Find out how to explain it in their existing culture. Curses, uncleanness, etc. There's probably SOMETHING responsible for misfortune, death, and disease.
Claim that there is a method which helps battle this, it involves purifying the water with a strong fire for this duration of an hour-glass. Once it is cooled it is, for a short time, safe to use.
Tell the truth, but dress it in marketing. Use the same tricks that advertisers use to sell damned near everything to damned near everyone.
Shitting in the river will make you sick, but it's also the low-class thing to do, it's unfashionable, it's unattractive to the opposite sex, it makes you look foolish. Toilets are respectable, they're modern, they're stylish, they're a status symbol. You don't have to invoke superstition to create a taboo.
If you want people to boil their water, just provide subsidised tea and let the addictive properties of caffeine do the work for you.
I am particularly hooked on this idea.Distributing free low concentrated sugar free tea :-)
I remember a case study i read some years ago about the difference in drinking culture between india and china.The Chinese factory workers often have a 24 hour boiling pot of low concentrated green tea parked where they constantly consume small portions of during their day.AFI remember around 70% of water intake for them was via this medium.
That could be an out of the box solution.:-)
One interesting attempt at this was a health intervention in Cambodia. Rural villagers were having iron deficiencies. A cheap countermeasure was to put lumps of iron into boiling broths or water that people were going to drink, but it was hard to convince locals to actually follow through.
So, they made iron lumps in the shape of fish (a local good-luck symbol) and handed them out - increased compliance a lot.
This wouldn't work in the case I mentioned because they actually already had a very strict definition of when water should be hot and cold. If this were vague or unclear in their culture, you could lie and say "Oh yeah, because you believe X and Y, Z is true" - ignoring the ethical questions this brings up.
But in their case water being boiled was something you did when you were sick. This already had them boiling water for sick people, the sick would get better (to some extent confirming their belief), and they'd go back to cold dirty water.
Find out how to explain it in their existing culture. Curses, uncleanness, etc. There's probably SOMETHING responsible for misfortune, death, and disease.
Claim that there is a method which helps battle this, it involves purifying the water with a strong fire for this duration of an hour-glass. Once it is cooled it is, for a short time, safe to use.