Particulate fouling in current practice is inhibited by themechanical
pre-treatment of the feedwater by the use of screens, sand filtration and
cartridge filters or membrane pre-treatment. Biological fouling, caused
by microorganisms sticking to the membrane producing a gel-like
layer, is a serious problemand has to be prevented for example by chlorination
during pre-treatment.
Fouling cannot be fully prevented evenwith optimized pre-treatment.
Therefore, periodical membrane cleaning has to be performed. Complete
removal is not possible and fouling has to be tolerated up to a decrease of
mass flux down to 75% of the original flux [106]. Good operating practice
calls for the chemical cleaning of themembranes if normalized permeate
flowdecreases by 10%, feed channel pressure loss increases by 15% or normalized
salt rejection decreases by 10% from the initial conditions during
the first 48 h of plant operation [107]. Different cleaning methods can be
applied to restore the membrane performance, such as alkaline solutions
for the removal of organic compounds and acid to remove inorganic salts
with low solubility. However, the cleaning procedure increases the cost
and the environmental impact due to the use of chemicals. Moreover,
most membranes cannot tolerate residual chlorine concentrations, introduced
in most desalination processes to prevent biological growth at the
membranes. The development of chlorine tolerant membranes has to
some extent improved the RO process.
Qaisrani and Samhaber [108] have evaluated different methods
for fouling reduction and the enhancement of trans-membrane flux
including enhanced cross-flow velocity, air bubbling, backflushing
and combination of backflushing and air bubbling. These methods
have been compared with dead-end filtration (Fig. 9). The combination
of backflushing and air bubbling followed by backflushing has been
proved to be the best method for fouling control due to reduced cake
layer deposition [108]. The authors also found that the membrane
cleaning time is reduced significantly and almost 100% ofmembrane restoring
can be achieved by combining backflushing and air bubbling
[108]. This study clearly reveals that, althoughmembrane fouling is difficult
to avoid completely, hydrodynamic conditions should be chosen
carefully to optimize the membrane process.
Estimation of critical flux is another important parameter for process
optimization. The concept of critical flux is based upon the certain value
of flux below which there is no increase in trans-membrane pressure
with time and where irreversible fouling is said to be non-existent.
However, in reality fouling is difficult to avoid, thus concepts of threshold
flux and sustainable flux have been introduced. The threshold flux is
the flux that divides high fouling regions from low fouling regions [109]
and sustainable flux is defined fromboth an economic and an operational
point of view [109]. Sustainable flux should be chosen in the regionwhere
fouling can be controlled giving an optimal balance between moderate
operating costs (opex) and moderate capital costs (capex) [105]. The
establishment of critical, threshold or sustainable fluxes is related to
membrane properties, process conditions and fluid properties [105] and
therefore changes for different applications.
Recently, quorum quenching has been applied to minimize biofouling
in MBR wastewater treatment, where the communication between
microorganisms (quorum sensing) is interrupted, thus minimizing the
potential of grouping together and creating biofilms on the membrane
surface [110]. The quorumquenching bacteria can either be encapsulated
in the lumen side of themembrane or on themembrane surface [110,
111]. Different bacteria have been tested such as AHL-lactonase and isolates
obtained from a real MBR plant operated at Okcheon (Korea).