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UV irradiation (using lamps in particular) provides a quick and effective way of disinfecting water supply from harmful bacteria without leaving a chemical residue, like the one chlorine leaves. Usually it is used as one step in the process of making water safe to drink. Other steps include filtration and/or reverse osmosis. But in general, UV light (whether by sun of lamps) has demonstrated efficacy against pathogenic organisms, including those responsible for cholera, polio, typhoid, hepatitis and other bacterial, viral and parasitic diseases.<ref> Trojan UV: [http://trojanuv.com/uv-disinfection Introduction to UV Disinfection]</ref> Special low-pressure mercury vapor lamps produce UV radiation at 253.7nm, the optimal wavelength for disinfection.<ref>[http://www.hitechultraviolet.com/ultraviolet.html HiTech Ultraviolet Pvt. Ltd.] </ref>