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All the options given so far are great, but still relatively expensive.

In my opinion the most tried and tested means of checking whether water is bacteriologically safe to drink, which has been used across the world, at every economic scale, is the presence of a chlorine residual.

If you chlorinate your water using relatively cheap calcium hypochlorite tablets(1) after filtration, then test using relatively cheap DPD tablets(2) comparing the colour of water + DPD with a standard chart at time of use(3), this will indicate a number of things:

A) If the water was recontaminated you will measure no chlorine(4) B) If the sand filter has failed & is letting through gross contamination then the chlorine (if correctly dosed) will be overwhelmed and you will measure no chlorine(4) C) If the anticipated chlorine dose is measured then from a bacteriological perspective, the water should be safe to drink.

This is much cheaper and less complex than any direct measures of bacteriological activity (P/A, H2S, etc.) it is in my opinion also somewhat safer as it is more broad ranging, covering many viruses and other pathogens too.

(1) or calcium hypochlorite powder or granules or cheaper, but less stable sodium hypochlorite liquid, or another chlorine based disinfectant.

(2) or even cheaper but slightly more complex DPD liquid or powder - DPD is a generic chemical that turns pink in the presence of chlorine, there are others available, but it's likely the cheapest & most accurate.

(3) time of use should be at least 20 minutes after application of chlorine - or check the instructions.

(4) or at least a big reduction from normal.

All the options given so far are great, but still relatively expensive.

In my opinion opinion, the most tried and tested means of checking whether water is bacteriologically safe to drink, which drink is the presence of a chlorine residual. This has been used across the world, at every economic scale, is the presence of a chlorine residual. scale.

If you chlorinate your water using relatively cheap calcium hypochlorite tablets(1) tablets after filtration, filtration(1), then test using relatively cheap DPD tablets(2) comparing the colour of water + DPD with a standard chart at time of use(3), this will indicate a number of things:

A) If the water was recontaminated you will measure no chlorine(4) B) If the sand filter has failed & is letting through gross contamination then the chlorine (if correctly dosed) will be overwhelmed and you will measure no chlorine(4) C) If the anticipated chlorine dose is measured then from a bacteriological perspective, the water should be safe to drink.

This is much cheaper and less complex than any direct measures of bacteriological activity (P/A, H2S, etc.) it is in my opinion also somewhat safer as it is more broad ranging, covering many viruses and other pathogens too.

I should also say that this overcomes the main problem with sand filters - they don't necessarily disinfect the water - as pathogens can pass through. Using chlorine disinfects the water - using sand filters before it removes particulates & increases the efficiency and effectiveness of the chlorine disinfection.

(1) or calcium hypochlorite powder or granules or cheaper, but less stable sodium hypochlorite liquid, or another chlorine based disinfectant.

(2) or even cheaper but slightly more complex DPD liquid or powder - DPD is a generic chemical that turns pink in the presence of chlorine, there are others available, but it's likely the cheapest & most accurate.

(3) time of use should be at least 20 minutes after application of chlorine - or check the instructions.

(4) or at least a big reduction from normal.