Changes in biofilm architecture with addition of membrane fouling reducer in a membrane bioreactor

doi:10.1016/j.procbio.2006.12.003

Process Biochemistry

Volume 42, Issue 4, April 2007, Pages 655-661

https://doi.org/10.1016/j.procbio.2006.12.003 Get rights and content

Abstract

Changes in biofilm architecture and membrane filterability were investigated in submerged membrane bioreactor (MBR) under various operating conditions. Using confocal laser scanning microscopy (CLSM) and image analysis techniques, the porosity and biovolume of a biofilm formed on a membrane surface was analyzed along the length of hollow fibers. The addition of a membrane fouling reducer (MFR), a type of cationic polymer, to a conventional MBR led to the flocculation of activated sludge, resulting in a more porous biofilm on the membrane surface, which substantially enhanced membrane filterability. Soluble foulants in the bulk phase of MBR, such as soluble COD and soluble extra-cellular polymeric substances (EPS) were also entrapped by the microbial flocs during the course of the flocculation, leading to an increase in the concentration of bound EPS. The porosity of the biofilm changed greatly along the length of the hollow fibers. The lowest porosity was observed at the potted ends of membrane fibers which can be easily compressed by suction pressure. The biovolume of the biofilm near the potted ends was greater than that near the free-moving ends. With the addition of MFR, porosities were increased whereas biovolumes were decreased along the length of the fibers. The spatial distributions of both porosities and biovolumes, however, became more uniform along the length of fibers.

Introduction

Membrane bioreactor (MBR) processes have been widely applied to wastewater treatment in the past decades due to their exceptional separation capabilities. Thousands of MBR plants are currently in operation over the world. However, biofouling, a significant problem in MBR, remains to be solved because frequent membrane cleaning and replacement are required, thereby increasing in operation and maintenance costs. Therefore, many studies have been focused on the alleviation of membrane fouling, for example, by backwashing [1], aeration [2], intermittent suction [3], module modification [4], [5], and addition of an inorganic coagulant [6] in attempts to control membrane fouling in MBR.

A membrane fouling reducer (MFR), a type of cationic polymer, was recently developed to control membrane fouling in MBR [7]. However, the details of how MFR reduces membrane fouling have not been fully clarified yet. To thoroughly elucidate the mechanism of how MFR alleviates membrane biofouling, an analysis of biofilm architecture is needed because the biofilm, which is formed on the membrane surfaces is mainly responsible for the loss of membrane filterability in MBR. However, very few studies have been reported, in which biofilm architecture has been examined [8], [9], [10].

The purpose of this study was to elucidate how MFR mitigates membrane fouling, with a particular focus on changes that occur in biofilm architecture. The spatial distributions of porosities and biovolumes along the length of membrane fibers were analyzed by confocal laser scanning microscopy (CLSM) and image analysis techniques.

Section snippets

MBR systems

Two MBRs, a control MBR (conventional type of MBR) and membrane fouling reducer-added MBR were operated over 2 months prior to membrane filtration. Fig. 1 shows a schematic diagram of the experimental setup used to evaluate the two MBRs. Compressed air (air flow rate = 2 L/min) was supplied through a bubble stone on the bottom of each reactor to provide dissolved oxygen and turbulence. Both MBRs were run in parallel under the same experimental conditions (Table 1), except that MFR was added only

Effect of MFR addition on membrane permeability

Fig. 3 shows TMP increase as a function of time for the control and MFR reactor. The TMP reached 30 kPa in 5.2 days in the MFR reactor, while only 0.7 days was required in the control reactor, indicating that the membrane filterability of the MFR reactor was 7.4 times higher than that of the control reactor. This improvement in membrane permeability with MFR has been previously reported [7].

To further study the effect of MFR on the membrane filterability, different doses of MFR were applied in

Conclusions

In this study, the mechanism of how MFR addition to an MBR reduced membrane fouling was investigated. The following conclusions were reached:

(1)
Adding MFR to a conventional MBR gave rise to the flocculation of activated sludge, creating a larger floc size and lower attached biomass on membrane surface and, as a result, membrane filterability was substantially enhanced.
(2)
Soluble foulants in the bulk phase of MBR, such as soluble COD and soluble EPS were entrapped in the microbial flocs during the

Acknowledgements

The authors wish to thank the Korean Ministry of Science & Technology for their financial support under Grant M6-0403-00-0098. The authors also thank the National Instrumentation Center for Environment Management (NICEM) for the use of the confocal laser scanning microscope system, and Nalco company for supporting MPE.

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Changes in biofilm architecture with addition of membrane fouling reducer in a membrane bioreactor

Abstract

Introduction

Section snippets

MBR systems

Effect of MFR addition on membrane permeability

Conclusions

Acknowledgements

Water Sci Technol

Water Res

Desalination

J Membr Sci

Water Sci Technol

J Microbiol Methods

Water Res

Water Res

Water Res

Process Biochem