TMP, Permeability and Recovery – Understanding Filtration Performance

TMP, Flux, Permeability and Recovery – How to Interpret Filtration Performance

Ground rule – read this first

In LiqTech ceramic membrane systems:

• Permeate quality is defined by membrane type and integrity

• Operating parameters do not change permeate quality

• TMP, flux, recovery and permeability describe hydraulic operating conditions and membrane loading

• Changes in these parameters affect fouling rate, cleaning frequency and operability, not separation performance

As long as membrane integrity is intact, permeate quality remains static for a given membrane type.

---

TMP – what it represents

TMP (Transmembrane Pressure) is the pressure required to drive liquid through the membrane at a given operating point.

In LiqTech crossflow systems, TMP reflects the pressure difference between feed and permeate across the membrane housings and is influenced by how the system is operated.

Important clarification

TMP is not equal to fouling.

TMP can be high because:

• a high flux is applied

• recovery is increased (leading to higher membrane loading)

• a large amount of liquid is forced through the membranes

Likewise, TMP can be low simply because:

• flux is low

• only a limited amount of liquid is pushed through the membranes

TMP alone does not describe membrane condition and does not indicate permeate quality.

TMP must always be interpreted together with permeability.

---

Flux – the applied load on the membranes

Flux describes how much liquid is forced through the membrane area over time (LMH).

In LiqTech crossflow systems, flux is not set directly.
Instead, it is the result of:

• the Feed flow setpoint

• the Recovery setpoint

Together, these determine how much of the feed flow is distributed through the membranes as permeate.

In practice, flux is therefore:

• a chosen operating condition

• a direct hydraulic load applied to the membranes

• governed by Feed setpoint × Recovery

While flux and recovery define membrane load, they do not alter permeate quality for a given membrane type.

---

Example – how flux is established

If a system is operated with:

• Feed flow = 10 m³/h

• Recovery = 90%

Then:

• 9 m³/h is produced as permeate

• 1 m³/h leaves the system as retentate

The 9 m³/h permeate flow, distributed across the installed membrane area, defines the applied flux.

Increasing either feed flow or recovery will typically result in:

• higher flux

• higher TMP

• faster fouling development, depending on feed quality

Reducing feed flow or recovery has the opposite effect and can improve long-term stability.

---

Permeability – the primary indicator of membrane condition

Permeability describes how easily liquid passes through the membrane material.
It relates flux to TMP and is typically evaluated with temperature correction.

Permeability is the most reliable indicator of membrane health.

Why:

• it normalizes for operating pressure

• it allows comparison over time

• it reflects fouling more clearly than TMP alone

General interpretation

• High permeability
  Indicates clean, healthy membranes

• Declining permeability
  Indicates developing fouling

• Low permeability
  Indicates that cleaning should be considered

TMP should never be evaluated without also considering permeability.

---

Reference behavior – new and clean membranes

As a general reference only:

New, clean membranes operated in clean, lukewarm water typically show:

• stable permeability around ~300 LMH/bar, with normal variation

This reference illustrates how permeable clean membranes can be under ideal conditions.

It is not a target for normal operation.

Actual operating values depend on:

• feed composition

• temperature

• applied flux

• recovery

• operating strategy

---

When low permeability can be acceptable

As a rule of thumb, LiqTech systems generally prefer not to operate at very low permeability levels (for example around 30 LMH/bar).

However, low permeability can be acceptable if:

• the system has previously operated at similar levels in a controlled manner, and

• CIP has consistently restored permeability to a significantly higher level (for example >200–240 LMH/bar)

In such cases:

• the absolute value is less important

• the ability to recover performance after CIP becomes the key reference

---

CIP and BW – different roles

CIP (Cleaning in Place)

CIP is the safest and most reliable way to:

• reduce TMP

• restore permeability

CIP is the primary tool for recovering membrane performance once fouling has developed.

Backwash (BW)

In some feed media, BW can be very effective at:

• keeping TMP under control

• maintaining permeability

• slowing fouling development

If BW has proven effective for a given application:

• increasing BW frequency (rather than duration) can be beneficial

• especially when using time-based BW

BW supports performance maintenance but does not replace CIP.

---

Using trends to interpret performance

Single measurements rarely tell the full story.
Trend behavior over time provides much stronger insight.

Sudden changes may indicate:

• a sudden change in feed composition

• upstream process disturbances

• changes in operating conditions such as flux or recovery

Gradually steepening trends may indicate:

• incomplete cleaning during previous CIP

• operation at a more demanding operating point

• a gradually more concentrated feed medium

Trend behavior is often more informative than absolute values.

