Frequently Asked Questions

Learn more about filtration and what Membranology has to offer.  Find answers to your questions here!

What is membrane filtration?

Membrane filtration involves separation of materials based on their size.  The pore size of the membrane is tailored to the specific application.

What are the differences between cross-flow and the HPC01 stirred cell?

Cross-flow equipment recirculates the process material across the surface of the membrane, as a result they have a permeate stream and a concentrate stream that can be sampled.  These can be operated at a range of temperatures, cross-flow rates and pressures to suit the application, with the ability to sample either stream.  The high pressure stirred cell (HPC01) is a batch process whereby a 300mL sample is placed into the assembled cell and pressurised with compressed nitrogen.  The stirrer inside the cell mimics the cross-flow action and the concentrate stream is only available post experiment, once the pressure has been carefully vented from the cell.

What is the difference between hydrophobic and hydrophilic membranes?

Membranes are characterised by the surface material that can be hydrophobic or hydrophilic in nature.  The surface tension of hydrophobic membranes repels the water from entering the pores.  Water will only enter the pores if the applied pressure is greater than the membrane’s water entry pressure.  Hydrophobic membranes can be pre-wetted using alcohol.

Hydrophilic membranes have a low surface tension and water enters the pores as soon as it comes into contact with the membrane (capillary action).

How do you store unused flat sheet membranes?

Flat sheet membranes are stored according to the manufacturer guidance.  Some are stored in cold environments, but in general, storage in a cool place away from direct sunlight is recommended.

Does the membrane disc need to be preconditioned in the HPC01 stirred cell?

Yes, flat sheet membranes for water or aqueous solutions require pre-conditioning prior to your experiment.  This involves flushing deionised water through the membrane at the pressure and temperature expected for the separation experiments.  This ensures that the membrane is compacted at the operational pressure and temperature and removes any residuals or preservatives.  A clean water flux determination should be completed on this wetted membrane to determine the performance prior to the separation experiments.

Do not allow the wetted disc to dry out, store in a suitable medium.

How do you store used flat sheet membranes?

Once the membranes have been used they must be kept wet.  For short term storage (1 week or less) they can be stored in deionised water.  For longer term storage the bacterial growth is controlled by storing in a 0.5% sodium metabisulfite solution, which is replaced at least every 3 months, but better replaced monthly.

Where can I get membrane sheets for the HPC01 stirred cell?

Membranology supply membranes to fit into the stirred cell.  There are a large range of membrane options available and a selection pack can be provided for process development purposes.  If you have your own membrane then you can cut it to fit into the cell using the support disc as the template.

What is a clean water flux determination?

Clean water flux determinations are used to monitor performance of the membrane.  This can be done on cross-flow or using the flat sheets in the HPC01.  This involves operating the system at the temperature of the expected separation experiments at a range of different pressures.  The specific flux through the membrane is correlated against the transmembrane pressure in order to determine the performance of the membrane.  The clean water flux determination should be completed prior to and immediately after the separation experiments.  This identifies the level of fouling on the surface and indicates whether a cleaning cycle is required.  The clean water flux can be completed post cleaning to determine if the membrane has recovered to the level of that prior to the experiments.  Plotting the clean water flux for the membrane over time identifies the performance loss over time and indicates when the membrane must be replaced.