A claim like that sorta sounds suspect.

Depending on how one gets their tapwater, it could be as simple as drilling a hole in the ground and getting the water, or putting a bunch of sand in a basin. The per liter cost of getting intermediate raw water from a slow sand filter is infinitesimal, pennies per cubic meter. Water from a slow sand filter must then be chlorinated for distribution, but so does water from a desalination plant because chlorination isn't just for treating the water coming out of the tap, but it's for protecting the water as it goes through water lines.