JWR Replies: The least expensive option is to make your own filter. In my experience, the much-touted field-expedient sand and clay filters are only effective for use as a pre-filter. Their output still has a brown-tinged pond water look to it, and since the filter media is so coarse, they do not remove all harmful bacteria. (So their output still has to be treated either chemically, or by boiling.) You can, however, buy Berky filter white ceramic filter elements by themselves from a number of vendors including Ready Made Resources and Lehman's. With these elements, you can build your own bargain basement "Berky Clone". This consists of a pair of food grade plastic buckets, stacked one above the other. The top bucket has one or more holes drilled in it, to accept the Berky spare filter elements. Each element by itself costs around $40 . To get decent volume production from your filter, I recommend that you buy at least two elements. (A set of four is best.)

Materials:
4 - Food Grade HDPE food storage buckets (three to six gallon capacity), with lids
1 to 4 - Big Berky White Ceramic Filter Elements
Construction:
Drill one to four 1/2-inch diameter holes near the bottom-center of the upper bucket. (The same number of holes as you have filter elements.) Space the holes at least two inches apart and no closer than 1-1/2 inches from the edge of the bucket perimeter.With clean hands (to avoid contaminating the filter pores), insert the filters in the holes, screwing down their nuts on the bottom of the upper bucket. The nuts are plastic, so do not over-tighten them. But they must be tight enough to compress the o-ring seal, or the seal may leak--and this would be a contaminating leaks. (The filters point upward into the upper bucket, to avoid damage and to allow them to be cleaned periodically.)

Using a jig saw, cut a 7-1/2-inch diameter hole in the center of the lid of the lower bucket.
A third bucket is used to carry water. The fourth bucket is used as a pre-filter. This has a piece of tightly-woven cloth that is wired or taped over the top. Since the cloth will be saturated and will drip over the edge the pre-filtering step is best done outdoors, or in a large laundry sink. If treating river, stream on pond water, be sure to use a pre-filter. Just using a couple of thickness of T-shirt material will greatly extend the useful life of your secondary filter element(s).

Use:

Set the bucket with the hole in the lid on a low, stable surface. Stack the bucket with the filter element(s) on top of it. Gently pour pre-filtered water into the upper bucket, until it is nearly full. Note: Be very careful not to spill any water down the exterior of the upper bucket, or you will contaminate the water in the bucket beneath. This is a slow filtering process, so be patient. Even with four filter elements, it will take a considerable time to filter six gallons.