I have just bought a wfp backpack system it came with a big blue cylinder which i think is resin, please tell me if im wrong. I was told just plug one end into tap with hose and the other end into the backpack, but when reading other posts i read about ro unit, is this something that i need to get going, if yes how much are they normally and where in my system do i put it. i am based in Glasgow incase that makes any difference. Can anyone tell me how many litres of water a large cylinder of resin should do. thanks