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The effects of temperature, organic matter and time-dependency on rheological properties of dry anaerobic digested swine manure
Waste Management xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
The effects of temperature, organic matter and time-dependency on rheological properties of dry anaerobic digested swine manure Gang-Jin Liu, Yi Liu, Zhi-Yong Wang, Yun-Hui Lei, Zi-Ai Chen, Liang-Wei Deng ⇑ Biogas Institute of Ministry of Agriculture, Chengdu 610041, PR China
a r t i c l e
i n f o
Article history: Received 20 May 2014 Accepted 16 December 2014 Available online xxxx Keywords: Dry anaerobic digestion Swine manure Rheology Effect Model
a b s t r a c t An efficient way to avoid the pollution of swine wastewater is the application of dry anaerobic digestion, which needs rheological parameter for stirring and pipe designing. The rheological properties of this kind of sludge have been studied for many decades, yet their effects only solid concentration has been investigated widely. In this paper, the influences of temperature, organic and time-dependency on the efficiency of anaerobic digested swine manure were studied. The viscosity decreased with temperature arranged from 10 to 60 °C which caused increase in protein from 7.18 to 8.49 g/kg. 60 °C can make the digested swine manure with TS from 16.6% to 21.5% reach to the same rheology state. The added peptone decreased the viscosity because of its function of water-reducing admixture and air entraining mixture. Time-dependent experiment showed the decrease of shear stress over time. The first and the second yield stress of dry anaerobic digested swine manure were evaluated through time-dependent model. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Anaerobic digestion would be a good choice for the treatment of swine manure. Considering the post-treatment of digested wastewater from conventional processes needs more energy and investment, dry anaerobic digestion is getting more attention because it has the benefits of smaller reactor and less effluent water (Abouelenien et al., 2009). However, the rheological properties of dry anaerobic digested swine manure need to be known clearly for the design and efficient operation of the pumping systems (Wolny et al., 2008). This kind of sludge shows shear-thinning behavior, plastic behavior, thixotropy and viscoelasticity (Seyssiecq et al., 2003). The rheological properties of these sludges most studies mentioned are low solid concentration, displayed some changeful characteristics like the yield stress and viscosity increase with solid concentration (Baudez et al., 2011), the viscosity decrease with increasing temperature (Mu et al., 2006), the viscosity and shear modulus increase with organic (Khongnakorn et al., 2010), etc. In addition, the property of time dependency of these rheological properties cannot be ignored (Coussot et al., 2002b; Read et al., 2011). The polytropy of rheological behavior indicates that accurate estimation of sludge rheological properties is essential. The rheological properties of low concentration digested sludge and active sludge are widely studied. Because their operations ⇑ Corresponding author at: Biogas Institute of Ministry of Agriculture, No. 13, 4th Section, South Renmin Rd., Chengdu, Sichuan, PR China. Tel./fax: +86 28 85236376. E-mail address: [email protected] (L.-W. Deng).
involving pumping, storage, dewatering, etc. linked to rheological behavior and low concentration sludges are more common. Less early work studied the rheology of semi-dry anaerobic digested municipal solid wastes (Battistoni et al., 1993, 1991). However, it rarely saw the researches about rheological properties of dry anaerobic digested swine manure let alone the effects on its rheological property. Dry anaerobic digested swine manure is heterogeneous and a type of complex mixtures with inorganic matter and organic matter from the physical perspective. Consequently, their rheological properties were affected by many factors, like temperature, solid concentration, surface charge, organic matter (protein and polysaccharide take major effect), bound water, pH etc. However, the main factor seems to be solid concentration (Baudez et al., 2011; Dak et al., 2007; Eshtiaghi et al., 2012; Mu et al., 2006) which leads to a more rigid structure of the sludge network and to an enhancement of the sludge cohesion (Khongnakorn et al., 2010). Actually, temperature and organic concentration still have significant effect on sludge rheological properties (Battistoni et al., 1993; Baudez et al., 2013; Khongnakorn et al., 2010; Mu et al., 2006), especially for high solid content sludge (Battistoni et al., 1993) which was less studied. In addition, time is also an important influencing factor on thixotropy, viscoelasticity and the first/second yield stress. The yield stress is the critical stress of fluid before flow, it should be clearly understood in practical biogas engineering. In this paper, the influences of temperature, organic concentration and time-dependency on the effect of anaerobic digested swine manure were displayed, as well as the relationship between
http://dx.doi.org/10.1016/j.wasman.2014.12.015 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Liu, G.-J., et al. The effects of temperature, organic matter and time-dependency on rheological properties of dry anaerobic digested swine manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.015
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G.-J. Liu et al. / Waste Management xxx (2015) xxx–xxx
the first/second yield stress and time-dependent characteristics. The purpose is to try to figure out that the impact factors of temperature, organic concentration and time-dependent are also important as solid concentration on rheological property of dry anaerobic digested swine manure. Furthermore, considering the significance of yield stress (especially the second yield stress) in practical application, a new method which is used to calculate the first/second yield stress through time-dependent characteristic was proposed.
and apparent viscosity of sludge, at a special shear rate, can be generally expressed by the Arrhenius relationship (Eq. (1)) (Baudez et al., 2013; Pevere et al., 2009). Fig. 1 showed the plotting of lng against 1/T at four solid concentration and the straight line was obtained with the slope of Ea/R. The high correlation between viscosity and temperature indicates that the temperature affected the rheological property of digested swine manure according to the Arrhenius equation (Table 1).
