To assess the effective effeciency of a filter, is it ok to use delagua kit and is there a standard procedure particulary with regards to the time when a raw water volume is poured in and the filtrate sample is taken (just ensure that it is the same water which has been tested as raw water and not water already standing in the filter)
The Delagua will show you the number of thermotolerant coliform bacteria in the filtrate – these are mostly E. coli I believe. These only indicate possible other pathogens in the water since they also come from the gut, but are used in a general way to say if a water is safe or not. So yes, it is a good field method to know if your filter is functioning well but there are issues related to reliability of E. coli correlating to other pathogens.
What we know from recent research is that the water standing in the sand pores is much better quality than water that is the first flush, even though this water is still pretty good. First flush water will give you a spike (or a reduction in efficiency). When you get first flush water depends on the pore volume of the sand – that depends on the porosity and volume of sand in your container. In the round concrete filter I believe it is something like 10 litres water.
When sampling, don’t forget to sterilize the outlet. Also the Delagua needs to be calibrated to a precise temperature every month – if not then this can give you freak results as well (either zero if over 44.5 degrees, or many bacteria if under 43.5 degrees).
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