Biomedical Engineering Reference
In-Depth Information
3.4. Nanofiltration
Nanofiltration is an appreciable special process when reverse osmosis and ultrafiltration
are not the ideal choice for separation. Nanofiltration is a relatively recent membrane process
frequently used with low total dissolved solids water such as surface water and fresh
groundwater, with the purpose of softening (polyvalent cation removal) and removal of
disinfection precursors such as natural organic matter and synthetic organic matter.
Nanofiltration can perform separation applications that are economically feasible, such as
demineralization, color removal, and desalination. Nanofiltration is also becoming more
widely used in food processing applications such as dairy for simultaneous concentration and
partial (monovalent ion) demineralization (Hillie and Mbhuti, 2007). In concentration of
organic solutes, suspended solids, and polyvalent ions, the permeate contains monovalent
ions and low-molecular-weight organic solutions, like alcohol. In general, application of
nanofiltration could be defined as three areas: (a) removal of monovalent ions from waste
water or reaction mixtures; (b) separation ions with different valences; (c) separation of low-
and high-molecular weight components.
Ferjani et al. (2005) tested nanofiltration membranes (molecular weight cut-off 550, 342
and 500 Da) for treatment of seafood processing wastewater. The obtained retention rates of
COD and protein nitrogen indicated that the nanofiltration process was efficient in terms of
retention of organic and biological matters. The retention rates of COD and protein nitrogen
increased with pore size diminution. Retention of COD ranged from 87% to 93% while that
of protein nitrogen ranged from 52 to 82.5%.
Yeong et al. (2002) tested a nanofiltration membrane (molecular weight cut-off 500 Da)
for treatment of fish and surimi wastewater.The permeate flux of nanofiltration remained
almost stable for about 270 min. This finding was a very promising, thus polyamide
membrane used in this study was suitable for the fish and surimi industries to treat the
wastewater. This nanofiltration membrane was capable of reducing COD and total suspended
solids up to 93 % and 87 %, respectively. A study on long-term flux decline indicated that
polyamide nanofiltration membrane fouled rather slowly.
Blue Whiting fish is mainly used for production of fish meal and fish oil. Recently, it has
been used for production of surimi. In order to concentrate the target bioactive peptides and to
avoid excessive concentration of salt, Vandanjon et al. (2007) applied a membrane enzymatic
bioreactor for concentration and purification of the peptides. The bioreactor involved both
ultrafiltration and nanofiltration membranes. The reactional mixture from bioreactor was sent
to the ultrafiltration membrane (molecular weight cut-off, 20 kDa). The step was well suited
for the separation of peptides and non-hydrolyzed proteins. Enzymes and non-hydrolyzed
proteins in the retentate were sent back to the bioreactor. The permeate from first
fractionation step was introduced to the ultrafiltration membrane (molecular weight cut-off, 4
kDa). The second step seemed promising for the fractionation of peptides, as their flux and
retention were good. At the end, nanofiltration membrane with molecular weight cut-off of
300 Da was applied to concentrate peptides. The fluxes and recovery rate were acceptable
and the excessive concentration of salts was avoided.
Nanofiltration membranes have also been evaluated and employed in pre-treatment
facilities for both reverse and thermal process (Al-Sofi et al., 1998; Criscuoli and Drioli,
1999). Walha et al. (2008) used a series of membrane including both ultrafiltration and
nanofiltration membranes as pretreatment process before reverse osmosis of high salinity
