Rheological Characterisation of blends of Primary and Secondary Sludge

Transcription

1 Rheological Characterisation of blends of Primary and Secondary Sludge Flora Markis 1, 2, Kevin Hii 1, Raj Parthasarathy 1, Jean-Christophe Baudez 1, 2, Paul Slatter 1, and Nicky Eshtiaghi 1 1 Rheology and Materials Processing Centre, Dept. of Chemical Engineering, RMIT University, Victoria, Australia, Irstea, UR TSCF, F Montoldre, France Abstract An understanding of sludge rheology during anaerobic digestion can improve digester performance, particularly with regards to mixing. However, the black box nature of digesters limits the knowledge on sludge flow behaviour, once it enters the digester. A study on the flow properties (i.e. apparent viscosity and yield stress) as well as the yielding and restructuring behaviour of primary, secondary and blends of primary and secondary sludge was carried out. Primary and secondary sludge feeds to the anaerobic digester were obtained from the Mount Martha wastewater treatment plant, Mornington Peninsula (Victoria, Australia). The Herschel-Bulkley model was used to determine flow properties of these individual sludge feeds at approximately same total solids concentration (5% TS). Creep tests (below and above yield stress) and shear stress sweep tests (at various times of rest from 60 to 3600s) were conducted to study the yielding and restructuring behaviour of the individual sludge types. Primary sludge (5% TS) was then mixed with secondary sludge (5% TS) at different ratios (0 80%) and their flow properties analyzed at 20 oc as sludge blends, mimicking conditions within an anaerobic digester. The Herschel- Bulkley model was used to model their flow properties. It was shown that primary sludge acted as a jammed material whose rheological parameters (apparent viscosity, yield stress and shear thinning properties) were lower than secondary sludge at approximately the same total solids concentration. The viscosity, yield stress and shear thinning behaviour of blends of inlet sludge feed increased with increasing ratio of secondary sludge to primary sludge. (Abstract) Keywords- municipal sludge; rheology; non-newtonian fluid; primary sludge; secondary sludge (key words) I. INTRODUCTION The rheology of sludge is of great importance in understanding the flow behaviour of sludge as it allows one to predict its behaviour during the various stages of sludge treatment such as pumping, chemical conditioning, mixing, storage, and dewatering (Slatter, 2008; Battisoni, 1997; Bhattacharya, 1981). However, the rheological properties and microstructure of sewage sludge are not well understood (Campbell & Crescuolo, 1982; Dentel, 1997; Dursun et al., 2004; Forster, 1982). Most literature focus on the rheological characterization of activated sludge (Dick & Ewing, 1967; Dursun et al., 2004; Seyssiecq et al., 2008; Tixier et al., 2003b; Tixier et al., 2003a; Xia et al., 2009) with few studies on primary (Bhattacharya, 1981; Moeller & Torres, 1997)and digested sludge (Battistoni, 1997; Baudez et al., 2911; Bhattacharya, 1981; Campbell & Crescuolo, 1982; Eshtiaghi et al., 2012b; Forster, 2002; Forster, 1982; Wang et al., 2011). These studies attempt to link operating parameters with the physico-chemical and non-newtonian flow properties (apparent viscosty, yield stress, thixotropy) of sludge using rheological models such as Ostwald, Sisko, Bingham, Herschel-Bulkley and Casson. Except for Battacharya (1981) and Moeller and Torres (1997), little knowledge is available on how to rheologically characterise primary sludge or its behaviour prior to and once it enters a digester. However, inconsistencies are found between both works, presenting a major knowledge gap. Whilst Battacharya (1981) described primary sludge as a shear thinning, yield stress material, Moeller and Torres (1997) detected no yield stress. Furthermore, Battacharya (1981) described digested sludge as a dilatant material, which contradicts current literature (Baudez et al., 2011; Eshtiaghi et al., 2012b; Slatter, 1997). Developing knowledge on the characterisation of primary sludge will provide industry with the correct procedure and estimation of its rheological properties at some given total solids concentration, shear rate and temperature. Bouallagui et al. (2010) studied the effect of different volume ratios of primary sludge to activated sludge and organic loading rate on digester performances at specific hydrolic retention time (20 days). However, no mention of how the rheological aspects change with blending primary and secondary sludge and its effect on anaerobic digester performance was made, highlighting another knowledge gap - which is the focus of this project. A "universal" process to characterise the rheology of sludge has been nearly impossible to determine due to a lack of consistency between literatures (Seyssiecq et al., 2003). This inconsistency is due to the nature of the material in question as

