behavior of sludge, are Bingham (eq 1), Power law (eq 2), Herschel−Bulkley (eq 3), and Casson models (eq 4). τ = τ0 + kγ ̇

(1)

τ = kγ ṅ

(2) © 2014 American Chemical Society

τ = τ0 + kγ ṅ

(3)

τ 0.5 = τ0 0.5 + k0.5γ 0.5 ̇

(4)

where, τ is shear stress, τ0 is yield stress, γ ̇ is shear rate, k is consistency coefficient, and n is flow index.11,12 In these models, k represents limit viscosity of fluid at an infinite shear rate, while flow index n varies from 0 to 1. The fluid properties deviate Newtonian fluid with the increase of the flow index. Viscosity was shown to depend on sludge origin, solid content, and soluble substances (such as extracellular polymeric substances (EPS)). EPS is one of the main components of sludge and has a strong affinity with water.13 However, sludge rheology could be modified by application of pretreatment methods which were usually used to improve the dewatering ability of sludge, including chemical conditioning, thermal hydrolysis, and mechanical disintegration.3,14,15 Ruiz-Hernando et al. reported effect of ultrasonic, thermal, and alkali conditioning on sludge rheology16 and indicated that the three treatment methods could significantly reduce sludge viscosity and thixotropy. Chen et al.17 showed that addition of polymer coagulants could significantly influence sludge viscoelasticity by small-amplitude-oscillatory-shearing tests. Also a few studies have explored the effect of lower temperature (lower than 105 °C) on sludge rheological characteristics.18−20 Received: Revised: Accepted: Published: 11185

April 10, 2014 June 7, 2014 June 12, 2014 June 12, 2014 dx.doi.org/10.1021/ie501488q | Ind. Eng. Chem. Res. 2014, 53, 11185−11192

Industrial & Engineering Chemistry Research

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Table 1. Physical Characteristics of Raw Sludge and Treated Sludgea

a

sludge types

TSS (%)

VSS (%)

COD (g/L)

D (μm)

S (m2/g)

calorific value (MJ/kg)

pH

raw sludge treated sludge

21.3 ± 0.11 19.3 ± 0.12

10.2 ± 0.09 7.9 ± 0.10

1.9 ± 0.08 35 ± 1.75

52.1 ± 0.9 30.6 ± 0.8

0.50 ± 0.01 0.90 ± 0.01

10.8 ± 0.4 10.4 ± 0.4

7.50 ± 0.04 7.3 ± 0.04

The deviation refers to standard deviation; D, the mean volume diameter of particles; S, specific surface area of particle.

Verma et al.21 reported the variation of sludge pseudoplasticity as solid concentration ranged from 10 g/L to 50 g/L after thermal-alkaline treatment (temperature, 121 °C; pH, 10.25). However, they did not refer to the change of viscoelasticity and thixotropy. Furthermore, the solid content of sludge was usually less than 80 g/L.1,22−25 It has been known for many years that thermal hydrolysis (holding time, about 1 h; temperature, 170 °C; pressure, 2 MPa) could significantly improve sludge dewaterability.26−29 However, rheology of the sludge (treated by high temperature) has been rarely studied. Therefore, in this research, our interest is to perform rheological characteristics of municipal sludge before and after thermal hydrolysis, and the solid content is extended to 187 g/L. Sludge rheological properties, obtained by steady and dynamic tests, including flow behavior, thixotropy, and viscoelasticity, not only could provide theoretical basis for designing heat exchangers, transport facilities in municipal sludge treatment plant, but also has an important effect on sludge dewatering.

