If you want to carbonate water but don't want to buy a countertop carbonator or its overpriced CO2 refills, you can get a ball lock valve cap that screws onto 1L or 2L soda bottles for around $8-16.

That valve will attach to a standard female fitting, which you can put on the end of a hose coming from a pressure regulator, which will attach to a full-size CO2 cylinder available from a brewing or gas supply shop. CO2 refills are a lot cheaper this way.

Put cold water in the bottle with some extra space at the top. Squeeze out the air and attach the valve cap. Set the pressure regulator, connect it to the bottle, open the regulator's output valve, and watch the bottle that was slightly crushed by your squeezing expand back to its normal shape. Slosh the water around with pressure applied for maybe 10-30 seconds. Close the output valve and disconnect.

Voilà. Carbonated water.

IIRC, PETE soda bottles are pressurized to about 50 psi for retail shelves. I don't think they're likely to burst until well beyond 100 psi, and they'll deform before they burst, so if you're careful, you can go a little higher than 50 and make fizzier water than what you can buy in the store. I have used 70 psi many times.

Read up on precautions for handling pressurized gas before doing any of this. Wear eye protection. Don't turn your bottle or gas cylinder into an unguided missile. :)

Sadly, I don't have any info on microplastics released by this process. (Nor by countertop carbonators and their rigid plastic flasks.) I wish I knew of a suitable steel bottle to use instead.

I found it cheaper and much, much more convenient to get an adapter to refill the countertop carbonator's CO2 cylinders from a standard 20 lb CO2 cylendar. That way, you can carbonate from the much smaller and easier to use countertop unit, you can service multiple countertop carbonators from a single larger tank, and you can leave the larger tank shut off and away from living areas so that a leak doesn't pose a hazard.

As a kid I built a pneumatic musket with both 20 oz and 2 liter bottles and filled them up to 80 psi repeatedly, and never had one pop.

I wouldn't recommend going that high for a carbonated drink though, unless you like to live dangerously while opening your soda.

I’ve done this for years and never ruptured a bottle, I set the regulator to 60psi.

I’d like a metal bottle too but haven’t found one - I presume spraying some co2 into it would be enough to get the plain air out since you obviously can’t squeeze the air out.

FWIW the counter carbonators aren't too bad if you use a third-party refill instead of the expensive branded ones. Also, you can just use dry ice to refill the bottles rather than swapping for new ones. If you don't want to geek out on a complete DIY setup, the countertop models are definitely a little more convenient.

I bought an Aarke (soda stream comp) and a CO2 hose. Now I use one of the 5lb tanks. It's great.

Could you do the math for the raw material cost of 355ml?