6th March 2005 at 12:00 pm #610
We normally extensively train local technicians, who are then responsible for teaching their customers at household level for instruction on maintenance. In all but a few cases this worked well. For instance, some ‘innovative’ cleaning procedures by some households have led to poor performance. Other invented methods that did not lead to measurable decrease in performance, for instance some families used to block the outlet of the filter, pour in lots of water while stirring up the top 10 cm of the filter sand (including the schmutzdecke). The resulting muddy water (containing most of the debris of filtered out material and schmutzdecke content) were then scooped out. Even though this goes against what everyone teaches, we found hardly any difference in performance in terms of coliform reduction, while it definitely restores the flow rate. It would be interesting to look into in more detail.
The normal cleaning procedure is to scoop out the top 5 -10 cm of sand, which after washing is put back (or else restored from new sand). We always preach that a household should allow time for the schmutzdecke to regenerate after maintenance (boil your water for 2-3 weeks), but in practice few people do so. It would be interesting to test, but it is my assumption that the schmutzdecke is actually active throughout a much larger vertical area of sand than just the few top centimeters as most books say.
As to when to clean: while we used to advise people to clean after a couple of months, in practice most households only clean their filter once the flow rate has become unacceptably low. While this might seem unscientific it is quite appropriate.