Nanomembranes - Nanotechnologies for Water Environment Applications - page 370

than calcium which is a consequence of the stronger hydration of the Mg 2+ ion.
Nanofiltration was also used for the elimination of one part of the temporary hardness of
Hamma water which provides Constantine City with drinking water. Both weak
pressures (0.5, 1, 2 bar) and relatively high pressures (4-16 bar) were studied (Figure
12.1). The results show that this membrane technique is of great interest in the process
of eliminating temporary hardness of water, which depends mainly on the applied
pressure and the tangential flow rate (Ghizellaoui et al., 2005).
Table 12.3 Comparison of retention characteristics between NF and RO.

RO NF

Monovalent ions (Na, K, Cl, NO 3 ) > 98% < 50%

Sodium Chloride 99% 0-70%

Sodium Sulfate 99% 99%

Hydrochloric Acid 90% 0-5%

Bivalent ions (Ca, Mg, SO 4 , CO 3 ) > 99% > 90%

Calcium Sulfate 99% 0-90%

Magnesium Sulfate > 99% > 99%

Sulfuric Acid 98% 0-5%

Bacteria and Virus > 99% < 99%

Microsolutes (MW > 100) > 90% > 50%

Fructose > 99% 20-99%

Sucrose > 99% > 99%

Humic Acid > 99% > 99%

Protein > 99% > 99%

Microsolutes (MW < 100) 0-99% 0-50%

In Florida, about 87% of public water is produced from groundwater (Bergman,
1995). Commonly, these water supplies are classified as being hard, having relatively
high concentrations of calcium. Many of the supplies also have substantial dissolved
organic color, hydrogen sulfide and iron. In 1995, a total membrane softening water
treatment capacity of 350,000 m 3 /d was installed. Figure 12.2 shows a typical membrane
softening plant in Florida. Sulfuric acid and, in many cases, antiscalants for pH and scale
control, are added to the raw water. Cartridge filters, usually rated at 5 microns, remove
particles that may foul the membranes. The softening membranes used are typically
spiral wound NF membranes. Permeate is sent to a degasifier for carbon dioxide (pH
adjustment) and hydrogen sulfide removal. Post-treatment chemicals are added to the
degasified water, including chemicals for disinfection (chlorine or chloramines), pH
adjustment (NaOH), and often corrosion control (inhibitor) and fluoride. The feed
pressure for NF is usually in the range of 6 - 9 bar. A majority of the membrane softening
plants in Florida is 2-stage plants with a recovery of 80 - 90%, and more than 90% of the
hardness can be removed. At the same time, a substantial part of the color is removed,
resulting in permeate with a color of less than 5. In 2000, a new membrane facility for
groundwater treatment for the City of Boca Raton, Florida was designed with a capacity
of 152,000 m 3 /d, which is the largest of its kind in the world (Suratt et al., 2000). The