Nothing technical....simple run off from shed roof,sheet hard plastic on roof, guttering, down pipes plastic gutter guards
either side of shed, muslin cloth strapped over openings into 4x water butts.In good downpours I have
water as low as 2ppm,very little crap ever gets through, I clean roof and guttering on reg basis.
I have more barrels coming off the house from downpipes front and back, diverters with guttermates
water not as good coming off tiled roof 14ppm onwards..to be honest I don't check guttering out
on reg basis
Mine was comming off a tiled roof and into the guttering then through a sock filter so the tds was high as 45 ish so I gave up in the end. I'm not on a water meter so figured it was less farting about and cheaper just to carry on using my static ro/di.
To be honest I like the farting about and going my own way 
I use my tapwater (swear word I know) which on average measures around 28-35ppm most
of the time and blade glass dry when hydrophobic....but when I get some low tds off shed
it give me a change from the norm,
You use tap water then blade the windows dry 
How come you don't just fill the van tank with tap water and use a di after the pump as would be so much easier than farting about like that surely?
No I use tapwater low tds and blade some windows from the ground when I have too,they are
normally hydrophobic glass which accounts for a small percentage of my round....its actually less
hassle for me, I only work part time and work from an estate. I carry 8 barrels and can plug in anywhere
where there is an outside tap and clean in my local area if need be.