Nanomembranes - Nanotechnologies for Water Environment Applications - page 392

(Choi et al., 2007) but the permeate productivity was lower. Cellulose acetate NF MBR
was demonstrated unsuitable for long-term operation (Choi et al. 2007). The dissolved
organic carbon (DOC) concentrations in the permeate of cellulose acetate NF MBR
deteriorated after 130 days of operation, in which only 10% of the influent TOC was
removed by the membrane. Similarly, salt rejection rates of monovalent and divalent
ions also reduced from 40-60% and 70-90%, respectively, to less than 10% after 80
days of operation. The NF membrane permeability increased by almost 6 times from
0.0006 L/m 2 ·h·kPa to 0.0035 L/m 2 ·h·kPa at the end of the operation due to an increase in
permeate flux and a decrease in transmembrane pressure (TMP). The increase in
hydrophobic (least polar and high molecular weight) and transphilic (intermediate
polarity and lower molecular weight) DOC fractions in the permeate indicated
hydrolysis of cellulose acetate had occurred, leading to an increase in the membrane
pore size and a decrease in the surface charge and hydrophobicity of membranes.
Atomic force microscopy (AFM) investigation showed large voids in submerged
cellulose acetate NF membranes treating synthetic domestic sewage after 71 days of
operation (Choi et al., 2002). This deterioration was suggested to be caused by
biodegradation.
The potential use of NF membrane has been demonstrated through its high flux
and lower energy requirement. The NF membranes tested for reclamation of tertiary
treated wastewater effluent at feed pressure of ~415-485 kPa provided a permeate flux
three times greater than the conventional RO membrane (Bellona and Drewes, 2007).
The energy cost could be hypothetically reduced by 2-4 times using NF at 415-485 kPa
compared with the RO membrane typically operating at 1000 - 2100 kPa to provide the
same permeate flux. Based on the power cost of $0.1 kW/h, NF membrane could
provide a cost saving of $0.03-0.08/ m 3 of reclaimed water (Bellona and Drewes, 2007).
12.3.2 Industrial Wastewater
Commercial application of nanomembranes in industrial wastewater reclamation
is still at an infancy stage. The potential and cost analyses are mainly at bench- to pilot-
scale levels. Many promising results have indicated the possible use of nanomembranes,
in particular NF membranes, for providing better effluent quality and resource recovery
from the industrial effluent, in addition to a lower energy requirement compared with
RO systems.
Table 12.11 summarizes the potential applications of NF in various industries.
Among the industries indicated in Table 12.11, nanomembrane is becoming a popular
option in reclaiming water from color dye industries such as printing and textile
industries in recent years. The main reason is that textile industry is one of the most
water consuming industries. Up to 500 L of process water is typically consumed per
kilogram of finished product (Capar et al., 2006). Thus, with the implementation of