Dual-objective for the mechanism of membrane fouling in the early stage of filtration and determination of cleaning frequency: A novel combined model
Graphical abstract
Introduction
For the past several decades, membrane bioreactor (MBR) has been gradually served as an alternative technology for treatment of domestic effluents and production of water suitable for reclamation [[1], [2], [3], [4]]. However, membrane fouling, which deteriorates membrane performance, increases energy consumption and shortens membrane lifetime, remains a stubborn obstacle to membrane separation [5,6]. In particular, the membrane fouling in the early stage of filtration has been widely concerned because of its high proportion of flux decay (30%–50%) in the whole membrane fouling process [7,8].
With regard to the classical filtration laws, the membrane fouling in the early stage of filtration presents various forms: cake filtration, standard, complete and intermediate pore blocking models [[9], [10], [11], [12]]. Foulants much larger than the pore size of membrane will lead to the growing of the filter cake [13], conversely, foulants smaller than the pore size will result in the standard pore blocking [10]. Both complete and intermediate pore blocking, which sizes of foulant particles are similar to the pore size, can be distinguished according to the probability that a foulant particle blocks an open pore. To be specific, it is assumed that the probability that a particle blocks an open pore is constant in the course of filtration for complete pore blocking, namely, the number of blocked pores is proportional to the filtrate volume per unit effective filtration area [14,15], while the probability that a particle blocks an open pore varies with filtrate volume for the intermediate pore blocking [16,17]. Nowadays, yet a single filtration model cannot be accurately described the fouling process, current combined models can be divided into two categories in terms of the sequence of deposition of foulant particles on the membrane surface. One believed that the fouling process generally proceeded with two steps consisted of pore blocking followed by cake formation [18,19], the others suggested that pore blocking and cake formation occurring simultaneously [20,21]. However, most of these models were established based on subjective assumptions or judgements and did not give theoretical explanations or verifications: e.g., how to determine the pore blocking is complete pore blocking or intermediate pore blocking in the combined models? Is it precise to predict the experimental data onto the neglect of larger particles depositing on the membrane surface in the early stage of filtration? What are the implications of the irregular distribution of membrane pores on the membrane fouling? As a consequence, it is essential to provide a reasonable conjecture and synchronous verification for the deposition mode of the foulant particles on the membrane surface before established the corresponding model.
Additionally, regardless of which model mentioned above, the filter cake would eventually be formed in the early stage of filtration. The filter cake, which is viewed as a granular bed depositing on the membrane surface and its thickness will increasing with the progress of filtration, ultimately, resulting in considerable additional resistance to flow [[22], [23], [24]]. As such, specific consideration must be given to how to remove the filter cake in time to ensure the sustainability of the application of membrane in the process of multi-cycle filtration.
In this study, we elucidate above issues by establishment of a novel model at constant flux mode and, in parallel, determination of the timing of the cake layer formation. The purpose of this study is to (1) speculate and confirm the deposition mode of foulant particles on the membrane surface in the early stage of filtration; (2) establish a new model to predict the variation of TMP in the filtration at constant flux mode based on the result of (1); (3) provide a guiding strategy for the determination of the cleaning frequency in terms of the timing of the cake layer formation calculated by the proposed model.
Section snippets
Analysis of the deposition mode of foulant particles on the membrane surface
The deposition mode of foulant particles on the membrane surface in the early stage of filtration determines the fouling behaviors and the selection of mathematical models [[25], [26], [27]]. However, it was may not be sufficient and comprehensive for the analysis of the deposition mode in previous studies. Firstly, the single deposition mode (pore blocking modes or cake filtration) was not rigorous to depict the whole fouling process [28,29] due to sludge suspensions were larger than the pore
Materials and experimental setup
Polyacrylonitrile (PAN) (0.1 μm and 100 kDa) and Polyvinylidene fluoride (PVDF) (0.1 μm) flat-sheet membranes purchased from Beijing Ande Membrane Separation Technology and Engineering (Beijing) Co., Ltd. were used in this study. Wherein the 0.1 μm PAN was applied in the main experiment, 0.1 μm PVDF and 100 kDa PAN were devoted to validation experiments. All new membranes were preconditioned by soaking with the DI-water for 12 h at 4 °C.
The feed suspensions in main experiment were derived from
Verification of the assumption of the deposition mode
In order to verify the hypothesis of deposition mode which proposed in section 2.1, various feed solutions (including sludge suspension derived from SBR3, sludge suspensions pre-treated by low and medium filter papers, extracellular polymeric substance (EPS) and various membranes (0.1 μm PAN and 0.1 μm PVDF) were employed in the dead-end constant flux filtration test (flux = 80 L/(m2∙h) and stirring speed = 200 rpm). The loss of effective filtration area (A/A0) was used to evaluate the fouling
Implications for membrane fouling control
It is worth nothing that cleaning frequency is a critical factor for membrane fouling control especially in the process of multicycle filtration [41,42]. However, currently the frequency in MBR cannot be determined but adopting experience values or choosing pre-determined scenarios based on experiments because the frequency is enslaved to operational parameters and foulant properties [[43], [44], [45]]. Generally, the higher cleaning frequency will greatly alleviate the membrane fouling but
Conclusions
In this study, a novel constant flux model based on the loss of the effective filtration area was established to explore the mechanism of membrane fouling in the early stage of filtration in MBR. The reliability of the model was evaluated under various conditions, the accuracy of the model was compared with other models and the wide adaptability of the model was validated by using different real wastewater as feed suspensions. Moreover, the timing of the cake layer formation calculated by the
CRediT authorship contribution statement
Wei Yao: Conceptualization, Methodology, Data curation, Formal analysis, Writing – original draft. Lei Hou: Data curation, Methodology. Fan Wang: Data curation. Zhan Wang: Supervision, Project administration, Funding acquisition. Hanmin Zhang: Supervision, Writing – review & editing, Project administration, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was financially supported by the National Natural Science Foundation of China (Project 52170026and 22078003).
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