Experimental analysis of coagulation of particles under low-shear flow
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, , by , with 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 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 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.
References (47)
Physical and chemical properties of activated sludge floc
Water Res.
(1993)- et al.
Activated sludge flocculation: on-line determination of floc size and the effect of shear
Water Res.
(2000) - et al.
Evolution of size distribution and transfer of mineral particles between flocs in activated sludges: an insight into floc exchange dynamics
Water Res.
(2002) - et al.
Changes in fractal dimension during aggregation
Water Res.
(2003) - et al.
Effectiveness of orbital shaking for the aeration of suspended bacterial cultures in square-deepwell microtiter plates
Biochem. Eng. J.
(2001) Effect of coagulation on a model planktonic food web
Deep-Sea Res. I
(2001)- et al.
Aggregation kinetics of small particles in agitated vessels
Chem. Eng. Sci.
(1997) - et al.
Modelling particle size distribution dynamics in marine waters
Water Res.
(2004) - et al.
The shear sensitivity of activated sludge: an evaluation of the possibility for a standardized floc strength test
Water Res.
(2002) - et al.
Evidence of shear rate dependence on restructuring and breakup of latex aggregates
J. Colloid Interface Sci.
(2001)