05-07-2013, 01:04 PM
Membrane Cleaning
Membrane Cleaning.pdf (Size: 458.78 KB / Downloads: 167)
Introduction
In last years, desalination by reverse osmosis (RO) process has experienced a significant
development, and it has become one of the major technologies for producing potable water
throughout the world. Despite this relevant growth, reverse osmosis, and membrane processes
in general, has several drawbacks to overcome. Specifically, fouling of membranes is the most
important problem in reverse osmosis desalination since economics of the process is still
highly influenced by membrane fouling rate and effectiveness of fouling control.
Fouling agents
Membrane lifetime and permeate productivity are primarily affected by concentration
polarization and fouling at the membrane surface. IUPAC defines membrane fouling as “a
process resulting in loss of performance of a membrane due to the deposition of suspended
or dissolved substances on its external surfaces, at its pore openings or within pores”. The
main mechanisms of membrane fouling are the following (Goosen et al., 2004):
- Adsorption, due to chemical affinity or interaction between solutes and membrane
material. This can happen at membrane surface or inside the pores.
- Pore blockage, when solutes go inside membrane pores.
- Gel formation, as a consequence of molecule accumulation at the film layer of the
membrane. This is very typical in solutions containing proteins.
- Biofouling, cause by bacterial adhesion and growth at membrane surface, besides the
production of extracellular polysaccharides (EPS) by some genera of bacteria, which in
fact are the substance responsible for the biofilm (Baker & Dudley, 1998).
In desalination by RO membranes, the most common fouling types are: organic fouling due
to natural organic material (NOM) such as humic and fulvic acids, protein and
carbohydrate; inorganic fouling due to depositions of inorganic scales (mainly BaSO4,
CaSO4, CaCO3); and biofouling due to microbial attachment to membrane surface followed
by growth and multiplication (Chesters, 2009; Al-Amoudi & Lovitt, 2007). Biofouling is one
of the most serious forms of membrane fouling because once it forms; biofilm is very
difficult to remove.
Pretreatment
It is essential to establish a good pre-treatment to avoid or minimize fouling, so productivity
loss would be lower (Figure 1). Nevertheless, in spite of a good pre-treatment, membranes
have to be periodically cleaned to remove reversible fouling. In case pre-treatment was
inadequate, higher frequency of cleanings will be necessary, and the restoration of
membrane performance will be worse (Sadhwani & Veza, 2001).
Modes of fouling detection and characterization
It is vey important to know the characteristics of the foulant deposited on the membrane in
order to select the most effective cleaning procedure. The best method of foulant
identification is membrane autopsy after fouling, which is considered as an off-line method
for fouling characterization. However, this is a destructive and expensive method that may
be considered to enhance system performance when fouling is complex, when cleaning fails
to restore membrane performance (Pontié et al., 2005; Al-Amoudi & Lovitt, 2007), and when
membranes result damaged after cleaning.
Conventional methods of cleaning
Membrane cleaning methods can be divided into physical, chemical and physio-chemical. In
practice, physical cleaning methods followed by chemical cleaning methods are widely used
in membrane applications. However, only the chemical cleaning methods are widely
applied for RO desalination.
Physical cleaning methods
Physical cleaning methods use mechanical forces to dislodge and remove foulants from the
membrane surface. Physical methods include sponge ball cleaning, forward and reverse
flushing, backwashing, air flushing (also called air sparging, air scouring or air bubbling)
and CO2 back permeation (Ebrahim, 1994; Al-Amoudi & Lovitt, 2007). Ultrasonic, electrical
fields and magnetic fields are other physical cleaning methods that are described in detail in
non-conventional cleaning methods point of this work.
Forward and reverse flushing
Forward flushing consists in pumping permeate water at high cross-flow velocity through
the feed side in order to remove foulants from the membrane surface (Ebrahim, 1994).
Because of the more rapid flow and the resulting turbulence, particles absorbed to the
membrane are released and discharged. In the reverse flushing method the direction of the
permeate flush is alternated for a few seconds in the forward (feed to brine) and for a few
seconds in the reverse direction (brine to feed) (Figure 2). Forward flush techniques are
particularly useful in removing colloidal matter.
