What is the flux of the BW 8040 reverse osmosis membrane?
In recent years, reverse osmosis membranes have attracted increasing attention in the field of water treatment. As one of the most important indicators of reverse osmosis membranes, membrane flux is still not well understood by many users. Today, we will provide a detailed introduction to membrane flux.
1.What is membrane flux?
In simple terms, membrane flux refers to the water production capacity, meaning the amount of water that passes through the reverse osmosis membrane within a certain period of time. In other words, it represents the permeate output of the RO membrane during a specific time period.
Membrane flux is usually measured on an hourly basis, with the unit expressed as L/(m²·h), abbreviated as LMH. It refers to the number of liters of liquid passing through each square meter of membrane area per hour.
So how do we calculate membrane flux? Let's take our BW 8040 membrane as an example.
the product specification of the BW 8040 membrane
|
TYPE |
Effective Membrane Area (ft² / m²) |
Permeate Flow Rate (GPD / m³/d) |
Stabilized Salt Rejection (%) |
Operating Pressure (PSI) |
|
YIME-MP-BW-8040-(FR) |
400 (37.2) |
10500 (39.7) |
99.7 |
225 |
Flux=Effective Membrane Area/Permeate Flow Rate
Flux=37.16/39.76≈1.07 m³/(m²⋅day)
1 m³/(m²⋅day)=24/1000 LMH
Flux≈45 LMH
Generally speaking, high-quality reverse osmosis membranes should have the following properties:
① High water permeability per unit area and high salt rejection rate;
② Good mechanical strength with minimal compaction of the porous support layer;
③ Excellent chemical stability, resistant to acid and alkali corrosion as well as microbial attack;
④ Uniform structure, long service life, and slow performance degradation;
What is the flux of a reverse osmosis membrane?
In general, for a 40-inch membrane element, the permeate flow rate is approximately 0.0625 m³/h per inch of diameter.
In other words, for a 40-inch long membrane:
- A 4-inch diameter membrane (commonly 4040) produces about 0.25 tons per hour (0.25 m³/h) of permeate.
- An 8-inch diameter membrane (commonly 8040) produces about 1 ton per hour (1 m³/h) of permeate.
However, this value also depends on salt rejection and temperature. Under the same brand and same membrane size, a membrane with higher salt rejection usually has slightly lower permeate flow compared to one with lower rejection.
Membrane flux is an important operating parameter in reverse osmosis systems and is also one of the key indicators for evaluating membrane performance. It is influenced by multiple factors, including feed pressure, feed temperature, feed pH, feed salinity, and system recovery rate.
Among these factors, temperature has a particularly significant impact on membrane flux. Generally, the lower the temperature, the lower the membrane flux (permeate production). When the temperature drops below 25°C, membrane flux typically decreases by about 3% for every 1°C reduction. Temperature also has a strong influence on operating pressure, salt rejection, and pressure drop, making it one of the most critical variables in system design and operation.
As temperature increases, water permeability improves, and the net driving pressure (NDP) required to maintain the same flux decreases, resulting in lower operating pressure. However, higher temperatures also accelerate solute diffusion, leading to increased salt passage, which is reflected as a higher conductivity in the permeate water, meaning slightly reduced water quality.
Overall, membrane flux is one of the most important parameters in reverse osmosis membrane selection and evaluation. Therefore, it is a key factor that users pay close attention to during procurement. It should be noted that the flux values provided in product datasheets are typically reference values measured under standard test conditions set by manufacturers. In actual operation, selection should be based on a comprehensive evaluation of feed water quality, operating pressure, and system design.
Therefore, before selecting membrane elements, it is recommended to conduct a detailed analysis of the raw water quality to ensure the most suitable membrane model is chosen, achieving stable long-term operation and optimal economic performance.