Nanomembranes - Nanotechnologies for Water Environment Applications - page 373

sulfate and phosphate salts of divalent ions, metal hydroxides, sulfides and silica.
Scaling frequently leads to physical damage of membranes due to the difficulty in scale
removal and irreversible membrane pore plugging. It is economically preferable to
prevent scale formation, even if there are effective cleaners available for scale removal.
Several methods of scaling control are employed in NF, including: 1) changing
operational parameters; 2) acidification; and 3) antiscalant dosage.
Operational parameters that can be changed are recovery, crossflow velocity,
temperature and pressure. Reducing the recovery will reduce the concentration ratio and
thereby the risk of precipitation. Increasing the pressure and reducing the crossflow will
enhance CaSO 4 -nucleation and precipitation. However, a reduction of the recovery may
be in conflict with the production target of the facility.
Antiscalants are surface active materials that interfere with precipitation
reactions in three primary ways: threshold inhibition, crystal modification and dispersion.
Common antiscalants are sodium hexametaphosphate (SHMP), diethylenetriamine-
penta-methyl phosphonic acid (DTPMPA) and 1-hydroxyethylidene-1,1-diphosphonic
acid (HEDP). Antiscalant dosages range from 2-5 mg/L. Due to the high molecular
weight and negative charge, the antiscalant will have a high rejection.
12.2.2 Natural Organic Matters
NOM is a polydisperse mixture of individual particles in natural water
originating from degraded and partly re-synthesized plant residuals. Humic andic (HA)
and fulvic acid (FA) were found to contribute to the natural color of water, which
becomes visible if the dissolved organic carbon (DOC) concentration is higher than 5
mg/L. It is for that reason that natural organics removal is often referred to as color
removal. Surface water DOC contains about 45% FA, 5% HA, 25% low molecular
weight (LMW) acids, 5% neutral compounds, 5% base and 5% contaminants (Thurman,
1985). Natural concentrations of natural organics are low, usually less than 20 mg/L, but
their removal is important to avoid undesired interferences in drinking water treatment
processes and impaired water quality as they are precursors of carcinogenic disinfection
by-products. Rapid advances in membrane technology in recent years have resulted in
improvement in membrane performances and increased rejection to contaminants.
Nanofiltration was found to be one of the most promising techniques for NOM removal.
Taylor and Mulford (1995) found total organic carbon (TOC) removal in NF to
be sieving-controlled, and thus, independent of pressure and recovery. The rejection of
inorganic solutes was diffusion limited. Bowen et al. (1997) suggested different
mechanisms for small ions and uncharged solutes. While Donnan partitioning described
ion rejection well, steric effects were important for uncharged solutes such as organic
molecules. It was found that the effective pore size determined with uncharged organic