How much campden ?

I need to treat 2 - 10 gallon igloo coolers of tap water to remove chlorine and chloromines from the water to brew with. I have read that I should use 1 campden tablet per 20 gallons. Considering it 10 gallons is it safe to assume use a half a tablet or should I just one tablet per 10 gallons and that is ok ? Also I have never used campden in beer brewing before only for wine. Will the sulfites in campden effect the taste at all ? Also what I the contact time for treating my water ? Thanks

Matching the metabisulfite dose to the dechlorination need is a good idea. But given the inaccuracy in knowing the current disinfectant concentration and ability to measure out the metabisulfite, erring on the side of adding a little too much is preferable. In the case above, the excess dose would add a little extra sulfite and Na or K. The excess would only be a few more ppm of any of those ions. I don't think that this would be notable in taste.

Some people are sensitive to sulfites. When dosed properly, the sulfite in metabisulfite is all converted to sulfate. When excessively dosed, a few ppm of sulfite could be left in the water. I'm not sure if that would be enough to affect people sensitive to sulfites. When metabisulfite is used in wine production, I understand that a much higher dose is used.

Luckily I am not sensitive to sulfites as I do drink wine with no ill effects. So I guess I should use 1 tablet per 10 gallons and err on the side of safety. I wish I actually knew the amount of chloride in my water.... it's frustrating because I never use anything to remove chlorine or chloramines and I NEVER have bad results in my beer actually I have won or placed many competitions. I am mainly looking to add the campden as I was told that chlorine/chloramine is especially important to remove in my upcoming beer a smoked maple brown ale as the chloramine will strongly clash with the by products of the smoked grain. IDK maybe I should just use Poland spring water as I was originally thinking but, if I don't start to treat my own water I feel like I'm not really growing as a brewer.

In home wine making and mead making the dose is one campden tablet per gallon. In brewing it is one per 20 gallons which is enough to take care of 3 mg/L equivalent chlorine as chloramine. Thus overshooting isn't a big concern from the sulfite sensitivity POV. Furthermore, sulfite which doesn't react will either be driven off by heating in the mash or boil as sulfur dioxide gas (that's why you smell SO2 when using campden tablets and why they are sometimes referred to as 'solid sulfur dioxide') if it doesn't first find something to reduce. Beer usually contains some sulfite (whether the water was treated with campden tablets or not) but the levels are minimal.

If you absolutely do not want to add anything you do not have to in order to deal with the chloramine crush a campden tablet in warm water, stir it up and add bits of this solution to the water being treated. When the water stops smelling of chlorine you have added enough.

Also you are confusing chloride ion with chlorine/chloramine. Chloride ion, Cl- lends sweetness and body but is odorless. Chlorine gas, Cl2, dissolved in water gives hydrochloric acid (i.e. chloride ion) and hypochlorite ion (OCl-) which is bleach and smells like it. Chloramine is NH2Cl and smells like bleach (but slightly different).

If you really want to know your chloramine content you can buy very simple test kits (add a drop of DPD to a sample and compare color to a color chart) which test free and total chlorine and are inexpensive.

The reasoning behind avoidance of chloramine with smoked beers sounds solid. Chlorinated phenols are the bete noire here and smokey flavors come from phenols.