A good simple alternative is a Doulton gravity-fed portable water filter. They can treat clear and turbid water from lakes, rivers, ponds and mud holes, and are able to filter for microbiological, chemical and heavy metals. However, they are more small scale. I did find an article some time back on how to construct a powerless filtration system for large scale filtration (130 gallons/hour). Check the link below
http://www.breadandbutterscience.com/GFWTS1.pdf
Best of luck, and remember, aim for the brain.