diameter of 0.6 mm to diminish the error caused by large particles. After loaded, raw sludge was presheared at 400 s−1for 10 min to eliminate the effect of aging history of the sample and then left at rest for 1 min. This procedure allowed us to obtain reproducible results.1,18 Steady and dynamic tests were carried out to analyze rheological behaviors of the raw and treated sludge. 2.3.1. Steady Test. Steady test, also called flow test, was used for studying viscosity, thixotroptic characteristics, and the relationship between shear rate and shear stress (rheogram) by shear rate sweep and thixotropy tests. (1) Shear rate sweep: shear rate logarithmically increased from 0.01 to 400 s−1, in order to obtain the viscosity and rheogram of sludge. (2) Thixotropy test: several methods could be used for investigating thixotropy of the material, including hysteresis technique, stepwise changes in shear rate, and time sweep at a constant shear rate. The hysteresis loop method is fraught with difficulties due to the presence of particles and floc (in the sludge). Sedimentation, time effects, shear history prior to experiment start, maximum shear rate, and acceleration rate all could introduce errors.30 To avoid the drawbacks of the hysteresis loop method, the latter two methods were employed. For time sweep, the raw and treated sludge with different solid contents were sheared at 100 s−1 until the steady states were reached, to investigate the dependence of viscosity on time. For the last test (stepwise changes in shear rate): shear rate started from 0.1 s−1, and stepwise increased to 1 s−1, 10 s−1,100 s−1, and 400 s−1. Each shear rate was maintained for 2 min. When the shear rate suddenly stepped up, the transient variation of viscosity reflected the changes of sludge microstructure. 2.3.2. Dynamic Test. Dynamic rheological test (also referred to as “dynamic mechanical test” or “oscillatory test”) has been proved to be a better method for the viscoelastic measurement of sludge, by applying sinusoidal stress (τ = τ0 sin ωt; τ0, amplitude of applied stress; ω, angular frequency (rad/s)).6 As shear stress was exerted, viscoelastic material could respond with a combination of fluid and solid behaviors. Dynamic test, including shear strain sweep and frequency sweep, could differentiate these fluid and solid responses. In this study, shear strain sweep tests were carried out to determine linear viscoelastic regions at a constant frequency of 1 Hz, while frequency sweep tests could reveal relationships between viscoelastic parameters and frequency at linear viscoelastic regions for raw and treated sludge.

2. MATERIALS AND METHODS 2.1. Sludge Samples. Activated sludge sample was taken from the discharge of a horizontal spiral filter centrifuge in a municipal wastewater treatment plant in Zibo, China. Primary characteristics of the sludge (such as total suspended solid content (TSS), volatile suspended solid content (VSS), and pH) were measured by a series of tests according to CJ/T 2212005 (China standard for municipal sludge analysis) and are listed in Table 1. Afterward, the activated sludge was stored at 4 °C to minimize impact of biological activity. 2.2. Thermal Treatment Test. A sludge sample with a volume of 1.4 L was batch treated in a high-pressure reactor (effective volume, 2L). The reaction was carried out with the nitrogen protection under a pressure of 2 MPa to avoid water evaporation and reaction. Then, the sludge was treated at 170 °C for 60 min, and the treatment process was named thermal hydrolysis. During thermal hydrolysis, the sludge was stirred continuously at 200 rpm in order to ensure uniform heat in the reactor. As thermal hydrolysis was finished, the sludge was cooled by freshwater inside a tube in the reactor, and then was taken out of. The sludge, treated by thermal hydrolysis, was called treated sludge and was used for the following rheological tests, while the sludge without thermal treatment was named raw sludge. The primary characteristics of treated sludge are also listed in Table 1. The raw and treated sludge were diluted to different solid contents with distilled water for the following tests. 2.3. Rheological Test. Rheological properties of the raw and treated sludge were investigated using a commercial rheometer (DHR-2, from TA Instruments) equipped with concentric cylinder geometry (cup diameter, 30.39 mm; bob diameter, 27.98 mm; length, 41.90 mm). Measurement temperature (varied from 10 to 60 °C) was controlled by a Peltier system. To avoid water evaporation during measurement, a plastic ring was fitted around the measuring geometry. Before each test, sludge was screened by a sieve with pore

