I'm trying to determine the proper dosage of Campden Tablets to remove chloramine from water. I keep seeing 1 tablet per 20 gallons posted all over brewing forums, but I never see a source for this information. Is this really a good recommendation or does it just keep getting reposted and passed about as common knowledge without anything to back it up?
The tl;dr version is: 30mg of sodium metabisulfite or 35mg potassium metabisulfite per gallon of water.
For my usual 10gl ± 3gl of brewing water (10gl batches) I often over-compensate and just add 1g of k-meta to each (mash and sparge) water salts.
Campden tablets typically contain 0.44 g each of sodium metabisulfite (plus filler), or 440mg, enough for nearly 15 gallons of water at the 30mg rate.
Thanks, jsled. Here is the PDF that outlines the specifics from your link. The discussion of Campden Tablets begins on Page 23. Page 24 is where the 20 gallon recommendation appears.
An important note here is that this is assuming the tablet is "695 mg of, presumably, nearly pure potassium metabisulfite."
Also, from the article "Our recommendation of 1 Campden tablet per 20 gallons is designed to cover this nominally “worst case” of 3 mg/L chlorine all as chloramine."
When I look up water quality information on my local water company's website they state that chloramine is present between 1.5-2.8 ppm (or 1.5-2.8 mg/L) in my water.