Chemical Engineering July 2014 - 42

Seawater RO
Capex
Energy
O&M
Total
Table 2. presenT Value CosTs for a 38,000 m3/d (10 million gal/d) seawaTer ro planT for 25 Years
5% Reduced energy consumption
Typical energy consumption
Present Value,
25 years
$60,004,542
Cost,
gal/d
$6.00
$216,953,081 $21.69
$138,571,724 $13.86
$415,529,347 $41.55
produce 1,000 gal from traditional
water supplies ranges from $6 to
15 ($1.52-3.88 per m3) [17]. A comparison
of these figures to the costs
listed above makes it clear that RO
can be a cost-competitive means of
water supply.
Potential technology advances
Technology for RO desalination continues
to improve. Here are some
possible future advances for the key
components of RO operations, and
their potential value.
Membranes. Ultrahigh-permeability
membranes, including graphene
membranes or TFC membranes
made with carbon nanotubes or
aquaporins, have recently received a
Cost per
1,000 gal
$0.66
$2.38
$1.52
$4.55
Percent
of total
14%
52%
33%
100%
Present Value,
25 years
$60,004,542
Cost,
gal/d
$6.00
$209,569,720 $20.96
$138,571,724 $13.86
$408,145,986 $40.81
lot of attention for their potential to
reduce the pressure needed to drive
permeation, thereby reducing the
energy demand of RO [18]. However,
high permeability typically reduces
salt rejection. Assuming salt rejection
can be improved, the amount of
energy that can be saved by using
ultrahigh-permeability membranes
is likely to be very small. Current
RO plants already operate near the
thermodynamic limit, with the applied
pressure being only 10 to 20%
higher than the osmotic pressure of
the concentrate. Some of this excess
pressure is needed to drive crossflow
past whatever membrane material
is used. Although these membranes
might make it possible to use less
Cost per
1,000 gal
$0.66
$2.30
$1.52
$4.47
Percent
of total
15%
51%
34%
100%
membrane area and therefore save
on capital expense, this would require
a redesign of membrane elements
because concentration polarization
induced by high water fluxes
already hinders the performance of
current thin-film composite membrane
elements. Additionally, membrane
fouling is exacerbated at
higher water fluxes, further diminishing
the value of ultrahigh-permeability
membranes for RO.
Reducing or eliminating pretreatment
could reduce the cost
of RO, but this would require the
development of fouling-resistant
membranes or membrane coatings.
Advances in membrane technology
can also reduce the need for the
Pure performance
The science of inorganic scale
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Chemical Engineering July 2014

Table of Contents for the Digital Edition of Chemical Engineering July 2014