Progressing towards placement of low cost, pottery purifier systems in vulnerable communities, final decisions will soon be made on system design, silver content, etc.  The most recent purifiers under flow test, candles of 8.6 cms. diameter and 12.0 cms. height, are giving an average flow rate of 2.0 liters per hour, a target amount for the household systems.  For these systems the water column height above the candles is 15.0 cms.  But by comparison with these systems, the column is being increased in order to determine other test variables, such as:  percentage of e coli removal, increased turbidity and the effective lifetime of the purifiers.

For the smaller sized candles shown above, 6.0 cms. diameter by 10.0 cms. height, the column height was increased in order to observe changes in these same variables.  The six candles shown were placed in the bottom of the fifty liter plastic container, giving column height of 38 cms.  These candles, previously tested in the household system, indicated an average flow rate of about 0.5 liters per hour, given 15 cms. column height.  By contrast at the increased column height of 38 cms. the candles averaged about 1.0 liter per hour.  So the flow rate was about double.

As to e coli removal at 38 cms., for a raw water sample testing 48 bacteria colony units each and every candle indicated nil, i.e. virtual 100% removal.  However it should be noted that the 2.5 cm. wall thickness of the candles appears to be adequate to this removal rate, even without the silver saturation.  

Such accelerated flow is also being undertaken as an industry technique for the determination of purifier effective lifetime over a shorter period.  For example, in a step-wise way:

1.  Double flow rate for the purifier(s) by increasing column height, in this case to 38 cms. 
2.  After six months return the purifier(s) to the household column height, 15 cms.
3.  Undertake microbiological tests on the filtrate.
4.  In the event of 100% removal of e coli on this filtrate, and a lesser percentage for control purifiers (containing no silver), it can then be said that silver purifiers of this type should remove virtually 100% of e coli for not less than twelve months. So given this example, where column height gives double flow, it is possible to determine effective lifetime for removal due to silver in a period of half of that lifetime.

It is not yet possible to predict how increased column height will effect turbidity, but on occasion it may be desirable to design systems having column height that are above the 15 cms. of the household system.  A recent test using the larger of the two candles indicated that for six of these in a fifty liter container, and with 35 cms. column height, the flow rate was 36 liters per hour.  (Note that these six larger candles had been fabricated with greater permeability than the six smaller ones cited above.)  Low cost systems coming out of this design, at the greater column height, could be considered desireable for institutions.   

The lesson may be an obvious one, but interesting nonetheless as a new possibility, that increasing water column height leads to increased flow.  And it is now more clear that pottery purifiers are very responsive to column height, that permeability can be altered so as to make the purifier appropriate for deeper containers.