Nanomembranes - Nanotechnologies for Water Environment Applications - page 399

The general pre-treatment used in seawater desalination prior to RO systems is
illustrated in Figure 12.13. These include coarse screens before the seawater supply
pumps to the gravity-driven media filtration columns. Hypochlorite is generally added to
control biofouling at the intake and as part of the pretreatment. As most types of RO
membrane are sensitive to chlorine, dechlorination is essentially required prior to RO
membranes. Chemicals are usually added before the media filtration columns to enable
coagulation and flocculation of suspended solids and colloids to enhance the
performance of the filtration process. Inorganic coagulant such as ferric chloride and
organic polyelectrolyte are used to flocculate colloidal and fine particles in the
feedwater. Sulfuric acid is added prior to the filtration column to enhance the
coagulation process and as an anti-scalant in preventing the formation of calcium
carbonate scale (Khawaji et al., 2007). These pretreatment steps and a dual media
filtration system are commonly used in RO desalination plants. The applications are
reported at the RO desalination plants in Yanbu Industrial City, Saudi Arabia (Khawaji
et al., 2007) and Addur Seawater RO (SWRO) plant, Bahrian (Alawadhi, 1997).

Hypochlorine
generating unit

Coagulant

Seawater
intake

Coarse
screen

Seawater
supply pumps

Coagulant aid

Seawater intake

Media
filters

Dechlorination

1 st pass RO
train

2 nd pass RO
train

Anti-scalant

2 nd pass RO
booster
pump

1 st pass RO
booster
pump

Cartridge
filter

Pretreatment

Desalination

Figure 12.13 RO desalination using media filtration as pretreatment.

The performance of the dual media filters was reported to be highly variable. The
SDI values after a dual media filtration column at the RO desalination plant for Yanbu