ln g ¼ ln A0 þ 2. Materials and methods 2.1. Dry anaerobic digested swine manure The sludge samples used in this study were obtained from a laboratory scale cylindrical anaerobic reactor. The ratio of height to diameter of this reactor is 1. It has a treatment capacity of 20 L and working volume of 15 L. The total solid (TS) and volatile solid (VS) of swine manure were 26% and 20% respectively. Meanwhile, the inoculum were 17% and 11% respectively. The swine manure to inoculum ratio is 2. It is being operated in batch mode at temperature of 30 ± 2 °C and lasted for 80 days. In this study, the TS concentration of anaerobic digested swine manure was lower than 22%. 2.2. Analytical methods Rheological experiments were performed on a rotational controlled share rate rheometer (NXS-11B rotational Viscometer, Chengdu Instrument Co.), a coaxial cylindrical measurement device (system C) with a gap of 2.7 mm. The volume of the sample used for each test is 9 mL. The automatic discharging of sludge from digester creates a low shear rate in the pipe under the gravity. Thus, the shear rate below 163.1 s1 was considered. The measurement protocol consists of an increase of the shear rate from 2.5 to 163.1 s1 in 90 s and an increase of the time from 0 to 1000 s with the shear rates remaining unaltered. System C will be instead by system D (the gap is 5.7 mm; the sample volume is 10 mL; shear rate is from 1.4 to 96.6 s1 when the measurement range be exceeded. From day 20, the rheological properties of sludge samples were measured every 15 days and the sludge showed different solid concentration. The variation of effects was set as follows. Temperature: The temperature increased from 10 to 60 °C through a water jacket which was heated by a thermostatic bath (HS-4 thermostatic bath, Chengdu Instrument Co.); Organic: Peptone was added to the sludge from 1% to 5% volatile solid in samples and then the samples were stirred for 1 min and relaxed for 2 h. Time: the value of viscosity was read once every two seconds under every invariant shear rate until the viscosity reached to a constant value. The TS and VS were measured by standard methods (APHA, 2005). Carbohydrates were analyzed according to the anthrone method (Zhang et al., 2013). The amount of protein was analyzed by Thermo Scientific Varioskan Flash spectral scanning multimode reader (Varioskan, Thermo Electron Co, Waltham, MA) with SkanIt Software (wavelength: 562 nm). Volatile fatty acids (VFA) were analyzed using a gas chromatograph (GC-7AG, Shimadzu, Japan).
Ea RT
ð1Þ
where A0 is the pre-exponential factor (Pa s1), Ea the Activation energy of flow (kJ/mol), R the universal gas constant (8.314 J/mol), T the absolute temperature (K). The viscosity decreased with a temperature increase at different shear rates under four solid concentrations (Fig. 2) agree with the activation energy theory (Mayr, 2006). On a molecular level, the higher temperature makes the thermal motion of the solid particles more violent, which weakened the network strength between the particles or broken some chemical bonds of long molecule chains and make them dissolvable. A direct proof was the increasing of soluble protein in digested swine sludge after heating treatment. However, the other soluble macromolecule organic content including polysaccharide and volatile fatty acids have no significant changes (Table 2). This might be caused by the larger amount of protein than polysaccharide and volatile fatty acids in dry anaerobic digested swine manure or the temperature was high enough for protein dissolving. An interesting phenomena was that temperature, when it reached to 60 °C, seemed to have less effect on sludge viscosity at the solid concentration ranged from 16.6% to 21.5% (Fig. 3). This phenomena can be deduced from Arrhenius equation which showed a constant viscosity when temperature was high enough. The theory of the relationship between reaction rate and activation energy indicates that the relatively high temperature is beneficial to the reaction with a higher activation energy. Which means, in this study, the temperature about 60 °C can more easily reduce the viscosity of dry anaerobic digested swine manure which has a TS of 21.5%. Moreover, 60 °C was the common temperature which made the digested swine manure with TS ranged from 16.6% to 21.5% reach to a similar rheology state.
3. Results and discussion 3.1. Influence of temperature on rheology The influence of temperature on digested swine manure was analyzed in four different TS. The correlation between temperature
Fig. 1. Viscosity as a function of 1/T at a constant shear rate of 22.41 s1 in a different fermentation stage which showed different solid concentrations.
Please cite this article in press as: Liu, G.-J., et al. The effects of temperature, organic matter and time-dependency on rheological properties of dry anaerobic digested swine manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.015
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G.-J. Liu et al. / Waste Management xxx (2015) xxx–xxx Table 1 Simulation results of digested swine manure using the Arrhenius equation at various solid concentrations. Solid concentration (%TS)
A0
Ea (kJ mol1)
R2
21.5 20.1 18.5 16.6
7.86 5.82 5.67 3.97
25.91 19.99 18.84 13.91
0.961 0.963 0.986 0.967
3.2. Influence of organic matter on rheology The variety of organics like protein, polysaccharide, fatty acid etc., especially protein and polysaccharide (Forster, 1983), contained in EPS have a significant effect on sludge rheology. However, the protein in digested swine manure was significantly higher than polysaccharide as it has a higher content and increase with temperature (Table 2). The polysaccharide and volatile fatty acids do not have obvious changes in various temperatures, which does
not means that they did not have the effects on rheological behavior. The research on pasty sewage sludge studied by Baudez and Coussot (2001) showed that the rheological parameters are mainly depend on the organic fraction. They extracted the main solid components (minerals, proteins, lipids, carbohydrates) and showed that the behavior evolution is governed by the synthesis of volatile fatty acids (Baudez and Coussot, 2001). In this study, the materials were different and proteins seem to be playing a leading position. Thus, 1–10 times of peptone were added in digested swine manure and their rheological properties were tested. However, there was an interesting phenomenon that viscosity curves showed a decreased tendency to the increase of peptone, even the added organic increased the solid concentration (Fig. 4). Usually the increasing solid concentration will increase the viscosity of fluid. The reasonable explanation was that peptone have the function of water-reducing admixture which contains – SO3, H–COOH, –NH2, –OH hydrophobic groups (Hofstee, 1975). These hydrophobic groups can disperse sludge particles and release some water contained between particles through electrostatic
Fig. 2. Viscosity curve under different temperature for four solid concentrations of digested swine manure: 21.5% (a), 20.1% (b), 18.5% (c), 16.6% (d).
