Turbulent Flocculation Research Articles

Experimental approaches for identifying the impact of enhanced flotation technology using hollow microspheres

Hollow microspheres should be characterized in terms of physicochemical aspects to understand the flotation effect principle. There have been insufficient studies on the effect of hollow microspheres in water treatment in terms of flotation. In this study, various analytical and experimental approaches were utilized to identify the flotation characteristics and understand the effect of hollow microspheres on flotation. These approaches included analytical methods such as particle count and zeta potential measurements, scanning electron microscopy-energy dispersive X-ray spectroscopy analysis, X-ray photoelectron spectroscopy, a floc breakage experiment, and a collision efficiency model. The hollow microspheres were spherical shape with an average size of 50 μm. The microspheres were identified to have a higher negative charge (−69.1 mV) than microbubbles (−51.1 and −30.5 mV). The binding energy of Si–O from the hollow microspheres showed the highest peak at 103.18 eV and 61,503.7 counts/s. Si–O binding structure and the molecular structure of the SixOx series were considered to be a structure in which Si32− is bonded to oxygen ion. The optimum floc breakage condition was determined to be 15 min. The number of particles increased because the average particle size is increased with the concentration of hollow microspheres injection. The turbulent flocculation (TF) model was confirmed to be consentient to the experimental data in comparison with the white-water bubble blanket (WWBB) model. As a result, the hollow microspheres had the characteristic of increasing floc size so that flocs can be floated easily.

Viscoelasticity and yielding properties of flocculated kaolinite sediments in saline water

The effect of salinity on the yielding properties and viscoelastic behaviour of flocculated kaolinite sediments was studied through stress growth and creep-recovery tests. Turbulent flocculation was performed on a detachable cylinder using the plunger method to promote efficient and controlled mixing. The suspensions were prepared at natural pH (between 5.8–6.0) with salinities in the range 0–0.5M NaCl. Once a suspension was prepared, it was allowed to settle for 2.5h, during which time the variation in sediment height was small. The sediment was removed with minimal disturbance and subjected to stress growth or creep-recovery tests on a rheometer operated in control rate and control stress modes, respectively. Kaolinite suspensions in the presence of flocculant and salt in concentrations larger than a small although critical concentration (NaCl 0.001M) exhibit an inverse relationship between yield stress and settling rate with salt concentration, and direct relationship between shear strain and salt concentration. It is concluded that the particle network becomes denser and the crosslinking weaker with increasing salt. At the critical concentration of 0.001M NaCl, flocculation is best, the initial settling rate and yield stress are the highest, and shear strain the lowest. The response of the floc network to salinity changes is the result of two mechanisms competition: (i) charge screening of both anionic particles and flocculant by salt counterions, and (ii) shielding of the active sites on the flocculant by salt cations that causes the polymer to fold into compact structures adopting balled-up conformations. The first mechanism promotes flocculation while the second supresses flocculation.

Theoretical Foundation and Test Apparatus for an Agent-Based Flocculation Model

Abstract A predictive, mechanistically based performance model for hydraulic flocculation could potentially enable improved design for hydraulic flocculators. Such a model would take characteristics of the flocculator and the suspension as inputs so that designers and operators could rationally consider effects of changing flocculator dimensions or influent water conditions on performance. A performance model exists for laminar flocculators, but a model applicable to turbulent flocculators is still needed, as real scale flocculators operate in the turbulent regime. This article outlines a theoretical approach that suggests the form of two dimensionless composite parameters that account for the influence of raw water turbidity, coagulant dose, flocculator hydraulic residence time, and energy dissipation rate on settled water turbidity. The utility of these parameters in describing turbulent flocculation will require testing and validation. Thus, this article also describes the design of a suitable laborato...

Numerical Simulation of Turbulence-Induced Flocculation and Sedimentation in a Flocculant-Aided Sediment Retention Pond

A model combining multi-dimensional discretized population balance equations with a computational fluid dynamics simulation (CFD-DPBE model) was developed and applied to simulate turbulent flocculation and sedimentation processes in sediment retention basins. Computation fluid dynamics and the discretized population balance equations were solved to generate steady state flow field data and simulate flocculation and sedimentation processes in a sequential manner. Up-to-date numerical algorithms, such as opera tor splitting and LeVeque flux-corrected upwind schemes, were applied to cope with the computational demands caused by complexity and nonlinearity of the population balance equations and the instability caused by advection-dominated transport. In a modeling and simulation study with a two-dimensional simplified pond system, applicability of the CFD-DPBE model was demonstrated by tracking mass balances and floc size evolutions and by examining particle/floc size and solid concentration distributions. Thus, the CFD-DPBE model may be used as a valuable simulation tool for natural and engineered flocculation and sedimentation systems as well as for flocculant-aided sediment retention ponds.

