Can water unused for one and half years be purified?

We have 10000 litres of water tank and we are not using this water from last one and half years, so how can I use this 10000 liters water?

How much chlorine or bleaching powder should I dissolve for a 10000 litre tank? Give me simple processes as early as possible on my email makarandmvispute@yahoo.co.in pune

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