Table 2 Organic matter concentrations in the thermally-treated digested swine manure. Temperature (°C)
Soluble proteins (g kg1)
Soluble carbohydrates (mg kg1)
Acetic acid (mg kg1)
Propionic acid (mg kg1)
Butyric acid (mg kg1)
10 20 30 40 50 60
7.18 7.29 7.46 7.75 7.89 8.49
2.55 2.60 2.72 2.48 2.33 2.48
835 880.7 797.2 904.6 778.8 842.1
267.5 307.2 347.9 329.1 235.4 312.8
197.8 209.2 184.1 212.4 190.5 202.8
Please cite this article in press as: Liu, G.-J., et al. The effects of temperature, organic matter and time-dependency on rheological properties of dry anaerobic digested swine manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.015
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out to a constant value. The time-dependency of digested swine manure with the TS concentration of 20.1% was shown in Fig. 5. When the shear rate was greater than 22.41 s1, the shear stress showed the stress–strain lagging in the first 10 s, whereas this lasted for 100 s with the shear rate between 5.825 and 22.41 s1. That is because the structures of sludge could not respond quickly and the structures showed different response frequencies under different shear rates. The shear stress decreased with time after stress–strain lagging. Finally, the shear stress reached to a constant value after about 1000 s. In viscoelasticity theory, stress relaxation was usually used to measure yield stress. Baudez and Coussot’s results (Baudez and Coussot, 2001) showed that s < s1 (the first yield stress) would be linear viscoelastic. According to their research, the behavior of anaerobic digested swine manure also can be summarized as follows:
Fig. 3. The viscosity curves for the digested swine manure with four solid concentrations at 60 °C.
Fig. 4. Viscosity versus the peptone concentration and the solid concentration.
force. The effect of water-reducing admixture on the rheology of gypsum plaster was studied by Peng et al. (2005) and they showed the decrease of fluidity of gypsum paste when water-reducing admixture was added. In addition, the added peptone might also plays the role of air entraining mixture. Because peptone has some content usually used in air entraining mixtures. Kalyon et al. (1991) showed that the air entraining mixture decreased viscosity of concentrated suspensions during continuous processing. The air entraining mixture can bring in more microbubbles when continuous stir was processed, which can reduce the viscosity of fluid, especially showed in the research field of concrete (Beaupré et al., 1999).
s < s1 linear viscoelastic. s1 < s < s2 non-linear viscoelastic. s > s2 purely viscous. When shear rate was 2.509 s1 the shear stress versus time was unchanged, because the longer time did not make the digested swine manure show the significant aging. Based on earlier results, this stress was obviously equal to the first yield stress. Thus, the first yield stress was about 121 Pa (s1 sshear rate = s2.509/s = 121 Pa). It was an inaccurate value because the more interval stress was not detected. In practical application, the second yield stress (s2) would be the considerable parameter in the pipe design of transportation systems. When the shear rate was greater than 5.825 s1, a linear relationship between shear stress and time at the ending of stress–strain lagging was shown and the shear stress would eventually maintain a relatively constant value after few minutes shear. The steady shear stress can be thought as s2 but a little greater than s2 for maintaining pure viscous shear, since the yield stress is also a time-dependent rheological property (Cheng, 1986). Furthermore, the shear stress versus time curve can be simulated by Eq. (2) and only when the shear stress is greater than s2 this equation can be effective. This equation is similar to the rebuilding equation which was investigated by Heymann et al. (1996) about the build-up after shearing of the yield stress. The high regression coefficients (Table 3) indicate that this equation could adequately describe the time-dependent variation of shear stress. The change of shear stress rate (h) (Fig. 5) calculated through second derivative
3.3. The relationship between the first/second yield stress and timedependency property The rheological property of time-dependent is connected to viscoelasticity and thixotropy concept which are both time-dependent properties. Barnes (1997) detailed many definitions of thixotropy. Thixotropy can also was analyzed as a special case of more general non-linear viscoelasticity or the non-linear region of the viscoelasticity (Labanda and Llorens, 2005). When place a thixotropic material at a constant shear rate, the measured shear stress or viscosity will decrease with time and eventually steady
Fig. 5. Stress relaxation above and equal to the critical shear stress.
Please cite this article in press as: Liu, G.-J., et al. The effects of temperature, organic matter and time-dependency on rheological properties of dry anaerobic digested swine manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.015
G.-J. Liu et al. / Waste Management xxx (2015) xxx–xxx Table 3 Simulation results for the dry anaerobically digested swine manure using the established model at various shear rates. Shear rate (s
1
)
163.10 89.62 40.33 22.41 12.25
2
s0 (Pa)
k
n
R
150.65 151.10 151.35 141.76 132.07
101.06 64.63 54.99 38.08 29.61
0.00299 0.00343 0.00285 0.00275 0.00192
0.982 0.975 0.982 0.967 0.932
Eq. (3) can be used to evaluate s2. When h is small enough (define h = 1E 5 in this study), the corresponding shear stress which is thought as s2, become stable and then can be calculated. When time is long enough, then s2 = s0. The second yield stress was 132–151 Pa calculated by this method (see Fig. 6).