3. RESULTS AND DISCUSSIONS Rehological behaviors of sludge contain four types: liquid characteristic, usually determined by viscosity; solid characteristic (evaluated by yield stress or storage modulus); thixptroptic characteristic; and viscoelastic characteristic (described by storage and loss modulus).31 3.1. Effect of Thermal Hydrolysis on Viscosity of Sludge. Sludge viscosity represented liquid characteristic and was a measure of resistance generated by the movement between two adjacent layers of a fluid. Theoretically, viscosity, determined by ratio of shear stress to shear rate, reflected 11186

dx.doi.org/10.1021/ie501488q | Ind. Eng. Chem. Res. 2014, 53, 11185−11192


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Figure 1. Rheograms of raw and treated sludge with different solid contents at 20 °C.

Figure 2. Correlations between rheological parameters and solid content for raw and treated sludge at 20 °C.

internal and external interaction forces occurring within the sludge flocs and fluids. 3.1.1. Effect of Solid Content on Viscosity. Solid content was an important factor to affect sludge rheology. The shear rate at the range of 0.01−400 s−1 was used to study the overall rheological history from low to high shear. As shown in Figure 1, the treated sludge still presented shear-thinning behavior and non-Newtonian characteristics gradually increased with the increase of solid content. In order to accurately study the difference between the raw and treated sludge, various rheological models (such as Bingham, Power law, Herschel−Bulkley models) were employed to fit these rheograms. Results showed that Herschel−Bulkley model was more appropriate than other models for the two types of sludge with different solid contents, which was in accordance with the previous works.23,32,33 Furthermore, the raw sludge of 120 g/L exhibited little fluidity, and there were no appropriate rheology model to accurately describe it. It was interesting that the Newtonian fluid model was found to be quite exact for the treated sludge expect for that of 187 g/L. As shown in Figure 2a, for the raw sludge, as the solid content increased from 20 to 120 g/L, consistency index (k) increased from 0.013 to 12.18 Pa s, whereas flow index (n) decreased from 0.69 to 0.28, which indicated that nonNewtonian flow characteristics of sludge were strengthened at higher solid content. The higher the solid content, the more viscous and less flowable was the fluid. For the treated sludge, k increased from 0.003 to 0.206 Pa s, and n decreased from 1 to

0.56, respectively, when the solid content increased from 54 to 187 g/L. Also, the correlation between k and solid content could be expressed by an exponential equation (eqs 5 and 6), while there was no obvious correlation between flow index and solid content. k = 0.108 exp(0.025φ)

(raw sludge, R2 = 0.972)

(5)

k = 2.39 × 10−6 exp(0.061φ) (treated sludge, R2 = 0.999)

(6)

In addition, for the sludge with solid content of 100 g/L, k decreased significantly from 5.9 to 0.0039 Pa s, and n increased from 0.3 to 1 after thermal hydrolysis, which clearly indicated that treated sludge was much closer to the Newtonian fluid. For the raw sludge, electrostatic and gel-like interactions (attributed to EPS) were closely related to non-Newtonian behavior. Hydrolysis of organic substances led to destruction of macromolecules colloidal properties. As a result, organic substances lost their natural shapes and sometimes precipitated irreversibly out of solution in an inactive form,34 corresponding to better fluidity. Figure 2b illustrated effects of solid content on viscosity (η) of the raw and treated sludge at a shear rate of 100 s−1 and indicated that the viscosity of treated sludge was much smaller than that of raw sludge. For the raw sludge, when solid content was larger than 80 g/L, the viscosity increased dramatically, while the critical value was 150 g/L for the treated sludge. At a high volume concentration of particles, a network of particles 11187



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Figure 3. Effect of temperature on viscosity at the shear rate of 100 s−1.

