Understanding TMP and backwash sequence in pool systems

Purpose

This article explains what trans-membrane pressure (TMP) represents in LiqTech pool systems and how a backwash sequence works physically.

Understanding TMP and the sequence of air scour and water flush is important for interpreting system behaviour correctly and for understanding why backwash is a central part of normal operation.

What is TMP?

TMP stands for trans-membrane pressure and describes the pressure difference across the membrane during filtration.

In practical terms, TMP reflects how much resistance the system experiences as water is pushed through the membrane.

In LiqTech pool systems, TMP is one of the most important indicators of membrane condition during operation.

What influences TMP?

TMP is influenced by several operating conditions, including:

Flow rate
Water temperature
Fouling level on the membrane surface
Fouling inside membrane pores and channels

Because several factors influence TMP, it should not be viewed as an isolated value.
However, when flow and temperature are relatively stable, TMP becomes a very useful indicator of how much fouling has accumulated on the membranes.

Why TMP matters in pool applications

In pool applications, suspended solids, organics, and other contaminants are continuously introduced into the system through bathers and general pool use.

As these solids are retained by the membrane, resistance to flow increases and TMP gradually rises.

This means TMP is not only a pressure value — it is also a practical reflection of how much fouling the system is currently handling.

For this reason, TMP is one of the key parameters used to evaluate:

filtration performance
backwash effectiveness
whether the system is operating normally

How filtration works in the membrane housing

During normal filtration:

Pool water enters the membrane housing
Water is pushed through the membrane material
Solids are retained on and within the membrane structure
Permeate exits as filtered water
Retained solids gradually form a fouling layer

As this layer builds up, flow resistance increases and TMP rises accordingly.

This is a normal part of operation and is the reason periodic backwashing is required.

What happens during backwash?

Backwash is performed to remove accumulated fouling before it becomes too difficult to remove.

A backwash sequence in LiqTech pool systems consists of two distinct steps:

1. Air scour

Air is introduced to the membrane housing to loosen solids and fouling from the membrane surface.

This step is the primary cleaning mechanism of the backwash sequence.

Its purpose is to:

break up the fouling layer
dislodge retained particles
prepare the housing for flushing

2. Water flush / backwash

After the air scour, water is flushed through the housing in reverse direction to carry the loosened material out of the membrane channels and to drain.

This step removes the fouling that has been loosened by the air step.

Its purpose is to:

discharge solids from the membrane housing
restore lower flow resistance
prepare the membrane to return to filtration

Why the two steps serve different purposes

It is important to understand that the two parts of the sequence do not contribute equally in the same way.

The air scour is what primarily loosens and detaches fouling
The water flush is what removes that loosened material from the housing

This means that backwash performance is often governed more by:

how effective the air step is
how often backwash is performed

…than by simply increasing the flush duration.

Backwashing one membrane at a time

In systems with multiple membrane housings, backwashing is typically performed sequentially, one housing at a time.

This allows:

continued filtration on the remaining housings
stable operation during the cleaning sequence
controlled discharge of loosened solids

The system therefore alternates between:

housings in filtration mode
one housing temporarily in backwash mode

What should happen to TMP after backwash?

After a successful backwash, TMP should drop back toward a relatively stable level.

This is often referred to as the baseline after backwash.

In stable operation:

TMP rises gradually during filtration
TMP drops after backwash
TMP returns to approximately the same baseline over time

This pattern indicates that fouling is being removed effectively and that the backwash sequence is functioning as intended.

What it means if TMP does not recover

If TMP does not return to its previous baseline after backwash, it may indicate:

fouling is accumulating faster than it is being removed
backwash is no longer sufficient
membranes are approaching the need for CIP
backwash settings may no longer match the operating conditions

This does not necessarily mean something is wrong mechanically — but it does indicate that the system is graduadly fouling and it should be reviewed if a CIP soon will be needed.

Practical interpretation of TMP trends

When reviewing TMP in operation, it is often more useful to look at the pattern over time than a single number.

Useful observations include:

whether TMP rises gradually or rapidly
whether TMP returns to baseline after BW
whether the baseline itself is drifting upward
whether TMP-triggered BW occurs more frequently than normal

These observations are often more valuable than focusing only on a single TMP reading.

Summary

TMP is one of the most useful indicators for understanding how a LiqTech pool system is performing.

It reflects the resistance created by fouling and therefore provides valuable insight into:

membrane condition
filtration behaviour
backwash effectiveness

A backwash sequence works by combining air scour and water flush, where the air step loosens fouling and the flush step removes it from the housing.

Understanding this relationship is key to interpreting system trends correctly and to operating the system in a stable and controlled way.

For guidance on how to use TMP and BW behaviour to set up a practical backwash strategy, see:
Backwash strategy in pool applications