172
Franken et al. explained that presence of natural organic matters (NOM) in high
concentration in the feed stream decreases the hydrophobicity of the membrane
through adsorption that causes wetting of membrane (Franken et al. 1987).
Eventually, deterioration of membrane contact angle and increase in organic content
in permeate stream were observed. In other words, the rate of wetting purely
depends on the type of foulant (based on characteristics of feed stream) (Naidu et al.
2016). Figure 6.7 indicates the membrane distillation fouling-wetting mechanism
based on organic fouling, and it also elaborates the consequences.
6.2.5 Scaling Phenomenon
Scaling of membrane takes place when inorganic salts precipitate and accumulate
on the surface of the membrane, thereby clogging the pores for vapor to diffuse
through the membrane and resulting in decline of permeate flux. CaCO 3 , CaSO 4 ,
and silicates were analyzed to be the prominent cause of permeate flux decline
while recovering reusable water from natural streams including seawater and brine
water stream. Membrane scaling falls into the category of membrane fouling.
Typically, there are two types of membrane scaling, namely, (a) inorganic scaling
and (b) particulate or colloidal fouling. In general, it is observed that the pressuredriven reverse systems assist the formation of compact cake scales, unlike membrane distillation process, driven by temperature difference (Martinetti et al. 2009;
Warsinger et al. 2015).
In the recent years, extensive investigations have been performed for inorganic
scaling in membrane distillation processes. Typically, inorganic scaling in both
reverse osmosis and membrane distillation processes is categorized into (a) alkaline
scaling, (b) non-alkaline scaling, and (c) uncharged molecule scales. The alkaline
salts (basic) when added through hydrolysis make the solution basic, correspondingly increasing the hydroxide ion concentration. In general, reducing the pH below
7 limits the scale formation by alkaline salts like calcium carbonate. Non-alkaline
salts constitute of readily dissolving charged ions that do not alter the pH of water.
Table 6.1 Mechanism of organic fouling and biofouling in membrane distillation operation due to
presence of natural organic matters and microorganisms in water sources
Type of
fouling
Foulant sources
Typical mechanism involves
References
Organic
fouling
Presence of natural
organic sources in water
sources
Organic fouling occurs via
adsorption of natural organic
sources on the membrane that
causes a layer of gel formation of
macromolecular substances
Naidu et al.
(2014) and
Srisurichan
et al. (2005)
Biofouling
(biofilm
formation)
Microbial growth
includes fungi, algae, and
other microorganisms in
water sources
Biofouling takes place when there
is a formation of biofilm on the
membrane surface
Goh et al.
(2013b) and
Krivorot et al.
(2011)
S. S. Ray et al.
Franken et al. explained that presence of natural organic matters (NOM) in high
concentration in the feed stream decreases the hydrophobicity of the membrane
through adsorption that causes wetting of membrane (Franken et al. 1987).
Eventually, deterioration of membrane contact angle and increase in organic content
in permeate stream were observed. In other words, the rate of wetting purely
depends on the type of foulant (based on characteristics of feed stream) (Naidu et al.
2016). Figure 6.7 indicates the membrane distillation fouling-wetting mechanism
based on organic fouling, and it also elaborates the consequences.
6.2.5 Scaling Phenomenon
Scaling of membrane takes place when inorganic salts precipitate and accumulate
on the surface of the membrane, thereby clogging the pores for vapor to diffuse
through the membrane and resulting in decline of permeate flux. CaCO 3 , CaSO 4 ,
and silicates were analyzed to be the prominent cause of permeate flux decline
while recovering reusable water from natural streams including seawater and brine
water stream. Membrane scaling falls into the category of membrane fouling.
Typically, there are two types of membrane scaling, namely, (a) inorganic scaling
and (b) particulate or colloidal fouling. In general, it is observed that the pressuredriven reverse systems assist the formation of compact cake scales, unlike membrane distillation process, driven by temperature difference (Martinetti et al. 2009;
Warsinger et al. 2015).
In the recent years, extensive investigations have been performed for inorganic
scaling in membrane distillation processes. Typically, inorganic scaling in both
reverse osmosis and membrane distillation processes is categorized into (a) alkaline
scaling, (b) non-alkaline scaling, and (c) uncharged molecule scales. The alkaline
salts (basic) when added through hydrolysis make the solution basic, correspondingly increasing the hydroxide ion concentration. In general, reducing the pH below
7 limits the scale formation by alkaline salts like calcium carbonate. Non-alkaline
salts constitute of readily dissolving charged ions that do not alter the pH of water.
Table 6.1 Mechanism of organic fouling and biofouling in membrane distillation operation due to
presence of natural organic matters and microorganisms in water sources
Type of
fouling
Foulant sources
Typical mechanism involves
References
Organic
fouling
Presence of natural
organic sources in water
sources
Organic fouling occurs via
adsorption of natural organic
sources on the membrane that
causes a layer of gel formation of
macromolecular substances
Naidu et al.
(2014) and
Srisurichan
et al. (2005)
Biofouling
(biofilm
formation)
Microbial growth
includes fungi, algae, and
other microorganisms in
water sources
Biofouling takes place when there
is a formation of biofilm on the
membrane surface
Goh et al.
(2013b) and
Krivorot et al.
(2011)
S. S. Ray et al.