**4. Particle dynamics and aggregate disruption**

Turbulence is the main driver of particle interactions in many practical applications. Consequently, the particle dynamics in terms of the particle collisions, coalescence/aggregation, growth, and breakage is primarily controlled by the fluid turbulence. The aggregate stability under the influence of hydrodynamic force has been suggested to be a function of the binding or cohesive force FB and the hydrodynamic breaking force FH. While the binding force is determined by the aggregate structure and physicochemical attributes discussed Section 2, flow turbulence is the principal factor in the case of the hydrodynamic force. Therefore, the dynamics of particle behavior depends on an interplay of collision-induced particle aggregation and cohesive force and the rate of aggregate breakage due to the hydrodynamic stress.

The aggregate cohesive strength τ is a function of the physicochemical and biological conditions as well as the aggregate properties, while the hydrodynamic stress σ depends on the design of the aggregation unit and the prevailing process conditions. Several empirical and theoretical formulations are available for predicting the aggregate cohesive force and the maximum hydrodynamic breaking force. The global hydrodynamic stress σ due to the shearing action of the fluid motion on the aggregate as well as the overall cohesive strength τ of the aggregate resisting the hydrodynamic loading assuming a uniform shape and constant porosity can be expressed mathematically in Eqs. (10) and (11) [25, 26]. An equilibrium of particle dynamics is reached at the steady-state condition. In this state, a continued particle or micro flocs/aggregate attachment to the larger flocs/aggregate is prevented, or the breakup kinetics is equal to the turbulence-induced collision rates.

In assessing aggregate strength and resistance to hydrodynamic-induced breakup, two common approaches are normally followed namely: limiting growth and limiting strength. The former relies on the determination of the maximum floc size before rupture, while the latter is based on the micromechanical analysis of aggregate strength. Many empirical and theoretical formulations based on the mentioned approaches are available in literature (Eqs. (12) and (13)). Liu et al. [27] presented the yield stress approach for calculating maximum aggregate tensile strength τ*y* at which breakage is likely to occur in the inertia range of turbulence (Eq. (12)). Similarly, Attia [28] presented a model for predicting the critical fluid velocity above which there will be aggregate disruption by estimating floc yield stress σ*y* resulting from the dynamic pressure acting on the floc (Eq. (13)).

$$
\mathbf{G} = \mu \mathbf{G} = \mu \sqrt{\frac{\varepsilon}{\mathbf{v}}} \tag{10}
$$

$$\boldsymbol{\pi} = \frac{(\mathbf{1} - \mathbf{p})\mathbf{F}\_{\mathbf{b}}}{\mathbf{p}\mathbf{d}\_{\mathbf{p}}^{\;\;\;\;\mathbf{a}}} \tag{11}$$

$$\boldsymbol{\tau}\_{\rm y} = \frac{\mathbf{F\_{B}}}{\mathbf{d\_{p}}^{2}} \left(\frac{\lambda\_{\rm o}}{\mathbf{d\_{F}}}\right)^{\rm k} \tag{12}$$

$$
\sigma\_{\mathbf{y}} = \frac{\mathbf{1}}{\mathbf{2}} \rho\_{\mathbf{f}} \mathbf{v}^{2} \tag{13}
$$

**11**

*The Role of Micro Vortex in the Environmental and Biological Processes*

It should be noted that improvements in the performance of the engineered processes (e.g., stirred tanks, shear reactors and photobioreactors etc.) in the identified areas of applications continue to shape the research focus in the field of environmental process engineering [29]. In this respect, studies have been conducted to determine how to accurately quantify the impact of hydrodynamic characteristics on the infectivity of bacteriophage MS2, a norovirus surrogate. Several studies also involved the development of bioreactors for testing the effect of hydrodynamic characteristics on microalgae and human enteric viruses [29–33]. The results obtained from the studies indicated that the hydrodynamic cavitation could trigger the inactivation waterborne viruses to levels defined in water quality directives. This was reportedly due to OH-radicals that form an AOP during the cavitation process and high shear forces inside the cavitation structure. Also, flow structures in a hydrodynamic filter have been numerically investigated [34]. In this study, tangential component of velocity was defined, and the three-dimensional pattern of the flow current/streamlines was obtained using their two-dimensional projection in the meridian cross-section of the filter, which allows one to discover the vortex structures. It was concluded that the optimal flow regime can be implemented by selecting the optimal correlation between the flow of liquid regime to be processed and the rotation frequency of the filtering baffle in the hydrodynamic filter. The remaining sub-sections describe how hydrodynamics, turbulence, and vortex dynamics are applied to achieve the desired process efficiency in other identified areas of applications—water purification, sludge dewatering, food process-

*Selected studies on computational and experimental studies of turbulence applications in environmental and* 

**Technical application Flow regime Study type References** Biofouling/biofilms Turbulent Experimental/CFD [25–30] Water disinfection/irrigation Turbulent Experimental/CFD [31–35] Water self-purification Turbulent Experimental [43–46] Solid-liquid separation Turbulent Experimental [2, 36–38] Food and paper processing Turbulent Experimental [39–42]

In fluid engineering problems, research has consistently focused on identifying parameters that improve engineered processes including water purification and inactivation of pathogens [35]. While the conventional technique of disinfection by chlorination has been employed to kill pathogenic microorganisms in raw water, recent studies have shown that chlorine reacts with organic compounds in water and generates disinfection byproducts (DBPs), such as trihalomethanes (THMs), haloacetic acids (HAAs), etc. As a result, turbulence-induced inactivation has been

The effect of hydrodynamic parameters such as orifice size, orifice number and orifice layout of multi-orifice plate, cavitation number, cavitation time and orifice velocity on the microbial population have been investigated to determine how the desired process efficiency can be achieved [36]. Experimental results have shown that cavitation effects increased with decrease in orifice size and increase in orifice number, cavitation time and orifice velocity. Flow hydrodynamics and pipe material have also been shown to influence biofilm development in drinking water distribution systems (DWDS). Furthermore, biofilm development was inhibited at higher

*DOI: http://dx.doi.org/10.5772/intechopen.93531*

**Table 2.**

*biological processes.*

ing, and self-purification of the water bodies.

studied as an alternative approach.

**5.1 Hydrodynamics in water purification process**
