When I first set mine up I had a lot of head scratching trying to figure out how the holding tank could only have one line and had to look up how it works
If I understand my set up right, the bladder is like a balloon inside the metal tank that fills with water. The rubber bladder compresses the air in the metal tank when it fills from the RO unit (the line should be before the DI resin chamber). When you open your valve to fill your water bucket (I keep a 20 gallon with float valve full in addition to the small tank) the compressed air in the holding tank will push out the water in the bladder. Once the pressure is less than the water coming from the RO unit, flow reverses and the tank fills. Keep in mind that the bladder is smaller by about half than the physical tank so my 2 gallon only holds about 1 gallon of water in the bladder, the rest of the space is air trapped between the bladder and the metal walls.
A note on RO/DI units. Unless you have a pump, your household pressure will not be enough for the 100+ gallon/day units. It will work but you will only get about around 20/day. The RO filter needs to match your system so check pricing (this is the expensive part that needs replacing). Usually you are better off with a slower rated filter that is less expensive since it will provide as much RO water as the higher rated, more expensive units.
Stages are another thing to think about. Adding as much prefiltration as you can will help preserve the life of your RO filter. Most 6 stage units include an after RO carbon filter for taste that is pretty useless unless you plan to use it for drinking water.
I also like the clear cannisters for the particulate filter stage (no reason to have it clear for the carbon but it hurts nothing if all the cannisters are clear and it allows you to change one to a finer particulate filter if you decide you need it). We have a lot of mud in our water so the clear cannister lets me see when the first stage needs changing (we filter our house water but the mud is still a problem and we would have no water pressure if we used finer house filtration).
The prefilter cannisters are all the same so you can experiment with what you want to put in them. I use a fine particulate filter and two carbon stages before the RO unit. Just today I removed my refillable DI nightmare and connected a new (bought but never installed probably a year ago - added it today after 30 gallons of water on the garage floor because of a leak in the - ugly words - DI component) cannister style DI stage. I also removed the post RO carbon filter that had never been changed and is only for drinking water (and really only for taste). So now my 6 stage is a 5 stage system (1 particulate, 2 carbon, 1 RO and 1 DI). If I wanted to add an additional stage it would be a second particulate before the carbon but nothing after the RO.
