Environmental Engineering Reference
In-Depth Information
membranes are available at lower lux 24.6 and 34 m
3
/day (6500 and 9000 gfd) to meet the
demands of desalination plants with warm high-salinity waters and hybrid designs.
16.3.1.1 Performance Data: Las Palmas III Pilot Test Example—Increased Capacity
In most seawater RO systems, six to eight modules are loaded in series. As puriied water is
withdrawn from each element, the feed water becomes progressively more saline, increas-
ing the osmotic pressure of the solution to be desalinated. At the same time, feed pressure
drops slightly owing to the energy consumed promoting mixing in the feed channel. In
combination, both of these lead to a decrease in net driving pressure with each subsequent
element. When lower-energy systems are designed, the same increase in osmotic pressure
is observed and approximately the same pressure drop is observed. With a lower feed
pressure, this leads to a larger proportional difference in net driving pressure down the
length of the housing, which can lead to very high lux rates for the lead element (which
can exacerbate fouling). Use of an internally staged design strategy, where elements of
differing permeability are used to balance the permeate lux down the length of the pres-
sure vessel, can then be used to prevent this accelerated fouling potential. Such a design is
undergoing pilot testing by EMALSA (Empresa Municipal de Aguas de Las Palmas, S.A.)
in the Canary Islands.
The pilot run by EMALSA is a second-stage system used to increase recovery by desali-
nating the brine from an SWRO system. The pilot with TFN membranes was installed
in late 2011, the pilot test utilizes commercial nanocomposite Qfx 24.6 m
3
/d and Qfx 34
m
3
/d elements in the same pressure vessel. As a result of the higher permeability, more
water was permeated at the same incoming pressure. More speciically, water production
increased by 50% and both water quality and energy consumption improved over the
elements that were previously running in the plant. Energy savings increased by approxi-
mately 4%. Data for the irst 4 months of operation is shown in Figure 16.6 and demon-
strates stable performance over the test period.
EMALSA system
Permeate flow rate
System pressure
10
9
8
7
6
5
4
3
2
1
0
0
80
75
70
65
60
55
50
20
40
60 80
Days of operation
100
120
140
160
FIGURE 16.6
Stable permeate low rate and system pressure over a 130-day period at an EMALSA pilot in the Canary Islands.
