Frequently Asked Questions

Polymem - Membanes et fibres d'ultrafiltration

Why to choose Inside/Out or Outside/In ?

Fibre Polymem fd5d8Inside/Out filtration was originally developed for fluids concentration (when the valuable stream is the concentrate). This mode is recommended when cross flow filtration is required due to high concentration of organic matter and when the concentrate is the final product (concentration process).

The Outside / In filtration is particularly advantageous for natural water treatment (fresh water or sea water) and for wastewater (membrane bioreactors or tertiary treatment) because it offers:

– Higher membrane area in a smaller volume
No risk of fiber plugging
Easier backwashing (with air scour)
Simpler process thanks to dead end filtration mode (successive steps of filtration and backwash)
Simpler modules with only two connections leading to simpler and cheaper systems with less piping

Ultra or Micro Filtration ?

For short term applications (disposable cartridges), microfiltration offers the advantage of a higher filtration flux (liters of water passing through the membrane per hour and per square meter).

For long term applications (municipal, industrial) ultrafiltration is preferred to microfiltration because for the same investment and operating cost, it offers:

– similar fluxes (after a few weeks or months of operatio, ultrafiltration and microfiltration fluxes become similar).
– 4 log and more viruses removal in all cases (regardless of viruses size) when microfiltration will remove 1 to 4 log.
– a SDI removal which is much better with ultrafiltration than with microfiltration.

Which membrane for which filtration ?

Objective Process pressure Filtration type Pore size
Clarification, primary disinfection Low pressure Microfiltration 0.1 µ to 1 µ
Clarification, virus disinfection, Nanofiltration or Reverse Osmosis pretreatment Low pressure Ultrafiltration 0.005 µ to 0.1 µ
Organic molecules and large ions retention, COD removal, color removal, partial desalination, softening, desulfation,.. High pressure Nanofiltration <0.005 µ
Desalination, demineralization High pressure Reverse Osmosis Water is passing by diffusion through a dense membrane while ions and small organic species are rejected