s2 ¼ s0 þ kent h¼
@2s 2 ¼ kn ent @t 2
ð2Þ ð3Þ
where s2 is the second yield stress (Pa), s0 the constant (Pa), k and n the coefficients, t the time (s). Actually, for the constant shear rates the shear stresses will not be the second yield stress, which is affected by many factors (Møller et al., 2008, 2006). However, the second yield stress has similar value in an appropriate range of the shear rates. The evidence in Table 3 showed the second yield stresses approached to 151 Pa when shear rates ranged from 40.33 to 163.1 s1. Thus, considering the synthetic influencing factors, 141 ± 10 Pa would be more authentic as the second yield stress of anaerobic digested swine manure with solid concentration of 20.1%. Additionally, this result was according with the values of yield stress, which were 139 ± 8 Pa and 147 ± 11 Pa, calculated through slump test (Eq. (4)) (Garcia-Bernet et al., 2011) and inclined plane test (Eq. (5)) (Coussot et al., 2002a), respectively. Certainly, this value was not a very accurate or standard one because yield stress was hard to be evaluated and yield stress was a great dispute issue, in rheological literature for two decades, on whether the yield stress exists (Bonn and Denn, 2009). In order to gain more accurate results, the more corresponding areas of shear rates should be tested.
s ¼ H þ z0
2s qgðH þ z0 Þ 1 þ ln 2s qg
ð4Þ
where s is the slump (m), H the initial bed height (m), z0 represents the equivalent height of the added mass (m), s the yield stress (Pa), q the density of tested sample (kg m3), g the gravitational acceleration (m s2).
Fig. 6. The second derivative of shear stress with respect to time.
s ¼ qgysinðiÞ
5
ð5Þ
where y is the asymptotic thickness in the center region of a deposit of a finite volume of material poured over the inclined plane (m), i the angle of inclination of the plane. 4. Conclusion The effects, including temperature, organic and time-dependent, on rheological properties of dry anaerobic digested swine manure were found to be equally important as solid concentration. Arrhenius equation can simulate the correlation between viscosity and temperature. When temperature reached to a critical value the digested swine manure with high solid concentration showed the same rheology state as the digested swine manure with low solid concentration. The soluble protein like peptone can reduce viscosity of digested swine manure, which might be caused by its potential function of water-reducing admixture and air entraining mixture. Yield stress, especially the second yield stress, need more accurate value in engineering design. Time-dependency property of digested swine manure showed the stress relaxation over time and this property might be an effective way to evaluate yield stress. However, further work using viscoelasticity theory to investigate the yield stress and the act mechanism of organic would be more precise. Acknowledgement The authors acknowledge financial support from China Agriculture Research System (CARS-36). References Abouelenien, F., Kitamura, Y., Nishio, N., Nakashimada, Y., 2009. Dry anaerobic ammonia–methane production from chicken manure. Appl. Microbiol. Biotechnol. 82, 757–764. APHA, 2005. Standard Methods for the Examination of Water and Wastewater, 21st ed., American Public Health Association, American Water Works Association and Water Environment Federation, Washington, DC, USA. Barnes, H.A., 1997. Thixotropy—a review. J. Non-Newtonian Fluid Mech. 70, 1–33. Battistoni, P., Fava, G., Stanzini, C., Cecchi, F., Bassetti, A., 1993. Feed characteristics and digester operative conditions as parameters affecting the rheology of digested municipal solid wastes. Water Sci. Technol. 27, 37–45. Battistoni, P., Pava, G., Cecchi, F., Pavan, P., 1991. Rheology of sludge from semi-dry anaerobic digestion of municipal solid waste. Environ. Technol. 12, 897–905. Baudez, J.-C., Coussot, P., 2001. Rheology of aging, concentrated, polymeric suspensions: application to pasty sewage sludges. J. Rheol. 45, 1123. Baudez, J.C., Markis, F., Eshtiaghi, N., Slatter, P., 2011. The rheological behaviour of anaerobic digested sludge. Water Res. 45, 5675–5680. Baudez, J.C., Slatter, P., Eshtiaghi, N., 2013. The impact of temperature on the rheological behaviour of anaerobic digested sludge. Chem. Eng. J. 215–216, 182–187. Beaupré, D., Lacombe, P., Khayat, K., 1999. Laboratory investigation of rheological properties and scaling resistance of air entrained self-consolidating concrete. Mater. Struct. 32, 235–240. Bonn, D., Denn, M.M., 2009. Materials science. Yield stress fluids slowly yield to analysis. Science 324, 1401–1402. Cheng, D.C., 1986. Yield stress: a time-dependent property and how to measure it. Rheol. Acta 25, 542–554. Coussot, P., Nguyen, Q.D., Huynh, H., Bonn, D., 2002a. Avalanche behavior in yield stress fluids. Phys. Rev. Lett. 88, 175501. Coussot, P., Nguyen, Q.D., Huynh, H., Bonn, D., 2002b. Viscosity bifurcation in thixotropic, yielding fluids. J. Rheol. 46, 573. Dak, M., Verma, R.C., Jaaffrey, S.N.A., 2007. Effect of temperature and concentration on rheological properties of ‘‘Kesar’’ mango juice. J. Food Eng. 80, 1011–1015. Eshtiaghi, N., Yap, S.D., Markis, F., Baudez, J.C., Slatter, P., 2012. Clear model fluids to emulate the rheological properties of thickened digested sludge. Water Res. 46, 3014–3022. Forster, C.F., 1983. Bound water in sewage sludges and its relationship to sludge surfaces and sludge viscosities. J. Chem. Technol. Biotechnol. Biotechnology 33, 76–84. Garcia-Bernet, D., Loisel, D., Guizard, G., Buffière, P., Steyer, J., Escudié, R., 2011. Rapid measurement of the yield stress of anaerobically-digested solid waste using slump tests. Waste Manag. 31, 631–635. Heymann, L. et al., 1996. In: Ait-Kadi, A. et al. (Eds.), Proc. XIIth Int. Congress on Rheology, Laval University, Quebec City, Canada, pp. 451.
