This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
Dr. Langfeldt has experimented with a number of substances in studying their applicability to the purpose of destroying microscopic life in drinking-water. The most striking results he obtained from citric acid. Upon the addition of one part to two thousand, life ceased in from one-half to two minutes. Microscopic examination showed that those forms of animalculae supplied with a thick epithelial covering, are not affected by this dilute citric acid, but only those with delicate coatings. But as the greater portion of these unwelcome visitors belong to the latter category, and as those of the former variety are visible to the naked eye, a solution of the above-mentioned strength (1 - 2,000) will suffice as a safeguard. In about one minute after their death, these animalculae settle to the bottom of the vessel containing the water, and can always be found in abundance in the sediment. As the solution of citric acid spoils so readily, Langfeldt advises that it should be freshly prepared every day.
This experiment may prove valuable for domestic purposes; for purifying water for industrial establishments its use is impracticable, as a more thorough purifying agent must be employed.