Sugar yeast solution generally dies after a week or 2 depending on recipe. This happens because yeast produce alcohol along with co2, the alcohol content eventually kills the yeast. Citric acid method doesn't have this issue because it's chemical not biological, however, it does supposedly cost more. I have used sugar and yeast however I bought citric acid this morning. Got fed up with replacing sugar and yeast in 2 2L every week or so.