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Please cite this article in press as: Liu, G.-J., et al. The effects of temperature, organic matter and time-dependency on rheological properties of dry anaerobic digested swine manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.015
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
The effects of temperature, organic matter and time-dependency on rheological properties of dry anaerobic digested swine manure Gang-Jin Liu, Yi Liu, Zhi-Yong Wang, Yun-Hui Lei, Zi-Ai Chen, Liang-Wei Deng ⇑ Biogas Institute of Ministry of Agriculture, Chengdu 610041, PR China
a r t i c l e
i n f o
Article history: Received 20 May 2014 Accepted 16 December 2014 Available online xxxx Keywords: Dry anaerobic digestion Swine manure Rheology Effect Model
a b s t r a c t An efficient way to avoid the pollution of swine wastewater is the application of dry anaerobic digestion, which needs rheological parameter for stirring and pipe designing. The rheological properties of this kind of sludge have been studied for many decades, yet their effects only solid concentration has been investigated widely. In this paper, the influences of temperature, organic and time-dependency on the efficiency of anaerobic digested swine manure were studied. The viscosity decreased with temperature arranged from 10 to 60 °C which caused increase in protein from 7.18 to 8.49 g/kg. 60 °C can make the digested swine manure with TS from 16.6% to 21.5% reach to the same rheology state. The added peptone decreased the viscosity because of its function of water-reducing admixture and air entraining mixture. Time-dependent experiment showed the decrease of shear stress over time. The first and the second yield stress of dry anaerobic digested swine manure were evaluated through time-dependent model. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Anaerobic digestion would be a good choice for the treatment of swine manure. Considering the post-treatment of digested wastewater from conventional processes needs more energy and investment, dry anaerobic digestion is getting more attention because it has the benefits of smaller reactor and less effluent water (Abouelenien et al., 2009). However, the rheological properties of dry anaerobic digested swine manure need to be known clearly for the design and efficient operation of the pumping systems (Wolny et al., 2008). This kind of sludge shows shear-thinning behavior, plastic behavior, thixotropy and viscoelasticity (Seyssiecq et al., 2003). The rheological properties of these sludges most studies mentioned are low solid concentration, displayed some changeful characteristics like the yield stress and viscosity increase with solid concentration (Baudez et al., 2011), the viscosity decrease with increasing temperature (Mu et al., 2006), the viscosity and shear modulus increase with organic (Khongnakorn et al., 2010), etc. In addition, the property of time dependency of these rheological properties cannot be ignored (Coussot et al., 2002b; Read et al., 2011). The polytropy of rheological behavior indicates that accurate estimation of sludge rheological properties is essential. The rheological properties of low concentration digested sludge and active sludge are widely studied. Because their operations ⇑ Corresponding author at: Biogas Institute of Ministry of Agriculture, No. 13, 4th Section, South Renmin Rd., Chengdu, Sichuan, PR China. Tel./fax: +86 28 85236376. E-mail address: [email protected] (L.-W. Deng).
involving pumping, storage, dewatering, etc. linked to rheological behavior and low concentration sludges are more common. Less early work studied the rheology of semi-dry anaerobic digested municipal solid wastes (Battistoni et al., 1993, 1991). However, it rarely saw the researches about rheological properties of dry anaerobic digested swine manure let alone the effects on its rheological property. Dry anaerobic digested swine manure is heterogeneous and a type of complex mixtures with inorganic matter and organic matter from the physical perspective. Consequently, their rheological properties were affected by many factors, like temperature, solid concentration, surface charge, organic matter (protein and polysaccharide take major effect), bound water, pH etc. However, the main factor seems to be solid concentration (Baudez et al., 2011; Dak et al., 2007; Eshtiaghi et al., 2012; Mu et al., 2006) which leads to a more rigid structure of the sludge network and to an enhancement of the sludge cohesion (Khongnakorn et al., 2010). Actually, temperature and organic concentration still have significant effect on sludge rheological properties (Battistoni et al., 1993; Baudez et al., 2013; Khongnakorn et al., 2010; Mu et al., 2006), especially for high solid content sludge (Battistoni et al., 1993) which was less studied. In addition, time is also an important influencing factor on thixotropy, viscoelasticity and the first/second yield stress. The yield stress is the critical stress of fluid before flow, it should be clearly understood in practical biogas engineering. In this paper, the influences of temperature, organic concentration and time-dependency on the effect of anaerobic digested swine manure were displayed, as well as the relationship between
http://dx.doi.org/10.1016/j.wasman.2014.12.015 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Liu, G.-J., et al. The effects of temperature, organic matter and time-dependency on rheological properties of dry anaerobic digested swine manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.015
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the first/second yield stress and time-dependent characteristics. The purpose is to try to figure out that the impact factors of temperature, organic concentration and time-dependent are also important as solid concentration on rheological property of dry anaerobic digested swine manure. Furthermore, considering the significance of yield stress (especially the second yield stress) in practical application, a new method which is used to calculate the first/second yield stress through time-dependent characteristic was proposed.
and apparent viscosity of sludge, at a special shear rate, can be generally expressed by the Arrhenius relationship (Eq. (1)) (Baudez et al., 2013; Pevere et al., 2009). Fig. 1 showed the plotting of lng against 1/T at four solid concentration and the straight line was obtained with the slope of Ea/R. The high correlation between viscosity and temperature indicates that the temperature affected the rheological property of digested swine manure according to the Arrhenius equation (Table 1).
