hi guys cheers for reply, ive been tol the resin should last around 6 months with my ppm and i'll only be using about 300-400 litres a week as i build my wfp work.
once my tds starts to rise i will add anouther 11 litre after the used one to try and last abit longer.
think i'll leave the prefilters now then untill i get an 4040 ro in the future as at the moment with setting up a van with wfp system on a budget the easiest thing for me is to have two 11 ltr di's set up!