What is membrane treatment?
To better understand flux, it is best to explain what membrane treatment is in the water industry. Membrane treatment is widely used for various applications within the industry, including water purification, wastewater treatment and desalination.
The membranes used in water treatment are semi-permeable and allow contaminants to be separated from water. The membranes are designed to be selective, meaning they allow certain molecules to pass through while blocking others. This selectivity is based on the pore size of the membrane, the charge of the molecules or other interactions.
Water is passed through the membrane, either with pressure or by utilizing energy sources such as an electrical field, where contaminants are contained by the membrane.
Popular membrane treatment technologies include:
- Microfiltration (MF): Removes smaller particles like bacteria
- Ultrafiltration (UF): Removes larger particles and dissolved solids
- Nanofiltration (NF): Removes smaller molecules and ions
- Reverse Osmosis (RO): Removes dissolved solids and impurities
- Electrodialysis (ED): Separates ions using an electrical field
Membrane treatment has a certain amount of throughput, which is where flux comes in.
What is flux?
Flux is the amount of liquid or other material that permeates through a membrane per unit area and per unit time. Flux is essentially the measure of a membranes performance and how much liquid can be filtered or processed.
Two units are typically used to describe flux in membrane treatment: gallons per square foot per day (GFD) and liters per square meter per hour (Lmh).
Flux rates vary due to several factors including treatment technologies, membrane type and environmental conditions. For example, in UF membranes the flux can vary between 50 and 200 GFD at an operating pressure of about 50 pounds per square inch (psig), while RO membranes can have a flux between 10 to 20 GFD at 200 to 400 psig.
This variability makes it crucial for treatment facilities to properly calculate flux rates to determine their end goals. Flux rates are also used to ensure liquids are not moving too fast or too slow through a membrane to maximize effectiveness.
How is flux calculated?
Flux can be calculated by using equations to determine the GFD or Lmh.
To determine GFD, the following equation can be used:
As an example of calculating flux from the University of Tennessee, Knoxville, “suppose 200,000 gal/d are flowing through a membrane with an area of 4,000 ft2. The flux would be (200,000 gal/d)/(4,000 ft2) = 50 gfd (85 Lmh).”
What is flux in fluid mechanics?
Flux in fluid mechanics, like flux in membrane treatment, is the rate at which a quantity flows through a given surface. This definition is broad and encompasses different quantities such as volume, mass or energy.
- Volumetric flux: The volume of fluid flowing though a surface per unit time and unit area.
- Mass flux: The mass of fluid flowing through a surface per unit time and unit area.
- Energy flux: The rate at which energy is transferred through a surface per unit area.
For membrane treatment, volumetric flux is most commonly used.
What does flux mean in reverse osmosis (RO)?
In RO, flux refers to the rate at which water passes through the RO membrane. The flux rate is directly related to the amount of water produced during treatment and the quality of the treated water.
- High flux: More water produced but can lead to faster membrane fouling or scaling.
- Low flux: Less water produced but gentler on membranes.
