3.2. Microfiltration of brining wastewater after
chemical precipitation
Microfiltration of the effluent from the precipitation
tank was necessary to further purify the chemicallytreated wastewater to be suitable for the reuse at the brining
step. As the pH of the effluent was 10, it should also
be neutralized for reuse. However, the selection of the
neutralization point in this recycling system, e.g. before
or after MF, would be closely related to the MF performance.
In this context, two kinds of feeds with pH=10
(non-neutralized) and pH=7 (neutralized) were applied
to MF, respectively, and comparison was made with
respect to flux, backwashing interval and permeate quality.
Fig. 4 shows the results of the experiments with the
two feeds in both dead-end and crossflow filtration
modes.
With the dead-end filtration the flux was about two
times greater but the backwashing interval was less than
those with the crossflow filtration regardless of the feed
pH. The difference in flux between dead-end and
crossflow filtration was attributed to the difference in
trans-membrane pressure. As the pump in the CMF unit
was a centrifugal type, the retentate flow rate was controlled
only with the back-pressure valve. Therefore, as
the retentate flow rate (or feeding rate in the dead-end
mode) decreased, the trans-membrane pressure increased
and thus higher flux was obtained. However, the shorter
backwashing interval was observed in the dead-end
mode due to the thicker cake layer formed on the membrane.
In summary, the lower the retentate flow rate, the
higher the trans-membrane pressure, the higher the flux
but the shorter the backwashing interval.
The quality of permeate was monitored and is shown
in Table 2. The suspended solid was removed completely
and turbidity removal was over 99.8%, while
TOC removal was around 30%. The permeate TOC and
turbidity were almost similar in the two operation
modes, but the dead-end mode was a little better than
the crossflow one in terms of turbidity.
Fig.As the magnesium concentration in brining water is
known to affect the taste of the final product (Kimchi),
the lower the magnesium removal, the better the brining
wastewater reuse system. Through the hybrid
MF/chemical precipitation, the loss of magnesium was
only 10–15% resulting in no significant difference in the
taste of Kimchi.