• Fouling is the build-up of unwanted deposits or scales on a membrane that result in a reduction of flux through the membrane .

Fouling can be caused by :-

• The slow build up of precipitates over extended periods of time .
• The precipitation of initially dissolved species as a result of :
• The concentration process.
• Changes in the feed stream's composition.
• Failure of a pre-treatment system.
• Inadequate flushing of the system after a shutdown .
• The adsorption of organics on the membrane surface .
• The formation of bacterial slimes on the membrane surface .
  

Fouling is commonly reduced by :

• Using higher flow-rate , which create turbulent flow conditions and prevent large particulates from settling on to the membrane.
• Selecting membrane materials that do not promote adsorption of foulants .
• Pre-treating the feed stream to remove or " inactivate " potential foulants by , for example , filtering , pH adjustment , or adding a dispersant .
• Back flushing .
• Generally , high pressure tend to promote fouling . In Ultrafiltration this is due to compaction or thickening of the gel layer adjacent to the membrane that forms to compensate for the higher flux
• Appropriate hardware design of a membrane system can be help to minimize fouling .
  Thin flow channels and high flow velocities are advantageous in preventing particulate deposition .
• Low or stagnant flow areas anywhere in the flow path should be avoided as such regions promote the growth of micro-organisms.
• Pre-treatment used for commonly encountered foulants are listed in Table 2 .
• Selection of a suitable pre-treatment is governed by :
  • The composition of the feed stream , including pH , total dissolved solids ( TDS ) , temperature , and particulate size distribution .
  • The chemical compatibility of the membrane unit with residuals that may carry over from the pre-treatment stage .
  • The results of thorough bench-scale and pilot-scale testing .

CLEANING :-

• All membrane systems need regular to maintain productivity (flux) and performance ( rejection or retention ) .
• The frequency of cleaning can vary from hours too months . Generally , bench-scale and pilot-scale testing will indicate how often cleaning is likely to be required .
• Fouling reduction methods can and should be used to reduce the amount of cleaning required and the associated loss of production .
• Cleaning procedures and routines vary widely from application to application but will usually consist of at least one of the following:-

Rinsing and Recycles :

• Rinsing can be done with or without a chemical additive .
• Rinsing is used to remove loosely adhering surface foulants .
• Make sure that any additives used are compatible with the membrane .
• A final rinse without additives should be done to remove all additive residues from the system before resuming production .

Back flushing :

• Back flushing cleans the membrane by forcing permeate-quality water backward through the membrane . This loosens and lifts foulant from the membrane .
• Back flushing is generally more effective than simple rinsing and is especially useful in removing pore-clogging foulants from UF and MF membranes.
• Back flushing , like rinsing , may not use an additive .
  

Mechanical cleaning :

• Mechanical cleaning is usually a method of last resort as it is often labour intensive and costly.
• Tubular modules can be cleaned mechanically by passing sponge balls through them . An Electrodialysis stack is cleaned mechanically by disassembling the stack and manually cleaning the membrane surfaces .
• In some cases , " clean-in-place " (CIP ) systems are available . They can be programmed to initiate rinse or back flush cleaning cycles automatically .
• The choice of chemicals to use during cleaning is dictated by :

• The nature of the foulant that has to be removed .
• The compatibility of the chemical with the membrane material .
• The cost of the chemicals .

 

• No matter what cleaning solution is chosen , it must be tested for compatibility with the membrane . Failing to do so could damage the membrane irreparably , resulting in substantial costs and lost production .

Fouling And Cleaning By Chemix Systems