viscosity with time when a high shear rate is applied on a sample, which is termed “shear rejuvenation”. However, in general the viscosity increases in time at low or zero shear rates (termed “aging”).30 Municipal sludge mainly consists of water and organic particles. Strong colloidal forces tend to rebuild the solid structure (aging), and hydrodynamic forces tend to maintain the solid structure broken (shear rejuvenation). These phenomenons are closed to mechanisms of flocculation and deflocculation.38 Municipal sludge often presents thixotropic properties, and its destruction and reconstruction are not instantaneous processes. The relative rate of viscosity change, calculated by the following formula: RV = (ηs − ηr)/ηs, is shown in Figure 4,

was formed due to the rapidly increased interactions and the floc size of sludge may be larger and closer to each other.35 Consequently, flow resistance of sludge increased, corresponding to larger viscosity. The relationship between η and solid content (φ) at the shear rate of 100 s−1 can be represented by the following expressions (analyzed by regression analysis using experimental data): η = 2.75 exp(0.046φ) η = 0.15 exp(0.028φ)

(raw sludge, R2 = 0.999)

(7)

2

(treated sludge, R = 0.992) (8)

The extremely high value of R2 indicated that the correlation between viscosity and solid content could be accurately expressed by the exponential equation, which was in agreement with some previous works.3,5,36 The higher exponent value indicated that viscosity of the raw sludge changed much faster than that of treated sludge. Temperature was another important factor affecting the viscosity of sludge, and the correlation between temperature and viscosity is shown in Figure 3. Thermal motion of particles was more violent at higher temperature, and then network strength among the particles was weakened, resulting in a decrease of viscosity.37 The influence of temperature on viscosity can be described well by an Arrhenius type equation:

η = A e Ea / RT

(9)

where A is the pre-exponential factor; T is absolute temperature (K); R is the universal gas constant (R = 8.3145 × 10−3 kJ K−1 mol−1), and Ea is the activation energy of flow (kJ mol−1). Table 2 lists the regression values of A and Ea for the raw and

Figure 4. Relative rate of viscosity change at stepwise shear rate.

when the shear rate stepwise increased, where RV is relative rate of viscosity change, ηs is instantaneous viscosity at initial shear, and ηr is the viscosity at a certain shear rate. After thermal hydrolysis, the relative rate of viscosity change declined, indicating that the treated sludge exhibited a more stable state. As the shear rate increased from 0.1 s−1 to 1 s−1, viscosities of raw sludge (120 g/L) and treated sludge (187 g/ L) decreased by 86.8% and 74.5% and then decreased by 97.4% and 94.5% at 10 s−1, respectively, which illustrated that floc structure of the two kinds of sludge was broken down at initial shearing and the destruction degree of raw sludge floc was greater. Sudden changes of shear rate caused instantaneous variation of aggregate shape and orientation but noninstantaneous changes in aggregate size.39 Therefore, evolution of viscosities of the raw and treated sludge over time was explored at a

Table 2. Regression Model Parameters of Equation 9 sludge

R2

A(mPa s)

Ea(kJ mol−1)

raw sludge treated sludge

0.992 0.997

0.162 11.11

5.39 6.62

treated sludge both with solid content of 80 g/L. The high values of R2 (0.992 and 0.997 for raw and treated sludge) suggested that the Arrhenius equation could accurately describe the relationships between viscosity and temperature. Also the activation energy of the two kinds of sludge was approximate, implying that destruction of the sludge network slightly modified the dependence of sludge viscosity on temperature. 3.2. Effect of Thermal Treatment on the Thixotropy. Thixotropy should be defined as the continuous decrease of 11188



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Figure 5. Evolution of viscosity with shear time at shear rate of 100 s−1.

