Applied pilot-scale studies on ceramic membrane processes for the treatment of wastewater streams

Asymmetric multilayer Al2O3 ceramic membranes with pore sizes ranging from 3 to 500 nm
are synthesized in tubular form with external diameter of 14mm, internal diameter of 8mm and
length of 340mm. The membrane synthesis took place on commercially available supports,
with the dip-coating technique either from aloumina particle suspensions or from boehmite
sols.
The membranes are subsequently mounted in a pilot-scale module able to accept six
specimens, and used in micro- and ultra- filtration experiments for the purification of aqueous
streams from suspended solids. The experimental module is equipped with a back-flushing
circuit that can be activated on demand to prevent the fouling of the membranes and the
associated reduction in permeability.
For the microfiltration experiments membranes with 100nm pore size are used. In the case of
an aqueous solid suspension with a concentration of 0.1 to 1 wt.% consisting of solid particles
with an average size of 0.5 μm, complete solid rejection is observed. The process has a
capability of treating 0.8 m3 of feed per hour per square meter of membrane surface under an
average pressure difference of 3 x 105 Nt m-2. The fouling of the membranes can be quite
effectively reduced by back flushing at regular time intervals. Under complete retentate
recycling conditions, more than 95% of the feed volume can be recovered as microparticle
free water.
For the ultrafiltration experiments membranes with 3 nm pore sizes are used. In the case of
an aqueous solid suspension of nanoparticles with a concentration of 0.1 to 1 wt.% consisting
of solid particles with sizes of 20-30 nm the rejection was also almost complete. The process
has in this case a capability of treating 0.16 m3 of feed per hour per square meter of
membrane surface under an average pressure difference of 3 x 105 Nt m-2. Fouling appears
not to cause serious permeability drop in this case probably because even after the
nanoparticle deposition the membrane hydraulic resistance is the permeability determining
step. Almost the entire feed volume can be recovered as nanoparticle free water under
complete retentate recycling conditions.
Purification experiments are also performed in olive oil mill wastewater. Best results are
achieved by using a two step membrane process with gradually decreasing pore size.
Although complete rejection of solids and significant reduction of the BOD5 and phenol
content of the wastes is achieved, the very low permeability is the main draw back of the
process.