loosing 7kg of water through water filter.

The water PURIFIER has an element filter on its intake which destroys anything that doesn't please it.
Spent way longer than I should have looking for a liquid filter in that screenshot.
 

The purifier uses sand to filter contaminants from the contaminated water.
It should let water (clean, blue kind) pass through without using any of the sand, as there is nothing to filter out.

Edited March 15, 2017 by Masterpintsman

It is not destroying causing you to lose 7kg of water, the purifier can only take 3kg at a time. the pipe holds 10kg per tile/node etc. as the purifier take 3kg from the first node the rest of the nodes cascade to fill the 3kg difference in each node and move forward which then leaves the last node with only 7kg in it. The pump will then suck u only 3kg to fill in the difference. If you look in pipe view it shows you blue/yellow balls just to show what water is in the pipe, a ball does not stand for 10kg even 1g will make the ball.