See all these RO machines allover nowadays? they are very cheap to run if your water is not salty or hard the filters actually run very long if you are treating 'clean' water like driveway runoff or settled muddy water,
for a household of 6-10 in Nairobi with about 4 Girls who take long showers a 10x3x3 water tank is more than enough to run for about 4 months.
But I read that the water is not healthy drinking since it has all the salts removed, some machines re-add the salts like calcium etc.
To Each His Own