6th March 2005 at 11:58 am #609
More generally, what instructions are given to the user on how to operate the filter? For example, are they instructed to run the filter to waste until the schmutzdecke if formed? How do they tell when sufficient time has passed to allow schmutzdecke development after cleaning? What is the cleaning procedure and how should the user know when cleaning should be done?
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.
6th March 2005 at 12:02 pm #612
Through my water analyses I found out after four weeks all filters have established a schmutzdecke to such an extend that drinking water quality can be obtained. Some filters are ready earlier. It depends on certain factors. Users are instructed to continue boiling the filtered water during the first four weeks of filter usage, because after 4 days the water is at least physically clean and it would be a waste to discard this water.
Generally the users know it is time to clean the filter, when the flow rate to so little, that is does not meet the water needs of the family anymore. Only then it is really necessary, because the more time the water has to pass the water to cleaner it will become. In raining seasons the filter sometimes blocks completely, because the raw water was too turbid.
The user is thought how to clean the filter himself, because before they buy the filter they undergo a seminar about maintenance. They will remove the top 5 cm of the filter matrix and was it in water several times. After this they apply the sand carefully again and start with the four weeks cycle. In some cases filter start to develop a crack and therefore leakage. The user will have to remove all the sand and seek a technician for help.
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