Backwashing
This is a reversed filtration process in which permeate is flushed through the membrane to
the concentrate side. In porous membranes, when backward flush is applied, the pores are
flushed inside out. The pressure on the permeate side of the membrane is higher than the
pressure within the membranes, causing the pores to be cleaned (Figure 3).
In reverse osmosis membranes, backwash is based on flow induced by osmotic pressure as
direct osmotic cleaning. This cleaning process is based on negative driving pressure between
the operating pressure and the osmotic pressure of the water solution in the feed side. This
can be done either by reducing operation pressure below the osmotic pressure of the feed
solution or by increasing the permeate pressure (Sagiv & Semiat, 2005). Backflow from the
permeate to the feed side of the membrane expands the thickness of the fouling layer and
fluidizes it. After this, a forward flush is usually used to wash out the detached layer or
dilute the fouling layer. The best cleaning performance is generally reached optimizing the
two flows (forward flow and backflow). Some significant factors affecting physical cleaning
when combining forward and backward flushing are production interval between cleans,
duration of backwash and pressure during forward flush (Chen et al., 2003).
Air flushing
The air flushing or air sparging method generates a two phase flow to remove external fouling
and thus reduces the cake layer deposited on the membrane surface. There are several flow
patterns possible depending on the superficial liquid and air velocities. It has been showed
that slug flow is the most effective pattern to enhance mass flow (Psoch & Schiewer, 2006;
Mercier et al., 1997). The type of gas used for the sparging also seems to have an influence on
the cleaning efficiency. In a recent research, it has been shown that water/CO2 mixture
performes better cleaning results in comparison to water/N2 mixture (Ngene et al., 2010).
Air sparging can be applied either during the course of filtration to reduce fouling
deposition (Cui & Taha, 2003; Mercier et al., 1997; Cabassud et al., 2001) or periodically to
remove already formed deposits. Air sparging is typically applied in MF and UF
membranes, and it seems to work best for tubular and flat sheet membranes and to a lesser
extent in hollow fiber and spiral wound modules (Cui & Taha, 2003). Anyway, it is clear that
the use of air leads to an enhancement of flux in MF and UF. This positive effect is due to the
presence of air bubbles which increase turbulence in the feed side of the membrane, thus
increasing permeate flux as well as solute separation efficiency (Cabassud et al., 2001;
Ducom & Cabassud, 2003).
CO2 back permeation
This is a method traditionally used for hollow fiber configuration in which CO2 gas is forced
from the permeate side through the internal fiber and out through them (Ebrahim, 1994).
More recently, (Fritsch & Morau, 2008) applied a CO2 backpulsing system for cleaning a
tubular MF membrane used in a dairy industry. They state that this is a very promising
technique for maintaining a greater and more stable permeate fluxes.
Chemical cleaning methods
Membrane fouling can be classified as physically reversible fouling which can be totally
eliminated by physical cleaning or certain pretreatment, and physically irreversible fouling
which cannot completely removed by physical cleaning or pretreatment (Hiroshi et al., 2007,
as cited in Gao et al., 2011). Such irreversible fouling can only be overcome by chemical
cleaning, which has to be limited to a minimum frequency since repeated chemical cleaning
may affect membrane life (Kimura et al., 2004).
Chemical cleaning is the most common membrane cleaning method, especially in reverse
osmosis membranes. In this type of cleaning, the choice of the cleaning agent is critical. The
optimal selection of the cleaning agent depends mainly on membrane material and type of
foulant. These agents must be able to dissolved most of the deposited materials on the
surface and removed them from the surface but not damaging membrane surface, thus
maintaining membrane properties. Commercial cleaning products are often, Most chemical
cleaning agents are commercially available, they are often mixtures of compounds, and
many of them are recommended by membrane manufacturers according to the type of
foulant, although in most cases the actual composition is not clearly specified (Ang et al.,
2006). Anyway, in general acid (nitric, phosphoric, hydrochloric, sulphuric and citric) are
often used to remove precipitated salts or scalants, while alkaline cleaning is suitable for
organic fouling removal. Other categories of chemical cleaning agents are: metal chelating
agents, surfactants and enzymes (Mohammadi et al., 2002). In addition, disinfectants (O3),
oxidants (H2O2, KMnO4) or sequestration agents (EDTA) are often used for chemical
cleaning of membranes (Lin et al., 2010).