A simple water filter design
 Need to filter 50 gallons of water per day?
Use a 6 foot long section of 4" diameter Schedule 40 PVC (Rigid white plastic pipe) Cover the bottom with wire or something to hold the filter material in place. Fold a piece of fiberglass cloth two or three times and put into the bottom of the pipe. Follow that with a two-foot layer of activated charcoal (either home-made or from a fish aquarium supply store). On top of the charcoal is a two-foot layer of fine sand, then the top-most layer is two foot of pea gravel.
 Need to filter 10,000 gallons per day?
The same method scales up nicely. Instead of a PVC pipe, use 55 gallon food grade plastic barrels of filter medium, where the inlet pipe opens at the bottom of the barrel, and the outlet pipe is near the top of the barrel. Use a separate barrel for pea gravel, sand, and fiberglass cloth.
Filtration (especially of pathogens such as bacteria and viruses) is improved be added with a sufficiently established culture (see Biofilter). Bacterial filtration will help remove some chemicals, like ammonia and phosphates, but will not be able to remove everything.
Note: This is mechanical filtration for removing dirt and particulate matter. It will remove some of the bacteria, minerals, and heavy metals, as all of these are almost entirely attached to particles. A disinfection step (e.g. chlorination or solar distillation) is highly recommended to complete, i.e. kill pathogens in the water.
If there is arsenic in the water, this is a different matter, and does require a special filter design.