In this post, Luka Safaric, ATBEST researcher at Linköping University discusses the importance of an understanding of the rheological properties of AD sludge.
When operating a biogas reactor, we generally follow several parameters to make sure it is running as efficiently as possible. Rheology is however often not one of them. But should it be? Surely everything seems to be running just fine without worrying about this aspect of anaerobic digestion, so why should we spend our time, effort, and money on it?
And what exactly is rheology anyway?
Well, the “official” definition is that rheology describes the deformation of a body under the influence of stresses. Bodies in this context can be either solids, liquids, or gasses (Schramm, 2000).
So let us focus on fluids (as that is mainly what our reactors contain). When describing their behaviour, we use terms such as viscosity, shear rate, and shear stress. Viscosity is used to describe the fluid’s resistance against irreversible positional change – therefore the higher it is, the more energy is needed to make the fluid flow. If we now imagine an example when the fluid is positioned between two parallel plates, then the shear stress would be the force applied tangentially to the liquid through one of the plates as it moves in a parallel direction in relation to the other one, divided by the area this force is acting upon (i.e. the area of the plate-liquid interface) (Schramm, 2000). In more practical terms, shear stress is the force exerted on the liquid when we stir it. Shear rate on the other hand, describes the drop in flow speed across the gap size (between the both plates in our example) as the liquid will develop thin layers across this gap, flowing at different speeds (decreasing from the moving, towards the stationary plate) (Schramm, 2000). This can therefore be considered as analogous to the speed with which we mix the fluid.
Generally we can separate liquids into two distinct categories – Newtonian and non-Newtonian. The former, also known as ideal fluids, are characterized by the fact that their viscosity does not change with changes in shear rate. In other words – their viscosity is unaffected by how intensively we mix them. We are quite used to this type of liquid behaviour because we deal with water on a daily basis. In reality Newtonian fluids are rare in comparison with non-Newtonian ones (Schramm, 2000). Some examples (besides water) are methanol, olive oil, and glycerol (Björn et al., 2012).
Non-Newtonian fluids on the other hand, can behave in many interesting ways and are thus further classified into several different categories. For example, we have the pseudoplastic fluids, which are characterized by drastic viscosity decreases at increasing shear rates. The more intensively we mix them, the less viscous they are (up to a certain point). Examples of such fluids are corn syrup and ketchup (Björn et al., 2012).
Then there are liquids, showing a dilatant-flow behaviour, which is opposite from the previous type, as their viscosity increases significantly at increasing shear rates. Therefore the more intensively we mix them, the more viscous they become. This can be seen with honey, cement, and ceramic suspensions (Björn et al., 2012).
Fluids may also exhibit plasticity. In this case they behave like pseudoplastic fluids, only that they additionally feature a so-called yield point. Such a liquid will initially behave as a solid, until a certain threshold in shear stress is reached. After that, the internal forces of the liquid will no longer be strong enough to resist the outside forces and it will start to flow. Blood and some sewage sludges can behave in this way (Björn et al., 2012).
There are even some liquids whose behaviour is affected by their shear history. In other words, they are capable of “remembering” if and how much they were mixed recently. They are called thixotropic and rheopectic liquids and they are in a way opposites of one another. Thixotropic liquids solidify if left alone for a sufficient amount of time due to interactions between particles and/or molecules developing in the liquid. These are often hydrogen or ionic bonds, which are relatively weak, so they rupture easily if the dispersion is subjected to shear over a sufficient period of time, transitioning back into their fluid state. The solid structure is then reformed if the liquid is allowed to rest again for a while. What this means is that the shear history of the liquid affects its rheological properties, i.e. the longer you mix it, the lower the viscosity (to a certain level). This can be observed with some paints, soap, wastewater, and sewage sludge (Björn et al., 2012). On the contrary, the rheopective liquids are characterized by a viscosity increase related to the duration of shear stress. When they are allowed to rest, they will recover their original low viscosity. Such liquids are very rare (Schramm, 2000).
But why should we care about any of this while running biogas reactors? In most common reactor designs, the anaerobic sludge inside them is a complex dispersion of solid particles and soluble molecules in water. Most often it can be classified as one of the non-Newtonian fluid types. Which one and what viscosity it has is very much dependent on many different parameters in the process, and can change due to significant shifts in said parameters. It has even been observed that two reactors of the same type (e.g. CSTR), digesting the same type of substrate, can exhibit different fluid behaviour (Björn et al., 2012). This has important implications as it might happen that a reactor was designed with a different fluid type in mind than the one it actually contains, thus leading to possible operational problems.
For example imagine mixing a dilatant-flow type of sludge very intensively. It would resist your attempts by increasing its viscosity and causing you to spend extra energy while losing mixing efficiency. Another example would be using an intermittent mixing regime on a sludge that exhibits thixotropic behaviour. By stopping the stirring, you would allow the viscosity to increase, again increasing your energy consumption and decreasing mixing efficiency when the stirrers are turned back on. Different sludges therefore need to be treated differently.
Currently, when biogas reactors are being designed, liquid viscosity is often only estimated based on reference data that is based on the relationship between the total solids concentration of sewage sludge digester fluids and their viscosity. This can be problematic when used for reactors that will be digesting other substrates, which may impose different rheological characteristics on the sludge despite having similar total solids contents. The stirring equipment and the stirring intensity might therefore not be optimal for the type of sludge in question. This can be problematic because stirring is an important part of the anaerobic digestion process as it brings the microorganisms into contact with new feedstock, facilitates the release of biogas from the sludge, and helps with temperature distribution. Inadequate stirring therefore has detrimental effects on the general efficiency of the reactor, and can potentially lead to severe operational problems such as foaming. Because of this, more attention should be directed to actual rheological characteristics of the sludge than it currently is. (Björn et al., 2012)
We can accurately determine these characteristics through rheological analyses. We take samples of the sludge and analyse them with a rheometer. These come in many different shapes and sizes, but the most suitable for anaerobic sludge characterization, are the rotational rheometers, which spin a cylinder in a cup with the sample to determine its rheological properties. The result is a graphical representation of the relationship between shear stress and shear rates, called a rheogram. Alternatively, viscosity may also be plotted against shear rates, thus creating a viscosity curve (similar to the example images for different fluid types above). Based on the shape of these curves, we can determine which type of fluid we are dealing with. We can also use values attained by our measurements and compare different sludges to one another.
A more advanced use of rheological results can be done through computational fluid dynamics, which allows us to model and predict mixing behaviour of the sludge in relation to our specific reactor design. We can then optimise mixing time, power consumption, flow patterns and velocity profiles (Latha et al., 2009).
We can therefore conclude that rheology is an important, yet often overlooked parameter of anaerobic digestion that can have big implications for reactor performance.
Björn A., Segura de La Monja P., Karlsson A., Ejlertsson J., Svensson B.H. 2012. Rheological characterization. In: Sunil Kumar (Ed.), Biogas. Chapter 3 (63-76). Tech publisher, Rijeka, Croatia. ISBN 979-953-307-221-9.
Latha, S, Borman, DJ and Sleigh, PA (2009) CFD multiphase modelling for evaluation of gas mixing in an anaerobic digester. In: Aqua-enviro, TT, (ed.) UNSPECIFIED 14th European Biosolids and Organic Resources Conference and Exhibition, 9‐11th November 2009, The Royal Armouries, Leeds, UK.
Schramm G. 2000. A practical approach to rheology and rheometry. 2nd Ed., Thermo Haake Rheology, Germany.