Thixotropic behaviour of thickened sewage sludge
© Trávníček and Junga; licensee BioMed Central Ltd. 2014
Received: 26 April 2013
Accepted: 12 April 2014
Published: 23 April 2014
The aim of the work is a description of the rheological behaviour of thickened sewage sludge. The sample of thickened sludge was collected from the wastewater treatment plant, where pressure flotation unit is used for a process of thickening. The value of dry matter of collected sample was 3.52%. Subsequently the sample was diluted and the rheological properties of individual samples were obtained. Several types of rheological tests were used for the determination of the sample. At first the hysteresis loop test was performed. The next test was focused on the time-dependency, i.e. measurement of dependence of dynamic viscosity on the time at constant shear rate. Further dependence dynamic viscosity on the temperature was performed. Then the activation energy was obtained from measured values. Finally, the hysteresis areas were counted and measured values were evaluated with use of Herschel-Bulkley mathematical model.
KeywordsRheology Mathematical model Herschel-Bulkley Sewage sludge Pressure flotation
The Rheological measurements of substances are very important and find applications in many fields of human activity. Determination of the rheological behaviour of substances is particularly important for designing of equipments for transport, pumping and storage of substances. Survey of the rheological behaviour also play very important role in the food rheology, where rheology among other things related with quality control and sensory properties . The other applications of the rheology are for example the polymer industry , the building industry , metallurgy , geology and mining industry . Equally important application is use of the rheology in the waste management. These include wastewater treatment, and sewage sludge utilization. Determination of rheological parameters of sewage sludge is the base for its characterization. Information about rheological properties is very important for processes, which relate with the utilization of sewage sludge. These are especially transport, dewatering, drying and landfilling of sewage sludge . The rheological properties of various kinds of sewage sludge can be very different. These properties are different in various steps of wastewater treatment [7, 8]. It is known that suspensions of biological sewage sludge are non-Newtonian fluids . However, from the rheological point of view very thin layers behave as Newtonian fluid. When the suspension is concentrated the sludge starts to behave as non-Newtonian fluid . The dependence of total solid soluble (TSS) on rheological properties of the activated sewage sludge was described by other authors [9–11].
Problematic of pumping and transportation of thickened sludge was described for example in works of author Slater [12, 13]. But rheological behaviour of thickened sludge is very little discussed in professional works. However for example Baudez et al. pointed out that rheological behaviour of activated (raw) and anaerobically digested sludge at low shear and a shear thinning at high shear stress are viscoelastic [14–16]. But these types of sludges have the thixotropic behaviour at intermediate shear stress . It can be expected that the rheological behaviour of thickened sludge is very similar. As was mentioned previously the rheological behaviour of sewage sludge depends on total solid soluble, but also content organic and inorganic substances per unit volume, number of particles and particle size distribution. It is also proved that the change of a particle size distribution change also rheological properties of sewage sludge, for example after disintegration [18, 19]. The goal of the paper is to extend the knowledge about rheological behaviour of thickened sewage sludge, which is not sufficiently presented in professional articles. For the reason the change of the rheological properties of thickened sludge can be observed, the researched sample was diluted in various ratios. By diluting data were obtained for various thickened sewage sludges with various dry matters.
Material and methods
Description of samples
Description of samples
Diluted in rate 1:1
Diluted in rate 1:2
Diluted in rate 1:9
Measurement of rheological behaviour
There are several methods, which are designed for measurements of the rheological behaviour of substances with different types of measuring geometry such as concentric cylinders, cone and plate or parallel plates . An extensive overview about measurement techniques for rheological testing is given in paper . Rheological measurement of substances for this paper was performed using rheometr Anton Paar MCR 102 (Austria) with measuring geometry plate–plate. The diameter of plate was 50 mm. The constant shear test was performed with value of shear rate 50 s-1.
The rheological experiments were performed with non-dilute sample and with dilute samples in various rates. The flow curves were modelled by using the following model:
τ – shear stress [Pa]
τ 0 – yield stress [Pa]
K – consistency coefficient [-]
n – flow behaviour index [-]
– shear rate [s-1]
The change of dynamic viscosity in dependency on temperature was measured at temperature range 5 – 40°C. Shear rate was constant with value 50 s-1.
η 0 – initial value of dynamic viscosity [Pa∙s]
E A – activation energy [J]
R – universal gas constant [J∙K-1∙mol-1]
T – thermodynamic temperature [K]
This model was used for an evaluation of dependence of the dynamic viscosity η on the temperature and for an evaluation of the activation energy E A . The rate of the thixtotropy was determined by equation:
All measurements were performed in three repetitions. Subsequently arithmetic mean has been evaluated from measured data. The data were tested by Grubbs test for remoteness values.
Results and discussion
Base characteristics of measured samples
Dry matter (%)
Water content (%)
Loss on ignition (%)
Residue on ignition (%)
Hysteresis area (Pa · s-1)
Evaluating Herschel-Bulkley model for non-diluted sewage sludge
Yield stress (Pa)
Flow index (-)
Activation energy of individual samples
Activation energy(J · mol-1)
The table shows that activation energy is dependence on the dry matter of the sample. It means that in case of decreasing of dry matter values of activation energy increases. For comparison for example Yang et al. presents that the activation energy of activated sewage sludge from membrane technology was 9217 J∙mol-1 and activation energy of activated sewage sludge was 6000 J∙mol-1. The values are thus very similar.
The rheological measurements were performed by rotational rheometer with geometry plate–plate. It has been showed that thickened sewage sludge has the similar rheological properties as activated sewage sludge or digested sewage sludge. Similarly as these types of sludges the thickened sewage sludge has thixotropic behaviour in intermediate shear stress. The rate of a thixotropy decreases with an increase of water content in the sample. It was also showed that Herschel-Bulkley mathematical model is very suitable for evaluating of shear stress dependence on the shear rate too. Determination coefficient was ranged mostly about 0.99. The calculation of activation energy was performed by using Arrhenius mathematical model. The values of activation energy were very similar as other types of sewage sludges, such as activated sludges.
This study was financed by the Sixth Framework Programme of the European Community for research, technological development and demonstration activities (2007–2017), No. 7AMB12SK076 Production of biogas from non-traditional biodegradable material and supported by project CZ.1.02/5.1.00/10.06433 Acquisition of Instrumentation for BAT Centre at MZLU in Brno for Categories of Food Processing Activities and Categories of Facilities for Disposal or Destruction of Animal Bodies and Animal Waste.
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