The EPA set the safe reference dosage for silver (RfD) as 5E-3 mg/kg/day (.005mg per kg body weight). So a 150 lb person has 68 kg. 68 x .005 = .34mg silver max dosage daily.
1 tablespoon water = 14.786764797 grams (14,787mg)
20ppm silver in 1 tablespoon C.S. = .296mg silver
1.15 tblspn of 20ppm C.S. is needed to ingest .34mg silver
4.6 tblspn of 5ppm C.S. is needed to ingest .34mg silver