6th March 2005 at 11:23 am #591
Flow rate: we never made vertical flow rate an issue in our project design and have not evaluated the performance of the filters in this regard. When the sand is new (or recently washed), the flow rate will be high. Depending on the amount of water filtered and the turbidity of the water, the flow rate will fall progressively. However, it is important to note that a slower flow rate will have no negative impact on the quality of the water, rather to the contrary: a slower flow rate will increase the time during which the schmutzdecke can be active on the water passing through. In all cases in the field (rural Africa), users of the filter did not find flow rate an issue to bother with and maintenance (washing of the sand) was done mostly when the output of the filters became too slow for convenience. Often, ‘slow for convenience’ meant little more than a trickle that over the course of several hours still provided sufficient water for the family.
Adriaan