Membrane fouling control method
Some common preventative measures to avoid membrane fouling are
1. Scheduled cleaning
2. Pretreatment
3. System design
Scheduled cleaning A systematic cleaning regimen can help to prevent foulants from building up on the membrane. Cleaning cycles should be scheduled monthly or at other regular intervals to provide the greatest benefit. Maintenance strategies can vary depending upon the membrane filtration system design and the types of contaminants involved, and can employ one or more cleaning methods, such as:
(i). Mechanical cleaning involves the use of physical force to loosen contaminants from the membrane and flush them out of the system. Typical approaches include vibration, as well as backward or forward flushing, where water or a cleaning solution is run through the unit at a faster speed or higher pressure than in a normal service cycle, resulting in turbulence that removes foulants from the membrane. In a related process known as air scouring, air is added to the backwash/forward flush solution to further increase turbulence.
(ii). Chemical cleaning involves the application of detergents, caustics, acids, antiscalants, or dispersants to loosen and remove foulants from the membrane surface. Cleaning chemicals are selected based on the type of contaminants present, with consideration also given to the membrane material to ensure that the chemicals used do not damage it.
Pretreatment
RO/NF membranes have smaller pores than MF/UF membranes, therefore, they are more likely to require some form of pretreatment to avoid membrane fouling or other issues. Streams with high concentration of contaminants may also demand pre-treatment ahead of membrane filtration units in order to minimize the risk of membrane fouling. Pre-treatment options can include coagulation if colloidal particles are present, as well as gravity settling(sedimentation), flocculation and media filtration for the removal of larger or coagulated particles. Other types of pre-treatment can include chemical pH adjustment and ion exchange to prevent adsorption or deposition of foulants on the membrane.
System design
Preventing membrane fouling is best accomplished by good planning and design. There are many variables that play a role in proper system function for a membrane filtration system, each of which should be considered when replacing a membrane or installing a new system. These include: (i) Membrane material: Filtration membranes may be fabricated from a wide variety of synthetic polymers, ceramic, and metallic materials. Properties of the membrane material, such as its surface ionic charge, hydrophobicity, and pH tolerance range, determine whether the membrane will be resistant to certain types of fouling, and how well it will withstand process conditions and the necessary maintenance regimen.
(ii) Membrane pore size: Pore size is the key factor to ensuring efficient removal of targeted contaminants by a membrane filtration unit. Additionally, selection of the proper membrane pore size can help to avoid fouling by optimizing permeate flux in light of other factors, such as feed water quality, temperature, and salt concentration.
(iii) Operating conditions: Membrane fouling can be exacerbated by certain ranges of temperature, pH, transmembrane pressure, and flow rate. A welldesigned system will balance these variables to ensure that foulants do not collect on the membrane surface. Several approaches can be taken to minimize membrane fouling:
a. Optimize pH and ionic strength of the feed solution to minimize the adsorption or deposition of the feed materials.
b. Select an appropriate pre-filtration procedure or other means to remove large molecules, since the presence of larger molecules or particles could cause a steric hindrance to the passage of smaller molecules through the membrane.
c. Select a membrane with an optimum pore size to result in good separation performance as well as optimized permeate flux.
d. Optimize the operating conditions. This includes increasing transmembrane pressure to maximize flux without introducing more fouling potential.
e. Increase the cross-flow velocity, which generally results in an improvement in permeate flux.