zetapotentialAFM is used for the tertiary treatment of municipal sewage effluent.  In this industry sand lasts from 6 months to maybe 2 years before it needs to be replaced, When AFM is back-washed all of the solids including bacterial cell biomass  are removed.  If a media was going to biofoul it would happen in this wastewater, yet we have AFM in these systems for over 15 years.
Bacteria stick themselves onto sand or most surfaces by secreting an extra-cellular polysaccharide alginate. The alginate acts like a glue and through a process of natural selection the glue becomes stronger as the biofilm ages and develops.  After about 1 to 2 years the biofilm starts to mineralize and this gives the impression that the sand is becoming round.
AFM is hydrophilic, it has a negative surface potential as well as a catalytic surface.  The negative surface attracts di and trivalent positve ions such as aluminium, calcium and magnesium.  Surrounding the cations you will have a layer of divalent anions, such as sulphate and then another layer of  cations.  Between the outer cation anion layer is called the stern layer or slip-zone.  Bacteria can approach the surface of AFM but the stern layer prevents the bacteria from actually making a connection with the AFM, so on back-wash the bacteria simply slip off,  The bacteria and solids are held on to AFM by the charge,  but this attractive force is completely broken on back-wash.
The charge or zeta potential is important,  but actually the most important aspect is the surface area.  The greater the surface area the more room there is for the attraction of organics and small particles.  In this respect AFM is 300 times greater than un-activated media.
The surface is also loaded with metal oxides that act as catalysts that cause the dissociation of oxygen and well as hypochlorous to form free radicals and a zone of high oxidation potential on the surface of AFM.  The high oxidation potential also makes it difficult for bacteria to become established on the surface

Latest Articles