2 well as a lack of uniformity between rheometric techniques associated with sludge (Mori et al., 2006; Novarino et al., 2010; Seyssiecq et al., 2003; Wang et al., 2011, Spinosa and Lotito, 2003; Baudez and Coussot, 2oo1; Baudez et al., 2011; Eshtiaghi et al, 2012b; Eshtiaghi et al., 2012a). This shows that sludge is generally a difficult material to characterise quantitatively. The current study focuses on developing methods to rheologically characterise individual inlet sludge feeds to the digester (primary, secondary and digested) in order to give a true picture of what is happening in the digester. This allows the prediction and estimation of the right blending ratio required to achieve particular sludge rheological properties during mixing. II. EXPERIMENTALS A. Sample Preparation Samples of inlet sludge feeds (primary and secondary) to the anaerobic digester were obtained from the mount Martha wastewater treatment plant (Mornington Peninsula, Victoria, Australia). Initial total solids concentration (%TS) of primary sludge was 5.0% TS. Secondary sludge in the form of dissolved air flocculated thickened sludge at 4.0% TS was thickened to 5.0% TS by vacuum filtration. Mixtures of 5% TS primary sludge and 5% TS secondary sludge were prepared at different ratios (0-80%). B. Rheometric Technique Rheological measurements were conducted using a controlled stress rheometer (DSR200, TA Instruments) equipped with the concentric cylinder geometry (D c = 32.0 mm; D b = 29.4 mm, L b = 44 mm). Figure 1 - Plot of flow curve (top) and viscosity curve (bottom), (5.0 % primary sludge, 20 o C). The tests conducted as follows: Flow curve (decreasing ramp starting at high stress corresponding to shear rate of 1000 s -1, 20 o C). Creep test (below and above yield stress, 20 o C). Shear stress sweep test (at various times of rest from 60 to 3600 s, 20 o C). Prior to each measurement, the sample is pre-sheared for 600 s at high stress corresponding to a shear rate of 1000 s -1 and allowed to rest for 60 s. Shear stress sweep tests were conducted only for primary and secondary sludge. III. RESULTS AND DISCUSSIONS Primary and secondary sludge were found to be complex, shear-thinning materials exhibiting a yield stress (Figure 2, Figure 3). The rheological behaviour of secondary sludge was consistent with current literature on activated sludge (Dick & Ewing, 1967; Dursun et al., 2004; Forster, 2002; Forster, 1982; Guibaud et al., 2004; Mikkelsen, 2001; Mori et al., 2006; Seyssiecq et al., 2008; Tixier et al., 2003b; Tixier et al., 2003a, Xia et al., 2009). The rheology of primary sludge was consistent with the works of Battacharya (1981); however, it was inconsistent to the works of Moeller and Torres (1997). Figure 2 - Plot of flow curve (top) and viscosity curve (bottom), (5.0% primary sludge, 20 o C, 37 o C and 55 o C).