ln g ¼ ln A0 þ 2. Materials and methods 2.1. Dry anaerobic digested swine manure The sludge samples used in this study were obtained from a laboratory scale cylindrical anaerobic reactor. The ratio of height to diameter of this reactor is 1. It has a treatment capacity of 20 L and working volume of 15 L. The total solid (TS) and volatile solid (VS) of swine manure were 26% and 20% respectively. Meanwhile, the inoculum were 17% and 11% respectively. The swine manure to inoculum ratio is 2. It is being operated in batch mode at temperature of 30 ± 2 °C and lasted for 80 days. In this study, the TS concentration of anaerobic digested swine manure was lower than 22%. 2.2. Analytical methods Rheological experiments were performed on a rotational controlled share rate rheometer (NXS-11B rotational Viscometer, Chengdu Instrument Co.), a coaxial cylindrical measurement device (system C) with a gap of 2.7 mm. The volume of the sample used for each test is 9 mL. The automatic discharging of sludge from digester creates a low shear rate in the pipe under the gravity. Thus, the shear rate below 163.1 s1 was considered. The measurement protocol consists of an increase of the shear rate from 2.5 to 163.1 s1 in 90 s and an increase of the time from 0 to 1000 s with the shear rates remaining unaltered. System C will be instead by system D (the gap is 5.7 mm; the sample volume is 10 mL; shear rate is from 1.4 to 96.6 s1 when the measurement range be exceeded. From day 20, the rheological properties of sludge samples were measured every 15 days and the sludge showed different solid concentration. The variation of effects was set as follows. Temperature: The temperature increased from 10 to 60 °C through a water jacket which was heated by a thermostatic bath (HS-4 thermostatic bath, Chengdu Instrument Co.); Organic: Peptone was added to the sludge from 1% to 5% volatile solid in samples and then the samples were stirred for 1 min and relaxed for 2 h. Time: the value of viscosity was read once every two seconds under every invariant shear rate until the viscosity reached to a constant value. The TS and VS were measured by standard methods (APHA, 2005). Carbohydrates were analyzed according to the anthrone method (Zhang et al., 2013). The amount of protein was analyzed by Thermo Scientific Varioskan Flash spectral scanning multimode reader (Varioskan, Thermo Electron Co, Waltham, MA) with SkanIt Software (wavelength: 562 nm). Volatile fatty acids (VFA) were analyzed using a gas chromatograph (GC-7AG, Shimadzu, Japan).
Ea RT
ð1Þ
where A0 is the pre-exponential factor (Pa s1), Ea the Activation energy of flow (kJ/mol), R the universal gas constant (8.314 J/mol), T the absolute temperature (K). The viscosity decreased with a temperature increase at different shear rates under four solid concentrations (Fig. 2) agree with the activation energy theory (Mayr, 2006). On a molecular level, the higher temperature makes the thermal motion of the solid particles more violent, which weakened the network strength between the particles or broken some chemical bonds of long molecule chains and make them dissolvable. A direct proof was the increasing of soluble protein in digested swine sludge after heating treatment. However, the other soluble macromolecule organic content including polysaccharide and volatile fatty acids have no significant changes (Table 2). This might be caused by the larger amount of protein than polysaccharide and volatile fatty acids in dry anaerobic digested swine manure or the temperature was high enough for protein dissolving. An interesting phenomena was that temperature, when it reached to 60 °C, seemed to have less effect on sludge viscosity at the solid concentration ranged from 16.6% to 21.5% (Fig. 3). This phenomena can be deduced from Arrhenius equation which showed a constant viscosity when temperature was high enough. The theory of the relationship between reaction rate and activation energy indicates that the relatively high temperature is beneficial to the reaction with a higher activation energy. Which means, in this study, the temperature about 60 °C can more easily reduce the viscosity of dry anaerobic digested swine manure which has a TS of 21.5%. Moreover, 60 °C was the common temperature which made the digested swine manure with TS ranged from 16.6% to 21.5% reach to a similar rheology state.
3. Results and discussion 3.1. Influence of temperature on rheology The influence of temperature on digested swine manure was analyzed in four different TS. The correlation between temperature
Fig. 1. Viscosity as a function of 1/T at a constant shear rate of 22.41 s1 in a different fermentation stage which showed different solid concentrations.
Please cite this article in press as: Liu, G.-J., et al. The effects of temperature, organic matter and time-dependency on rheological properties of dry anaerobic digested swine manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.015
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G.-J. Liu et al. / Waste Management xxx (2015) xxx–xxx Table 1 Simulation results of digested swine manure using the Arrhenius equation at various solid concentrations. Solid concentration (%TS)
A0
Ea (kJ mol1)
R2
21.5 20.1 18.5 16.6
7.86 5.82 5.67 3.97
25.91 19.99 18.84 13.91
0.961 0.963 0.986 0.967
3.2. Influence of organic matter on rheology The variety of organics like protein, polysaccharide, fatty acid etc., especially protein and polysaccharide (Forster, 1983), contained in EPS have a significant effect on sludge rheology. However, the protein in digested swine manure was significantly higher than polysaccharide as it has a higher content and increase with temperature (Table 2). The polysaccharide and volatile fatty acids do not have obvious changes in various temperatures, which does
not means that they did not have the effects on rheological behavior. The research on pasty sewage sludge studied by Baudez and Coussot (2001) showed that the rheological parameters are mainly depend on the organic fraction. They extracted the main solid components (minerals, proteins, lipids, carbohydrates) and showed that the behavior evolution is governed by the synthesis of volatile fatty acids (Baudez and Coussot, 2001). In this study, the materials were different and proteins seem to be playing a leading position. Thus, 1–10 times of peptone were added in digested swine manure and their rheological properties were tested. However, there was an interesting phenomenon that viscosity curves showed a decreased tendency to the increase of peptone, even the added organic increased the solid concentration (Fig. 4). Usually the increasing solid concentration will increase the viscosity of fluid. The reasonable explanation was that peptone have the function of water-reducing admixture which contains – SO3, H–COOH, –NH2, –OH hydrophobic groups (Hofstee, 1975). These hydrophobic groups can disperse sludge particles and release some water contained between particles through electrostatic
Fig. 2. Viscosity curve under different temperature for four solid concentrations of digested swine manure: 21.5% (a), 20.1% (b), 18.5% (c), 16.6% (d).
