Suspended Particles

What Are Suspended Particles?

Suspended particles are small solid materials dispersed throughout swimming pool water rather than dissolved in it or settled on the pool floor. They include fine dust, pollen, algae cells, organic fragments and mineral precipitates.

Depending on their size, density and water movement, these particles may remain within the water column for extended periods before settling or being removed by filtration. Some are individually visible, while microscopic particles may only be noticeable through their combined effect on water clarity.

Suspended particles are different from two other forms of material found in swimming pools:

• Dissolved solids: Salts, minerals and other substances present as dissolved ions or molecules. Ordinary mechanical filtration cannot remove these dissolved constituents.

• Settled debris: Solid material that has accumulated on the pool floor, steps or ledges, where it can often be removed by vacuuming or other suitable cleaning equipment.

Common Sources of Suspended Particles in Pool Water

Suspended particles can enter a swimming pool from the surrounding environment, be introduced by swimmers or form directly within the water through biological and chemical processes.

Australian outdoor swimming pools can experience increased particle loads during dry, windy weather, eucalyptus leaf fall, pollen seasons and heavy rainfall.

Not all particles originate outside the pool. For example, calcium carbonate can precipitate directly from the water when factors such as pH, calcium hardness, total alkalinity and temperature create conditions that favour mineral formation.

This distinction matters because particles entering from the environment and particles forming within the water may require different corrective actions.

How Suspended Particles Affect Pool Water

Water Clarity and Light Scattering

Suspended particles can scatter or absorb light travelling through swimming pool water. This reduces underwater visibility and can make the pool appear hazy or milky.

The extent of the visual effect depends on several factors:

• Particle size: Fine particles can scatter light effectively even when they are too small to see individually. The relationship depends on particle size relative to the wavelength of light.

• Particle concentration: Higher concentrations generally increase turbidity, although the relationship is not necessarily linear.

• Particle composition: Mineral crystals, organic fragments and biological particles have different optical properties.

• Particle distribution: Circulation and turbulence can keep particles dispersed throughout the water instead of allowing them to settle.

Two pools containing similar masses of suspended material may therefore look quite different if their particle sizes and compositions differ.

Suspended Solids and Turbidity

Turbidity describes the optical cloudiness of water, commonly measured by the scattering of light and expressed in nephelometric turbidity units (NTU).

Total suspended solids (TSS) describe the mass concentration of suspended material, typically expressed in milligrams per litre (mg/L).

Although related, turbidity and TSS are not interchangeable measurements.

For example, fine clay particles can produce substantial light scattering at relatively low mass concentrations. Coarser mineral particles may contribute more mass but settle quickly, reducing their effect on persistent water cloudiness.

Turbidity is therefore useful for assessing the optical impact of suspended material, while TSS provides information about the amount of particulate matter present.

Filtration Load and Water Hygiene

As a filter captures suspended material, particles accumulate within or on its filtration media.

This increases the filter's particle load and may eventually restrict water flow or change operating pressure, depending on the equipment design. Filters must be cleaned, backwashed or serviced according to their operating requirements.

Some particles may also carry microorganisms or organic matter that increases sanitiser demand. However, suspended mineral dust does not automatically indicate unsafe water, and clear water does not guarantee effective disinfection.

Water clarity and microbiological safety are related but separate considerations.

Poor visibility is itself a safety concern. If the deepest part of the pool floor cannot be clearly seen, swimming should stop until visibility has been restored and the cause investigated.

Particle Size and Pool Filtration

Particle size influences how suspended material behaves in water and how effectively it can be captured by filtration equipment.

Particle dimensions are commonly expressed in micrometres (µm). One micrometre is one-thousandth of a millimetre.

Larger Suspended Particles

Relatively large particles, such as visible soil grains and plant fragments, are generally easier to collect through suitable filtration or remove after settling.

However, particle size alone does not determine whether material remains suspended.

A lightweight organic fragment may remain afloat or drift in moving water, while a denser mineral particle of similar dimensions may settle more quickly.

Fine Suspended Particles

Fine silt, clay and biological fragments often settle slowly, particularly when circulation currents keep them moving through the water.

Their removal depends on the effective retention performance of the installed filter.

Some fine particles may pass through the filter media during a single circulation cycle. Repeated circulation can increase the opportunities for capture, although removal efficiency depends on filter design, operating conditions and particle characteristics.

Very Fine and Colloidal Particles

Colloidal particles are very fine materials dispersed in water that can remain suspended for prolonged periods.

Their stability may be influenced by Brownian motion, surface charge, particle interactions and water chemistry.

These effects can limit natural aggregation and gravitational settling, making some colloidal material difficult to remove through conventional filtration alone.

Suitable coagulation or flocculation treatment may improve removal by encouraging fine particles to form larger aggregates.

Not every microscopic particle behaves as a stable colloid, and colloidal behaviour cannot be defined by one universal particle-size threshold.

Understanding Filter Micron Ratings

A filter's stated micron rating provides information about its particle-retention characteristics, but it should not automatically be interpreted as a guaranteed physical cut-off.