On-line monitoring of floc formation in various flocculants for piggery wastewater treatment

In this study photometric dispersion analyzer (PDA) was used to monitor floc size variation during the flocculation process for piggery wastewater treatment. Three types of flocculants, alum, Fe2(SO4)3 and FeCl3, were used to investigate the effects of flocculant type and dosage, mixing intensity and temperature on floc formation. From the analysis of the floc size index (FSI) at the optimum dosage, FeCl3 was proved to produce the larger floc formation and better settling efficiency than Fe2(SO4)3 or alum. It took 7.5 min in FeCl3, 18.5 min in Fe2(SO4)3 and 22.0 min in alum for complete floc settlement. FeCl3 produced the most easily settled flocs. As the mixing intensity increased from 200 to 400 rpm, FSI value reduction rate was 50% in alum, 21% in Fe2(SO4)3 and 10% in FeCl3. As well, floc formation produced by FeCl3 had lower degree of disaggregation at high mixing intensity. Alum and Fe2(SO4)3 were less effective in floc settling at low temperature while FeCl3 showed no significant effect related to temperature. It could be concluded that FeCl3 flocs did not affect from high shear force and low temperature due to the unaltered characteristics of turbulent flocculation, fluid dynamics and solubility of FeCl3. It made FeCl3 a better flocculant compared to alum and Fe2(SO4)3. In terms of particle removal, turbidity and TP removals increased with the decrease of mixing intensity and with the increase of temperature. Mixing intensity and temperature have shown no significant effect on the removals of COD and NH3-N in piggery wastewater.

A two-scale PBM for modeling turbulent flocculation in water treatment processes

A population balance model (PBM) that incorporates two scales of turbulent motion in the breakup frequency function has been presented. The breakup frequency function is designed such that particles smaller than the impeller-region Kolmogoroff microscale will erode according to a critical velocity related to the local energy dissipation rate. Particles larger than the impeller-region Kolmogoroff microscale will fracture according to a critical velocity related to the impeller tip speed. The two-scale model was found to better predict the experimental steady-state particle size distribution in 5, 28, and 560 l tank sizes and with a Rushton turbine and A310 fluid foil impellers. The two-scale PBM was also used to investigate the most appropriate scale-up law for drinking water flocculation processes. In addition, the impact of higher tank average energy dissipation rate, primary particle concentration, and coagulant concentration on the volume mean particle size with increasing tank size and different impeller types was also presented.

The Effects of Temperature on Turbulent Flocculation: Fluid Dynamics and Chemistry

One of the primary objectives of the research described here was to identify the best available measure of the intensity of turbulence with a view to correcting the mixing intensity of the flocculation process to compensate for temperature effects. It was concluded, however, that no one measure of turbulence intensity is better than another. The authors observed that flocculation efficiency at 20°C is not sensitive to impeller geometry, but at 5°C impeller geometry is much more important. A turbine impeller caused much more floe breakup than a stake and stator impeller. With metal coagulants, the use of constant pOH is appropriate for correcting system chemistry for temperature effects. Alum floc was significantly weaker than iron floc under all conditions tested.

Critique of Camp and Stein's RMS Velocity Gradient

The analysis of relative fluid motion by Camp and Stein, which led to their well-known formulation of particle collision rate in flocculation, is examined using standard tools from continuum mechanics. As a result of certain apparent conceptual errors, including the notion that a three-dimensional relative velocity field can be represented in general by a single velocity gradient, several recommendations are offered, including the abandonment of Camp and Stein’s terms “absolute velocity gradient” and “root mean square (RMS) velocity gradient.” Revisions in nomenclature and an alternate theory of particle collisions are discussed, as well as some implications of inhomogeneity in the turbulent flocculation problem.

Is Velocity Gradient a Valid Turbulent Flocculation Parameter?

The familiar Camp-Stein equation for the root-mean-square velocity gradient G is widely used in the design of rapid mixing and flocculation facilities. The literature related to turbulent phenomena, power required in turbulent mixing, turbulent flocculation modeling, paddle configuration, and the impact of temperature on flocculation was reviewed and re-analyzed, leading to the conclusions that G is only a valid parameter for the flocculation of particles smaller than the Kolmogoroff microscale of turbulence, which is not typical of common water or wastewater flocculation practice. Mean power input per unit mass to the two thirds power is a more appropriate flocculation parameter than G for common water and wastewater flocculation practice because the important turbulent eddies are larger than the Kolmogoroff microscale. This region of turbulent eddy flocculation should be independent of temperature, but dependent on paddle configuration.

Physical aspect of flocculation process—II. Contact flocculation

Kinetic equations which describe the process of contact flocculation in solids contact clarifiers were proposed. Decrease of micro-flocs with the contact of high concentration and large diameter well-grown-flocs both in a turbulent flocculation chamber and in a floc blanket of upflow clarifier were discussed. First order equations with the micro-floc concentration were derived theoretically for both types of contact flocculation. By experiments, these kinetic equations were verified and their coefficients were evaluated. Finally, practical equations for the design of micro-floc removal process with the contact of well-grown-flocs both in a turbulent flocculator and in a floc blanket were proposed.