---

Recovery – its influence on membrane stress

Important clarification – permeate quality

Recovery does not change permeate quality for a given membrane type.

Permeate quality is defined by membrane pore structure and material and remains unchanged as long as membrane integrity is intact.

Recovery influences:

• membrane stress

• fouling rate

• TMP development

• permeability decline

It does not influence separation performance.

---

Recovery describes how the incoming feed flow is split between:

• permeate

• retentate

Higher recovery:

• forces a larger share of the feed through the membranes

• increases membrane stress

• typically accelerates fouling

Lower recovery:

• allows more liquid to leave as retentate

• reduces membrane stress

• can improve long-term stability

---

Why small recovery changes matter

Example 1 – 90% recovery

Feed flow: 10 m³/h
Recovery: 90%

• Permeate: 9.0 m³/h

• Retentate: 1.0 m³/h

Approximately a 1:10 concentration ratio.

Example 2 – 95% recovery

Feed flow: 10 m³/h
Recovery: 95%

• Permeate: 9.5 m³/h

• Retentate: 0.5 m³/h

Approximately a 1:20 concentration ratio.

What this means

Increasing recovery from 90% to 95%:

• increases permeate production only marginally

• doubles the concentration load on the retentate side

As a result:

• particle concentration in the crossflow loop increases

• fouling typically accelerates

• TMP rises faster

• permeability declines faster

This impacts operability and cleaning frequency, not permeate quality.

---

Practical takeaway

If permeability declines faster than expected or TMP rises rapidly:

• reviewing recovery setpoint is often one of the most effective first steps

• even a small reduction in recovery can significantly reduce membrane stress

---

When to look further

Further investigation may be required if:

• permeability does not recover after CIP

• CIP effectiveness declines over time

• performance trends deteriorate faster than previously observed

Refer to:

“Improving CIP Effectiveness When Cleaning Results Are Poor”

---

Key takeaways

• Permeate quality is defined by membrane type and integrity

• TMP alone does not indicate fouling

• Flux is governed by Feed setpoint × Recovery

• Recovery affects membrane stress, not permeate quality

• Permeability is the primary indicator of membrane condition

• CIP is the safest way to restore performance

• BW can be effective as a maintenance tool in suitable media

• Trends provide earlier insight than absolute values

---

Alarm ID reference – TMP / Permeability / Filtration Performance (examples)

The following alarm IDs are examples of alarms related to filtration performance, including high TMP, low permeability, and conditions where BW or CIP should be evaluated.

The alarm IDs are provided as references only, to help identify the alarm type when a customer mentions a specific alarm number.
Other alarms with similar texts may exist depending on system generation and project configuration.

---

MK6 – Numeric alarm IDs

(Filtration performance, TMP and permeability related)

Examples include alarms with texts such as:

• High TMP

• TMP too high

• Filtration performance low

• Low permeability

• CIP recommended / CIP required

Examples include (non-exhaustive):

• 38 – High TMP

• 39 – TMP high warning

• 40 – TMP high alarm

• 41 – TMP too high / filtration limited

• 83 – Filtration performance low

• 84 – Filtration performance very low

• 85 – Low permeability warning

• 86 – Low permeability alarm

• 141 – CIP recommended (performance related)

• 142 – CIP required (performance related)

(MK6 alarms typically repeat per filtration module.)

---

MK8 – Project-based systems (D-series)

Examples include alarms with texts such as:

• High TMP

• TMP increasing

• Low permeability

• Filtration performance degraded

• Cleaning recommended

Examples include:

• D0205 – TMP high

• D0206 – TMP high alarm

• D0301 – Filtration performance degraded

• D0302 – Low permeability detected

• D0401 – CIP recommended (performance related)

• D0402 – CIP required (performance related)

• D0503 – Performance outside expected range

• D0504 – Filtration limitation due to fouling

---

MK8 – FM01 platform (F-series)

Examples include alarms with texts such as:

• TMP high

• Low permeability

• Filtration performance reduced

• Cleaning recommended

Examples include:

• F0124 – TMP high

• F0125 – TMP too high

• F0133 – Low permeability warning

• F0134 – Low permeability alarm

• F0142 – CIP recommended (performance related)

---

Scope note

The guidance in this article may also apply to other alarms with texts indicating:

• high or increasing TMP

• low permeability

• reduced filtration performance

• cleaning recommended or required

Exact alarm numbering and wording depend on system generation, project configuration, and membrane setup.