Homemade Water Filter (step by step) for a School Science Project – A step-by-step process of building a water filtration device using simple and readily available materials. By Amelia Dani source Chapter 4
I would suggest coffee filters or some other fibrous material other than sand, for a couple of reasons, (a) as you point out, sand becomes saturated and lowers output (b) sand contains SALT which can NOT be removed from the filtered water. You can get a whole pack of coffee filters for a dollar or so at any 99 cents store. The charcoal will need to be very tightly packed and also I would not use gravel (stones) with high surface area since they also will tend to catch water. If however such stones are the only type available they should be smoothed as much as possible for use. On the other hand your water source should contain a high number of small smooth stones. Lastly your output container should be glass and not plastic as plastic cannot and should not be reusable.