Two terms are commonly used:

• Nominal micron rating: Indicates a manufacturer's stated particle-retention performance at a specified size. The associated efficiency and testing methods can vary between products.

• Absolute micron rating: Indicates a particle size at which a filter achieves a specified retention efficiency under defined test conditions. It should be interpreted alongside the test method and efficiency specification.

Actual capture performance depends on filter media structure, flow rate, particle properties, filter loading and maintenance condition.

A filter advertised with a smaller micron rating is not necessarily more suitable for every swimming pool application.

For a more detailed explanation, see Filtration Micron Rating.

How to Remove Suspended Particles from Pool Water

1. Mechanical Filtration

Mechanical filtration removes suspended solids by passing pool water through filtration media that retain particles while allowing water to continue through the circulation system.

Three commonly used filter types are:

Sand and glass-media filters

These filters pass water through a granular media bed, where particles may be retained through interception, attachment and other filtration mechanisms.

Their performance depends on media grading, bed condition, flow rate and equipment design.

Cartridge filters

Cartridge filters use pleated or other porous filtration materials to retain particles as water passes through the filter element.

Their large effective surface area can support fine-particle capture when the filter is correctly sized and maintained.

Diatomaceous earth (DE) filters

DE filters use a porous layer of diatomaceous earth supported by an internal filtration structure.

They can provide fine-particle retention but require equipment-specific cleaning, media replacement and disposal procedures.

No single filter category is universally superior. Effective removal depends on the system's rated performance, operating conditions and maintenance.

2. Circulation and Water Movement

Pool circulation transports particle-laden water towards the skimmer, suction outlets and filtration system.

Adequate circulation helps reduce poorly mixed areas where suspended material may otherwise accumulate.

However, circulation moves particles; filtration or physical collection removes them.

Flow rate also influences filtration performance. Excessive flow relative to the filter's design can reduce particle retention or contribute to operating problems, while inadequate flow may limit the amount of water reaching the filter.

Channelling in granular media can occur when water follows preferential pathways through the filter bed. It may be associated with media condition, installation, hydraulic operation or maintenance problems rather than flow rate alone.

Variable-speed pumps can provide flexibility in balancing circulation, filtration and energy consumption. The appropriate operating speed and duration depend on the installed system and should follow manufacturer guidance.

3. Coagulation, Flocculation and Clarification

Some very fine particles are difficult to remove through mechanical filtration alone.

Suitable water-treatment aids may improve capture by encouraging smaller particles to combine into larger structures.

Coagulation destabilises particles, often by reducing the electrostatic forces that prevent them from aggregating.

Flocculation encourages destabilised particles to form larger aggregates known as flocs. Depending on the treatment, these may be captured through filtration or allowed to settle for physical removal.

Clarification refers to improving water clarity. Pool clarifiers may use particle aggregation to help the installed filter retain fine suspended material.

The terms are related, but the products and processes are not interchangeable.

Some settling flocculants are unsuitable for cartridge or DE filtration systems and may cause blockage or other operational problems if used incorrectly.

Always check the treatment product's instructions and the filter manufacturer's compatibility requirements before use.

4. Vacuuming and Settled Particle Removal

Once particles settle onto the pool floor, vacuuming can remove them before swimming activity or return currents disturb the sediment.

Manual vacuum systems can collect settled material through the pool filtration system. Where suitable equipment and plumbing are available, some sediment or floc may be removed directly to waste in accordance with the treatment instructions.

Robotic pool cleaners can collect settled dust and debris from pool surfaces using their onboard filtration systems.

However, their filtration capability varies between models, and they should not be relied upon to remove all microscopic particles dispersed throughout the water column.

Why Suspended Particles Remain in Pool Water

Persistent suspended material generally results from inadequate particle removal, continued particle entry or new particles forming within the pool.

The key diagnostic distinction is whether particles are not being removed effectively or are continuing to enter or form within the pool.

For example, cleaning a functioning filter may provide little improvement if calcium carbonate continues to precipitate because of unfavourable water chemistry.

Similarly, persistent algae growth requires the underlying sanitisation problem to be addressed rather than relying solely on mechanical particle removal.

Practical Inspection and Maintenance

If suspended particles remain visible or pool water develops a persistent haze, Australian residential pool owners can follow these checks:

1. Assess underwater visibility. Check whether the deepest part of the pool floor can be clearly seen. Do not swim when visibility is inadequate.

2. Test relevant water chemistry. Check free chlorine, pH and other appropriate parameters to investigate algae growth or mineral precipitation.

3. Inspect circulation and skimming. Confirm the pump is operating correctly, water is flowing through the filtration system and skimmer baskets are clear.

4. Check filter condition. Examine pressure or flow indicators where fitted, and clean or backwash filters according to manufacturer requirements.

5. Remove environmental debris. Collect leaves and larger fragments before they break down into smaller particles.

6. Investigate recurring contamination. Identify the particle source and filtration limitations before repeatedly applying clarifiers or flocculants.

After strong winds, bushfire-related ash deposition or heavy rainfall, additional inspection and debris removal may be necessary.

If suspended material persists despite appropriate filtration, circulation and water chemistry management, seek advice from a qualified pool service professional.