Have you heard of the WAPI? It’s very low-tech, costs about $9 or so at www.sunoven.com. It’s a water pasteurization indicator. It won’t filter your water but will let you know when water (and milk) have reached the temperature for pasteurization.
Berkeys can filter thousands of gallons of water before the filter needs to be replaced. There are also directions on the internet for making your own ceramic filter.
I’ve never seen the WAPI. I’ve always planned on a full rolling boil to be safe. I guess it would save fuel to use a device like this.