Agriculture Reference
In-Depth Information
rapid development, resulting in broader prospects for the application of NF
technology.
In advanced water treatment, to achieve separation and purification, selectively
permeable membrane is used as a separation medium, and driving forces are
applied on both sides. As a result, the components of the raw material on one side
of the membrane get selectively penetrated into the other side. However, during the
penetration process, the membrane resistance increases and the membrane flux
declines. As a pressure-driven process, NF faces the problems of fouling and
purge control in practical applications similar to RO and UF. The two main reasons
for membrane flux decline are as follows: (1) concentration polarization effects—
the interception of some solvents through the membrane results in accumulation of
components on the surface of the membrane, forming a high concentration layer on
the membrane surface, namely, the concentration polarization layer. This layer
decreases the permeability of water and further reduces the permeation flux because
of higher osmotic pressure. Nevertheless, this effect is reversible by reducing the
feed concentration or improving the hydrodynamic fluid conditions of the feed
liquid, such as increasing the flow rate, using turbulence promoter, or designing
more rational flowing structure, to decrease the concentration polarization effect
and restore the separation performance of the membrane. (2) Solvent adsorption
and particles deposition—a high concentration of solvent is deposited to form a gel
layer, and the suspended particles migrate to the surface of the membrane with
sediments, resulting in clogging of the membrane pores. This gel layer reduces the
hydraulic permeability and permeation flux, promoting the formation of long-term
and irreversible contamination. Unlike pollution, concentration polarization is often
an important factor in the formation of membrane fouling. Studies have shown that
factors affecting the fouling of NF membrane include properties of the pollutants,
inflow conditions, membrane surface properties, and hydraulic conditions, and the
contribution of these factors on membrane fouling is interrelated.
Although numerous studies have been carried out to develop methods to reduce
membrane fouling, this pollution is still inevitable. Some of the physical methods to
reduce membrane fouling include washing the contaminated membrane, lowering
the operating pressure, increasing the circulation of the liquid material, flushing the
surface of the membrane, etc. In addition, chemical cleaning methods, such as acid
and alkali solution cleaning, surfactants cleaning, enzymes cleaning, changing the
surface properties of the membrane (e.g., transforming the membrane to hydro-
philic membrane), etc., are also employed to reduce membrane fouling.
12.12 Membrane Bioreactor
Traditional aerobic biological treatment methods such as the activated sludge
process have been widely used for a long time in sewage and agricultural waste-
water treatment. However, as the solid-liquid separation of water and micro-
organisms is based on gravity sedimentation, these methods face the following
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