Chemical and Biomolecular Engineering - Theses
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ItemQuantification of wastewater dewaterability for understanding sludge dewatering and fouling during membrane bioreactor operation( 2018)This thesis investigated the dewatering properties of wastewater treatment sludges, and biofouling layers in membrane bioreactors. An experimental and data analysis methodology was developed that unified existing procedures for lab-scale filtration, centrifugation and gravity settling tests. This unified dewaterability characterisation methodology was used to quantitatively compare fifteen wastewater sludge samples. The comparison highlighted a correlation between lower volatile suspended solids and improved filterability. Further modelling of the extreme compressibility of biofouling layers has provided insights into optimisation of water recycling.
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ItemAggregate densification behaviour in sheared suspensions( 2012)Analysis of batch settling tests incorporating shear indicates that the aggregate densification rate increases in time as the test proceeds, nominally as the concentration of solids in the settled bed increases. The concentration dependent behaviour of the rate of densification has been investigated and a semi-empirical relation proposed that describes the rate of densification as a function of solids concentration. This semi-empirical relation is an application of collision rate theory. Modelling and analysis were performed by further generalisation of the Extended Kynch Method to include the effect of concentration on the densification term. While the Extended Kynch Method provides a macroscopic view of aggregate densification, a study of micro-scale behaviour provided insight into the fundamental cause of aggregate densification. To achieve this, simulations of aggregate-aggregate collisions were performed using the Discrete Element Method (DEM). DEM simulations provide some insight into such factors as bonding stiffness, collision velocity, and the effect of multiple impacts on colliding aggregates. Finally, consideration of the effect of raking a networked suspension shows that whilst aggregate densification may be achieved at higher rates, ultimately shear enhanced dewatering is less likely to be effective at high concentrations, despite the concentration dependence of the rate. It was found that for the systems studied, the material properties reveal that the rate of consolidation is flux limited. Through the incorporation of densification functional forms into multi-dimensional simulations, future work would bridge the gap between laboratory scale measurements and predictions of full-scale thickener behaviour.
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ItemEffect of flocculation conditions on the dewaterability of hematite and red mud suspensions( 2005)Solid-liquid separation plays an important role in many industrial applications. One such example is the Bayer process, which is used to extract alumina from bauxite. During this process aluminium containing species present in the bauxite ore are extracted with hot, concentrated sodium hydroxide. The soluble sodium aluminate that is formed is then separated from the insoluble red mud residue in a gravity settling device referred to as a thickener. While the sodium aluminate undergoes further processing to form alumina, the red mud is washed in a counter-current washer train prior to disposal in a tailings dam. To hasten the solid-liquid separation process flocculants are added to the gravity settling devices. Of particular interest was the use or these tools to investigate the effects of flocculant type, dose, mixing conditions and temperature on suspension dewaterability and how this in turn affects the performance of a hypothetical thickener or washer. Due to the complex nature of red mud suspensions, the majority of the characterisation work was carried out using hematite as a model system. Limited investigations were performed on red mud suspensions to ensure the suitability of hematite as a model system. Flocculation and suspension dewaterability characterisation tests were carried out on four flocculants, namely Alclar 665, W50, HXPAM and Starch. (From abstract)
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ItemControlling clay behaviour in suspension: developing a new paradigm for the minerals industry( 2011)Clays, which are often associated with mineral deposits, can be the source of significant problems in mineral processing operations. Their presence as impurities in low grade ores can lead to issues such as high pumping energy, high water consumption and large volume of tailings. Current strategies for dealing with clay related issues in mineral processing are all end-of-pipe in nature, in the sense that they all attempt to solve the problems well after they have been created through clay dispersion. However, the effectiveness of such strategies will always be limited due to the physical and chemical nature of clays. The purpose of this study is to develop a new paradigm for the minerals industry to control clay behaviour in suspensions using a start-of-pipe, or up front strategy that reduces the level of dispersion and break-up of the clays throughout the process. Through such a strategy, the occurrence of clay-associated problems will be predicted and moderated at the front end of a processing plant. This could result in significant improvements in dewaterability over the current end-of-pipe practice. The investigation of the start-of-pipe strategy for minimising in process clay break-up, referred to as controlled dispersion, was conducted on two clay minerals that are commonly associated in mineral ore bodies: kaolinite (non-swelling clay) and montmorillonite (swelling clay). In both cases, the focus was on controlling the dispersion and break-up of the clays during initial hydration, but due to differences in their physical and chemical properties, different strategies were investigated for each clay mineral subgroup. For the swelling clay minerals, the investigation was focused on as-mined Volclay bentonite swelling control through monovalent (Na+) and divalent (Ca2+) ion exchange and double layer compression. For the non-swelling clay minerals, the investigation was focused on the implementation of cementation via aluminium hydroxide precipitation on as-mined Skardon River kaolin. The outcomes from the investigation demonstrated that effective dispersion control could be achieved with the application of different controlled dispersion strategies. The resultant quantitative improvements