Filling backpack with pure

Hi guys newbie question I'm afraid. I have a 350ltr wcw compact Di system how can I fill my back pack with pure water from the system with out it taking hours?

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You could fit a small submersible pump and get it linked up to battery , would probable need to have small hole  cut in the lid or top of the tank so you can feed the power wire in and have the 1" hose coming out , or take it back to wcw and see if they can place a fast will outlet on the bottom of the tank , may cost you £40-50 but I personally think the 1" fast fill outlet is better 

is it pure in the tank, if so then a submersible pump will work, if not then then your best bet is to fill a few barrels with pure the night before for the backpack

I use a backpack for a few jobs and as above I fill the backpack and 2 containers if needed the night before then its a simple quick job refilling backpack. 25ltr containers can be bought of ebay for cheap.