Nanomembranes - Nanotechnologies for Water Environment Applications - page 398

The up-coming desalination plants covering the coastline of California is another
example of pressing need for water. California's population is projected to increase to 48
million by 2030 which would require over 4 Mm 3 /day of water supply. In order to meet
this demand, California has carried out a desalination initiative that is expected to yield 2
dozen projects which would supply 5-10% of the urban demand along the coastal cities
(Voutchkov, 2007). It is expected that by year 2020, desalination will be supplying
about 10-20% water to Southern California Utilities. The two biggest desalination plants
located at Huntington and Carlsbad, Southern California with a capacity of 200,000 m 3 /d
each are expected to be completed by year 2010. The proposed desalination plants are
also able to provide a competitive cost for freshwater supply at $0.70/m 3 to $1.30/m 3 as
compared to an average of $0.86/m 3 for drinking water for Californian residents
(Voutchkov, 2007). The lowest proposed desalinated water cost ($0.70/m 3 ) is only
marginally lower than the cost for drinking water.
Approximately 44-50% of the cost for seawater desalination is the energy
component (Dawoud, 2005; Hinkebein and Price, 2005). Energy is required to generate
a high pressure to overcome the osmotic pressure caused by the high TDS (generally
above 30,000 mg/L) in the seawater. The energy consumption for seawater is within the
range of 3-3.4 kWh/m 3 , which is at least 3 times more energy consumption compared
with surface, brackish or wastewater (Dreizin, 2006). Besides energy, pretreatment,
chemical addition, cleaning, maintenance and capital makes up of the rest of the other
costs (Dolnicar, 2006).
A suitable pretreatment technology will reduce the rate of fouling, and hence, the
frequency and the cost of chemicals for cleaning. Fouling in seawater desalination can
be caused by: (1) scaling due to the presence of calcium carbonate; (2) colloidal fouling
caused by the presence of colloids such as ferric hydroxides, aluminum hydroxides and
silica; (3) biofouling caused by deposition of microorganisms on the membrane surface
which eventually forms biofilm; and (4) organic fouling due to the presence of organics
in seawater (Chua et al., 2003). In addition, pretreatment could also remove some of the
TDS prior to RO membrane which will further reduce the energy requirement at the RO
stage. The RO membrane fouling index (usually measured by using Silt Density Index,
commonly known as SDI) recommended by most manufacturer is less than 3.0. The
water quality of the seawater varies significantly from one location to another. As an
example, the SDI values for the seawater in Singapore ranges from 6.1 to 6.5 (Chua et
al., 2003) compared to almost 3 times higher SDI values reported in Bahrain (Alawadhi,
1997). Thus, the pretreatment required would be dependent on the location which
governs the feed water quality to the desalination plant. Sections 12.4.1 and 12.4.2
provide a discussion on the currently used pretreatment methods for RO and the up-
coming NF-RO system for desalination.
12.4.1 Conventional Pre-Treatment-RO Systems