3.3. Particle size distributions in various MF feeds
The SS concentrations of non-neutralized and neutralized
feed are 37.5 and 27.5 ppm while turbidities are
10.0 and 9.1 NTU, respectively. Therefore, it was
expected that the non-neutralized feed would result in
greater membrane fouling than the neutralized feed. As
seen in the backwashing intervals (Fig. 4), however, the
neutralized feed showed worse filtration performance
than the non-neutralized feed in both MF modes. A
reasonable interpretation of these unexpected results was
made by analyzing particle size distributions of the two
feeds, presented in Fig. 5.
As shown in Fig. 5, most of the particles of size over
10 mm in raw brining wastewater were removed through
the chemical precipitation process. After the chemical
precipitation, the neutralization of the supernatant stillcaused the fraction of larger particles (30–40 mm) to
decrease while the fraction of smaller particles (around
1 mm) increased. This is probably because the suspended
magnesium hydroxide crystals that had not settled in the
precipitation tank were redissolved after being neutralized.
During this process, the adsorbed organic matters
on the magnesium hydroxide crystals would also be
desorbed. It is well known that smaller particles cause
more serious membrane fouling than larger particles in
the membrane process according to the Carman–Kozeny
equation:
J5
e3d2p
DP
180(1-e)3h
(1)
In this equation, the flux (J) is proportional to the square
of the particle size (dp). Therefore, the reason why the
neutralized feed demanded more frequent backwashing
to maintain a certain range of trans-membrane pressure
compared with the non-neutralized one is that neutralization
made the particles smaller in the MF feed.