AFM Comparison With UF

Pre-treatment Of Ground Water Prior To Membranes

By aeration and AFM media bed filtration

Ground water is often used as a source of water to be treated by membrane filtration, which may include UF (Ultra filtration) or RO (Reverse Osmosis). UF is a technique used when absolute filtration down to approx. 0.03 microns is required. RO membrane filtration is much finer and is designed to remove just about everything from water. Typically RO is used to treat water containing a high concentration of salts such as seawater at approximately 35g/l down to brackish water at 1.5g/l.Ground water refers to spring water, or bore-hole (tube-well) water. Boreholes may go down to 150m to reach a suitable water supply.

The water often has the following conditions

  • no oxygen and low redox potential
  • elevated partial pressure of nitrogen gas and carbon dioxide
  • dissolve sulphide, methane and VOC’s ( volatile organic carbon)
  • iron, manganese & arsenic
  • heavy metals such as chromium, lead, mercury
  • colloidal silica
  • fluoride

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Aeration & Oxidation

For the oxidation of metals, gas stripping, and dropping zeta potential

As a very first step of water treatment process, the water must be gas stripped and aerated. Gas stripping will blow off volatile components and stabilize the alkalinity, this is essential for efficient filtration and to allow the zeta potential of the water to drop. The aeration phase should last at least 30 minutes. During aeration, the oxidation potential will increase and most heavy metals will precipitate out of solution. With regards to iron, it will change from Fe2+ to Fe3+, arsenic changes from AS3+ to AS5+. For chromium a second stage is required, aeration is recommended as the first stage followed by a reduction phase when Fe2+ is added to water at a high pH to reduce Cr6+ to Cr3+

During the oxidation stage with Fe2+ to Fe3+, the small ferric particles will be suspended by the aeration system and over a period of 30 to 60 minutes they will continue to grow and react to co-precipitate other heavy metals from solution. The longer the aeration phase the larger the ferric particles which makes it much easier to remove them by AFM (Active Filter Media). In some case we recommend the addition of ferric to the water in order to assist with co-precipitation and oxidation reactions. This is a practical solution to many water issues relating to dissolved organics, Arsenic and Chromium because AFM is very efficient at removing ferric from water.

AFM Filtration, Pre-treatment Prior To Membranes

For the removal of iron, manganese, silica, bacteria, and dissolved organics

UF and RO membranes work best when the membranes are new and clean. Fouling of the membranes by inorganic and organic contaminants as well as biofouling are some of the main issues effecting the performance and cost of running membrane water filtration systems.AFM is an activated mesoporous aluminosilcate with glass as a structural substrate. The surface area is over 1,000,000m2 per tonne of filter media of a particle size 0.5 to 1.0mm. The very large surface area has a negative –ve charge for the adsorption of positive +ve particles that are too small to be mechanically removed from the water. The pre-coagulation and flocculation stage will drag some chemicals out of solution to form small positive charge particles that are easy to remove by AFM

AFM is used prior to UF and RO, however AFM filtration can actually out-perform UF filtration when combined with pre coagulation and flocculation via a ZPM static mixer prior to AFM filtration. AFM is a much lower cost than UF and can remove most particles down to 0.1 microns and many particles that are much smaller by adsorption reactions. While UF will give absolute filtration down to 0.03 microns, AFM can give nominal filtration down to at least 0.1microns and sub 0.01 micron particles by electrical adsorption reactions. AFM will also remove many chemicals and pollutants from solution by selective molecular sieve adsorption. Indeed we can now activate AFM to remove specific dissolved priority substances from water. AFM can therefore perform better than UF and is a more cost effective and efficient option as a pre-treatment prior to RO.

Regeneration of AFM is by a simple back-wash using filtered water, and the process is 100% sustainable, the media should last for the life of the filtration system.You don’t need to use chlorine with AFM, so no THM (trihalomethane) production with organic matter such as organic acids. In addition to dissolved natural organic matter TCP (trichlorophenol) can act as a precursor for the production TCA trichloroanisole. TCA has a terrible taste even at single-digit parts per trillion. Dryden Aqua manufactures APF which is a multi-spectrum coagulant and flocculent. APF contains some PAC (polyaluminium chloride) but most PAC contains TCP, at Dryden Aqua we manufacture all of the chemicals so we are sure there is no TCP in our products.In order to insure efficient use of the coagulants and flocculants, the products must be injected via a ZPM static mixer. The equipment is also manufactured by Dryden Aqua.

AFM performance factors

As a pre-treatment prior to RO or UF, the SDI after AFM will be at least 50% to 75% lower than the SDI from a sand filter. IFTS independent accredited laboratory have conducted a very detailed analysis of AFM in comparison to sand other media. The results confirm the superior performance of AFM.

Performance Comparison Of AFM With UF

Parameter AFM UF at 0.03 microns
Remove dissolved silica Yes No
Remove phosphate Yes, when combined with NoPhos No
Remove dissolved organics Yes, especially when combined with APF precoagulation and flocculation using ZPM No, UF cannot remove chemicals in solution.
Biofouling No, AFM does not biofoul Yes, membranes need to be regularly cleaned
Prevention of RO membrane biofouling Yes, because it removes phosphate and dissolved nutrients required by bacteria Phosphate, and dissolved nutrients will react the RO membranes
Scaling No Yes, with carbonates, calcite, struvite, ferric etc
chlorination Not required, AFM does not biofoul. So no chlorine reaction products such as THM`s, TCA, or hydrobromous acid May be required as part of the cleaning schedule
Filtration level 90% down to 0.1micron, approx. 80% down to 0.01micron 0.03 to 0.01 microns absolute, unless the membrane ruptures
Sustainable 100% sustainable, Circular Economy VIBES winner in the UK Not sustainable, membranes need to be replaced at 1 to 10 year intervals
Operation costs Can operate at 50% higher flow rates, so much lower capital cost than sand filtration Much higher capital cost than AFM. Much higher pressures and running costs. Lower area required for installation

AFM Application Areas

Swimming pool

AFM® reduces the biological risk from bacteria and parasites as well as the chemical risk from water and air just above the surface. The result is a safer pool for children, a healthier work place for staff in public pools and a greatly improved swimming experience for everyone. Read More…

Pretreatment to membranes ( RO & UF)

UF, RO membranes  & DM plant resins work best when they are new and clean. Fouling of the membranes by inorganic and organic contaminants as well as biofouling are some of the main issues effecting the performance and cost of running  water treatment systems.Read More…

Arsenic Removal

Arsenic is often found in borehole / tube wells and ground water at varying concentrations depending upon the geology of the ground. In India western states along with certain locations in the south have fairly high contamination of Arsenic in the ground water. IN certain cases Arsenic is also found in the waste water effluent of certain industries. Read More…

Iron & Manganese removal

Iron and manganese are often found in borehole / tube wells and ground water at varying concentrations depending upon the geology of the ground. Read More…

Chromium removal

Chromium is occasionally found in ground water in trace amount from 1 to 10 ug/l however the occurrence of chromium is usually associated with pollution from industries such a leather tanning, wood preservation and textiles. Polluted ground water may have much higher concentrations. Read More…

Benefits at a glance:

  • Long life span
  • Very short pay back
  • 100% resistant to bio-fouling
  • Filters twice as good as sand
  • Ecological and economical solutions

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