Table 3. Regression Model Parameters in Thixotropic Kinetic Equation sludge types

treated sludge

solid content(g/L)

a

187

150

120

raw sludge 100

80

54

120

80

54

eq 11

R2 a b

0.91 8.91 26.2

0.997 5.34 4.66

0.979 3.22 2.18

0.998 2.57 0.53

0.982 2.16 0.31

0.92 1.4 0.38

0.80 293 248

0.95 155 46.6

100

0.99 81.2 16.4

0.99 23.5 2.18

eq 10

R2 K

0.91 9 × 10−5

0.82 3 × 10−4

0.82 0.001

0.85 3 × 10−3

−a

−a

0.97 8 × 10−6

0.92 3 × 10−5

−a

−a

The − means invalid regression.

constant shear rate of 100 s−1 (shown in Figure 5). Under shearing, floc structure of sludge was broken down gradually, and then viscosity decreased with time, causing flow acceleration due to an avalanche effect.40 Notably, the time for viscosity of treated sludge to achieve a stable value was shorter than that of raw sludge. Labanda et al.41,42 reported that evolution of viscosity over time could be simulated by the first-order or second-order thixotropic kinetic equation. In our paper, a second-order thixotropic kinetic equation (eq 10) and a referred first-order kinetic equation (eq 11) were employed to fit viscosity evolution. η = ηe +

for the raw sludge was quicker, maybe due to its looser floc structure. Table 3 lists regression model parameters of the thixotropic kinetic equation. For the raw sludge, when solid content was less than 100 g/L, the relationship between viscosity and time could be expressed by a referred first-order equation accurately. However, it was represented by a second-order thixotropic kinetic equation for the sludge with solid content of higher than 100 g/L. For the treated sludge, the relationship between viscosity and time could be simulated by a referred first-order equation, further indicating that thermal hydrolysis altered the floc structure, and with the increase of solid content, a secondorder thixotropic kinetic equation may be more accurate. In addition, the reduction degree of viscosity for the raw sludge was much larger than that of treated sludge as shown in Figure 6 for all samples, which suggested that the thixotropy of the treated sludge was weaker. Where, relative rate of viscosity change was calculated by the expression of (ηs − ηe)/ηs. 3.3. Effect of Thermal Treatment on Viscoelasticity. Viscoelastic material means that as applied stress is low, it will

ηs − ηe 1 + K (ηs − ηe)

⎛ t⎞ η = a × exp⎜ − ⎟ + b ⎝ ts ⎠

(10)

(11)

where η is observed instantaneous viscosity at different shearing time, ηs is the viscosity at initial shearing, t is the shearing time, ts is thixotropic time or equilibrium time, K is kinetic constant, and ηe is equilibrium viscosity at the applied shear rate. The value of (η − ηe)/ts decreased remarkably after thermal hydrolysis, further indicating that the treated sludge was more stable than the raw sludge. In raw sludge, EPS produced during metabolism and autolysis of sludge biomass could glue together to create a three-dimensional matrix by van der Waals forces, hydrophobic interactions, and bridging via electrostatic binding with bivalent cations. Therefore, the floc structure and floc functional integrity was stronger. However, EPS and other organic substances were destroyed during thermal hydrolysis, leading to the breakup of floc structure and the reduction of attractive forces among particles. As a result, the raw sludge needed more energy and more time to cleave the network for achieving a stable state. The change rate of viscosity over time

Figure 6. Relative rate of viscosity change at shear rate of 100 s−1. 11189



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Figure 7. Stress sweep for raw and treated sludge at 1 Hz.

Figure 8. Frequency sweep for raw and treated sludge at 1 Hz.

