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Depends on how dirty the water is. This type of filtration (physiochemical) is good for removing for removing suspended and dissolved particles, but will not disinfect. The activated carbon will absorb mostly everything and the gravel will keep everything in place and screen large stuff.
Also depends on how fast you want the filter to run. Sand will slow things down. Coarse sand/gravel and activated carbon will make run faster and will work for mostly clear water.
Source: I'm a water/wastewater engineer. I've never built a field water filter but I have process design experience.
