Membrane and Desalination Technologies - TCE Moodle Website

248 L.K. Wang et al.
3.2. Metal Oxides Fouling
The oxide or hydroxide forms of iron, manganese, and aluminum are the most common
problem causing compounds. Operation of the membrane system at acidic pH levels
and possibly, iron and manganese removal by oxidation are recommended pretreatment
alternatives.
3.3. Colloidal Fouling
Colloidal fouling is caused by the entrapment of colloids on the membrane surface caused
by the coagulation of the colloids in the membrane system. Reduction of the concentration of
colloids can be accomplished by the following pretreatment schemes (16):
1. Filtration, using sand, carbon or other filter media
2. In-line coagulation with iron, salts, and polyelectrolyte aids, followed by pressure filtration
3. Iron salt coagulation, flocculation, flotation (or sedimentation), and filtration
3.4. Biological Fouling
Biological fouling is caused by the growth of microorganisms in membrane modules. Two
major factors are considered:
1. Biological fouling or plugging which interferes with the hydraulic flow of the membrane modules.
2. Biological attack of the membrane, which is extremely serious when CA membranes are attacked
by the microorganisms’ enzyme system.
Periodically feeding a cleaning solution containing free chlorine residuals of 0.5–1.0 mg/L
is recommended to prevent detrimental biological growths on the CA membrane surface. Gas
chlorine and liquid sodium hypochlorite have been used successfully for this purpose. It
should be noted that thin-film composite membranes containing a polyamide separating
barrier on a polysulfone or polyethylene supporting layer, generally give better RO performance
with regard to temperature stability, pH stability, and cleanability, but have almost
zero chlorine resistance. Generally, hydrophobic membranes consisting of polysulfone and
polyamides may be more prone to biological fouling than hydrophilic membranes consisting
of celluloses.
3.5. Cleaning Agent Fouling
Sometimes, membranes “fouling” problems may actually be “cleaning” problems. Ideally,
the membrane material, and all other food contact surfaces, should be compatible with normal
food and dairy cleaners, such as mild caustic or acid solutions. Severe cleaning problems may
require the use of enzyme detergents.