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I belief GTZ published a book documenting existing traditional ways of treating water by nomadic people. I Know there is a copy of that book in the University at Loughborough and you could probably obtain copies of part of the book through the WELL document service at WEDC.
If you look for individual treatment it might be worth considering a ceramic filtering straw. The idea is based on the guinea worm filtering straws used in South Sudan. For more centralised systems you will have to give more info as none of the small plants are simple. Even slow-sand-filter plants which are simple will require a basic understanding of the underlying principles.