Abstract
Abstract. Natural particles are frequently applied in drinking water treatment processes in fixed bed reactors, fluidised bed reactors, and sedimentation processes to clarify water and to concentrate solids. When particles settle, it has been found that, in terms of hydraulics, natural particles behave differently when compared to perfectly round spheres. To estimate the terminal settling velocity of single solid particles in a liquid system, a comprehensive collection of equations is available. For perfectly round spheres, the settling velocity can be calculated quite accurately. However, for naturally polydisperse non-spherical particles, experimentally measured settling velocities of individual particles show considerable spread from the calculated average values. This work aims to analyse and explain the different causes of this spread. To this end, terminal settling experiments were conducted in a quiescent fluid with particles varying in density, size, and shape. For the settling experiments, opaque and transparent spherical polydisperse and monodisperse glass beads were selected. In this study, we also examined drinking-water-related particles, like calcite pellets and crushed calcite seeding material grains, which are both applied in drinking water softening. Polydisperse calcite pellets were sieved and separated to acquire more uniformly dispersed samples. In addition, a wide variety of grains with different densities, sizes, and shapes were investigated for their terminal settling velocity and behaviour. The derived drag coefficient was compared with well-known models such as the one of Brown and Lawler (2003). A sensitivity analysis showed that the spread is caused, to a lesser extent, by variations in fluid properties, measurement errors, and wall effects. Natural variations in specific particle density, path trajectory instabilities, and distinctive multi-particle settling behaviour caused a slightly larger degree of the spread. In contrast, a greater spread is caused by variations in particle size, shape, and orientation. In terms of robust process designs and adequate process optimisation for fluidisation and sedimentation of natural granules, it is therefore crucial to take into consideration the influence of the natural variations in the settling velocity when using predictive models of round spheres.
Highlights
The settling behaviour of natural grains in drinking water treatment processes is of great importance (Camp, 1946; Cheremisinoff, 2002; Edzwald, 2011; Howe et al, 2012; Crittenden et al, 2012)
Examples include pellet softening in fluidised bed reactors (Graveland et al, 1983), sedimentation, flotation and flocculation, filtration processes (Amburgey, 2005; Tomkins et al, 2005), backwashing of filter media, and washing columns in which fine material and impurities are separated from seeding material (Cleasby et al, 1977; Soyer and Akgiray, 2009)
Like granular activated carbon (GAC) filtration, where bio-based raw materials are receiving more attention compared to fossil fuel-based materials, the settling behaviour is important during filter bed backwashing
Summary
1.1 Deviations in the prediction of settling in water treatment processesThe settling behaviour of natural grains in drinking water treatment processes is of great importance (Camp, 1946; Cheremisinoff, 2002; Edzwald, 2011; Howe et al, 2012; Crittenden et al, 2012). Examples include pellet softening in fluidised bed reactors (Graveland et al, 1983), sedimentation, flotation and flocculation, filtration processes (Amburgey, 2005; Tomkins et al, 2005), backwashing of filter media, and washing columns in which fine material and impurities are separated from seeding material (Cleasby et al, 1977; Soyer and Akgiray, 2009). In processes such as pellet softening (Rietveld, 2005; van Schagen, 2009), it is important to always keep the particles in a fluidised state, i.e. to prevent a fixed bed state (which sets the minimum superficial velocity) or flushing state (which sets the maximum superficial velocity). The physical properties of bio-based grains are often different compared to conventional grain types, which affects the settling behaviour in backwashing processes as well
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