in the physical properties of the clays, such as their dewaterability, were in excess of one order of magnitude in many cases. The benefits of the strategy to a mineral processing operation were illustrated via model simulations of the operation of an arbitrary steady state thickener. With the implementation of the controlled dispersion strategy, improvements were observed in either throughput or required equipment size in excess of one order of magnitude. Further investigations were then conducted on mixed clay mineral systems to illustrate the additive nature of the dispersion control strategies. The results highlighted the effectiveness of each controlled dispersion strategy in controlling the dispersion of the respective clay minerals, and also highlighted the importance of adequate clay mineralogical data for the processing plant feed to allow continuous adjustment and optimisation of the controlled dispersion strategy for systems with a clay composition that varies. For this reason, the potential use of Quantitative Scanning Electron Microscope system (QEMSCAN) technology as a clay mineral characterisation tool was investigated. However, due to its limitations in quantifying different clay minerals, the existing QEMSCAN technology needed development by customising the existing sample preparation method for clay-rich samples and updating the Species Identification Protocol (SIP) with the mineral database from a number of swelling and non-swelling clays. As a validation study, the QEMSCAN measurements of a range of clay-spiked samples were compared with the analysis from X-ray Diffraction. In general, the results demonstrated the potential for use of QEMSCAN for providing basic clay mineralogical data. However, further work needs to be completed to improve and quantify the accuracy of the results.
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ItemCharacterisation of shear upon dewaterability of colloidal suspensions( 2004)Solid-liquid separation is an important unit operation in many industrial processes. Research on the process optimisation and technical advancement of this operation is crucial to ensure a reliable and economical process. Work include developments in filtration theory and understanding of suspension behaviour are constantly investigated to ensure the process reaches the target. In this work, enhancement in dewatering was quantified with the use of shear. Shear, in this context, is a mechanism used to deform a suspension network such as the action of raking in thickeners. In order to understand the inter-play between shear and dewatering for colloidal networked suspension, the relationship between shear and compression rheology was investigated. The effect of shear was investigated in two situations, which are the presence of shear during and before dewatering. The former was achieved by applying an electric field to a suspension during (in-situ to) dewatering and the latter was achieved through varying shear condition for flocculated suspensions prior to dewatering. The analysis of dewatering properties was then performed by applying the consolidation theory of Landman and White (Landman et al. 1995; Landman and White 1997). The materials used in the study were two types of colloidal metal oxide particles namely AKP-30 alumina and Ajax kaolin. They were chosen due to their physical differences such as particle size and shape. The dispersion state of these suspensions from coagulated to dispersed, was fully controlled by manipulation of the particle surface chemistry. This changed the suspension micro-structure. Flocculated suspensions were prepared by the addition of a non-ionic, high molecular weight polyacrylamide to the coagulated AKP-30 alumina suspension. The study on shear rheology was investigated by employing steady shear and small amplitude oscillatory shear (SAOS) measurements. The use of these two methods allowed quantitative interpretation of a network deformation of a suspension which involved the transition between solid-like behaviour (before yielding) and liquid-like behaviour (after yielding). The network deformation can be characterised as either being of a brittle or ductile type. The use of Lissajous figures also aided the understanding of the deformation. The shear rheology of the metal oxide suspensions was found to depend critically on the extent of the inter-particle interactions. Both dispersed and coagulated suspensions show different rheological properties. In the case for coagulated suspensions, rheological parameters such as the shear and compressive yield stresses, and elastic modulus plateau value, all scaled, which indicate that these parameters arise from particle interaction. This is in contrast to the understanding of shear rheology on freshly flocculated suspension which is still limited, particularly for concentrations at close the gel point. The shear rheology of flocculated suspension was then compared to that of coagulated suspensions. Various degrees of flocculation conditions were investigated. Network deformation was found to show an identical pattern to that of coagulated suspensions but the network strength was found to increase with shear rate. The dewatering properties of dispersed and coagulated AKP-30 alumina and Ajax kaolin suspensions were compared. It was found that dispersed suspensions have lower compressibility (i.e. produce a higher final solids concentration at the same applied pressure) and permeability compared to coagulated suspensions. Comparison between the two model suspensions shows that AKP-30 alumina suspensions have better dewatering qualities compared to kaolin suspensions. The effect of flocculation conditions for AKP-30 alumina suspension was found to dramatically affect the settling rate but had only a small effect on dewatering at higher solids. A combination of dewatering methods was used and they gave excellent results for the prediction of the dewatering characteristics of suspensions for a wide range of solids concentration from close the gel point up to close to the maximum close packed concentration. Finally, electrically enhanced dewatering (EED) for Ajax kaolin suspension was compared to that of normal dewatering. Results showed significant dewaterability enhancement at pressures below 10 kPa. The application of EED was also investigated for an industrial sample of water treatment sludge. Similar results were noted as for Ajax kaolin, with an increase of equilibrium solids concentration and permeability with EED at a given pressure. In this context, EED delivered promising results in improving the dewatering properties of difficult-to-dewater suspensions.
