Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Civil Engineering


P.L. Dold


This study focuses on sludge settleability and SVI-type measures. Quantification of sludge settleability is crucial. This usually is done via empirical relationships. Parameters used in these relationships require measurement of sludge zone settling velocity in column settling tests over a wide range of concentrations. Due to the extensive experimental effort involved, several alternative measures (e.g. SVI, DSVI, SSVI) have gained favour for monitoring sludge settleability at full-scale treatment facilities. There is considerable debate over which SVI-type measure is best. The study also includes a section on one-dimensional settling tank models. These models often are incorporated into wastewater treatment process simulators. Many of the one-dimensional settling tank models proposed to date are plagued by numerical instability and solution problems. Background for this thesis is provided in Chapter 2. Included in this section is background on settling tests used to quantify the effect of suspended solids concentration on sludge settling velocity. Chapter 2 also provides background on the development of secondary settling tank modelling. The main body of this thesis is presented as a series of four papers. The first paper (Chapter 3) addresses the considerable confusion which exists as to the best SVI-type parameter and experimental technique to use. A simple mechanistic model was developed and used to evaluate the effects of biosolids characteristics and test parameters on SVI-type indices. The model explains many of the artifacts associated with SVI and questions the validity of correlations for zone settling parameters based on SVI-type measures. The second paper (Chapter 4) examines the approach of correlating SVI-type measures with zone settling velocity (ZSV) parameters for use in flux theory analysis (design or operating charts). Correlations were assessed using the model developed in Chapter 3. The results show that use of the correlations may lead to erroneous results. The third paper (Chapter 5) presents experimental data to demonstrate that differences in column height and sludge concentration can lead to large differences in calculated SVI for a given sludge. The model developed in Chapter 3 was used to further highlight these potential problems and evaluate the effects of sludge characteristics and test parameters on SVI-type indices. The paper raises considerable doubt regarding the validity of correlations for zone settling parameters based on SVI-type measures. An alternative SVI-based method was proposed for determining zone settling parameters. The fourth paper (Chapter 6) outlines the approaches commonly used in one-dimensional layered secondary settling tank models. Two cases were examined: steady-state for a continuous flow secondary settling tank, and unsteady-state for a batch settling test. These cases were used as a basis to provide a rational explanation of numerical solution and stability problems that historically have plagued the one-dimensional layered modelling approach. The results show that the approach of introducing flux constraints into one-dimensional models should be avoided.

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