content increased from 80 to 120 g/L, G′ increased from 16.6 to 228.9 Pa, which indicated that colloidal forces and network strength of floc were stronger at higher solid content. Consequently, the linear viscoelastic region was expanded (the critical shear strain: 55% (80 g/L) and 115% (120 g/L)). As shown in Figure 7b, G′ of treated sludge (17 Pa, at solid content of 187 g/L) was much smaller than that of raw sludge (228.9 Pa, at solid content of 120 g/L), indicating that energy stored in the treated sludge structure was less, which may be due to the destruction of EPS and hydrolysis of polysaccharides and proteins after thermal hydrolysis. Also the critical shear strain values were about 3% and 1% for treated sludge with solid content of 187 g/L and 150 g/L, respectively, implying that the linear viscoelastic region shrinked after hydrolysis. Transition from a solid-like to a liquid-like behavior for the raw sludge of 120 g/L was abrupt (the shear strain declined sharply from 33 to 115% as the stress increased from 25 to 34 Pa), while the treated sludge gradually yielded, which meant that the treated sludge was much closer to an equilibrium material. Notably, for the treated sludge, as solid content was less than 80 g/L, G′ < G″ was observed (Figure 7b), implying that the solid-like regime disappeared and the sludge only exhibited viscous behavior. 3.3.2. Frequency Sweep. Figure 8 illustrates the evolution of storage and loss modulus during a frequency sweep in linear viscoelastic regions for raw and treated sludge with different solid contents. For the raw sludge with solid contents of 100 g/ L and 80 g/L, relationships between storage modulus, loss modulus and frequency could be expressed by eqs 12 and 13, respectively. However, for the treated sludge, G′ and G″ could be expressed by the same eq 12. Parameters in equations are

exhibit solid behavior (elastic characteristic). As the applied stress is high, it starts to flow (viscous characteristic).When the applied stress reduces to zero, a partial elastic recovery is observed which may be related to the storage of elastic energy in interparticle bounds.43 Complex modulus (G*), its real part G′ (storage modulus, ratio of elastic stress over strain), and its imaginary part G″ (loss modulus, ratio of viscous stress over strain) are the main parameters to describe sludge viscoelasticity. Complex modulus represents deformation resistance of particles arrangement. The larger the G*, the stronger the deformation resistance. Storage modulus expresses the elastic storage capacity during deformation, while loss modulus, also called viscous modulus, is a measure of dissipation during deformation. Municipal sludge like other kinds of suspensions, such as mayonnaise (an emulsion), shaving foam, and kaoline, exhibits viscoelastic properties, which could be investigated by dynamic test, including shear strain sweep test and frequency sweep test. 3.3.1. Strain Sweep. For the raw and treated sludge with different solid contents, evolution of storage and loss modulus during shear strain sweep at a constant frequency of 1 Hz is shown in Figure 7. For the raw sludge, storage modulus G′ and loss modulus G″ were constant until the shear strain achieved about 30%, and G′ was larger than G″, which suggested that a linear viscoelastic region was observed (Figure 7a). In the linear viscoelastic region, the elastic behavior was dominant until a turning point appeared and then G″ > G′ was obtained. Generally, phenomenon of G′ > G″ is expected for solid and paste. However, municipal sludge also presented G′ > G″ in the linear viscoelastic region, implying that sludge exhibited a gellike structure.44 Furthermore, for the raw sludge, as solid 11190



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Table 4. Regression Parameters in Equations 12 and 13 G′ = a + b ln( f)

raw sludge

G = a + b ln( f)

treated sludge

G″ = a + b × f n 100(g/L) R2 a b n

80(g/L)

187(g/L)

150(g/L)

120(g/L)

