AFM® (Activated Filter Media) is a specialist filtration media manufactured by Dryden Aqua, designed to provide improved particle removal and resistance to biofouling compared with conventional sand filtration.
Used in domestic, commercial and industrial water treatment systems, AFM is particularly effective for reducing turbidity, suspended solids and fine particulate contamination.
But what makes AFM different from ordinary sand or recycled glass, and is it the right choice for your water treatment system?
In this guide, we explain how AFM works, the different grades available and the importance of correct filter sizing and backwashing.
What Is AFM Activated Filter Media?
AFM is manufactured from specially processed and activated recycled glass.
Unlike ordinary crushed glass media, AFM undergoes a controlled manufacturing and activation process that modifies the surface characteristics of the particles.
This produces filtration media with:
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A large active surface area.
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Consistent particle size and shape.
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Improved fine-particle retention.
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Resistance to biological fouling.
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Effective backwashing characteristics.
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Good mechanical durability.
AFM is primarily a mechanical filtration media, designed to capture suspended particles as water passes through a filter bed.
It is commonly used as an alternative to conventional silica sand in suitably designed pressure and gravity filtration systems.
How Does AFM Work?
In a conventional sand filter, suspended particles become trapped between the grains of sand as water passes through the bed.
AFM uses a similar basic principle but benefits from its engineered surface properties and controlled particle size distribution.
This can improve the capture of fine particles and help the filter maintain more consistent performance between backwashing cycles.
Dryden Aqua produces two main AFM types:
AFM Standard (AFM®s)
AFM Standard has a hydrophilic surface with a negative electrical charge.
It is designed for general filtration applications, including the removal of suspended particles and certain positively charged contaminants or flocs.
AFM Next Generation (AFM®ng)
AFM Next Generation has modified hydrophobic surface properties.
This makes it particularly suitable for applications involving fine particles, organic material, oils and other hydrophobic contaminants.
Both types are engineered to resist biofouling, but they have different surface characteristics and should be selected according to the intended application.
How Does AFM Compare with Sand?
One of AFM's main advantages is its ability to retain finer particles than conventional sand under comparable operating conditions.
In published comparative tests conducted at a filtration velocity of 20 metres per hour without flocculation, Dryden Aqua reports the following results:
| Filtration media | Reported particle removal |
|---|---|
| AFM®ng Grade 1 | 95% of particles larger than 1 micron |
| AFM®s Grade 1 | 95% of particles larger than 4 microns |
| High-quality silica sand | 95% of particles larger than 20 microns |
These results demonstrate the potential improvement in filtration performance.
However, they represent specific test conditions rather than guaranteed particle removal ratings for every installation.
Actual performance depends on the flow rate, filter design, bed depth, water quality and operating conditions.
Understanding the Different AFM Grades
AFM is available in several grades, each with a specific purpose within the filtration bed.
Choosing the correct combination is essential for achieving good filtration and effective backwashing.
| AFM grade | Particle size | Main function |
|---|---|---|
| Grade 1 | 0.4–0.8 mm | Main filtration layer |
| Grade 2 | 0.7–2.0 mm | Secondary filtration and support |
| Grade 3 | 2.0–4.0 mm | Coarse support layer |
AFM Grade 1 – Main Filtration Layer
Grade 1 is the principal working layer in many AFM filtration systems.
Its relatively fine particle size provides effective removal of suspended solids and turbidity.
AFM®ng Grade 1 is especially suitable where fine-particle removal is important.
AFM Grade 2 – Secondary Filtration Layer
Grade 2 is a coarser media that can provide additional filtration while supporting the finer working layer.
In an appropriately designed multi-grade bed, it contributes to flow distribution and filtration performance.
AFM Grade 3 – Support Layer
Grade 3 is the coarsest of these grades.
It is primarily used at the bottom of pressure filters fitted with lateral distribution systems.
The purpose of this layer is to cover and protect the laterals while supporting effective water distribution during service and backwashing.
Not every vessel requires Grade 3. The correct media arrangement depends on the internal distribution system and manufacturer's recommendations.
Why Is Backwashing So Important?
Like sand and other granular filtration media, AFM gradually accumulates suspended solids during operation.
These particles must be removed periodically through backwashing.
During backwashing, water flows upwards through the filter bed at a sufficient velocity to expand and clean the media.
Effective backwashing is essential to:
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Remove accumulated suspended solids.
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Prevent excessive pressure loss.
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Reduce the risk of channel formation.
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Maintain filtration performance.
