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The removal of Cryptosporidia from water

One of the major applications for AFM is  the removal of the Cryptosporidium oocysts from water by using rapid gravity and pressure sand filters.  

There is an excellent paper by Dr.R.Gregory, Volume 1 Issue 1 February 2002 Journal of Environmental Health Research that is worth further reading,  the following section covers some key points from the paper that have relevance with respect to AFM.

"In applying the potable water standards to C parvum oocysts,  it follows that filtration for the removal of oocysts takes priority over treatments to inactivate oocysts. However this does not mean that the treatments to inactivate oocysts is much less important, because of its contribution to reducing infectivity of oocysts in pool water or captured within the filters and not discarded during back-washing".

We know that AFM performs better that sand in terms of its removal of small particles from the water.  It therefore follows that AFM will remove more of the oocysts   in comparison to a standard sand filtration media. We also know that AFM is a great deal easier to back-flush clean  in comparison to sand,  so not only should the AFM remove more of the oocysts from the water,  but there will be less chance of the pressure filter infecting the product water.

"Ballantyne et a (1999) AWWA annual conference Chicago  found that the impact of filtration stress weakened Bacillus subtilis spores and rendered them 1.7 times more susceptible to chlorine dioxide. It  is assumed that the shear stress resulting from the contact with the granular filtration media damaged the spore coat and allowed easier access of the chlorine dioxide into the cell."

Shear forces also have an impact of C.parvum oocysts,  now if it is a contact issue  the effectiveness of the process will be related to the surface of the filtration media. We know that sand has a coat of fats, bacteria and muco saccharide jelly, and we also know that AFM stays much cleaner and is self sterilising than sand. The surface of the sand may well have a cushioning effect, while impact of the oocyst on the surface of AFM would be like the oocyst hitting a concrete block..  This aspect of AFM needs to be confirmed,  but logic dictates that it will be the case.

AFM has a high negative zeta potential which attracts the oocyst onto the media surface,  the removal of  the oocysts is therefore a combination of physical as well surface adsorption, coupled with the greater degree of damage inflicted on the oocyst on passage through the AFM bed further improves the effectiveness of the process

However the over-riding key point is that sand filters, and indeed any media that does not actively surface sterilise,  will  be subject to bacterial fouling which will result in filter bed channelling. The channelling allows un-filtered  to pass through the filter bed, the oocysts therefore have a conduit though which they can infect the product water.  In contrast AFM does not support bacteria and is resistant to channelling and is therefore a much more effective means of removing oocyst,  especially when combined with good flocculation.

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