RO is not really confusing. I have been using one of two models available in the states. I am hoping there is something similar available in the UK or the EU. There are only 3 connection involved.
1. From the water source (your faucet) into the RO unit.
When the water goes through the unit it comes out in two streams.
2. One stream is the water you want which will have low mg/l numbers for many things.
3. The other stream is the waste water which you throw away i.e. down the drain. You can use it for lawns and plants outside.
It is necessary for me to have a modified faucet in the sink from which I feed the water into my unit. My unit has 3 lines as above and they are 3 different colors:ed is input, Yellow is waste and Blue is the RO water. I have a garden hose type connection on red input line. The faucet on the sink has a converter that makes it compatible with a garden hose. The unit is sold in a section the supplier labels as Reverse Osmosis Systems. From there they have several options, The first is Aquarium RO DI Systems. I do not go there, Instead I got to the section on the bottom of the list for Portable RO Systems. There are several options. All have the same color coded lines.
The is a second option for the input line and that is for one used only in one place that allows the system to be plumbed into a water line. This is normally the input line for cold water and you have to tap into that line. I preferred to be able to make my water, store it in 1 gal jugs and 5 gal. cans with lids. I used about 11 gallons a week. So. I ran the unit about once a month and stored about 45 gallons.
I batch the water into a 20 gal. Rubbermaid garbage can. Front there it gets pumped (with a small pump) into the storage containers. I fill the can for under 24 hours to make 4 weeks worth of water. I let it run over night.
I got a faucet that accepted an adapter so it would become compatible with garden hose connections. I also would bring the unit with me when I was selling in the vendor room of a weekend fish event. I brought tanks with cycled filters for this and I needed to mic the hotel water 50/50 with my RO/DI water. The DI part means I got really pure water.
Most folks only do the RO and that is more than enough. I wanted to go the extra mile (kilometer?) as I was using for wild fish which came from waters with almost nothing it them and a pH of about 4.0. I have all of the Python hose adapted even though I do not use the rest of the Python system save their hose which ahs garden hose compatible connections.
You can see the two systems I had. I started with the:
The 3rd stage was also the first in order of flow and it contained carbon. I replaced the carbon and RO modules once but the next time I oopted to upgrade to the:
I suggest you would be porefectly happy if you got an RO without the DI systems they offer here: look at the Mikro Epsilon or Mikro Alpha systems you can see here
https://store.afwfilters.com/portable-ro-systems/?CategoryTree=Portable__RO__Systems You can ignored the the remineralizatiion mentioned in the name of the 3 stage- it is just a plain RO unit and anyremineralization would require tou to buy the minerals separately and fron and aquarium type vendor. I alwys just mixed my RO/DI with my tap water.
One note here. When the water goin into the unit is in the low 70sF (about 22.22C), it make the output faster. Out cold water is very chilly. Vecause I am connedted to a caucet and not in input pipe line for coled water, I can mix both hot and cold to get the temp for max. output. SOmetime I will turn off the hot entirely for the overnight run to make sure if I oversleep or forget I have it running, there is less chance it overflows before Ithe UH OH alarm does go off in my head.
I no longer have my unit as I sold my Altums about 2 years ago to a member in my fishclub who already kept some. I sold my slightly used 4 stage RO/DI to another member in the club.