Flux describes the rate at which the liquid passes through the membrane. In Australia it is almost always quoted in units of LMH, or litres per square meter per hour.
Selecting the appropriate flux is one of the most important decisions when designing a membrane system. We typically want a high flux, because this reduces the number and hence cost of the membranes. In the case of reverse osmosis, it will also result in a better quality product. However, if the flux is too high, then it will lead to operational problems – frequent chemical cleans, short membrane life, or simply failure to produce the amount of product needed. If you are tempted to push your system capacity just a little, then proceed with caution and keep a close eye on operating pressures. A small increase in flux can lead to a big drop in performance.
How is flux determined?
The value of the flux will be determined based on lots of parameters, including:
- the type of membrane (if it is reverse osmosis, or ultrafiltration, or membrane distillation, etc)
- the designer’s experience with the solution in question
- the amount of risk we are willing to take
- the amount and type of dissolved substances that are known to be present
- the risk of foulants being present
- what type of pretreatment is used before the membrane
- lots of other parameters
Historically, the cost of the membranes has been a big part of the cost of the overall system. However, for standard RO and MF/UF membranes, the costs of the membranes have gradually reduced, so the benefit of choosing an aggressive flux is much less, while the risks of high flux remain as high as ever.
Typical flux rates
Here are some typical flux rates for a variety of membranes. Keep in mind that there are a lot of variables that designers will consider. But if the flux of your system is at the higher end of the range given below, you should check the designer’s experience and confidence.
RO with tap water or low salinity ground water: 20-25LMH
RO with sea water: 12-16LMH
RO with recycled water: 15-18LMH
Polymeric MF/UF with clean water: 50-100LMH
Polymeric MF/UF with dirty or fouling water: 20-60LMH
Ceramic filtration: 50-1,000LMH