Elsevier

Water Research

Volume 39, Issue 13, August 2005, Pages 2994-3000
Water Research

Experimental analysis of coagulation of particles under low-shear flow

https://doi.org/10.1016/j.watres.2005.04.076Get rights and content

Abstract

The aggregation and breakup of particle flocs were investigated by monitoring the size distribution of a suspension of aggregates, with diameter do, under shear flow created by two mixing systems. The aggregation behavior was studied in 63 experiments under various conditions of induced shear rate and particle volume concentration for particle aggregates smaller than the Kolmogorov scale. Despite small shear rates being characteristics of natural systems, only experiments with comparatively high shear rates have been conducted to date. Because of this reason, in this study, the shear rates were chosen to mimic those found in natural systems. In the first set of experiments the aggregate size, d, was analyzed by changing the mean shear, G¯ (ranging from 0.70 to 27.36 s−1) created in a tank with a grid oscillating through the whole suspension volume. In the second set of experiments, a spherical flask was placed in an orbital shaking table where G¯ ranged from 0.45 to 2.40 s−1. In all the cases there was an increase of d at increasing G¯. The dependence on d was found to be identical for the particle volume concentrations investigated, φ=0.2,0.8,2,4,6,8 and 10×10−5, with the stable aggregate size shifting towards aggregate growth as particle volume concentration increased. These results demonstrate that shear provided a means to keep the particle number count high for collisions to occur but it is small enough that the aggregation–breakup balance is dominated by aggregation.

Introduction

The intensity, extent and duration of the relevant mixing processes and the concentration of suspended particles are key parameters to understand the fate of microorganisms and inorganic particles in a body of water. It has been recognized that the intensity and the spatial and temporal characteristics of turbulence are important factors determining the dynamics of particles, especially for particles smaller than the scale of the smallest eddies dissipating turbulent energy (Kiørboe et al., 1994; Reynolds, 1994; Huppert et al., 1995; Li et al., 2004).

Among systems to generate sheared flows such as paddles, impellers and couette devices, this study focuses on laboratory experiments conducted in mixing boxes, where a characterizable turbulence is generated by (a) a vertical oscillation of a horizontal grid moving inside a container or (b) a container moving in an orbital shaker table. When the fluid is laden with particles, they are in a continuous process of aggregation and disaggregation until eventually a steady state is reached with a given average aggregate size. Shear facilitates to some extent aggregation, but as shear rate increases it causes limiting growth to aggregates (Spicer et al., 1996; Spicer and Pratsinis, 1996a; Serra et al., 1997; Yukselen and Gregory, 2004). Floc structure and particle concentration of primary particles are important since they determine floc size and density (Spicer and Pratsinis, 1996a; Spicer et al., 1998; Mikkelsen and Keiding, 2002; Chakraborti et al., 2003; Selomulya et al., 2004).

The increase in floc size is found to be especially relevant in lakes and seas where aggregates can account for the removal of particles as they form. Seasonal changes in the intensity of turbulent mixing in a lake may produce shifts in phytoplankton succession and the population composition of the algal assemblage (Berman and Shteinman, 1998). Turbulence can also be highly significant to phytoplankton by affecting their swimming motion (Karp-Boss et al., 2000) and to grazing on bacteria (Peters et al., 2002). Biological rates related to the ingestion of particles or the uptake of dissolved substances are, on average, favored by turbulence although there is considerable variability related to growth, taxa and organism sizes (Peters and Marrasé, 2000). Flocs comprised of dead and living cells of green algae were found to form as a result of shear. At the same time shear produced growth inhibition (Gervais et al., 1997; Hondzo et al., 1998; Hondzo and Lyn, 1999; Juhl et al., 2001). Finally, turbulent motion has been found to affect bacterial growth and respiration (Bergstedt et al., 2004; Malits et al., 2004).

In this study, we experimentally address the importance of turbulence intensity on floc aggregation and breakup with primary particles of a given small size where the only relevant mechanism in the collision frequency is the shear stress. In general, turbulence levels are designed to maximize the effects of both particle concentration and shear rates on particle coagulation. Special attention is made to reduce the intensity of turbulence in the experiments compared to those used in other similar experiments (Spicer and Pratsinis, 1996b; Serra and Casamitjana, 1998; Liem et al., 2000; McAnally and Mehta, 2000). This results in a closer match of turbulence assessed in the laboratory and naturally occurring field turbulence. As pointed out by Hondzo and Lyn (1999), since shear rate (proportional to energy dissipation, ε, by G=(ε/ν)1/2, with ν the kinematic viscosity) largely determines the steady-state diameter of any particle population, laboratory results are relevant to natural systems. Grid mixing systems have characteristics to maximize mixing intensity (therefore high particle contacts) while minimizing floc breakup rate (Liem et al., 2000). It has been found to be an alternative mixing device to traditional impeller systems, with an excellent performance for particle removal for flocculation mixing experiments (Liem et al., 2000). Also, containers placed in orbital shakers have been extensively found to account for the effects of turbulence on microorganisms (Savidge, 1981; Berdalet, 1992; Duetz and Witholt, 2001).

Since small shear rate conditions are dominant in aquatic environments (turbulence ranges from 10−6 to 100 cm2 s−3 in terms of dissipation of energy, or from 10−2 to 101 s−1 in terms of mean shear) here, low shear rate conditions will be carefully studied. Generally, previous studies have focused on G¯ larger than 20 s−1 because of technical constrains (Spicer et al., 1996; Serra et al., 1997; Liem et al., 1999, Liem et al., 2000) and did not account for the coupling between the low shear rate and particle concentration-dependent regimes of the aggregation/breakup processes. As the median is less than the Kolmogorov microscale η=(ν3/ε)1/4 it is likely that the breakage mechanism is erosion (Biggs and Lant, 2000) and validates the use of G as an appropriate parameter for turbulence characterisation (Mikkelsen and Keiding, 2002).

Section snippets

Particles

We used monodisperse sulfate polystyrene latex particles (Interfacial Dynamics Corporation, Portland, Oregon, USA) of 2.1 μm in diameter (standard deviation of 0.037 μm) as primary particles. Because of the sizes of the particles, Brownian motion would be relevant in the first states of the aggregation process. The particles had a density of 1055 kg m−3. Thus, during coagulation experiments, a density-matched aqueous solution, created by adding 99.5% purity NaCl to ultrapure water (Milli-Q-water,

Results and discussion

Aggregate formation rate increases with increasing energy dissipation. The growth in the aggregate size was faster at higher shearing rates in the beginning of the aggregation process (Fig. 1). As the characteristic diameter, d, we use the median of the size distribution (with a confidence of 95%) with respect to aggregate volume. For all cases, at larger t*, the breakup of aggregates is more pronounced until it balances the aggregation, and the steady state is then reached.

Usually, the

Acknowledgments

We thank Teresa Serra and Elisa Berdalet for assistance in measurements and valuable discussions. Research funding throughout this study was provided to Jordi Colomer by the ‘Ministerio de Educación y Ciencia’ with projects CGL2004-02027/HID and PR2002-0113. Francesc Peters and Cèlia Marrasé acknowledge the European project NTAP (EVK3-CT-2000-00022) and the Spanish projects TURFI (REN2002-01591/MAR) and VARITEC (CTM2004-04442-C02/MAR). This is ELOISE contribution no. 515/40.

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