Table 2 Organic matter concentrations in the thermally-treated digested swine manure. Temperature (°C)
Soluble proteins (g kg1)
Soluble carbohydrates (mg kg1)
Acetic acid (mg kg1)
Propionic acid (mg kg1)
Butyric acid (mg kg1)
10 20 30 40 50 60
7.18 7.29 7.46 7.75 7.89 8.49
2.55 2.60 2.72 2.48 2.33 2.48
835 880.7 797.2 904.6 778.8 842.1
267.5 307.2 347.9 329.1 235.4 312.8
197.8 209.2 184.1 212.4 190.5 202.8
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out to a constant value. The time-dependency of digested swine manure with the TS concentration of 20.1% was shown in Fig. 5. When the shear rate was greater than 22.41 s1, the shear stress showed the stress–strain lagging in the first 10 s, whereas this lasted for 100 s with the shear rate between 5.825 and 22.41 s1. That is because the structures of sludge could not respond quickly and the structures showed different response frequencies under different shear rates. The shear stress decreased with time after stress–strain lagging. Finally, the shear stress reached to a constant value after about 1000 s. In viscoelasticity theory, stress relaxation was usually used to measure yield stress. Baudez and Coussot’s results (Baudez and Coussot, 2001) showed that s < s1 (the first yield stress) would be linear viscoelastic. According to their research, the behavior of anaerobic digested swine manure also can be summarized as follows:
Fig. 3. The viscosity curves for the digested swine manure with four solid concentrations at 60 °C.
Fig. 4. Viscosity versus the peptone concentration and the solid concentration.
force. The effect of water-reducing admixture on the rheology of gypsum plaster was studied by Peng et al. (2005) and they showed the decrease of fluidity of gypsum paste when water-reducing admixture was added. In addition, the added peptone might also plays the role of air entraining mixture. Because peptone has some content usually used in air entraining mixtures. Kalyon et al. (1991) showed that the air entraining mixture decreased viscosity of concentrated suspensions during continuous processing. The air entraining mixture can bring in more microbubbles when continuous stir was processed, which can reduce the viscosity of fluid, especially showed in the research field of concrete (Beaupré et al., 1999).
s < s1 linear viscoelastic. s1 < s < s2 non-linear viscoelastic. s > s2 purely viscous. When shear rate was 2.509 s1 the shear stress versus time was unchanged, because the longer time did not make the digested swine manure show the significant aging. Based on earlier results, this stress was obviously equal to the first yield stress. Thus, the first yield stress was about 121 Pa (s1 sshear rate = s2.509/s = 121 Pa). It was an inaccurate value because the more interval stress was not detected. In practical application, the second yield stress (s2) would be the considerable parameter in the pipe design of transportation systems. When the shear rate was greater than 5.825 s1, a linear relationship between shear stress and time at the ending of stress–strain lagging was shown and the shear stress would eventually maintain a relatively constant value after few minutes shear. The steady shear stress can be thought as s2 but a little greater than s2 for maintaining pure viscous shear, since the yield stress is also a time-dependent rheological property (Cheng, 1986). Furthermore, the shear stress versus time curve can be simulated by Eq. (2) and only when the shear stress is greater than s2 this equation can be effective. This equation is similar to the rebuilding equation which was investigated by Heymann et al. (1996) about the build-up after shearing of the yield stress. The high regression coefficients (Table 3) indicate that this equation could adequately describe the time-dependent variation of shear stress. The change of shear stress rate (h) (Fig. 5) calculated through second derivative
3.3. The relationship between the first/second yield stress and timedependency property The rheological property of time-dependent is connected to viscoelasticity and thixotropy concept which are both time-dependent properties. Barnes (1997) detailed many definitions of thixotropy. Thixotropy can also was analyzed as a special case of more general non-linear viscoelasticity or the non-linear region of the viscoelasticity (Labanda and Llorens, 2005). When place a thixotropic material at a constant shear rate, the measured shear stress or viscosity will decrease with time and eventually steady
Fig. 5. Stress relaxation above and equal to the critical shear stress.
Please cite this article in press as: Liu, G.-J., et al. The effects of temperature, organic matter and time-dependency on rheological properties of dry anaerobic digested swine manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.015
G.-J. Liu et al. / Waste Management xxx (2015) xxx–xxx Table 3 Simulation results for the dry anaerobically digested swine manure using the established model at various shear rates. Shear rate (s
1
)
163.10 89.62 40.33 22.41 12.25
2
s0 (Pa)
k
n
R
150.65 151.10 151.35 141.76 132.07
101.06 64.63 54.99 38.08 29.61
0.00299 0.00343 0.00285 0.00275 0.00192
0.982 0.975 0.982 0.967 0.932
Eq. (3) can be used to evaluate s2. When h is small enough (define h = 1E 5 in this study), the corresponding shear stress which is thought as s2, become stable and then can be calculated. When time is long enough, then s2 = s0. The second yield stress was 132–151 Pa calculated by this method (see Fig. 6).
