Membrane flux is one of the most frequently discussed parameters in membrane bioreactor (MBR) design.
A higher flux can appear attractive because it may reduce the membrane area required for a given treatment capacity. However, not every quoted flux value represents the same amount of actual water production.
This is where the distinction between gross flux and net flux becomes important.
For engineers, EPC contractors, and project owners, understanding this difference can provide a more meaningful basis for membrane sizing and technology comparison.
Membrane flux describes the volume of permeate produced through a unit membrane area over a given period.
It is commonly expressed as:
For example, a membrane operating at 25 LMH produces approximately 25 liters of permeate per square meter of effective membrane area per hour during the relevant filtration period.
However, an MBR system does not necessarily produce permeate continuously throughout every minute of operation.
Depending on the operating strategy, the membrane may go through:
- Filtration
- Relaxation
- Backwashing, where applicable
- Cleaning
- Other operational periods
Therefore, the flux during active filtration may not represent the average production over the complete operating cycle.
Gross flux generally refers to the membrane flux during active filtration.
For example:
A membrane may operate at a gross filtration flux of 40 LMH during the active filtration period.
This can be a valid operating value.
However, it does not automatically mean that the membrane produces permeate continuously at 40 LMH throughout the entire operating period.
If the system includes relaxation or other non-production periods, the average production rate will be lower.
This is why a quoted gross flux should always be evaluated together with its operating cycle.
Net flux represents the effective average membrane production after accounting for periods when permeate production is reduced or stopped.
Depending on the system design, these periods may include:
- Relaxation
- Backwashing
- Cleaning
- Maintenance
- Other non-production periods
A simplified relationship is:
For example, if a membrane operates at a relatively high flux during active filtration but spends part of the cycle in relaxation, the effective average production will be lower than the active filtration flux.
The exact calculation depends on the operating strategy.
Two membrane suppliers may appear to offer significantly different flux values.
Before concluding that one membrane has substantially better performance, engineers should ask:
- Is the quoted value gross or net?
- Is it instantaneous or averaged?
- How long is the active filtration period?
- Are relaxation periods included?
- Is backwashing included?
- Are cleaning periods included?
- What wastewater temperature was used?
- What MLSS concentration was used?
- What wastewater quality was used?
- How long was the test conducted?
Without this information, direct flux comparisons can be misleading.
A higher quoted operating flux from one supplier may not necessarily result in a higher net daily production capacity.
Membrane area is commonly estimated using:
This makes the definition of flux particularly important.
If a gross filtration flux is used as though it were a continuous net production flux, the theoretical membrane area may be underestimated.
For example, consider a system requiring:
1,000 m³/day
If the design basis is:
20 LMH net flux
the theoretical membrane area is approximately:
But if the actual sustainable net production is lower than the value used for sizing, additional membrane area may be required.
This is why membrane sizing should be based on a clearly defined design flux rather than simply the highest number shown on a specification sheet.
For a detailed calculation:
→ How to Calculate Required Membrane Area for an MBR System
A membrane capable of achieving a high active filtration flux may also require:
- Relaxation periods
- More intensive aeration
- More frequent cleaning
- Different filtration cycles
- Additional operating controls
These factors can influence actual daily permeate production.
Therefore, the question:
“What is the maximum membrane flux?”
may be less useful than:
“What net production can the membrane reliably maintain under the intended operating conditions?”
For full-scale MBR projects, the second question is generally more relevant.
These three concepts should be distinguished.
Flux during active filtration.
Effective average production after accounting for non-production periods.
A realistic long-term design value selected according to:
- Wastewater characteristics
- Temperature
- MLSS
- Fouling behavior
- Cleaning strategy
- Operating cycles
- Required operating margin
A membrane may achieve a high gross flux while having a lower sustainable long-term design flux.
This does not necessarily indicate poor membrane performance.
Instead, it may reflect a more realistic approach to long-term MBR operation.
For ceramic flat sheet membrane applications, sustainable design flux should be evaluated together with the actual MBR operating conditions.
→ Ceramic Flat Sheet Membranes
Temperature is another important factor when comparing membrane flux data.
A flux value measured at a relatively high wastewater temperature may not represent actual winter operating conditions.
Lower temperatures generally increase water viscosity and filtration resistance.
Therefore, project teams should consider:
- Average wastewater temperature
- Minimum wastewater temperature
- Seasonal variation
- Winter treatment requirements
- Temperature used for membrane performance testing
For projects with significant seasonal variation, the minimum operating temperature can be an important design condition.
For a more detailed discussion:
→ How Temperature Affects Membrane Performance in MBR Systems
Consider two hypothetical membrane systems.
- Higher gross filtration flux
- Regular relaxation periods
- Lower effective production time
- Lower active filtration flux
- Longer effective production time
- Stable operating cycle
Looking only at the headline flux may suggest that System A provides better performance.
However, the two systems could have similar net daily production.
The comparison therefore needs to consider:
Gross Flux → Operating Cycle → Net Flux → Membrane Area → Daily Production
rather than looking at gross flux alone.
When evaluating membrane performance, the following questions can help establish a common comparison basis:
- Is the quoted flux gross, net, or instantaneous?
- What is the active filtration time?
- What relaxation periods are included?
- What cleaning periods are included?
- What wastewater temperature was used?
- What MLSS range was used?
- What wastewater quality was used?
- How long was the performance test?
- What operating cycle was used?
- What net design flux is recommended for long-term operation?
These questions can make comparisons between different membrane technologies much more meaningful.
Gross flux and net flux are not interchangeable.
A high flux measured during active filtration does not necessarily represent the average amount of water produced over a complete operating cycle.
For MBR system design, the more meaningful basis is often the sustainable net production capacity under realistic project conditions.
Engineers should therefore look beyond headline flux values and evaluate:
Flux + Operating Cycle + Temperature + MLSS + Wastewater Characteristics + Fouling Control
The objective should not simply be to select the membrane with the highest advertised flux.
The objective is to select a membrane system capable of providing reliable and sustainable water production throughout the expected operating life of the project.
For preliminary evaluation, useful project information includes:
- Treatment capacity
- Wastewater characteristics
- MLSS
- Average and minimum temperature
- Operating cycle
- Target production
- Required membrane area
These parameters can help establish a more realistic membrane sizing and operating basis.
