Investigation On The Effect Of Flow Fields On Flocculation Efficiency Of Microbial Flocculant GA1

A bacterial strain, named by Paenibacillus polymyxa GA1, which can producebioflocculant with high flocculating activity was screened from soil of YueluMountain by means of the common bacteria screening and purification method. Themicrobial flocculant GA1(MBFGA1), which generated in the metabolic processes ofGA1, has excellent flocculating capability in the wastewater treatment and shows agreat potential in industrial applications. The objective of this work was to assess theeffect of flow fields in the reactor on flocculation efficiency of MBFGA1.A lab-scale fully baffled flocculation reactor was designed according to industrystandards. A series of flocculation tests were performed to investigate the influence offlow fields provided via mechanical mixing of a Rushton turbine on flocculation.Meanwhile, a three-dimensional computational fluid dynamic (CFD) model with astandard k-model was employed to simulate hydrodynamic characteristics of theflocculation reactor. The multiple reference frames (MRFs) approach was used toincorporate the rotating impeller. MBFGA1was used for flocculation of kaolinsuspension in the reactor, and the flow fields were controlled by changing theinstallation height of the impeller and rotation speed of rapid mixing and slow mixing.The flocculation performance was evaluated using key parameters, viz. the mean flocsize, residual turbidity, flocculating rate and zeta potential. Visualization of fluidvelocity, shear rate, turbulent kinetic energy and turbulent viscosity helped to analyzethe flow fields. By integrating flocculation tests with CFD simulations, it was shownthat flow behaviors of the reactor significantly affected the flocculation capacity ofMBFGA1. When the impeller was centrally fixed50mm above the bottom, flow fieldsinduced by rapid mixing at400rpm then slow mixing at80rpm achieved the highestflocculation efficiency of MBFGA1for2g/L and3g/L kaolin suspension: the largestfloc size was527.8μm and538.2μm, respectively; the lowest residual turbidity was5.03NTU and3.03NTU, respectively; the maximum flocculating rate was97.8%and98.2%, respectively; and the lowest Zeta potential was-6.34mV and-6.03mV,respectively. The CFD simulations showed that the two-loop flow pattern in thereactor guaranteed homogeneous dispersion of Ca2+and MBFGA1at the stage ofrapid mixing (N=400rpm), which was beneficial to the flocculation reactions. Thehigh velocity was along the impeller discharging direction, while a low velocity emerged near the wall. The velocity of the mixture increased and the uniformity ofvelocity distribution was achieved as the impeller speed increased. The flow fields atthe impeller speed of80rpm not only satisfied the optimum hydrodynamic conditionsof flocs growth for flocculation but also guaranteed the small degree of dama ge toflocs. The lower loop associated with higher turbulent kinetic energy and lowerturbulent viscosity could promote flocs suspension. Hence, the flocs collisionfrequency was enhanced by flocs suspension resulting in increased flocculationefficiency. Based on the experimental observations, it could be concluded that theflocculation of kaolin suspension using MBFGA1was completed by chargeneutralization occurred at the stage of rapid mixing and adsorption bridging occurredat the stage of slow mixing.This work concentrated on assessing the effect of flow fields on flocculation ofkaolin suspension using MBFGA1by integrating flocculation tests with CFDsimulations. It’s expected that the research results could provide useful insights intothe optimization of flocculation processes and reveal the possible application of theCFD simulation to guide the practical design of flocculation reactors.