s2 ¼ s0 þ kent h¼
@2s 2 ¼ kn ent @t 2
ð2Þ ð3Þ
where s2 is the second yield stress (Pa), s0 the constant (Pa), k and n the coefficients, t the time (s). Actually, for the constant shear rates the shear stresses will not be the second yield stress, which is affected by many factors (Møller et al., 2008, 2006). However, the second yield stress has similar value in an appropriate range of the shear rates. The evidence in Table 3 showed the second yield stresses approached to 151 Pa when shear rates ranged from 40.33 to 163.1 s1. Thus, considering the synthetic influencing factors, 141 ± 10 Pa would be more authentic as the second yield stress of anaerobic digested swine manure with solid concentration of 20.1%. Additionally, this result was according with the values of yield stress, which were 139 ± 8 Pa and 147 ± 11 Pa, calculated through slump test (Eq. (4)) (Garcia-Bernet et al., 2011) and inclined plane test (Eq. (5)) (Coussot et al., 2002a), respectively. Certainly, this value was not a very accurate or standard one because yield stress was hard to be evaluated and yield stress was a great dispute issue, in rheological literature for two decades, on whether the yield stress exists (Bonn and Denn, 2009). In order to gain more accurate results, the more corresponding areas of shear rates should be tested.
s ¼ H þ z0
2s qgðH þ z0 Þ 1 þ ln 2s qg
ð4Þ
where s is the slump (m), H the initial bed height (m), z0 represents the equivalent height of the added mass (m), s the yield stress (Pa), q the density of tested sample (kg m3), g the gravitational acceleration (m s2).
Fig. 6. The second derivative of shear stress with respect to time.
s ¼ qgysinðiÞ
5
ð5Þ
where y is the asymptotic thickness in the center region of a deposit of a finite volume of material poured over the inclined plane (m), i the angle of inclination of the plane. 4. Conclusion The effects, including temperature, organic and time-dependent, on rheological properties of dry anaerobic digested swine manure were found to be equally important as solid concentration. Arrhenius equation can simulate the correlation between viscosity and temperature. When temperature reached to a critical value the digested swine manure with high solid concentration showed the same rheology state as the digested swine manure with low solid concentration. The soluble protein like peptone can reduce viscosity of digested swine manure, which might be caused by its potential function of water-reducing admixture and air entraining mixture. Yield stress, especially the second yield stress, need more accurate value in engineering design. Time-dependency property of digested swine manure showed the stress relaxation over time and this property might be an effective way to evaluate yield stress. However, further work using viscoelasticity theory to investigate the yield stress and the act mechanism of organic would be more precise. Acknowledgement The authors acknowledge financial support from China Agriculture Research System (CARS-36). References Abouelenien, F., Kitamura, Y., Nishio, N., Nakashimada, Y., 2009. Dry anaerobic ammonia–methane production from chicken manure. Appl. Microbiol. Biotechnol. 82, 757–764. APHA, 2005. Standard Methods for the Examination of Water and Wastewater, 21st ed., American Public Health Association, American Water Works Association and Water Environment Federation, Washington, DC, USA. Barnes, H.A., 1997. Thixotropy—a review. J. Non-Newtonian Fluid Mech. 70, 1–33. Battistoni, P., Fava, G., Stanzini, C., Cecchi, F., Bassetti, A., 1993. Feed characteristics and digester operative conditions as parameters affecting the rheology of digested municipal solid wastes. Water Sci. Technol. 27, 37–45. Battistoni, P., Pava, G., Cecchi, F., Pavan, P., 1991. Rheology of sludge from semi-dry anaerobic digestion of municipal solid waste. Environ. Technol. 12, 897–905. Baudez, J.-C., Coussot, P., 2001. Rheology of aging, concentrated, polymeric suspensions: application to pasty sewage sludges. J. Rheol. 45, 1123. Baudez, J.C., Markis, F., Eshtiaghi, N., Slatter, P., 2011. The rheological behaviour of anaerobic digested sludge. Water Res. 45, 5675–5680. Baudez, J.C., Slatter, P., Eshtiaghi, N., 2013. The impact of temperature on the rheological behaviour of anaerobic digested sludge. Chem. Eng. J. 215–216, 182–187. Beaupré, D., Lacombe, P., Khayat, K., 1999. Laboratory investigation of rheological properties and scaling resistance of air entrained self-consolidating concrete. Mater. Struct. 32, 235–240. Bonn, D., Denn, M.M., 2009. Materials science. Yield stress fluids slowly yield to analysis. Science 324, 1401–1402. Cheng, D.C., 1986. Yield stress: a time-dependent property and how to measure it. Rheol. Acta 25, 542–554. Coussot, P., Nguyen, Q.D., Huynh, H., Bonn, D., 2002a. Avalanche behavior in yield stress fluids. Phys. Rev. Lett. 88, 175501. Coussot, P., Nguyen, Q.D., Huynh, H., Bonn, D., 2002b. Viscosity bifurcation in thixotropic, yielding fluids. J. Rheol. 46, 573. Dak, M., Verma, R.C., Jaaffrey, S.N.A., 2007. Effect of temperature and concentration on rheological properties of ‘‘Kesar’’ mango juice. J. Food Eng. 80, 1011–1015. Eshtiaghi, N., Yap, S.D., Markis, F., Baudez, J.C., Slatter, P., 2012. Clear model fluids to emulate the rheological properties of thickened digested sludge. Water Res. 46, 3014–3022. Forster, C.F., 1983. Bound water in sewage sludges and its relationship to sludge surfaces and sludge viscosities. J. Chem. Technol. Biotechnol. Biotechnology 33, 76–84. Garcia-Bernet, D., Loisel, D., Guizard, G., Buffière, P., Steyer, J., Escudié, R., 2011. Rapid measurement of the yield stress of anaerobically-digested solid waste using slump tests. Waste Manag. 31, 631–635. Heymann, L. et al., 1996. In: Ait-Kadi, A. et al. (Eds.), Proc. XIIth Int. Congress on Rheology, Laval University, Quebec City, Canada, pp. 451.
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Please cite this article in press as: Liu, G.-J., et al. The effects of temperature, organic matter and time-dependency on rheological properties of dry anaerobic digested swine manure. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2014.12.015