Clean water will be just as important.
The best filters; http://doultonusa.com/HTML%20pages/ceramic_candles_gravity.htm
They are the ones everyone copied that are called Berkey style filters. Doulton, formerly Fairey Industries, are the inventor and created them for the UK military.
How to assemble the filters into two 5 gallon buckets to make a gravity drip water filtering system.
https://www.instructables.com/DIY-Berkey-Water-Purifiier/