Flocculation Dynamics Of Activated Sludge Based On Population Balance Model

Activated sludge process has been widely used in wastewater biological treatment.Study on the flocculation dynamics of activated sludge, which focuses on the evolutionand effect factors of Floc Size Distribution (FSD) during flocculation process, has greatsignificance to the development of activated sludge fluid dynamics, the understandingof the distribution and sedimentation behavior of activated sludge in reactor orsedimentation tank and the investigation of the aggregation and formation of flocs innatural rivers, lakes and coasts.Flocculation dynamics of activated sludge based on population balance frameworkaims at the evolution of FSD during flocculation process under the assumption of theconservation of mass and quantity of flocs. However, some problems in this fieldremain unresolved and further research is needed, including the effective model forcollision efficiency and daughter particle distribution function, the modelling of theevolution of FSD during flocculation process, and the relationship between kineticparameters of kernel structures for aggregation and breakage and the key factors (EPScontent and Zeta potential) that affect the flocculation. In this thesis, the followingwork was conducted and conclusions were drawn:①FSD of aerobic activated sludge during flocculation process was measured bylaser particle size analyzer, and the influence of velocity gradient, VSS/SS, EPScontent and Zeta potential on FSD was investigated. The results showed that the flocvolume-average size was negatively correlated to velocity gradient, and theorder-of-magnitude of the floc volume-average size was equivalent to that ofKolmogorov scale (their differences were depended on sludge VSS/SS, floc strengthand etc). As fixing velocity gradient, the floc volume-average size was positivelycorrelated to VSS/SS and EPS, whereas negatively to Zeta potential. Organic matterand EPS play important roles on the flocculation of activated sludge by enhancing thefloc strength and improving the flocculation effect. Compared with polysaccharide,protein in EPS seems to be more beneficial to the flocculation of activated sludge.②The equivalent and maximium diameter of aerobic activated sludge floc weredetermined using microscopy and image analysis and thus the2D fractal dimension D2were obtained by fitting the maximum diameter and equivalent diameter. It was foundthat2D fractal dimension was negatively related to VSS/SS (or EPS content), and seemed to be related to the level of floc structure (there are non-ignorable differencesbetween the fractal dimension of large floc and that of small ones). The3D fractaldimension of aerobic activated sludge was also negatively related to floc size,displaying a power function form.③Different from the previous studies, the population balance framework-basedmodel of flocculation dynamics were modified as follows:(i) according to theknowledge that the floc were form through the aggregation of basic particles, abinomial distribution function was proposed for daughter particle distribution due tobreakage and the parameter Cpof binomial distribution was used to characterise themain mechanism of breakage, erosion or splitting;(ii)“balance factor”, defined as theratio of breakage coefficient (A) to collision efficiency (α), was proposed for thecalibration of the model, which contributes to the reduce of the parameters needed tobe calibrated and the efficient simulationof the evolution of FSD. The simulationresults show that:(i) not only the FSD of activated sludge at stable states, but also theevolution of FSD during flocculation process could be simulated well;(ii) the balancedfactor displays power function relation to the average velocity gradient, increasing withthe increasing average velocity gradient,(iii) collision efficiency determines the speedof the flocculation process, and diaplays power function relation to average velocitygradient, decreasing with the increasing average velocity gradient;(vi) breakagecoefficient seems only related to the character of activated sludge not velocity gradient;(v) geometric grid factor (or number of classes) used in discretization equation of PBMhas not affect the calibrated value of balanced factor, and more accurate results isobtained with more geometric grids;(iv) compared with binary distribution, moreaccurate results was obtained with binomial distribution.④The models for collision efficiency and breakage frequency were investigatedand modified. It was concluded that:(i) compared with the rectilinear model, whichsupposed collision efficiency is constant, the curvilinear models treating collisionefficiency as the function of particle size ratio, proposed by Friedlander (1965) andPruppacher and Klett (1978), could improve simulation accuracy of FSD;(ii) themodified Pruppacher and Klett (1978) model could be used to simulate the evolution offloc average size during flocculation process;(iii) Kusters (1991) model for breakagefrequency could improve simulation accuracy of large-size floc distribution, andsimulate the evolution of floc average size well;(vi) By supposing that breakagefrequency has positive correlation with the maximum size of floc and increases with floc size until arriving at a maximum value, the modified Pandaya and Spielman (1983)model for breakage frequency was proposed and not only the evolution of FSD but alsothat of floc average size during flocculation process at different velocity gradient couldbe simulated well by this model.⑤The relationship between the parameters of flocculation dynamics and somekey factors (EPS content and Zeta potential) affecting the flocculation was investigatedusing the model of flocculation dynamics. It was found that:(i) The balanced factor(A/α) is positively correlated to EPS content or Zeta potential, decreasing with theincreasing EPS content or the decreasing Zeta potential (in absolute value); averagevelocity gradient with the later and EPS present significant linear inverse proportionrelationship; the smaller value of balanced factor, the more beneficial to flocculationand the lager floc average size obtained;(ii) the wide or narrower shape of FSD withdifferent EPS content or Zeta potential could be simulated very well using the binomialdistribution for daughter particle, and the parameter Cpof binomial distribution ispositively related to EPS content or Zeta potential, which indicates that breakagemechanism of different activated sludge may be not the same (activated sludge floccontaining high EPS content seems to represent splitting mechanism, whereas thatcontaining low EPS content seems to represent erosion mechanism; activated sludgefloc adding Al3+has strong stability and seems to represent erosion mechanism,whereas that adding Ca2+has weakly stability and seems to represent splittingmechanism).