Expanding the Toolbox

Hollow-fiber ultrafiltration (UF) membranes are an important, innovative addition to the water treatment dealer's toolbox. New UF-based point-of-entry (POE) platforms now make it affordable to combine this unique technology with existing filtration and treatment systems to solve a broad range of water problems.

UF systems provide three fundamental advantages over traditional filtration alternatives:

  • Submicron filtration down to 0.02 µm removes fine sediment, turbidity, cysts, bacteria and most viruses.
  • High flow rates and smaller installation footprints are ideal for both whole-house and commercial applications.
  • Forward- and back-flushable fibers offer extended filter life.

Many dealers have identified significant market opportunities for UF technology, especially when applied to waters where fine sediment, turbidity and cysts are of primary concern. Most dealer sales opportunities fall into three categories:

  • Challenge water: Residential and commercial POE sediment/ turbidity/cyst filtration (NSF standards 42 and 53).
  • Prefiltration: Installed prior to one-time-use specialized filters, reverse osmosis (RO) systems and other sensitive equipment to extend life.
  • Barrier filtration: Provides a barrier to clean water supplies to protect against compromises to the water system.

 

Operation

Hollow-fiber membranes can be constructed to allow either inside- out or outside-in flow in a dead-end mode (i.e. there is no continuous waste flow).

  • Inside-out flow: Feed water travels down the bore of the fiber and passes through the fiber to the outside, where it is collected as treated (filtrate) water.
  • Outside-in flow: Feed water travels on the outside and passes through to the inside of the fiber, where it is collected as treated (filtrate) water.

 

Flushing

Like all filtration media, a UF membrane becomes covered with the material it removes from water. Over time, the accumulation of that filtered material reduces membrane flow and increases pressure drop. Similar to the backwash and rapid rinse cycles employed by traditional multimedia filters, many UF systems are designed to flush regularly in order to remove material filtered out of the water.

A flush cycle flow path will be set in either a forward or reverse direction, depending on the configuration of the system. In the case of forward-flush systems, a flush valve is opened so that flow through the bore of the fibers (inside-out) or around the fibers (outside-in) is significantly increased. Retained solids are flushed from the membrane surface due to the velocity of the water flowing around it.

For back-flush systems, a flush valve is opened and feed water is shut off so filtrate is forced to reverse flow through the membrane. This process lifts retained solids from the membrane surface and flushes them to drain. The filtrate used for back flushing can be provided by a storage tank or by other UF membranes that remain in service.

 

Applications

UF systems are used for many applications requiring the removal of suspended solids, including particles, colloids, cysts, bacteria and viruses. Typical residential and commercial applications include:

Surface and well water treatment - UF is used as part of a multibarrier surface water treatment for reduction of:

  • Turbidity;
  • Fine silt not easily filtered by conventional media filters;
  • Total oxidized carbons (TOC) associated with high molecular weight organics; and
  • Cysts not easily filtered by conventional media filters or inactivated by chlorine.

Pretreatment of RO feedwater - Pretreating RO feedwater with UF increases RO membrane life and lowers operating costs.

This approach:

  • Lowers feedwater silt density index by removing turbidity and colloids;
  • Removes suspended solids that break through conventional pressure filters;
  • Increases cartridge filter life; and
  • Reduces biofouling by reduction of TOC and bacteria.

Prefiltration of water feeding sensitive equipment - UF can be installed prior to one-time-use specialized filters and other sensitive equipment to lower operating costs:

  • Food service: Prefiltration of chlorine taste, odor and cysts greatly extends filter service life.
  • Laboratories, pharmaceutical, life sciences: Extends the life of disposable membrane filters.

Postfiltering high purity water in special applications - UF can be installed after an RO/DI system to improve the quality of ultrapure water and lower operating costs in the following ways:

  • Pharmaceutical, life sciences: When installed in the pure water recirculation loop, it extends the life of disposable point-of-use (POU) filters.
  • Ultrapure water: When installed in the pure water recirculation loop, it reduces bacteria and TOC and extends the life of disposable POU filters.

Installed as part of a multibarrier system, UF provides added peace of mind. UF, when used on potable water supplies, acts as an additional barrier against compromises to the water system by reducing cysts, turbidity, bacteria and viruses.

To provide protective redundancy against pathogens when treating nonpotable water supplies, UF should be combined with alternative technologies (such as ultraviolet or chlorination) certified to meet the U.S. Environmental Protection Agency and NSF purifier protocols for cysts, bacteria and virus removal.

Due to the complexity and variability of local regulations, dealers and installers should have a thorough understanding of state and local codes for treating unsafe water supplies. They should provide both certified systems and routine maintenance programs to ensure that the treated water continuously meets those standards.

Dan Elko is marketing manager and Dave Averbeck is director of engineering for Pentair Water. They can be reached at 262.784.4490.

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