Environmental Engineering Reference
In-Depth Information
chapter discusses all these aspects and the future of nanomembranes in water
beneficiation.
Table 12.1
Comparison of various pressure-driven membrane processes
.
MF
UF
NF
RO
Application
separation of
particles
separation of
macromolecules
(bacteria, yeastes)
separation of low molecular
weight (MW) solutes (salts,
glucose, lactose, micro-
pollutants)
MWCO
#
> 500k Da
2-500k Da
500-2000 Da
< 500 Da
Material-
Rejection
Capacity (nm)
50 - 1000
5-100
1-10
0.1-1
Osmotic Pressure
negligible
negligible
high ( 1-25 bar)
Applied Pressure
low (< 2 bar)
low ( 1-10 bar)
high ( 10-60 bar)
Structure
Symmetric
Asymmetric
asymmetric
asymmetric
asymmetric
Thickness of
Actual Separating
Layer
Symmetric 1-
150 m
Asymmetric
1m
0.1-1.0 m
0.1-1.0 m
0.1-1.0 m
based on difference in solubility
and diffusivity
#
MWCO = Molecular weight (Dalton) cutoff of the membrane, where solutes of this weight are
rejected by 90%.
Separation
Principle
based on particle
size
based on particle
size
Natural water sources contain a wide variety of particle types, from highly
soluble low molecular solutes to macromolecules and large particles in the micron range.
Many raw water sources contain undesirable drinking constituents that require
specialized or nonconventional water treatment processes in order to produce acceptable
drinking water quality. Membrane filtration was introduced in drinking water treatment
in the 1950s, mainly for desalination of seawater, brackish water and groundwater. Since
the late of 1980s, the number of membrane plants used for treatment of surface water
has increased. Those are used for treatment purposes such as removal of infectious
species, turbidity, hardness, micropollutants, natural organic matters (NOM) and taste
and odor. Different treatment methods need membranes of different pore diameters, as
shown in Table 12.2 (extracted from Thorsen, 1999). This section discusses the
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