Nanomembranes - Nanotechnologies for Water Environment Applications - page 372

A softening plant (NF70 plant) with a capacity of 21,000 m 3 /d of NF permeate
and a blend capacity of up to 30,000 m 3 /d was installed in Bajo Almanzora, Andalusia
for the production of potable water (Redondo and Lanari, 1997). The NF softening
process aims at reducing Mg 2+ to less than 50 mg/L as ion and total dissolved solids
down to the levels allowed by the Spanish Sanitary Guidelines and to lower the sulfates
to less than 250 mg/L. Figure 12.3 shows a general flow sheet of the plant. The
configuration of each rack is in two different arrays of 44 and 20 pressure vessels,
respectively. Each PV contains six FilmTec NF membranes (type NF70-345). Each train
operates at a recovery rate of 70%. Filtered water from existing pre-treatment is subject
to dosing of antiscalant, sodium metabisulfite to reduce free chlorine. Hydrogen chloride
and antiscalant are added in order to prevent precipitation of sparingly soluble salts.
Each of the three trains has a 5-micron cartridge filter ahead of the NF membranes. The
concentrations of Ca 2+ and Mg 2+ in permeate water were reduced to 7.45 and 2.49 mg/L,
respectively, which showed > 97% removal compared with the feed concentration of
278 and 111 mg/L, respectively.

Train 1

Cartridge
filter
5 micron

Two-stage NF

Brine
disposal
to sea

Filtered water from
existing
pretreatment

Permeate
tank

Reserve

HCl
Bisulfite
Antiscalant

Train 2

Train 3

Postozonation
chamber

To regulation storage
and water distribution

Figure 12.3 General flowsheet of the NF70 plant, Bajo Almanzora.

A serious problem in NF systems and a limiting factor for its proper operation is
membrane scaling. Scaling or precipitation fouling occurs in a membrane process
whenever the ionic product of a sparingly soluble salt in the concentrate stream exceeds
the solubility product. Inorganic foulants found in NF applications include carbonate,