Ultrafiltration is an another way of membrane filtration in which pressure or concentration gradients lead water to a separation through a special membrane – semipermeable membrane. Suspended solids and solutes of high molecular weight are restrained in the so-called retentate. In this time water and low molecular weight solutes pass through the membrane in the permeate. This separation process is used in industry and research for purifying and concentrating macromolecular solutions. This method is really good for protein solutions.

Ultrafiltration is not fundamentally different from microfiltration. Both of them separate based on size exclusion or particle capture. It is fundamentally different from membrane gas separation, which separate based on different amounts of absorption and different rates of diffusion. Ultrafiltration membranes are defined by the molecular weight cut-of of the membrane used. Ultrafiltration is applied in cross-flow or dead-end mode.

 

Drinking water

Drinking water treatment 300 m³/h using ultrafiltration in Grundmühle waterworks (Germany)

UF can be used for removing particulates and some macromolecules from raw water to produce safe and drinkable water. It has been used to either replace existing secondary (coagulation, flocculation, sedimentation) and tertiary filtration (sand filtration and chlorination) systems employed in water treatment plants or as standalone systems in isolated regions with growing populations. When treating water with very high suspended solids, ultrafiltration is integrated into the process, utilizing primary and some secondary treatments as pre-treatment stages. Ultrafiltration processes are currently preferred over traditional treatment methods for the reasons. And these reasons are:

  • No chemicals required (aside from cleaning);
  • Compact plant size;
  • Capable of exceeding regulatory standards of water quality, achieving 90–100% pathogen removal;
  • Constant product quality regardless of feed quality.

Because of high cost of treating water by ultrafiltration, these processes are currently limited. Additional pretreatment of feed water is required to prevent excessive damage to the membrane units.

In many cases ultrafiltration is used as a prefiltration in reverse osmosis plants to protect the RO membranes.