- 3 July '03 Update -
On Candle Purifiers and Tests for Silver in Filtrate
Tests have recently been undertaken on ten pottery purifiers for the presence of silver in the filtrate.  In every case, for 100 ml. of the very first filtrate per candle the silver content has been under 1.0 part per million  (i.e. under 1.0 mg. per liter).  Other tests have indicated that following the initial 4.0 liters of flow per purifier the content of silver in the filtrate is no longer at detectable levels.

The system shown was used for collection of the filtrate to be tested for silver content and e coli.  The candle in the upper container resembles the two at bottom.  At right is a pottery cylinder, end view, prior to closing with plastic cover caps. 

The permeable pottery cylinder shown is 8.6 cms. diameter and 12.0 cms height, recent ones giving over 1.0 liter of flow per hour.  This design is intended to have a ceramic candle of minimum volume, in cubic cms., that is equal to that volume of filtrate that passes the candle over a twenty minute period.  Thus the pathogens have a 
        minimum of twenty minutes within the purifier.  By contrast to thinner walled purifiers the thicker ones increase the
        time period over which the pathogens can be trapped within the ceramic medium, as well as giving additional
        contact time with the silver.  Current thinking is that this kind of over engineering provides a better guarantee of
        effectiveness.

        These tests are thought to be the first ever of their kind for small sized purifiers that are made of the common pottery
        clays.  And whereas all continuous flow tests since May have been for purifier disks the candles are now going into
        continuous flow testing for the first time.  Further microbiological tests for the candles will hopefully give the
        necessary indications these are ready for trial use in vulnerable communities.

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