Agriculture Reference
In-Depth Information
limitations: (1) as the liquid separation efficiency in the sedimentation tank is not
high, the concentration of the sludge in the aeration tanks is not maintained at a high
level, resulting in low volumetric loading and large area; (2) the treated effluent is
nonideal and unstable; (3) low oxygen transfer efficiency and high energy con-
sumption; (4) high sludge yield; and (5) complex management operations. Thus, it
is difficult for conventional biological treatment processes to meet the increasingly
stringent standards for effluent discharge and wastewater reuse.
A new water treatment technique, known as MBR, exploiting NF membrane
separation technology, instead of traditional gravity sedimentation tank, has
attracted wide interest among various researchers. MBR was first used in the
microbial fermentation industry, and its application in the field of wastewater
treatment began in the 1960s in the USA. However, at that time, owing to limited
membrane production techniques, the membrane life was short with low water
permeability, thus hindering its practical application. In the late 1970s, Japanese
researchers vigorously developed the membrane separation technology, which
resulted in practical applications of MBR. In the 1980s, many studies on MBR
were carried out around the world. Japan
s Ministry of Construction developed a
large-scale research program called “Aqua Renaissance
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90,” which mainly
focused on membrane development, research on membrane reactor, etc. Similarly,
France, the USA, Australia, and other countries also devoted much effort to develop
MBR, making the research on MBR more comprehensive and exhaustive, and laid
the foundation for further application of this technology in the 1990s.
MBR processes generally consist of biological reactors and membrane compo-
nents. According to the position of the membrane components, the MBR processes
can be divided into segregated and integrated modes. Furthermore, based on the
presence of oxygen, the bioreactors can be divided into aerobic and anaerobic
MBRs. The membrane of the segregated-mode MBR generally employs a pressur-
ized pattern. A mixture enters into the bioreactor and passes across the membrane
after pressurization through a pump under pressure. Subsequently, solid and macro-
molecular substances are intercepted by the membrane and flow back to the
bioreactor with the concentrated liquid. On the other hand, the integrated-mode
bioreactor is characterized by stable operation, it is easy to manage and clean, and
its membrane can be easily changed. However, under normal conditions, to reduce
the deposition of contaminants on the membrane surface, the flow rate of water
supplied by the circulation pump is maintained at a high level, which results in
higher power consumption.
In the integrated-mode MBR combined technology, the membrane components
are placed into the reactor, and a vacuum pump or another suction pump is used to
obtain the filtrate. To reduce contamination of membrane surface and extend the
operating cycle, general pump suction is applied intermittently. When compared
with the segregated-mode bioreactor, the most important feature of the integrated-
mode bioreactor is its low operation costs; however, stability, operation manage-
ment, and cleaning are difficult to achieve. Currently, applications of segregated-
and integrated-mode MBR technology are rarely reported.
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