171
The fouling layer formed changes the thermal properties, minimizes the temperature difference across the membrane, and correspondingly increases the polarization temperature implying a lesser driving force (Hsu et al. 2002). One of the
most important layer characteristic contributing to the resistances is the porosity.
The secondary layer formed can be either porous or nonporous. Gryta explained the
formation of porous layer by the deposition of smaller crystals due to higher velocity, while thicker layers (“mountain-like” structures) are formed with lower velocity
(Gryta 2005). Although a nonporous layer increases both thermal and hydraulic
resistance, the porous layer only effects the thermal resistance. Furthermore, less
transport of water vapor and increased mass transfer resistance are observed for
nonporous layer. Similarly, the escalated thermal resistance reduces the permeate
flux for a porous cake layer (Gryta 2008b).
6.2.3 Particulate or Colloidal Fouling
Particulate or colloidal fouling is a common concern in all filtration technologies.
Although the larger particles can be separated by advanced filtration (i.e., ultrafiltration, microfiltration, nanofiltration), the smaller particle still poses complications in
membrane-based technologies. Generally, for pilot-scale membrane distillation,
desalination units, cartridge filters, or screens are installed for efficient pre- treatment
of particulate matter. These particulates are generally clay, humic substances, silt,
and silica, abundant in ponds, lakes, groundwater, and especially river water, compared to seawater (Meng et al. 2014). Silica being small in size is harder to remove
with pre-treatment stems such as microfiltration, contributing to a significant concern of silica fouling. Primarily, there are three forms of silica present in water supplies: colloidal silica, particulate silica, and dissolved silica (or monosilicic acid).
Although recent investigation notes silica fouling to be predominant in membrane
distillation systems, the flux decline is lesser than scaling. Furthermore, acid treatment to counter scaling is inefficient to remove the uncharged silica foulants (Drioli
et al. 2015; Gilron et al. 2013).
6.2.4 Organic Fouling and Biofouling
Most of the recent research articles discussed about inorganic fouling as well as
scaling in membrane distillation process. There are limited sources that elaborate
the mechanism of organic and biofouling in membrane distillation process. However,
biofouling or microbial growth shows minimal impact in membrane distillation process due to high saline concentration and high-temperature condition utilized in
membrane distillation operation. Based on various water and wastewater studies,
organic fouling as well as biofouling can also be considered as one of the main fouling categories. Table 6.1 indicates the typical mechanism of type of foulant including organic fouling and biofouling.
6 Fouling and Wetting: A Major Challenge for Membrane Distillation
The fouling layer formed changes the thermal properties, minimizes the temperature difference across the membrane, and correspondingly increases the polarization temperature implying a lesser driving force (Hsu et al. 2002). One of the
most important layer characteristic contributing to the resistances is the porosity.
The secondary layer formed can be either porous or nonporous. Gryta explained the
formation of porous layer by the deposition of smaller crystals due to higher velocity, while thicker layers (“mountain-like” structures) are formed with lower velocity
(Gryta 2005). Although a nonporous layer increases both thermal and hydraulic
resistance, the porous layer only effects the thermal resistance. Furthermore, less
transport of water vapor and increased mass transfer resistance are observed for
nonporous layer. Similarly, the escalated thermal resistance reduces the permeate
flux for a porous cake layer (Gryta 2008b).
6.2.3 Particulate or Colloidal Fouling
Particulate or colloidal fouling is a common concern in all filtration technologies.
Although the larger particles can be separated by advanced filtration (i.e., ultrafiltration, microfiltration, nanofiltration), the smaller particle still poses complications in
membrane-based technologies. Generally, for pilot-scale membrane distillation,
desalination units, cartridge filters, or screens are installed for efficient pre- treatment
of particulate matter. These particulates are generally clay, humic substances, silt,
and silica, abundant in ponds, lakes, groundwater, and especially river water, compared to seawater (Meng et al. 2014). Silica being small in size is harder to remove
with pre-treatment stems such as microfiltration, contributing to a significant concern of silica fouling. Primarily, there are three forms of silica present in water supplies: colloidal silica, particulate silica, and dissolved silica (or monosilicic acid).
Although recent investigation notes silica fouling to be predominant in membrane
distillation systems, the flux decline is lesser than scaling. Furthermore, acid treatment to counter scaling is inefficient to remove the uncharged silica foulants (Drioli
et al. 2015; Gilron et al. 2013).
6.2.4 Organic Fouling and Biofouling
Most of the recent research articles discussed about inorganic fouling as well as
scaling in membrane distillation process. There are limited sources that elaborate
the mechanism of organic and biofouling in membrane distillation process. However,
biofouling or microbial growth shows minimal impact in membrane distillation process due to high saline concentration and high-temperature condition utilized in
membrane distillation operation. Based on various water and wastewater studies,
organic fouling as well as biofouling can also be considered as one of the main fouling categories. Table 6.1 indicates the typical mechanism of type of foulant including organic fouling and biofouling.
6 Fouling and Wetting: A Major Challenge for Membrane Distillation