Add the water, ginger and sugar to a large saucepan. Bring to a gentle simmer, stirring until the sugar has completely dissolved.
Remove from the heat and leave the mixture to cool completely.
Strain out the ginger and pour the cooled liquid into a clean jug or large jar.
Stir in the 160ml active ginger bug liquid and lemon juice.
Pour into two clean 1-litre pressure-safe bottles, leaving a little headspace at the top.
Leave at room temperature for around 1–3 days to carbonate. The exact time will depend on the warmth of the room and how active your ginger bug is.
Once you have a good level of fizz, move the bottles into the fridge to slow the fermentation.
Chill thoroughly before opening and serve cold.