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ItemDrinking water treatment sludge production and dewaterabilityф(D. I. Verrelli, 2008)The provision of clean drinking water typically involves treatment processes to remove contaminants. The conventional process involves coagulation with hydrolysing metal salts, typically of aluminium (‘alum’) or trivalent iron (‘ferric’). Along with the product water this also produces a waste by-product, or sludge. The fact of increasing sludge production — due to higher levels of treatment and greater volume of water supply — conflicts with modern demands for environmental best practice, leading to higher financial costs. A further issue is the significant quantity of water that is held up in the sludge, and wasted. One means of dealing with these problems is to dewater the sludge further. This reduces the volume of waste to be disposed of. The consistency is also improved (e.g. for the purpose of landfilling). And a significant amount of water can be recovered. The efficiency, and efficacy, of this process depends on the dewaterability of the sludge.In fact, good dewaterability is vital to the operation of conventional drinking water treatment plants (WTP’s). The usual process of separating the particulates, formed from a blend of contaminants and coagulated precipitate, relies on ‘clarification’ and ‘thickening’, which are essentially settling operations of solid–liquid separation.WTP operators — and researchers — do attempt to measure sludge dewaterability, but usually rely on empirical characterisation techniques that do not tell the full story and can even mislead. Understanding of the physical and chemical nature of the sludge is also surprisingly rudimentary, considering the long history of these processes. The present work begins by reviewing the current state of knowledge on raw water and sludge composition, with special focus on solid aluminium and iron phases and on fractal aggregate structure. Next the theory of dewatering is examined, with the adopted phenomenological theory contrasted with empirical techniques and other theories.The foundation for subsequent analyses is laid by experimental work which establishes the solid phase density of WTP sludges. Additionally, alum sludges are found to contain pseudoböhmite, while 2-line ferrihydrite and goethite are identified in ferric sludges. A key hypothesis is that dewaterability is partly determined by the treatment conditions. To investigate this, numerous WTP sludges were studied that had been generated under diverse conditions: some plant samples were obtained, and the remainder were generated in the laboratory (results were consistent). Dewaterability was characterised for each sludge in concentration ranges relevant to settling, centrifugation and filtration using models developed by LANDMAN and WHITE inter alia; it is expressed in terms of both equilibrium and kinetic parameters, py(φ) and R(φ) respectively.This work confirmed that dewaterability is significantly influenced by treatment conditions.The strongest correlations were observed when varying coagulation pH and coagulant dose. At high doses precipitated coagulant controls the sludge behaviour, and dewaterability is poor. Dewaterability deteriorates as pH is increased for high-dose alum sludges; other sludges are less sensitive to pH. These findings can be linked to the faster coagulation dynamics prevailing at high coagulant and alkali dose.Alum and ferric sludges in general had comparable dewaterabilities, and the characteristics of a magnesium sludge were similar too.Small effects on dewaterability were observed in response to variations in raw water organic content and shearing. Polymer flocculation and conditioning appeared mainly to affect dewaterability at low sludge concentrations. Ageing did not produce clear changes in dewaterability.Dense, compact particles are known to dewater better than ‘fluffy’ aggregates or flocs usually encountered in drinking water treatment. This explains the superior dewaterability of a sludge containing powdered activated carbon (PAC). Even greater improvements were observed following a cycle of sludge freezing and thawing for a wide range of WTP sludges. Further aspects considered in the present work include deviations from simplifying assumptions that are usually made. Specifically: investigation of long-time dewatering behaviour, wall effects, non-isotropic stresses, and reversibility of dewatering (or ‘elasticity’).Several other results and conclusions, of both theoretical and experimental nature, are presented on topics of subsidiary or peripheral interest that are nonetheless important for establishing a reliable basis for research in this area. This work has proposed links between industrial drinking water coagulation conditions, sludge dewaterability from settling to filtration, and the microstructure of the aggregates making up that sludge. This information can be used when considering the operation or design of a WTP in order to optimise sludge dewaterability, within the constraints of producing drinking water of acceptable quality.