Environmental Engineering Reference
In-Depth Information
1
=
C
s1
RT
mol
58 g
2 ions
mol
82
(298 K)
10
g
L
10
−
3
atm
·
L
=
.
05
×
mol
·
K
=
8
.
4 atm
.
Similarly,
2
=
0.3 atm, so
10
−
2
kg H
2
O
m
2
1
.
2
×
N
w
kg H
2
O
m
2
·
s
10
−
4
K
=
−
=
=
2
.
6
×
atm
.
P
55 atm
−
8.1 atm
·
s
·
Applications and characteristics
RO is not 100% effective, and some particles will pass through the membranes. The
membranes will have a higher rejection efficiency for some particles than for others.
Multivalent ions have a higher rejection than univalent ions (Ca
2
+
vs Na
+
).
Undissociated or poorly dissociated substances have lower rejection (e.g., silica).
Acids and bases are rejected to a lesser extent than their salts.
Co-ions affect the rejection of a particular ion (e.g., sodium is better rejected as sodium
sulfate than as sodium chloride).
Undissociated low-molecular-weight organic acids are poorly rejected and their salts
are well rejected.
Trace quantities of univalent ions are generally poorly rejected.
The average rejection of nitrate is significantly below that of other common monovalent
ions.
Hollow fibers appear to be the best in terms of surface area
volume, but they have serious
hydraulic problems that cause the water flux to be much lower than other configurations.
The most common membrane is cellulose acetate. It can reject over 99% of salts, but
the flux is relatively low: about 10 gal
/
ft
2
day. It consists of a thin dense skin (2000 Å)
on a porous support. The skin is the rejecting surface; the porous support is spongy and is
2
3
water by weight. Another type of membrane is a polyamide membrane. It has a longer
life than a cellulose-acetate membrane because it has a higher chemical and physical
stability, and it is immune to biological attack. It is not prone to hydrolysis like cellulose
acetate, and so it may be operated over a wider pH range. However, any water to be treated
must be dechlorinated before contact with a polyamide membrane to avoid degradation.
Figure 9.11 illustrates generalized curves that show trends [12]. Figure 9.11(a) shows
that the membrane flux increases linearly with pressure. The water quality (solute rejection)
increases with pressure (Figure 9.11(b)) until concentration polarization builds up at the
feed membrane surface and causes precipitation. Figure 9.11(c) and (d) show the effect
of temperature on performance. As the temperature increases, the membrane material
relaxes and water permeates faster. Along with this increased flux, there is an increase
in the amount of salt that also permeates, reducing water quality. Figure 9.11(e) and (f)
point out that performance decreases as the quantity of water recovered increases. This is
/
·
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