Larger, visible particles if not removed from the RO feed water will
naturally plug the feed flow channels in the membrane elements. To
prevent such fouling, RO feed water needs to have turbidities of less
than 1 NTU, and Silt Density Index (SDI), a flow rate over time through
a 0.45micron filter, of less than 4.0. Turbidity and SDI don't detect colloidal
fouling potentials. To control scaling, anti-scalants are used to
bind to nascent seed crystals preventing themfromgrowing into scales
and safely discharged with the reject water. For this reason, it is commonly
called threshold inhibition mechanism. Antifoulants for controlling
colloidal fouling work on the principle of keeping the colloidal
particles fromcoagulating once formed on themembrane [67]. Colloidal
iron andmanganese compounds, due to their positively charged characteristics,
are particularly sticky on the negatively charged membranes
[67]. Special measures are required particularly on the mechanism of
fouling by colloidal silica and silicates due to the spontaneous polymerization
of monomeric silicic acid in all natural waters [69]. Silicic acid
[Si(OH)4] is the reactive silica species that can be detected by themolybdate
colorimetric assay. It is spontaneously polymerized by elimination
of water during RO concentration, generating in the RO concentrate a
reaction mixture of oligomeric silica and silicates. The silica in the ultimate
dehydrated state is found as SiO2 (e.g. sand, quartz). When hydroxides
of iron, aluminum, magnesium and calcium are involved in
copolymerization with silicic acid, complex silicate oligomers are
formed in the RO concentrate, some of which depending on their size
may be deposited on the membrane surface