When I was in Chad many years ago, we made a practical water prefilter out of a fatigue pant leg. I haven't tried it with the newer BDU's, but there's no reason why it won't work.

Tie off the bottom of the leg with string, then fill the leg with alternating layers of gravel, hay, straw, and cotton cloth or batting. Poor water into the top, and collect the cleaner water in a container at the bottom.

You will still have to do something about the bugs, of course. But, if you're using something like the Big Berkey, it will filter out the bigger stuff and extend the life of your filters.

Onward and upward,
airforce