Industrial Ultrafiltration (UF) is a water purification process in which water is forced through a semipermeable membrane.

Ultrafiltration filter has a pore size around 0.01 micron while microfiltration filter has a pore size around 0.1 micron. Ultrafiltration removes these smaller particles compared to micro filtration, thus is effective compared to micro filtration. It removes bacteria & viruses.

Depending on the feed water characteristics pre-treatment might be needed in combination with the UF process. Dissolved substances are not removed by UF membranes, so they must be transformed into particulate form if they are the target of the UF process.

Operating Modes:

Ultrafiltration systems in water treatment applications often work in the so-called Dead- End (or Direct Filtration) mode as opposed to the Concentrate Recirculation (or Cross- Flow) or Bleed Operation modes, which will be described below.

The outside-in flow configuration allows the use of highly effective air scour cleaning which enhances particle removal and improves recovery. A dead-end flow format achieves higher recovery and energy savings. The module housing design eliminates the need for separate pressure vessels while the vertical orientation allows gravity draining and facilitates the removal of air from cleaning and integrity testing processes.

In the Dead-End operation mode (also known as “Deposition Mode”), all the feed volume entering the UF elements passes through the membrane (there is no reject stream) and is collected on the filtrate side, so there is 100% recovery of the water. The contaminants that are not small enough to pass through the membrane are either trapped on the membrane surface or stuck inside the pore channels, leading to an increase in the TMP and a decline of permeability. At some point the system is taken off-line and the membranes are cleaned hydraulic or chemically.

In the Concentrate Recirculation mode (or Cross-Flow), the concentrate flow exceeds the filtrate flow passing through the membrane (typically in a ratio of 5:1 or higher). The concentrate stream is then typically recycled back to the feed tank or to the recirculation pump suction side. This allows increasing the flow velocity through the feed channels and therefore achieving a shear force effect.

Depending on the type of driving filtration force, the UF modules can be categorized as pressurized or vacuum driven. In the pressurized form, the membranes are placed inside pressure vessels (with vertical or horizontal orientation), and the UF modules are grouped in parallel to form skids, racks, or trains. Typical operating pressure of pressurized UF systems is up to 2.5 barg (35 psi). The vertical orientation allows easier drain, the use of air as an aid to increase cleaning efficiency and the elimination of extra vessels or housings.

Vacuum driven UF modules are typically submerged in a feed tank with no membrane housing, and can only operate at low suction pressures, typically below 0.8 barg (vacuum pressure). Due to this limitation in the maximum pressure that can be applied, they must operate at a lower flux than pressurized systems and are more sensitive to water temperature fluctuations. Also, the module installation, cleaning, fiber repair, isolation or maintenance are more laborious in submerged systems.

Ultrafiltration can be used as a dead-end filtration and has a 90-98% recovery rate and can be used to treat wastewater for reuse.

There are several reasons to consider Ultrafiltration as an excellent choice for pre-treatment as opposed to conventional technologies:

  1. Ability to cope with difficult and variable waters: Ultrafiltration membranes are a physical barrier against most particles, suspended matter, colloids, bacteria and even viruses, that can produce an excellent water quality independently of variations in the influent water quality.
  2. Improved and more consistent product quality: Due to their fine pores, ultrafiltration membranes can provide a very high-quality filtrate, with typical ultrafiltrate turbidity less than 0.1 NTU (independent of the raw water turbidity), SDI less than 3%/min and 6-log or more removal of pathogens such as Cryptosporidium and Giardia cysts.
  3. Smaller plant footprint and less weight: UF pre-treatment systems require a smaller footprint (up to 50% lower) and weight than media filtration systems. This can lead to a reduced cost of land acquisition, building design, and transport.
  4. Module and skid Integrity Testing can be done easily on line to detect potential leakages without significant plant downtime.
  5. Membrane modules can be individually isolated for repair, maintenance or replacement without compromising the plant output.
  6. Ease of design and operation: Despite requiring more focus on sustained permeability and productivity, ultrafiltration systems offer much more stable water quality than a multimedia filtration system, without the need to monitor filter ripening time or breakthrough, or the need of ensuring appropriate layering of multimedia after backwash. Therefore, process design is less complicated and control is more automated than with conventional pre-treatment.
  7. Lower environmental impact: Conventional systems typically require chemical pre-treatment such as coagulation and pH adjustment for the removal of silt and fine particles, but UF can remove these contaminants just by size exclusion due to the small size of the membrane pores. This can lead to lower chemical consumption and lesser environmental concerns for wastewater disposal.
  8. Lower RO stage cost: The potential for lower downstream cost, based on improved and more consistent water quality facilitated by the UF system, is a key aspect. UF as pre-treatment also allows higher design flux in the RO stage, as well as lower requirements for membrane cleaning and ultimately lower replacement rates, by facilitating a RO feed water with lower fouling tendency. In addition, cartridge filters use can be significantly reduced or eliminated (especially when there is no break tank in between UF and RO).