Kegging up RO water just doesn't come out right... need some backbone to co with the carbonic bite.
5G RO water
Add minerals to about a gallon of RO in the keg. Shake a bit. They won't all dissolve. Top up to 5G. Still won't all dissolve. Put keg in fridge an set reg to 35psi. I think my fridge is set around 40degF. Take out the next day and shake 'till the reg doesn't hiss too much. The minerals should all be dissolved now, as the carbonic acid lowers the pH enough to get the bicarbonate into solution. You don't need to worry about balancing line length, because even though the mineral water will come out of the tap pretty quickly, there's no head so you won't foam over.
This is a good mineral profile to start with. It's not too 'salty.' I haven't played around too much with recipes, but I can say that bicarbonate at 20g/5G makes the water kind of chalky. This is kind of middle ground for domestic mineral waters, and on the low end compared to some European brands.
There's not too much info on the net about this topic. Most info is in tsp, and usually involves just one or two minerals. You can use common brewing water adjustment chems to make a wide variety of mineral waters. If you want to get really fancy you can start thinking about adding silicates, iron, and other trace elements. If there's a specific brand you like, you can clone it. All the profiles are available online.
BTW I would recommend adding lime juice to the glass, not the keg. It will make some foam so be careful.
Also, if you use NaCl to get some saltiness without having to add bicarb, and it tastes too salty, keep trying it for several days. I thought my first batch was too salty, but came to like it. It will seem less salty after a few sips. You can open the keg and top up with RO after drinking a bit. The recipe above has basically no perceived saltiness in my opinion.
Great use for a spare tap!
I drink a lot more water with this stuff on tap, which is a good thing I believe!