Using the double di method way better than single di but a slow flow of water is very important as well .
How long is he willing to stand out in the cold filling containers , I take it he is working out of a car as he will only be using 6 containers and possibly tradding bottom's .
Still need to know tds i know a lad who wont listen to advice has a 6.5 ltr vessel with tap tds of about 45-60 ppm uses about 300-400 ltrs a day over 4 days and buys a bag of resin almost every month .
Because he fills direct to tank on almost full flow from tap , I keep telling him he needs double DI and static tank and he would save a fortune . Better off spending a bit more money at the start otherwise you end up spending way more . Mike