Question:
Most of the commercial water filters (Berkefeld, Katadyne, etc) have carbon elements with very small filtering capacities (usually less than 100 gallons per filter).

Question: One can buy activated carbon for about $1.75 per pound. Would it make sense to construct a homemade filter (perhaps out of sand and activated carbon) to pre-filter the water before one runs it though the Berkefeld or Katadyne...to extend the life of those devices?

Answer: Some PVC pipe, some aquarium charcoal, some clean sand, and you've made a water filter.

Just make sure to pack the stuff in so the water has enough contact time.

Mine uses 4 layers of sand and of charcoal in a pipe about 2 feet long. I put a double layer of nylon (panty hose material) at the bottom and one at the top.

1==> That is an interesting question, isn't it? ....I doubt if anyone has ever bothered to get any certified. Tell you what, though, if you go down to your local water treatment plant am sure you will be able to offer a jar full out of their rapid sand filter. That could, I suppose, be called "water grade" sand, but should be just as good.

2==> The more important question, though, might be where you are going to get the food grade panty hose.