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Restore the filter's capacity to capture further particles.
Backwash Flow Rate vs Service Flow Rate
An important consideration when selecting an AFM system is that the backwash requirements are different from the normal service flow requirements.
The system must have sufficient available water flow and pressure to expand the media bed correctly.
A filter may operate satisfactorily during normal service but fail to clean itself properly if the available backwash flow is inadequate.
The required backwash velocity depends on the media grades, water temperature, filter diameter and desired bed expansion.
For this reason, both the service flow rate and the available backwash flow should be checked before selecting a system.
What Can AFM Remove from Water?
AFM is particularly effective for reducing particulate contamination, including:
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Turbidity and suspended solids.
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Fine sediment.
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Silt and particulate debris.
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Precipitated iron and manganese particles.
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Certain organic particles and flocs.
AFM can also be used as a filtration stage following oxidation or other treatment processes.
For example, where dissolved iron has been successfully oxidised into an insoluble form, an appropriately designed AFM filter may be used to remove the resulting particles.
What AFM Does Not Remove
AFM is not a universal water treatment solution.
It should not be relied upon by itself to remove:
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Dissolved calcium and magnesium hardness.
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Nitrates.
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Dissolved arsenic.
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Dissolved iron or manganese that has not been converted into a filterable form.
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Dissolved salts.
AFM is also not a substitute for microbiological disinfection.
Although the media is designed to resist biofouling, this does not mean that it sterilises water or guarantees the removal of harmful microorganisms.
Where a private drinking water supply is microbiologically contaminated, an appropriate disinfection system, such as UV treatment, may also be required.
Where Is AFM Used?
Private Boreholes and Wells
AFM can be useful for reducing turbidity and suspended particles in groundwater supplies.
It is particularly relevant where the water contains fine sediment or where particulate contamination needs removing before subsequent treatment stages.
Surface Water Treatment
Surface water supplies can contain variable quantities of sediment, organic matter and other suspended solids.
AFM may form part of a multi-stage treatment system designed to improve water clarity.
Commercial and Industrial Water Treatment
AFM is suitable for a variety of commercial filtration applications, including process water treatment and larger pressure filtration systems.
Swimming Pools and Aquaculture
AFM is also widely used in swimming pool and aquaculture filtration, where effective removal of fine suspended particles is important.
These applications may require different filtration arrangements and operating conditions from drinking water treatment systems.
Can AFM Replace Sand in an Existing Filter?
Potentially, yes.
AFM is frequently considered as an alternative to sand in existing filtration vessels.
However, replacing sand is not necessarily as simple as exchanging one bag of media for another.
Before converting a filter, it is important to check:
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Vessel diameter and height.
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Internal distributor or lateral design.
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Existing filter bed depth.
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Required AFM grades and quantities.
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Available freeboard for bed expansion.
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Backwash flow rate and pressure.
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Control valve and drain capacity.
Because AFM has different physical and hydraulic characteristics from sand, the correct media quantities and backwash settings should be established using the manufacturer's guidance.
Does AFM Need Replacing?
AFM is designed for a long operating life and is resistant to physical degradation and biological fouling.
However, this does not mean it requires no maintenance.
Regular backwashing remains necessary, and the condition of the media should be assessed during routine servicing.
Media replacement may eventually be required because of physical contamination, chemical fouling, damage or changes in filtration performance.
Unlike certain adsorption media, AFM does not have a fixed chemical exhaustion capacity for removing ordinary suspended solids.
Its useful service life depends on the application and operating conditions.
AFM Filter Systems from GAPS Water Treatment
GAPS Water Treatment supplies genuine Dryden Aqua AFM media and complete AFM filtration systems incorporating Clack and Fleck control valves.
Our systems are available in different vessel sizes to accommodate a range of flow requirements.
We supply:
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AFM®ng Grade 1 – main filtration layer.
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AFM®ng Grade 2 – secondary filtration layer.
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AFM Grade 3 – support media.
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Complete automatic backwashing AFM filter systems.
View our AFM Filtration Systems
If you're considering an AFM system for a private water supply, commercial installation or existing filtration vessel, please email us with your water analysis, required service flow rate and available backwash flow.
Our team can help identify suitable equipment and media for your application.
Email: sales@gapswater.co.uk
GAPS Water Treatment supplies water treatment equipment and technical product advice. We do not provide on-site installation services.
Technical Reference
For additional technical information, including media grading, filtration velocities and backwash requirements, refer to the Dryden Aqua AFM Product Information.
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