10 H20 is 10 tons. (1 ton per H20)
10 H20 turns into 7 Drinking Water at the food processor, which is then 0.7 tons (.1 ton per DW).
10 tons turned into 0.7 tons when the water was purified! Thus 93% of the water was pollutants that needed to be removed apparently. 
(But more seriously):
I think the recipe should be 10->10. People aren’t producing drinking water even though the price is stuck at the MM price of 63 because its not very profitable.
Also, H20 should be .1 ton as well, if DW is .1 ton. This would make it shippable. At present it makes no sense to ship water because its too heavy, especially with the newly changed higher fuel costs.