This is a pdf file of a simple water filter that is marketed to folks in 3rd world countries. You don't have to actually buy one though; the instructions are sufficient to suggest how to make your own using a clay pot (without a hole, or with the hole plugged) inside a food-grade poly bucket.
Here's a link to the file.
It reminds me of the plans I saw for a filter that was built into a cistern. It was just a box built of red bricks, enclosing the potable water pickup. Same idea, in reverse.