Mechanism And Model Of The Cake Layer Formation And Rinsing Process In MBR

Membrane fouling is a major obstacle in the wide application of membrane bioreactors.The cake filtration is the main form of the membrane fouling.Thus it is of great significance to explore the mechanism of the cake layer formation in the fouling process and the cake removal in the rinsing process,and to establish the corresponding mathematical models for in-depth understanding of the operation process of the membrane bioreactor.Four principal achievements have been obtained in this thesis:1)A series of short-term dead-end microfiltration of sludge suspensions domesticated at three different ratios of carbon to nitrogen(C/Ns)(63:10,100:5,200:5)were conducted in constant pressure mode.Flux decline behaviors were analyzed and the structure of cake layer was characterized by 3D optical digital microscope(3D-ODM)to explore the mechanism of the cake layer formation.Results showed that the sludge floc was the key substance in the initial stage of the cake layer formation while colloid matters dominated later.Based on the inhomogeneous structure of the formed cake,a dynamic traversing mechanism was proposed to show the fact that colloid matters traveled through the hole formed by sludge flocs and accumulated on the membrane surface.Meanwhile,a piecewise function about the relationship of accumulated filtrated volume(v)and filtration time(t)was established to reveal this mechanism.Moreover,the model predictions showed good agreement with experimental data and its applications were also approved by other activated sludge suspensions,kaolin suspensions and real wastewater.2)Sludge suspensions with three different concentrations were conducted in a dead-end constant pressure cell,and then the formed cake layer on the membrane surface was taken out and rinsed by three different flow rates.Effluent suspensions were collected at different rinsing time.The interaction energies between foulant particles and the surface were analyzed according to the extended Derjaguin-Landau-Verwey-Overbeek(XDLVO)theory.The results showed that the mechanism of the rinsing process could be considered as a process of continuously increasing the separation distance between foulant particles and the surface.Meanwhile,a first-order kinetic model of variation of the energy consumption with rinsing time revealed good agreement between model predictions and experimental data.Moreover,the model application was also approved by the real wastewater.These findings could put forward a new insight into understanding the ex-situ rinsing process in membrane bioreactors(MBRs).3)It was found that cascade models are more accurate than single-order models in predicting the variations of hydraulic resistance in the rinsing process,especially the cascade model M₂₊₁ had a highest accuracy.Moreover,the regression model(M_2+1,REG)had a better applicability after optimizing the model M₂₊₁ parameters.4)Three different filtration-backwash modes((Mode 1:5min filtration/20s backwash),(Mode 2:6min filtration/20s backwash)and(Mode 3:8min filtration/30s backwash)were used to conduct multi-cycle filtration experiments with three different sludge suspensions.The results showed that the higher frequency and shorter backwashing Mode(Mode 1)had a relatively weak membrane fouling trend compared with the other two backwashing modes in term of flux decline.Moreover,it showed a high hydraulic cleaning efficiency(HCE)and low hydraulic irreversible fouling index(HIFI).On the contrary,lower frequency and longer backwash(Mode 3)has low efficiency and high energy consumption due to its long backwash time.