conventional treatment, microfiltration is used before reverse osmosis as
pre-treatment for protection of reverse osmosis membrane life, to reduce fouling as
well as operational cost of this process. The microfiltration membrane is porous,
isotropic and is usually made of hydrophobic polymers like polysulphone,
polyethersulphone, polypropylene, polyvinylidene fluoride. However, recently,
ceramic materials and inexpensive clay mixtures, such as ball clay, kaolin, feldspar,
quartz, pyrophyllite, calcium carbonate, are also employed for preparation of these
membranes. The hydrophobic nature of the membrane renders greater affinity
towards organic matter such as oil and grease. This might lead to membrane fouling
by oil and grease. Hence, pre-treatment with granular activated carbon is done to
limit the oil and grease content to less than 1 mg/L. The membrane module may be
either hollow or tubular type. The wastewater is passed through a series of membranes under pressure or vacuum. The wastes accumulate on the upstream side of the
membrane. When the thickness of fouling layer on the surface of the membrane
increases, the trans-membrane pressure will slowly increase. The accumulated
contaminants are eliminated periodically by use of backwash with disinfectants
such as sodium hypochlorite, etc. The operating pressure range and hydraulic
permeability range are 0.1–2 bar and greater than 1000 L/hr-m
2
–bar, respectively.
Scaling, fouling or biofouling may occur on the membrane due to the formation of
biological growth and long-term operation; therefore, flux as well as separation
factor may diminish. Hence, some biofouling inhibitors such as bioacids (chlorine,
chloramine) are added to protect the membrane. However, the accumulated contaminants are removed by using disinfectants like sodium hypochlorite. Spent granular
activated carbon or polymeric materials may require regeneration, and it depends on
quality and quantity of wastewater. The treated water after microfiltration is not
suitable for makeup water in boiler feed water and in cooling towers but may be used
as emergency firewater (International Petroleum Industry Environmental Conservation Association 2010; Shahryar 2017).
B. Ultrafiltration:
Ultrafiltration is used for the removal of colloidal proteins, pigments, oils,
organics, microparticles from wastewater and the size range of the retained particles
is 0.001–1 μm. The operating pressure is about 1–5 bar and the hydraulic permeability is 10–1000 L/m
2 hr-bar. Ultrafiltration like microfiltration may also be used
before reverse osmosis process as pre-treatment to maintain the performance of
reverse osmosis membrane. In this technology, tubular or hollow ultrafiltration
module is equipped with polymeric or inorganic porous asymmetric type membrane.
The ultrafiltration operating process is same as microfiltration. Fouling or biofouling
may arise on the membrane of ultrafiltration due to long time operation and generation of biological growth which may be removed by using bioacids as in
microfiltration membrane (International Petroleum Industry Environmental Conservation Association 2010; Shahryar 2017). However, both microfiltration and ultrafiltration are not suitable for removal of heavy metals and dissolved salts (Shahryar
2017).
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M. Samanta and D. Mitra
pre-treatment for protection of reverse osmosis membrane life, to reduce fouling as
well as operational cost of this process. The microfiltration membrane is porous,
isotropic and is usually made of hydrophobic polymers like polysulphone,
polyethersulphone, polypropylene, polyvinylidene fluoride. However, recently,
ceramic materials and inexpensive clay mixtures, such as ball clay, kaolin, feldspar,
quartz, pyrophyllite, calcium carbonate, are also employed for preparation of these
membranes. The hydrophobic nature of the membrane renders greater affinity
towards organic matter such as oil and grease. This might lead to membrane fouling
by oil and grease. Hence, pre-treatment with granular activated carbon is done to
limit the oil and grease content to less than 1 mg/L. The membrane module may be
either hollow or tubular type. The wastewater is passed through a series of membranes under pressure or vacuum. The wastes accumulate on the upstream side of the
membrane. When the thickness of fouling layer on the surface of the membrane
increases, the trans-membrane pressure will slowly increase. The accumulated
contaminants are eliminated periodically by use of backwash with disinfectants
such as sodium hypochlorite, etc. The operating pressure range and hydraulic
permeability range are 0.1–2 bar and greater than 1000 L/hr-m
2
–bar, respectively.
Scaling, fouling or biofouling may occur on the membrane due to the formation of
biological growth and long-term operation; therefore, flux as well as separation
factor may diminish. Hence, some biofouling inhibitors such as bioacids (chlorine,
chloramine) are added to protect the membrane. However, the accumulated contaminants are removed by using disinfectants like sodium hypochlorite. Spent granular
activated carbon or polymeric materials may require regeneration, and it depends on
quality and quantity of wastewater. The treated water after microfiltration is not
suitable for makeup water in boiler feed water and in cooling towers but may be used
as emergency firewater (International Petroleum Industry Environmental Conservation Association 2010; Shahryar 2017).
B. Ultrafiltration:
Ultrafiltration is used for the removal of colloidal proteins, pigments, oils,
organics, microparticles from wastewater and the size range of the retained particles
is 0.001–1 μm. The operating pressure is about 1–5 bar and the hydraulic permeability is 10–1000 L/m
2 hr-bar. Ultrafiltration like microfiltration may also be used
before reverse osmosis process as pre-treatment to maintain the performance of
reverse osmosis membrane. In this technology, tubular or hollow ultrafiltration
module is equipped with polymeric or inorganic porous asymmetric type membrane.
The ultrafiltration operating process is same as microfiltration. Fouling or biofouling
may arise on the membrane of ultrafiltration due to long time operation and generation of biological growth which may be removed by using bioacids as in
microfiltration membrane (International Petroleum Industry Environmental Conservation Association 2010; Shahryar 2017). However, both microfiltration and ultrafiltration are not suitable for removal of heavy metals and dissolved salts (Shahryar
2017).
264
M. Samanta and D. Mitra