Page 147 - Membranes for Industrial Wastewater Recovery and Re-Use

Industrial waters 12 1

In addition to the membrane material, the module structure and the operating
parameters have a significant impact on membrane process performance. In
general the higher the shear rate at the membrane surface, the higher the flux
and the lower the fouling rate. The cross-flow velocity should exceed 4 m s-l
(Jonsson and Wimmerstedt, 1985) in a tubular membrane and 3.2 m s-l in flat
sheet module to avoid fouling (Manttari et al., 1998). As already stated (Section
2.1.4), in the high-shear modules such as the CR and the VSEP filters the rotation
speed and the vibration amplitude have a direct impact on the flux (Figs. 3.23
and 3.24).
The concept of critical flux (Section 2.2.4) has been introduced for
nanofiltration of pulp and paper waters by Manttari and Nystrom (2000), with
respect to the spiral-wound DS-5DK membrane (Osmonics). The efficacy of
operation below the critical flux has been clearly demonstrated in the M-Real
Kirkniemi case described in Section 5.6. Because the CR filters are operated at 0.8
bar, which yields fluxes well below the critical flux (depends on the membrane
and we have not measured it in the CR only in the VSEP and C30F) for the
membranes, hardly any fouling has arisen for this plant. The membranes are
washed only once a week as a part of the routine operational procedure, rather
than on demand. The VSEP filter operates with almost no fouling in
nanofiltration applications (Fig. 3.2 5) whereas ultrafiltration membranes are
usually readily fouled in the VSEP filter.
The main foulants arising from diagnosis of membranes that have been used
for the treatment of mechanical pulp and paper mill waters are resins, fatty acids
and other wood extractives, sugars, and metal ion coagulated organic matter
(Carlsson et al., 1998; Ramamurthy et al., 1995; Nuortila-Jokinen, 1997;
Huuhilo et al., 2001). The adsorption of such materials is increased at higher
membrane hydrophobicities (Fig. 3.2 6).

300

250

= 200
N
E
150
G
3
ii 100

50

0
0 60 120 180 240 300 360 420
Time, min
Figure 3.23 Eflect ofrotor blade tip speed in the CRfilter on thepermeateflux for thefiltration ofacidic clear
filtrate from upapw machine. NF200 membrane from Dow-Film Tee, pH4.9,3 5°C 10 bar TMP