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The classical flux theory for gravity thickening of sludge can be formulated from a balance between viscous and gravitational forces on settling particles. The addition of a third force, the interparticle compressive force, allows simulation of additional behaviors, such as predicting sludge blanket height in underloaded thickeners, dynamic simulation of continuous thickening, and realistic simulation of batch settling. In the classical flux model, the settling velocity depends only on concentration. Empirical forms exist to describe this relationship, but estimation of the parameters can be difficult. Thus, some have sought to correlate simple measurements such as SVI to the model parameters.

The compressive thickening model is similar to the classical flux model, with the exception that settling velocity is a function of both concentration and concentration gradient. In the same way that the parameters of the classical flux model have been correlated to the 30-minute sludge volume index (SVI30), we have developed correlations to predict compressive thickening model parameters from simple batch thickening measurements.

The second purpose of this work was to use the correlations together with the compression model to determine if the model can predict the effect of initial concentration on SVI30.

We conclude: 1) The compressive thickening model can describe batch settling behaviors that cannot be modeled by the classical flux model without compression. In particular, the compressive thickening model is able to explain and predict the effect of initial TSS on the sludge volume index. 2) Because the initial total suspended solids concentration used in the SVI test affects the result, it should be used together with the SVI in any correlation for predicting sludge properties. 3) Sludge properties would be better represented by two separate measures that allow the effects of compactability and of settling velocity to be distinguished. The SVI30 and SVI60 are proposed for this purpose, with the constraint that the initial SV30 be between 33% and 67% of the initial volume.4) If only a single measure of compaction is desired, and the results will not be used in predicting settling velocity, then SVI30 should be used with the constraint that the thirty-minute settled volume should be less than 25% of initial. 5) Correlations were developed that make it possible to estimate the parameters of the compressive thickening model using the laboratory measurements of initial TSS, SVI30 and SVI60. 6) The SVI is constrained above a minimum value by either compressive effects or settling velocity effects. The concentration region where SVI is approximately independent of concentration corresponds to where the settling rate is limited by compression effects; the region where SVI increases with concentration occurs because the test approaches the limitation due to the maximum settling velocity. This work has the potential to make it easier for plant operators to control their thickening processes, either by predicting sludge blanket height, or by the use of model-reference control.
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Document Type: Research Article

Publication date: 01 January 2001

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