3 Figure 4 - Creep test (5.0% primary sludge, 20 o C) - yielding at approxximately 1.8 Pa. Figure 3 - Plot of flow curve (top) and viscosity curve (bottom), (5.0% secondary sludge, 20 o C, 37 o C and 55 o C). Moeller and Torres (1997) described primary sludge as a shearshinning material exhibiting no yield stress which cannot be the case as our reproducible results indicate that primary sludge does exhibit a yield stress. The Herschel-Bulkley model was used to characterise the rheology of primary and secondary sludge. This model has been used to characterise various sludge types because it takes into account the yield pseudo plasticity of sludge (Baudez et al., 2011; Slatter (1997). The rheological behaviour of primary and secondary sludge at various temperatures was measured in order to understand the temperature dependency of sludge at temperatures that mimic anaerobic digester conditions (37 C and 55 C). The rheological behaviour was modelled using the Hershel-Bulkley model. The yield stress and shear-thinning behaviour of both primary and secondary sludge decreased with increasing temperature. The fluid consistency of primary sludge increased; however, it remained almost constant for secondary sludge. In general, the apparent viscosity decreased with increasing temperature over a wide shear rate range. This temperature dependency of sludge is consistent with literature (Manoliadis, 1984; Baudez et al., 2013), however no correlation has been made to link the temperature dependency and sludge as this is out of the scope of the paper. Whilst the temperature dependency of sludge is considered, the experimental analysis on blends of sludge as well as individual sludge were conducted at 20 C as a reference in order to understand the material behaviour of blends of sludge. The results also indicate that under low shear, primary sludge acts similar to a jammed material where there is a fight between floc build up and break up (Baudez, 2008). Figure 5 - Creep test (5.0% Secondary sludge, 20 C) yielding at 4.1 Pa The yielding of primary and secondary sludge (Figure 4, Figure 5) at the same solids concentration (5%TS) was determined using creep tests by imposing a constant stress and measuring the resulting strain (%) over time. The slope of the creep curve decreases in time corresponding to stresses less than the yield stresses. This means that a critical deformation has not been reached and the sludge is in the viscoelastic regime. However, when the slope of the creep curve becomes constant and equal to one, the sludge is said to be flowing. Hence, the critical deformation has been overcome. In this case, primary sludge yields at a lower stress (1.8 Pa) than secondary sludge (4.1 Pa) due to the nature of the material. Primary sludge behaves as a two phase suspension whilst secondary sludge behaves as a homogenous paste. The network structure of primary sludge is easier to break than secondary sludge resulting in a lower yield stress.

4 The stress strain behaviour at different rest time (Figure 6) was measured for both primary and secondary sludge. This illustrated any structural changes over different periods of time and can be applied to digester mixing. The stress strain behaviour of primary sludge behaved accordingly with increasing strain at increasing rest time indicating that the sludge recovers its structure at prolonged rest time. The stress strain behaviour of secondary sludge did not follow this behaviour; with the strain decreasing at increased time of rest (900s, 1800s and 3600s). This phenomenon is impossible indicating error in the measurements. This behaviour was observed by Baudez et al. (2011) and was associated with error. able to rebuild itself resulting in the formation of dead zones over prolonged periods of time. Figure 7 - Evolution of stress at a constant strain (2%) of primary sludge (a) and secondary sludge (b) The apparent viscosity of blends of primary and secondary sludge was determined. These were modelled using the Herschel-Bulkley model. Blends of sludge displayed shearthinning yield stress behaviour. The rheological properties namely, the apparent viscosity, shear thinning behaviour and yield stress decreased with increasing percentage of primary sludge in the mixture. It was found that the addition of secondary sludge lead to the fluidification of primary sludge. This resulted in a more homogenous mixture. Figure 6 - Stress Strain behaviour at different rest time (60s to 3600s, 20 C) of 5.0% primary sludge (top), 5.0% secondary sludge (bottom). The evolution of stress at a single strain was studied in order to understand the stress strain behaviour at different times of rest. It was observed that the stress increases logarithmically with increasing time of rest indicating that the sludge structure recovers over time. When this is applied to anaerobic digester mixing, it means that the longer the sludge is kept in the digester at low shear rates; the sludge structure is Figure 8 - Apparent viscosity curve, blends of primary and secondary sludge (5% TS)

