Membrane and Desalination Technologies - TCE Moodle Website

Desalination of Seawater by Reverse Osmosis 589
some cationic polymers, although effective in silica scale inhibition, reduce calcium carbonate
scale inhibition by 30–40% (61). Humic and fulvic acid act as good calcium scale
inhibitors but they promote biofouling in the membrane system.
A membrane restoration program was conducted at the site of a large (>3 MGD) RO
brackish water desalting plant (62). The procedure applied consisted of low-pressure flushing
with acidic (pH 4.0) solution of 2% citric acid with ammonia, followed by alkaline (pH 10.5)
flush with NaOH solution. Before and after applying each cleaning solution, the elements
were thoroughly flushed with product water. After this a tannic acid solution (100 ppm) was
applied at the pressure of 300 psi for hollow fiber elements and at 400 psi for spiral-wound
elements. The average salt passage of hollow fiber elements was about 14% before cleaning.
Following restoration, it was reduced to 6–10% and was maintained at that level for several
thousand operating hours by applying continuous online treatment. The reduction of salt
passage was accompanied by a decrease of about 10% in productivity. Similar treatment
results were observed with the PA, spiral-wound elements, which had a salt passage in the
range of 14–24%. As a result of the treatment, an average decrease in salt passage of 40%,
accompanied by a tolerable (approximately 10%) decrease of productivity, was obtained. The
treated spiral-wound elements were reinstalled in the desalting unit and operated for over
1,000 h with no significant increase in salt passage.
The 28,800-gal/day RO system consisting of five cellulose triacetate hollow fine fiber
permeators in a 3/2 array was cleaned using a combination of FLOCLEAN 103, a low pH
liquid formulation for removal of carbonates and metal hydroxides, and FLOCLEAN 107, a
neutral pH liquid formulation designed to remove organic, silt, and other particulates from CA
membranes. It was found that differential pressure of the membrane system could be reduced
by 42% and the permeate flow was increased by 6.4%. The TFC membrane can withstand the
higher pH of this formulation without concern about hydrolytic degradation, and a high pH is
generally conducive to the removal of colloidal soils. Another RO system equipped with the
TFC membrane was cleaned by FLOCLEAN 411, a high pH formulation designed to remove
organics, silt, and other particulate deposits from PA, polysulfone, and TFC membranes.
It resulted in an excellent improvement in permeate flow. After cleaning, the membrane
produced desired recovery at a lower feed pressure. The differential pressure was reduced
by 16.7% and the permeate flow was increased by 23.8% (63). Another pilot-scale study
demonstrated the outperformance of some commercial cleaners over the membrane manufacturer’s
recommended recipe of 2% citric acid at pH 4.0 for cleaning seawater RO
membranes (64). For example, percentage improvement in nominal product flow achieved
with Floclean 403 and 411 from Pfizer was 11.6–30.8% higher than that with citric acid.
Systems with chlorination prior to the membrane processes were easier to clean because of
the presence of chloramines, which act as a disinfectant and reduce the tendency of biofouling.
However, membranes in such a system are more susceptible to structural damage
because of prolonged exposure to the aggressive action of the combined chlorine. CA
membranes are reported to become more brittle when chlorine dosage was in the range of
15–20 mg/L (65). It was also suggested that certain organochlorine derivatives might modify
the molecular structure of the membranes, thus resulting in flux decline and decreased salt
rejecting efficiency.