if you want to get the max out of your resin, use a twin set up as this will extend the life of your water
or you could get your self a big di vessel and get old resin off other wc fill this up and use this to fill your tank and and then use a smaller di in the van, this will make it last longer, plus you will get max use of the resin, that a lot bin after 005, u could easily use this up to 40-50, so the water in the tank would be 40-50 then just run this through a 1 di set up
this is what i do and saves me a fortune, plus i am a tight arse lol