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An experimental system has been found recently, a coagulated CaCO3 suspension system, which shows very variable yield behaviour depending upon how it is tested and, specifically, at what rate it is sheared. At P��clet numbers Pe > 1 it behaves as a simple Herschel Bulkley liquid, whereas at Pe < 1 highly non-monotonic flow curves are seen. In controlled stress testing it shows hysteresis and shear banding and in the usual type of stress scan, used to measure flow curves in controlled stress mode routinely, it can show very erratic and irreproducible behaviour. All of these features will be attributed here to a dependence of the solid phase, or, yield stress, on the prevailing rate of shear at the yield point. Stress growth curves obtained from step strain-rate testing showed that this rate-dependence was a consequence of P��clet number dependent strain softening. At very low Pe, yield was cooperative and the yield strain was order-one, whereas as Pe approached unity, the yield strain reduced to that needed to break interparticle bonds, causing the yield stress to be greatly reduced. It is suspected that rate-dependent yield could well be the rule rather than the exception for cohesive suspensions more generally. If so, then the Herschel-Bulkley equation can usefully be generalized to read (in simple shear). The proposition that rate-dependent yield might be general for cohesive suspensions is amenable to critical experimental testing by a range of means and along lines suggested., Ver. 3. The text has been further improved substantially too. All figures are now finalised. Submitted to J. non Newtonian Fluid Mech. Comments welcome (to RB)

A simple and popular constitutive model used to describe the compressional strength of a consolidating strongly cohesive particulate gel is tested further with new experimental data. Strong cohesive particulate gels have variously been described as being ratchet (poro) elastic, on the one hand, and as having a yield stress in compression, on the other, to the point where same groups of workers have used both descriptions at one time or another. The dichotomy is real though as such gels do show a hitherto somewhat puzzling elastic-plastic duality. This can be explained in part by the strong concentration dependence of the modulus since this leads to irreversible volumetric strain-hardening, in effect, the ratchet; but only in small part. The real problem seems to be that, until very recently, most work on consolidation has neglected what what Michaels and Bolger told us to do over 50 years ago, viz. to take into wall adhesion into account, most cohesive particulate gels being adhesive too. Since wall adhesive failure is plastic in character, the simplest non-linear elastic model of compressive strength can be combined with the simplest possible model of wall debonding to produce a approximate complete constitutive description. Examples of the use of such a description in detailed modelling of consolidation equilibrium can be found in refs 10-12. Consolidation dynamics with wall adhesion is a substantial modelling challenge remaining to be tackled., Comment: 13 pages with 6 figs. Comments (to RB) are most welcome. arXiv admin note: text overlap with arXiv:1410.5280

The yielding of concentrated cohesive suspensions can be deformation-rate dependent. One consquence of this is that a single suspension can present in one several different ways, depending upon how it is tested, or more generally, how it is caused to flow. We have seen variously Herschel-Bulkley flow, highly non-monotonic flow curves and highly erratic or chaotic yield, all in one suspension. In controlled-rate testing one sees a systematic effect of deformation rate. In controlled stress testing, matters are more subtle. Whereas step-stress creep testing will elicit reproducible behaviour, any attempt to determine a flow curve by, e.g. stepping up or sweeping stress at an inappropriate rate can lead to highly irreproducible behaviour., Comment: Updated and corrected version of a short informal article written for the Bulletin of the British Soc. Rheology. An alternative extended abstract on the same topic with an additional figure has been included

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., Comment: Revised submission to Rheol. Acta. with change of title. Corresponding author is A.D. Stickland, email: stad@unimelb.edu.au. The text has been improved substantially and there is further data analysis and modelling

Aerobic granular sludge can improve the efficacy of wastewater treatment in existing sequencing batch reactors (SBRs) compared to conventional floccular sludge. Despite the well known advantages of increased capacity, reduced reactor footprint and enhanced nutrient removal, many performance characteristics of granular and floccular sludges have not been directly compared, in particular for sludges generated from treating the same sewage source. In this study we investigated the microstructure, microbial composition, settling and dewatering properties of granular and floccular sludge generated from the same sewage sources. Cryo-scanning electron microscopy analysis indicated that the granular and floccular sludges had a significantly different internal network structure, with flocs being much more open and porous than granules. Sulphur deposits were clearly visible on the surfaces of granules, which was complemented by the enrichment of some sulphur oxidising bacterial species compared to floccular sludge, as assessed by Next Generation Sequencing (NGS). Dewatering measurements showed, as expected, that the settling velocity of granules was much faster than flocs at solids concentrations below the gel point. At at higher solids concentrations above the gel point, however, both flocs and granules form a similar gelled network structure with no significant difference in dewaterability. We used the dewatering properties in process models to predict the impact of granulation on thickener and filter throughput. Overall, this work highlights the biological, structural and physical differences and similarities between granular and floccular sludges with the outcome being improved settling for granules but no significant difference in downstream dewatering properties that pertain to filtration processes. Refereed/Peer-reviewed