Suspension Dewatering with Aggregate Densification

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Abstract

This thesis concerns design of two pieces of suspension dewatering equipment (i.e. transient batch settlers and steady state continuous gravity thickeners). In a transient batch settler, very slow densification of aggregates within the suspension is considered whilst the drag on the solids in the suspension is assumed to be negligible. The interface of the suspension is then determined by a balance between gravity and the gradient of the compressive yield stress of the gelled suspension. The compressive yield stress functional form in general could be either a weakly gelled formula, or a strongly gelled formula. These formulae differ in the way they behave for solids concentrations in the neighbourhood of the suspension gel point. The effects of the above two gel formulae, the evolution of the compressive yield stress functional form over time during aggregate densification, different initial suspension heights, and different initial feed solids volume fractions upon the predictions of consolidated bed structures and solids volume fractions obtained at the bottom of a batch settler, and upon the evolution of the heights of the suspension and the consolidated bed have been explored. A sufficiently tall initial suspension height might lead to insignificant increases in the solids volume fractions obtained at the bottom of batch settlers after time-dependent aggregate densification. The interfaces of the suspension and the consolidation zone coincide after aggregate densification if the gel point, which increases with time, is larger than the initial feed solids volume fractions. Moreover, the maximum permitted underflow solids fluxes predicted from steady state thickeners have been investigated and compared. Pre-shearing of aggregates which densifies aggregates to have smaller diameters upon entering the thickener is necessary if large underflow solids fluxes and small underflow solids volume fractions are specified. The solids volume fraction at the top of the consolidated bed which is the densified gel point is influenced by the extent of pre-shearing of aggregates. An algorithm for determining this densified gel point has also been developed. In reality, thickeners contain not just a consolidating bed, but also a hindered settling region above it. When the hindered settling region is considered in a thickener, the effects of the extent of aggregate densification that has occurred in the hindered settling region and how that impacts upon thickener performance and sludge rheological properties have been explored in this thesis. A new algorithm for predicting the densified gel point obtained at the top of the consolidated bed has been developed when the hindered settling region is present. The effects of underflow solids volume fractions, aggregate densification rate parameters and pre-shearing of aggregates upon the predictions of maximum permitted underflow solids fluxes, sludge rheological properties, and thickener performance have been explored. The predictions of thickener performance using both the weakly and strongly gelled formulae have also been achieved. In cases where it is possible to neglect the hindered settling region, substantial increases in the maximum permitted underflow solids fluxes, and substantial decreases in the consolidated bed heights and the total solids residence times have been achieved after aggregate densification for a comparatively small underflow solids volume fraction. The benefits arising from aggregate densification are more modest if the underflow solids volume fraction is larger. On the other hand, when the hindered settling region is included, more densification of aggregates occurring in the hindered settling region might lead to taller consolidated bed heights for a specified suspension flux and a specified aggregate densification rate parameter due to higher underflow solids volume fractions.

Keyword(s)

Dewatering;; Rheology; Suspension;

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