THESIS
2019
xvii, 154 pages : illustrations (some color) ; 30 cm
Abstract
Leveraging extensive studies on dynamic membranes, a new type of membrane
bioreactor (MBR) — a self-forming dynamic membrane bioreactor (SFDMBR) — has
been developed as a low-cost alternative to current MBR. Instead of the microfiltration
and ultrafiltration membranes applied in conventional MBRs, SFDMBRs use economic
and easily accessible membrane materials with large pore size ranging from 10 to 100
μm. This technology requires a dynamic layer formed by activated sludge to provide
effective filtration function for high-quality permeate production. However, the quality
of the SFDMBRs effluent tends to fluctuate during long-term operation due to the large
pore size of the supporting materials. This is mainly due to the lack of understanding
on the development of dynamic layer d...[
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Leveraging extensive studies on dynamic membranes, a new type of membrane
bioreactor (MBR) — a self-forming dynamic membrane bioreactor (SFDMBR) — has
been developed as a low-cost alternative to current MBR. Instead of the microfiltration
and ultrafiltration membranes applied in conventional MBRs, SFDMBRs use economic
and easily accessible membrane materials with large pore size ranging from 10 to 100
μm. This technology requires a dynamic layer formed by activated sludge to provide
effective filtration function for high-quality permeate production. However, the quality
of the SFDMBRs effluent tends to fluctuate during long-term operation due to the large
pore size of the supporting materials. This is mainly due to the lack of understanding
on the development of dynamic layer during long-term operation and its relationship
with the supporting membrane and effluent quality. In other word, the utilization of
dynamic layer in SFDMBR makes its filtration and fouling mechanism distinct from
conventional MBRs, but relevant information is seldom available in literature.
Therefore, this thesis study was designed to investigate the performance of SFDMBR
systems, the feasibility of applying SFDMBR technology for water reuse after system
optimization, the dynamic layer development during start-up, stable operation, and
fouling phases, the characteristics of dynamic layer and its relationship with the effluent
quality, foulant structure in SFDMBR system, the system maintenance or fouling
recovery with chemical cleaning, and the membrane module design based on the better
understanding of the SFDMBR mechanisms for commercialization and scaling-up.
Excellent solid-liquid separation could be achieved after development of a mature
dynamic layer in the lab-scale SFDMBR system operated in this study, as evidenced by
a low permeate turbidity of less than 2 nephelometric turbidity units (NTU). However,
the permeate kept deteriorating until the turbidity exceeded 10 NTU in fouling phase.
Investigations revealed that the majority of permeate particles were dissociated from
the dynamic layer on the back surface of the supporting material, which is caused by
the compression, breakdown, and dissociation of the dynamic layer. This phenomenon
was observed directly in experiment instead of model prediction or conjecture for the
first time. After the membrane fouled, chemical cleaning with sodium hypochlorite
could achieve 99.0% of permeability recovery and full trans-membrane pressure (TMP)
reduction, which was more efficient than other tested chemical cleaning reagents. The
foulants investigation before and after chemical cleaning revealed that the major foulant
of the SFDMBR was a complex mixture of sludge flocs with extracellular polymeric
substances (EPS). β-polysaccharides were the intrinsic reason for fouling, which
attached on the mesh fibers, acted as the binding substance between the foulants and
mesh fibers and caused irremovable inner pore fouling.
Based on these findings, the compression, breakdown and dissociation processes of
SFDMBR were proposed. The fouling mechanism of SFDMBR was also established
with the holistic consideration of dynamic layer transformation. These findings can
provide valuable theories to the scaling-up of SFDMBR system in next steps of study.
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