Purified water is prepared from water of potable quaity by membrane process followed by a electrodionized system.Membrane processes are being preferred over Ion exchange because, besides reduction of Ions,they aid in removal of organism. More over they are continious and do not require storage and handling of acids and alkales for regeneration. Based on the inlet ionic load,requirement of single pass/two pass RO can be decided . Sanitary fittings and joints should be considered for the Ro model. The RO system can be sanitized through chemical or heat process. Electrodeionisation (EDI) combing the well-established water purification technologies of electrodialysis, Ion selective membtranes and ion exchange resin deionisation. Through this revolutionary technique dissolved salts can be removed with low energy cost and without the need for chemical regeneration, the result is high quality water of 0.08 - 0.05us/cm conductivity, which can be produced continiously at substantial flow rates. Ultraviolet sterilisation of the water is done to kill all micro-organism and virus.