5 A master curve was developed in order to predict the behaviour of any sludge sample based on the data from the original primary, secondary and blends of sludge. This master curve developed based on a dimensionless form of the Herschel-Bulkley model. Research, 45, [5] BHATTACHARYA, S. N Flow characteristics of primary and digested sewage sludge. Rheologica Acta, 20, [6] CAMPBELL, H. W. & CRESCUOLO, P. J The use of rheology for sludge characterization. Water Science and Technology, 14, [7] DENTEL, S. K Evaluation and role of rheological properties in sludge management. Water Science and Technology, 36, 1-8. [8] DICK, R. I. & EWING, B. B The rheology of activated sludge. Journal of the Water Pollution Control Federation, 39, [9] DURSUN, D., AYOL, A. & DENTEL, S. K Physical characteristics of a waste activated sludge: conditioning responses and correlations with a synthetic surrogate. Water Sci Technol, 50, [10] ESHTIAGHI, N., MARKIS, F. & SLATTER, P. 2012a. The laminar/turbulent transition in a sludge pipeline. Water Sci Technol, 65, Figure 9 - Dimensionless master curve. CONCLUSIONS The rheological characterisation of blends of primary and secondary sludge was determined in order to predict the behaviour of sludge after mixing. Flow measurements on individual primary and secondary sludge as well as blends of sludge illustrated that the sludge displayed shear thinning, yield stress behaviour. The apparent viscosity and yield stress of primary sludge were lower than secondary sludge at the same total solids concentration (5%TS). Primary sludge behaved as a jammed material whilst the addition of secondary sludge resulted in the fluidification of primary sludge. Primary sludge also yielded at lower stresses than secondary sludge. The evolution of stress over prolonged periods of rest time indicated that the sludge structure was able to rebuild itself. A master curve was developed to determine the rheology of any sludge based on the rheology of the primary and secondary sludge. This master curve was developed based on a dimensionless form of Herschel-Bulkley model. References [1] BATTISTONI, P Pre-treatment, measurement execution procedure and waste characteristics in the rheology of sewage sludges and the digested organic fraction of municipal solid wastes. Water Science and Technology, 36, [2] BAUDEZ, J. C Physical aging and thixotropy in sludge rheology. Applied Rheology, 18, [3] BAUDEZ, J. C. & COUSSOT, P Rheology of aging, concentrated, polymeric suspensions: Application to pasty sewage sludges. Journal of Rheology, 45, [4] BAUDEZ, J. C., MARKIS, F., ESHTIAGHI, N. & SLATTER, P The rheological behaviour of anaerobic digested sludge. Water [11] ESHTIAGHI, N., YAP, S. D., MARKIS, F., BAUDEZ, J.-C. & SLATTER, P. 2012b. Clear model fluids to emulate the rheological properties of thickened digested sludge. Water Research, 46, [12] FORSTER, C. F SLUDGE SURFACES AND THEIR RELATION TO THE RHEOLOGY OF SEWAGE SLUDGE SUSPENSIONS. Journal of chemical technology and biotechnology, 32, [13] FORSTER, C. F The rheological and physico-chemical characteristics of sewage sludges. Enzyme and Microbial Technology, 30, [14] GUIBAUD, G., DOLLET, P., TIXIER, N., DAGOT, C. & BAUDU, M Characterisation of the evolution of activated sludges using rheological measurements. Process Biochemistry, 39, [15] MIKKELSEN, L. H The shear sensitivity of activated sludge: Relations to filterability, rheology and surface chemistry. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 182, [16] MOELLER, G. & TORRES, L. G Rheological characterization of primary and secondary sludges treated by both aerobic and anaerobic digestion. Bioresource Technology, 61, [17] MORI, M., SEYSSIECQ, I. & ROCHE, N Rheological measurements of sewage sludge for various solids concentrations and geometry. Process Biochemistry, 41, [18] NOVARINO, D., SANTAGATA, E., DALMAZZO, D. & ZANETTI, M Rheological Characterisation of sludge coming from a wastewater treatment plant. American Journal of Environmental Sciences, 6, [19] SEYSSIECQ, I., FERRASSE, J.-H. & ROCHE, N State-of-theart: rheological characterisation of wastewater treatment sludge. Biochemical Engineering Journal, 16, [20] SEYSSIECQ, I., MARROT, B., DJERROUD, D. & ROCHE, N In situ triphasic rheological characterisation of activated sludge, in an aerated bioreactor. Chemical Engineering Journal, 142, [21] SLATTER, P Pipe flow of highly concentrated sludge. J Environ Sci Health A Tox Hazard Subst Environ Eng, 43, SLATTER, P. T The rheological characterisation of sludges. Water Science and Technology, 36, 9-18.

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