I agree with Bob G, a sand filter in a 55-gallon drum.
I recently got a job at a water plant in a small town (avg 4-million gallons a day), and thats what we use. Pea gravel and sand, during the backflush out to the de-silting pond, the heavier gravel naturally stays in the bottom half. We bubble air from the bottom up to ensure that the impacted sand is completely broken up before introducing water flow. We start water flow from the bottom up, shut off air, and increase water flow to the point that the sand is tumbling as high as possible, but not so high that it leaves the container with the silt. When the water appears clear, we stop.
Normal flow is (of course) from the top down.