Juice the lemons with a lemon juicer and pour the juice into a half-gallon jar or a large bowl.
Add sugar and stir until fully dissolved in the fresh-squeezed lemon juice.
Add the filtered water and stir well.
Stir in your fermentation starter culture (whey, ginger bug, or water kefir).
Drop a couple of raisins into the bottom of the bottle. Seal them tightly and let them ferment at room temperature for about 72 hours.
Pour the liquid into bottles, (or you can use a half gallon mason jar with lid) leaving about 1 inch of headspace at the top.
Once a raisin rises and floats to the surface, it’s a good sign that the lemonade is ready to enjoy. This will take 2-3 days.
Place the bottles into the fridge to chill overnight. Discard the raisin (if using) and enjoy your homemade lemonade!