SODIS is a cheap method of disinfecting water by putting it in the sun. Like many things, it works better in physics than society, where its effects were not significant, according to a study in PLoS medicine recently…. [In] Rural Bolivia, where the study was done … the children studied usually get diarrhoea four times a year, which causes about fifteen percent of deaths of children under five. For the poorest quintile in Bolivia the under five death rate is about one in ten of those born alive. … The leader of the study, Daniel Mausezahl, suspects a big reason for this is that lining up water bottles on your roof shows your neighbors that you aren’t rich enough to have more expensive methods of disinfecting water.Assume that the story is true, that putting water in the sun to disinfect it signals your poverty because it demonstrates that you cannot afford the more expensive methods. Depending on how the utilitarian calculus works out, one of the solutions may be to heavily tax (perhaps even ban) expensive water treatments so that more people must resort to setting their water out in the sun.
The calculus would depend critically on how many people already have the more expensive systems. If only a few people have the more expensive systems, the calculus is more likely to favor the tax. If a relatively small portion of the population that cannot afford the water treatment, then the calculus probably favors a subsidy.