G′

G″

G′

G″

G′

G″

G′

G″

G′

G″

0.995 78.56 6.50

0.992 10.73 1.65 0.994

0.998 23.29 1.85

0.974 3.07 0.64 1.30

0.980 87.09 20.55

0.985 42.72 10.19

0.970 11.96 2.58

0.991 5.82 1.16

0.990 3.22 0.68

0.871 1.87 0.36

sludge, it could be simulated by a first-order thixotropic kinetic equation. (4) Dynamic tests indicated that viscoelastic behavior of sludge decreased remarkably (storage modulus decreased by 92.5%) after thermal hydrolysis. For raw sludge (solid content higher than 80 g/L), evolution of the storage modulus and loss modulus over sweep frequency was expressed by an exponential equation and a logarithm equation, respectively. For treated sludge, as solid content was larger than 120 g/L, relationships between viscoelastic parameters and sweep frequency were described by a logarithm equation. In order to improve the transport and treatment processes in the wastewater treatment plant, rheological characteristics of sludge are necessary to be properly understood, due to their importance in heat and mass transfer calculations. This research gains a new insight into the rheological behavior of thermal hydrolysis sludge and provides reliable flow property parameters to engineers for accurately designing the translated pipe, pump, and heat exchanger for engineers. Furthermore, these models are especially useful in hydrodynamic optimization and computational fluid dynamic (CFD) simulation. Also the study results are suitable for sludge, which has similar characteristics with municipal sludge. However, further study will be done, by comparing all the rheological behavior of the thermal hydrolysis sludge with other suspensions in a broader range of concentration.

listed in Table 4, which were obtained by curve fitting. The higher value of R2 indicated that these equations fitted these experimental data well. Notably, these results were not consistent with that of a previous work,17 which reported that G′ ∝ f n for several types of sludge (sludge concentration, 7.5− 11.6 g/L; measured by a Haake RS Rheostress rheometer equipped with cone and plate sensor). However, in our study, for the raw sludge with solid content of 54 g/L, expression of G′ ∝ f 0.2 (R2 = 0.989) was obtained in the linear viscoelastic region, indicating that correlation between G′ and frequency could be well expressed by a power-low function at a low solid content. The different results may contribute to the different solid contents (as the solid content increased, interactions among particles changed from colliding to fraction). G′ = a + b ln(f )

(12)

G″ = a + b × f n

(13)

4. CONCLUSION After thermal hydrolysis (temperature, 170 °C; holding time, 60 min), colloidal properties of organic macromolecules in sludge were destructed, and organic substances lost their natural shapes and sometimes precipitated irreversibly out of solution in an inactive form. As a result, sludge rheological behaviors were altered. The influence of thermal hydrolysis on sludge rheology was investigated by steady and dynamic measurements using a rheometer (DHR-2). Rheological behaviors of raw and treated sludge were determined, and the conclusions can be drawn as follows: (1) For the raw and treated sludge, the Herschel−Bulkley model could represent their flow behavior more accurately than other models. Notably, for the raw sludge, as solid content reached 120 g/ L, fluidity disappeared and all the rheological models could not describe it accurately. After thermal hydrolysis, the consistency index (k) of sludge decreased significantly while flow index (n) increased. Also the treated sludge could be expressed by the Newtonian fluid model. (2) Both the raw and treated sludge exhibited strong dependence on solid content and temperature. Correlations between solid content, temperature, and viscosity were expressed by an exponential equation and an Arrhenius type equation, respectively. Thermal hydrolysis altered the relationship between viscosity and solid content significantly and slightly modified the dependence of viscosity on temperature. (3) Analysis of thixotropic properties illustrated that the treated sludge presented a more stable state, and the floc structure was broken down quickly at the initial shear for the two types of sludge. For raw sludge, evolution of viscosity over time could be expressed by a first-order (solid content lower than 100 g/L) and a second-order thixotropic kinetic equation (solid content higher than 100 g/L), respectively. For treated

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ASSOCIATED CONTENT

S Supporting Information *

Tables listing rheological model parameters for raw sludge and treated sludge and parameters in the thixotropic kinetic equation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86 022 2740 8728. Fax: +86 022 2740 8728. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge both the testing support of Shandong Institute of Tianjin University and the provision of municipal sludge from the Zibo Municipal Wastewater Treatment Plant.

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REFERENCES

(1) Baudez, J. C.; Markis, F.; Eshtiaghi, N.; Slatter, P. The rheological behaviour of anaerobic digested sludge. Water Res. 2011, 45, 5675. (2) Spinosa, L.; Lotito, V. A simple method for evaluating sludge yield stress. Adv. Environ. Res. 2003, 7, 655.

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Effect of Thermal Hydrolysis on Rheological Behavior of Municipal

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