Anthony D. Stickland

Quantification of wastewater sludge dewatering

Quantification and comparison of the dewatering characteristics of fifteen sewage sludges from a range of digestion scenarios are described. The method proposed uses laboratory dewatering measurements and integrity analysis of the extracted material properties. These properties were used as inputs into a model of filtration, the output of which provides the dewatering comparison. This method is shown to be necessary for quantification and comparison of dewaterability as the permeability and compressibility of the sludges varies by up to ten orders of magnitude in the range of solids concentration of interest to industry. This causes a high sensitivity of the dewaterability comparison to the starting concentration of laboratory tests, thus simple dewaterability comparison based on parameters such as the specific resistance to filtration is difficult. The new approach is demonstrated to be robust relative to traditional methods such as specific resistance to filtration analysis and has an in-built integrity check. Comparison of the quantified dewaterability of the fifteen sludges to the relative volatile solids content showed a very strong correlation in the volatile solids range from 40 to 80%. The data indicate that the volatile solids parameter is a strong indicator of the dewatering behaviour of sewage sludges.

Metal–Phenolic Supramolecular Gelation

Materials assembled by coordination interactions between naturally abundant polyphenols and metals are of interest for a wide range of applications, including crystallization, catalysis, and drug delivery. Such an interest has led to the development of thin films with tunable, dynamic properties, however, creating bulk materials remains a challenge. Reported here is a class of metallogels formed by direct gelation between inexpensive, naturally abundant tannic acid and group(IV) metal ions. The metallogels exhibit diverse properties, including self-healing and transparency, and can be doped with various materials by in situ co-gelation. The robustness and flexibility, combined with the ease, low cost, and scalability of the coordination-driven assembly process make these metallogels potential candidates for chemical, biomedical, and environmental applications.

The effect of premature wall yield on creep testing of strongly flocculated suspensions

Measuring yielding in cohesive suspensions is often hampered by slip at measurement surfaces. This paper presents creep data for strongly flocculated suspensions obtained using vane-in-cup tools with differing cup-to-vane diameter ratios. The three suspensions were titania and alumina aggregated at their isoelectric points and polymer-flocculated alumina. The aim was to find the diameter ratio where slip or premature yielding at the cup wall had no effect on the transient behaviour. The large diameter ratio results showed readily understandable material behaviour comprising linear viscoelasticity at low stresses, strain-softening close to yielding, time-dependent yield across a range of stresses and then viscous flow. Tests in small ratio geometries however showed more complex responses. Effects attributed to the cup wall included delayed softening, slip, multiple yielding and stick–slip events, and unsteady flow. The conclusion was that cups have to be relatively large to eliminate wall artefacts. A diameter ratio of three was sufficient in practice, although the minimum ratio must be material dependent.

Compressional rheology: A tool for understanding compressibility effects in sludge dewatering

Water and wastewater treatment sludges exhibit compressible behaviour due to flocculation and aggregation. At a critical solids concentration called the gel point, which is as low as 1-2 v/v%, a continuous interconnected network of particles is formed that can resist an applied load. The applied load (mechanical filtration pressure or buoyancy in settling for example) must exceed the network strength in order to consolidate the network. The network strength increases with solids concentration such that the equilibrium extent of consolidation is a function of the applied load. Improved understanding of the nature of compressible suspensions can have a significant impact through optimising design and operation of sludge handling and dewatering processes. This work gives an overview of compressional rheology, which has proven to be a useful tool for describing the solid-liquid separation of compressible systems. This is followed by three examples where compressibility effects must be taken into account, namely the extraction of material properties for extremely compressible materials, consolidation and crust formation during constant rate evaporation, and the effect of bed height in thickening.

Viscoelasticity of coagulated alumina suspensions

The solid-to-liquid transition of a model coagulated alumina suspension at concentrations above the gel point was investigated to explore the critical parameter for describing network failure under shear forces. Static (creep and creep-recovery) and dynamic (small and large amplitude oscillatory) shear experiments were combined to examine shear softening in these systems and time-based dependence in the yielding dynamics. The particulate network structure exhibits failure and viscous dissipation under creep and oscillatory shear tests at stress values well below the conventionally defined yield stress. Results from strain recovery tests highlight a time-dependence for failure, where only partial recovery of strain energy was possible once a specific duration of creep was surpassed. The system was observed to fail at a common strain value across all methods of rheology testing. These results are self-consistent, showing a clear transition from the linear to non-linear viscoelastic region for a coagulated material under shear stress. It provides the starting point to incorporate mechanical viscoelastic models to extract time constants for yielding behaviour. This work also presents one of the first reported LAOS and creep results for particulate suspensions using a vane geometry.

