For info on refilling at home, go to https://www.bordsvatten.com/kolsyrefyllning/adapter-for-egen-kolsyrefyllning-kops-pa-internet-20/ and then use the translation script on that page.
On carbon dioxide bottles, it mentions how much Co2 they contain when full, normally 425 grams on newer machines and 300 grams on the older gray Sodastream canisters. You empty the bottle completely and then put it into the freezer for a few hours. Once this is completed the smaller bottle is attached to larger bottle, placed on weighing scales and filled until the weight is correct.
For this you will need a proper valve for the big bottle as well as quality rubber hoses and couplings designed for extreme pressure. Proper couplings and hoses are also required if you decide on large bottles which hook directly into your machine.
Retailing these couplings could be a business idea for make-your-own-wine stores. You could then refill your own bottles or maybe have them refilled by someone who normally refills fire extinguishers or carbonic acid for fish tanks or for pubs. Another idea would be to become a retailer for slightly larger bottles and for two to four-kilogram refills.
Perhaps someone will even challenge Sodastream. A luxury-model CO2 pump costs about 4300 Euros and a cheaper model perhaps half. With one of these, one could refill carbon dioxide canisters in 20 seconds while the customer is waiting. Through leasing refilling machines and distributing the cost of this over one or more years, a business such as this could get started without large cost.
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The business itself won’t lower prices. Wassermaxx is owned by AGA and Airliquide who has interests in other markets. Through everyone protecting themselves one provider can’t ever refill the bottles of another.
However, a new generation of soda machines is moving in with cooling systems and in-line carbonation which means no carbon dioxide is lost and large 2-7 kg Co2 bottles can be utilized. Such improvements simply makes buying such a system the only sensible choice over any other kind of flavoured drink.
These are actually small restaurant machines for home use. The water must be cold if you want a proper level of carbonation so current machines without cooling systems will probably soon disappear. Models with cooling often carbonate better than soda machines and even though they are quite expensive, the carbonic acid can be bought in large bottles making the carbonated water extremely cheap. Flavors can be ordered worldwide from allfreightfree.com.