Membrane and Desalination Technologies - TCE Moodle Website

582 J.P. Chen et al.
4.2.3. Coagulation/Flocculation
The process of coagulation and flocculation is applied to separate suspended solids
(colloids) in water when small particles aggregate to form larger particles, which settle
down as sediments. This is usually achieved by the addition of chemicals such as alum, ion
salts, and high molecular weight polymers. For example, aluminum sulfate and ferric chloride
as coagulants were examined for use in seawater (43). Results showed that the best clarification
was found using aluminum sulfate at dosages of 20–30 mg/L with an anionic polyelectrolyte
flocculant aid at 0.025–0.1 mg/L. Another study showed that seawater turbidity could
be effectively removed by all tested coagulants of aluminum sulfate, ferric chloride, and
electrolytic aluminum. Ferric chloride was recommended in the pretreatment of seawater
because less of it remains in the treated seawater than aluminum over a wide pH range.
Similarly, electrolytic aluminum was found to be ideal for turbidity removal of brackish
water (44).
Although coagulation/flocculation is a very effective pretreatment for removing colloidal
and suspended matter, the process is expensive as chemical dosage is required. The process is
also difficult to operate because optimum dosage depends on influent quality. Furthermore,
the coagulation–clarification process generates solid waste that requires additional handling
and disposal. In cases where overdosage occurs, high metal salt content of the pretreatment
water may result in metal hydroxide precipitation on the subsequent RO membranes (45).
4.2.4. Sedimentation and Filtration
In sedimentation, suspended solids in the water are removed by gravitational settling. The
settling rate of the particles depends on their size and density as well as the density of the
water. Filtration is the most common pretreatment to reduce membrane fouling by particular
matters. The most common media filters are silica sand and crushed coal (anthracite). The
selection of filter design depends very much on the required quality of effluent.
4.2.5. Dechlorination
Because of high sensitivity of polymide membranes to chlorine, dechlorination is usually
achieved by the addition of sodium bisulfite (NaHSO3). The problem related with this
approach is that the solution itself becomes an incubator of bacteria, causing biofouling of
the membranes. In some cases, granular activated carbon is used to remove chlorine, but it
can also serve as an incubator of bacteria because of its porous structure and nutrient-rich
environment. Recently, UV treatment has become an increasingly popular dechlorination
technology with none of the aforementioned drawbacks. UV light produces photochemical
reactions between 180 and 400 nm that dissociate free chlorine to form hydrochloric acid.
The peak wavelengths for dissociation of free chlorine range from 180 to 200 nm, whereas the
peak wavelengths for dissociation of combined chlorine range from 245 to 365 nm. The UV
dosage required for dechlorination depends on the adsorption of UV in the water, total
chlorine level, ratio of free vs. combined chlorine, background level of organics, and target
reduction concentrations.