Comparison of Geotechnical Engineering Consolidation and Physical Science Filtration Testing Techniques for Soils and Suspensions

Traditionally, there have been two approaches to the modelling and prediction of the extent and rate of dewatering of particulate networks: consolidation theory and filtration theory, developed by geotechnical engineers and physical scientists, respectively. The physical situations and governing equations for Terzaghis consolidation model (Terzaghi and Peck 1967) and Landman and Whites filtration model (Landman and White 1997) are essentially the same. However, their methods of determining the relative dewatering parameters differ. The consolidation method matches experimental data from oedometer testing to the theoretical predictions of the model in order to determine the coefficient of consolidation, cv. The filtration method determines a solids diffusivity coefficient, D, based upon the experimental data from a filtration rig, which is then used in modelling to make predictions. This work aims to highlight the similarities between the two approaches, initially by demonstrating the theoretical relationship between the two parameters, cv and D, and then through experimental determination. The material characteristics of a kaolin sample undergoing one-dimensional (zero lateral strain) compression are determined using both oedometer and filtration testing and equated using the developed theoretical relationship. The results indicate that the two testing methods are essentially the same, and that their relevant analysis techniques give similar outcomes. Consequently, geotechnical engineers can use filtration methods and physical scientists can use consolidation methods.

Wall adhesion and constitutive modeling of strong colloidal gels

Wall adhesion effects during batch sedimentation of strongly flocculated colloidal gels are commonly assumed to be negligible. In this study, in-situ measurements of gel rheology and solids volume fraction distribution suggest the contrary, where significant wall adhesion effects are observed in a 110 mm diameter settling column. We develop and validate a mathematical model for the equilibrium stress state in the presence of wall adhesion under both viscoplastic and viscoelastic constitutive models. These formulations highlight fundamental issues regarding the constitutive modeling of colloidal gels, specifically the relative utility and validity of viscoplastic and viscoelastic rheological models under arbitrary tensorial loadings. The developed model is validated against experimental data, which points toward a novel method to estimate the shear and compressive yield strength of strongly flocculated colloidal gels from a series of equilibrium solids volume fraction profiles over various column widths.

Computational models of populations of bacteria and lytic phage

Abstract The use of phages to control and reduce numbers of unwanted bacteria can be traced back to the early 1900s, when phages were explored as a tool to treat infections before the wide scale use of antibiotics. Recently, phage therapy has received renewed interest as a method to treat multiresistant bacteria. Phages are also widely used in the food industry to prevent the growth of certain bacteria in foods, and are currently being explored as a tool for use in bioremediation and wastewater treatment. Despite the large body of biological research on phages, relatively little attention has been given to computational modeling of the population dynamics of phage and bacterial interactions. The earliest model was described by Campbell in the 1960s. Subsequent modifications to this model include partial or complete resistance, multiple phage binding sites, and spatial heterogeneity. This review provides a general introduction to modeling of the population dynamics of bacteria and phage. The review introduces the basic model and relevant concepts and evaluates more complex variations of the basic model published to date, including a model of disease epidemics caused by infectious bacteria. Finally, the shortcomings and potential ways to improve the models are discussed.

The non-monotonic shear-thinning flow of two strongly cohesive concentrated suspensions☆

Abstract The behaviour in simple shear of two concentrated and strongly cohesive mineral suspensions showing highly non-monotonic flow curves is described. Two rheometric test modes were employed, controlled stress and controlled shear-rate. In controlled stress mode the materials showed runaway flow above a yield stress, which, for one of the suspensions, varied substantially in value and seemingly at random from one run to the next, such that the up flow-curve appeared to be quite irreproducible. The down-curve was not though, as neither was the curve obtained in controlled rate mode, which turned out to be triple-valued in the region where runaway flow was seen in controlled rising stress. For this first suspension, the total stress could be decomposed into three parts to a good approximation: a viscous component proportional to a plastic viscosity, a constant isostatic contribution, and a third shear-rate dependent contribution associated with the particulate network which decreased with increasing shear-rate raised to the −7/10th power. In the case of the second suspension, the stress could be decomposed along similar lines, although the strain-rate softening of the solid-phase stress was found to be logarithmic and the irreducible isostatic stress was small. The flow curves are discussed in the light of recent simulations and they conform to a very simple but general rule for non-monotonic behaviour in cohesive suspensions and emulsions, namely that it is caused by strain-rate softening of the solid phase stress.

Compressive Strength and Capillary Pressure: Competing Properties of Particulate Suspensions that Determine the Onset of Desaturation

The drying of particulate suspensions is important to many industries such as paints, ceramics, minerals processing, and pharmaceuticals. Cakes or films first consolidate due to capillary pressure and, at a critical concentration, stop consolidating and begin to desaturate. Desaturation occurs once the compressive strength of the particulate network is greater than the maximum capillary pressure at the air-liquid interface. This work combines existing descriptions of the compressive strength and the maximum capillary pressure to give the dependencies of volume fraction, particle size, interparticle bond strength, surface tension, and contact angle on the breakthrough pressure and critical concentration. Understanding the interplay of these system parameters explains the point of desaturation in filtration and drying processes, allowing optimization of these processes, including mitigation of cracking. Air-driven filtration results are presented for the direct measurement of breakthrough pressure of coagulated calcium carbonate.