Extensive studies of membrane autopsies show the physical damage of the membrane, such as increasing salt
passage, changes in flux, and so on. After studying innumerable membranes, we found silica plays a vital role
in most of the physical and irreversible damage of the membrane among the crystallized minerals.
In such cases, the foulant composition contains silica near about 10% – 50% . Aluminum and iron in
Association with silica accelerates the phenomenon.
Silica is one of the major culprits that can cause irreversible damage to the membrane surface. It may
affect the membrane surface in different ways, such as by forming deposition/fouling on the membrane or by
causing an impact on the flux and increasing the trans membrane pressure on the membrane, etc.
The presence of reactive and colloidal silica in feed water plays an important role in the pretreatment of
water. It has been observed that while designing the system, colloidal silica is rarely analysed. Most of the
time, the parameter analysed is reactive silica but not the colloidal silica. It is not necessary that colloidal silica may have a similar trend to calcium, magnesium, and other minerals and metals. The presence of
colloidal silica in water depends on various factors like temperature, pH, etc.
Simultaneously, this factor affects the solubility of reactive silica. But. Unfortunately,y it is very difficult to
obtain the trend of Colloidal Silica in water. Though the sample has been drawn in similar conditions and
period, we can observe variability in colloidal silica content. Thus, the variation and deviation of silica
forms take place not only as per the pH and temperature but also the solubility of silica, the saturation level of silica, the content of different minerals and metals in water, and the source of water.
The presence of colloidal silica is commonly found in river water, sea water, followed by pond water and
well water. In these circumstances, the pretreatment of feed water plays an important role.
Apart from raw water clarification followed by ultrafiltration also plays a major role in reducing the
colloidal silica. The ultrafiltration process removes the colloidal particles larger than 0.01μm, but the
size of colloidal silica particles can be smaller up to 0.001μm. So here we understand the limitation of the UF
filter. The uncertain trend of colloidal silica and the limitation of UF make pretreatment more crucial. Hence, to get rid of colloidal silica in feed water, good design and healthy operation practices of pretreatment is
essential.
In the case of the reverse osmosis processes, while the separation of salts, some salts may react with silica,
which enhances the accumulation of other forms of minerals and silica on the membrane surface and causes
the silica fouling formation. This silica causes an impact on the flux and increases the trans membrane
pressure. Thus, higher concentrations of silica along with changes in pH may increase fouling possibilities
on the membrane surface.
The presence of silica foulant leads to panic and harsh cleaning, which are responsible for other
types of physical damage. The acid and alkali cleaning has an impact on silica. The possibility of metal
silica fouling increases due to the frequent acidification of the feed water. As pH increases above neutral, silicic acid dissociates into the silicate anion (SiO32-)n. This can react with calcium, magnesium, iron,
manganese, or aluminum to form insoluble silicates. Among all these, aluminium is one of the most
powerful elements that causes precipitation of silica. The presence of both Al3+ and Fe3+ in the
pretreated feed water causes the precipitation of silica even below the saturation concentration level of
silica.
Hence, to maintain the concentration of both Al3+ and Fe3+ in feed water, it is very important, as
both Al3+ and Fe3+ salts are used as coagulants in water treatment processes. So, post-coagulation
concentration of Al3+ and Fe3+ should be below 0.05mg/l or less. Silica forms a complex with the hydrated form of calcium, aluminium, magnesium, and iron elements. This complex form polymerizes and creates colloids. In the presence of calcium carbonate and calcium sulphate, amorphous deposits of silica are formed. The colloid forms of silica form the bridge between organic and inorganic matter, a gel-like layer on the membrane surface.
Apart from the above, silica also reacts with organic components, including different types of proteins, humic
acids, and polysaccharides. The presence of silica along with organic compounds in the feed water causes
comparatively more impact on flux as compared to individual organic or inorganic fouling. The deposition of silica on the membrane causes the formation of a thin film on the membrane, which also causes an impact on the polyamide layer. The hydrolysis of trimesoyl chloride (which is a part of polyamide membrane) causes the formation of the carboxyl group . The silica may form a bond with carbon in the carboxyl group by substituting the oxygen. The decrease in the carboxyl group may have an impact on flux. These changes in the carboxyl group cause irreversible damage.
To get rid of the reactive and colloidal silica problem, understanding the effective pretreatment is essential
and to design effective pretreatment, adequate data on reactive and colloidal silica are required.
So, negligence of the reactive silica and colloidal silica will affect CapEx and OpEx.
#membraneautopsy
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