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For freshwater COD level ranges between 15 mg/L to 30-40 mg/L it depends on the water. Drinking water should be around 3 to 7.5 mg/L, distilled water should have 0 mg/L of COD, high quality water (after GAC treatments) has level of COD around 0.1 to 1 mg/L. I would not be surprised if the COD concentration of rain water is low, but everything depends on the limit of detection of your kit, you might be able to find other kits with lower detection limit.