The problem

Reverse Osmosis membranes for desalination or TDS reduction will always be subject to fouling from biological contamination, organic and inorganic chemical precipitation

There are three options to treat the water prior to RO membranes;

  • UF ultra filtration down to 0.03um
  • sand filter followed by cartridge filters
  • AFM filtration is standard sand filters, may be used without cartridge filters

UF filtration

UF has better mechanical filtration performance than sand / cartridge filter combination, but UF will not remove dissolved organics or chemicals from solution. UF is purely a mechanical filtration process, dissolved components, or particles smaller than 0.03 microns will pass trough the membranes. The dissolved organics lead to biofouling of the membranes.  The inorganic components such as free silica or phosphate will form a precipitate and scale up the membranes.

Sand filtration

Sand will become a biofilter, and are effective at removing particulates and dissolved biological nutrients, but the filter will generate bacterial cell biomass, which will foul the membranes.  Sand filters also suffer from biodynamic instability leading to transient wormhole channelling and passage of unfiltered water which blocks the cartridge filters. This process takes about 6 months before it starts to impact on system performance. Coagulants and flocculants maybe used prior to sand filters to remove phosphate but sand has free silica and this will cause a blockage of the membranes, especially if there aluminium is in the water or if aluminium used as a flocculent.

AFM filtration

AFM is a direct replacement for sand in standard sand filters. Operational criteria are similar to sand filters. Run phase water flow depending upon influent water quality is up to 15m/hr as a velocity.  Backwash water flow must be in excess of 40m/hr for a period or 5 minutes.  Air scour is not required unless treated grossly contaminated water.

AFM has been independently verified by IFTS (Institute if Filtration and Technical Separation) in France to remove 97% of all particles down to 5 microns. The best that can be achieved by a sand filter is 20 microns.

When AFM is combined with pre coagulation and flocculation using a ZPM static mixer, mechanical filtration performance is improved by 100 times and we have nominal filtration down to less than 0.1 microns.  In addition to removing solids, the coagulation reactions will remove dissolved organics such as proteins, lipids, amino acids and inorganic components including phosphate and free silica.  AFM is also excellent at removing hydrocarbons.

AFM does not support the growth of bacteria, so the media is not subjected to bio coagulation and it does not suffered from transient wormhole channelling.  AFM is a secure, stable and 100% predictable water filtration media that is at least twice as good as sand and surpasses the performance of UF in many aspects

 

Water chemistry and quality

Seawater or high TDS water abstracted from the ground via a borehole (tube-well) water, often has the following conditions;

  • no oxygen and low redox potential
  • high zeta potential and colloidal suspension of silica
  • elevated partial pressure of nitrogen gas
  • dissolved sulphide, methane and VOC's (volatile organics carbon)
  • iron, manganese & arsenic
  • heavy metals such as chromium, lead, mercury
  • phosphate
  • colloidal silica
  • Fluoride
  • Ammonium, in combination with phosphate and magnesium if forms struvite

Aeration & Oxidation

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

If the water has dissolved oxygen content below 5mg/l, or a redox potential under 200mv, the very first step should be aeration to gas strip the water and restore the chemical equilibrium.

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 be from 10 to 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+. 

During the oxidation stage with Fe2+ to Fe3+, the small ferric particles will be suspended by the aeration will grow and react to co-precipitate other heavy metals from solution.  The longer the aeration phases the larger the ferric particles making them easier to be remove by AFM (Active Filter Media).    In some case we recommend the addition of ferric to the water in order to assist with co-precipitation reactions.

 

AFM filtration characteristics and performance benefits

AFM is an activated mesoporous aluminosilcate with glass as a structural substrate.  The surface area is 500,000 to 1,000,000m2 per tonne of filter media of a particle size 0.4 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 chemicals out of solution to form small positive charge particles that are easy to remove by AFM.

Regeneration of AFM is by a simple backwash using filter water, and the process is 100% sustainable, the media should last for the life of the filtration system.

Key performance benefits

  • Filtration performance verified by IFTS, 97% down to 5 microns with grade 1, 99.7% down to 3 microns with grade 0. When combined with flocculation, nominal filtration performance is better than 0.1 microns.
  • AFM will deliver a 50% to 75% lower SDI than a sand filter
  • Unlike sand there is no free silica in AFM, so less fouling of membranes
  • Performance comparable or better than UF
  • Does not biofoul of channel
  • Minimises biofouling of membranes, because of more efficient filtration
  • When combined with coagulation AFM removes more dissolved components
  • AFM directly adsorbs organics like activated carbon
  • Can remove phosphates when combined with NoPhos injection
  • Chlorination is not required, so no TCA, THM or hydrobromous acid production
  • Regenerated by back-washing with water
  • Sustainable and predictable process.

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 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.  Dryden Aqua also manufactures the equipment.

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