Membrane Fouling
Fouling can be defined as the potential deposition and accumulation of constituents in the feed stream on the membrane.
Fouling can take place through a number of physic-chemical and biological mechanisms
which are related to the increase deposition of solid material onto the membrane surface.
The main mechanisms by which fouling can occur, are:
1) Build-up of constituents of the feed-water on the membrane which causes a resistance to flow.
This build-up can be divided into different types:
· Pore narrowing, which consists of solid material that it has been attached to the interior surface of the pores.
· Pore blocking occurs when the particles of the feed-water become stuck in the pores of the membrane.
· Gel/cake layer formation takes places when the solid matter in the feed is larger than the pore sizes of the membrane.
2) Formation of chemical precipitates known as scaling
3) Colonization of the membrane or bio-fouling takes place when microorganisms grow on the membrane surface.
- 1. Fouling Control and Mitigation
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Since fouling is an important consideration in the design and operation of membrane systems,
as it affects pre-treatment needs, cleaning requirements, operating conditions, cost and performance,
it should prevented, and if necessary, removed. Optimizing the operation conditions is important
to prevent fouling. However, if fouling have already take place, it should be removed by using physical or chemical cleaning.1-1 Physical Cleaning
includes membrane relaxation and membrane backwashing.
· Back-washing or back-flushing consist of pumping the permeate in the reverse direction through the membrane.
Back-washing removes successfully most of the reversible fouling caused by pore blocking.
Backwashing can also be enhanced by flushing air through the membrane. Backwashing increase
the operating costs since energy is required to achieve a pressure suitable for permeate flow reversion.
· Membrane relaxation consists of pausing the filtration during a period of time, and thus,
there is no need for permeate flow reversion. Relaxation allows filtration to be maintained
for longer period of time before the chemical cleaning of the membrane.
· Back pulsing high frequency back pulsing resulting in efficient removal of dirt layer.
This method is most commonly used for ceramic membranes.
· Recent studies have assessed to combine relaxation and backwashing for optimum results.1-2 Chemical cleaning.
Relaxation and backwashing effectiveness will decrease with operation time as more irreversible
fouling accumulates on the membrane surface. Therefore, besides the physical cleaning,
chemical cleaning may also be recommended. They include:
· Chemical enhanced backwash, that is, a low concentration of chemical cleaning agent is added during the backwashing period.
· Chemical cleaning, where the main clVeaning agents are sodium hypochlorite (for organic fouling)
and citric acid (for inorganic fouling). It should be point out, though, that every membrane supplier proposes
their own chemical cleaning recipes, which differ mainly in terms of concentration and methods.
- 2. Optimizing the Operation Condition.
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Several mechanisms can be carried out to optimize the operation conditions of the membrane to prevent fouling, for instance:
· Reducing flux. The flux always reduces fouling but obviously it impacts on capital cost since
it demands more membrane area. It consists of working at sustainable flux which can be defined as the flux for which
the TMP increases gradually at an acceptable rate, such that chemical cleaning is not necessary.
· Using cross-flow filtration instead of dead-end. In cross-flow filtration only a thin layer
is deposited on the membrane since not all the particles are retained on the membrane, but the concentrate removes them.
· Pre-treatment of the feed water is used to reduce the suspended solids and bacterial content
of the feed-water. Flocculants and coagulants are also used, like ferric chloride and aluminum sulfate that,
once dissolved in the water, adsorbs materials such as suspended solids, colloids and soluble organic.