I have several times referred to my DIY gravity water filter. It's lighter than any pump filter and most gravity setups, and no pumping is needed. I finally located my parts list, so I can furnish the how-to information. I'll add pictures of my setup later, but in the meantime, here's a link to pictures of a similar setup: There are several DIY gravity filters described in this BPL thread.
The one I made is shown most of the way down on page 2, the post by Cola Vaughan showing the bright orange silnylon bag. Parts
Katadyn Hiker Pro replacement filter cartridge
Bag to hold the dirty water, Sea-to-Summit Ultra-Sil dry bag, 8 liter. I don't fill it full, no more than 4-5 liters at a time. I hang it by the loop formed by the dry bag closure.
From US Plastics (www.usplastic.com
#54033 Silicon tubing 1/4"ID x 38" OD (4 feet)
#62169 Female adapter nylon fitting 1/4" x 1/4"
#62174 Male adapter nylon fitting 1/4" x 1/4"
#50200 Dura-Clamp tubing clamp fits up to 1/2" OD
2 flat rubber washers with 3/8" or 1/2" hole, from hardware store, Lowe's or Home Depot
4 feet of cord to tie dry bag closure loop to a tree should you be in a heavily used campsite with large old growth trees and no lower branches or even stubs of branches. I've had to do this only a few times in the last 5 years, but the cord was desperately needed at those times!
First, trim most of the unneeded plastic housing off the Katadyn Hiker Pro filter cartridge, being sure to leave the nozzle intact. This will lighten the filter weight by about an ounce. Not having the recommended Dremel tool, I used an old kitchen knife, heated on the stove burner, which worked better than a hacksaw. Before trimming, I put a short piece of plastic hose over the nozzle opening of the filter to protect it, and was super careful not to let the hot knife touch that vital part. Don't expect to have a pristine butcher knife afterwards! In the grand scheme of things, this step may not be worth the ounce of weight saved!
I used a piece of metal (actually the round handle end of an all-metal whisk, about 1/2" in diameter) to melt a hole in the bottom of the dry bag, and then coated the area around the hole with McNett Silnet. (What I didn't do, and should have, was to put the hole at one side of the bottom so the filter will lie flat in the bottom of the bag.) With the flat rubber washers on each side of the hole, fasten together the male and female adapters, one on each side of the hole. Cut a short (2 1/2 inches) piece of plastic tubing to connect the filter cartridge nozzle to the adapter nozzle inside the bag. Cut 3 feet of tubing to connect the adapter nozzle on the outside the bag to your clean water container. Thread the long piece of tubing through the clamp so you can start and stop the water flow from the filter. Test the setup in your bathtub or shower stall. A new filter will bleed carbon dust the first time it is used, so let the water run through until it is clear--do this before your first trip.
If you still have a (now discontinued) Platypus filter connector, you can connect it to the other end of your tubing--the Platy top fits soda water bottles, Evernew bottles and, of course, Platy bottles. At present, the only way to get such a connector is to buy a Platy tube kit. Platypus tubing is heavier and much stiffer than the nice flexible tubing from US Plastics, so remove the Platy tubing and just use the bottle top and connector.
The Katadyn Hiker Pro filter cartridge has two advantages: It has a built in prefilter covering wrapped around the filter, which can easily be removed and rinsed, and it contains activated charcoal which helps remove bad tastes. The charcoal isn't enough to remove chemical pollutants in the water, though. If the water is really nasty, you will need an additional prefilter, such as a bandanna or coffee filter.
I've used this setup for the past two years. It works quite fast, taking about 10 minutes to filter 5 liters of water. For mid-day refills, you can easily filter 2 liters during a 5-minute rest/snack period.
The most wonderful thing about this setup is that you no longer need to stand or sit alongside a buggy stream pumping away! You fill the bag, carry it to camp and hang it up, connect it with your water bottle, undo the clamp and then do other chores while the water is filtering. You do have to keep an eye on the process to avoid overflowing or having the water flow stop when air pressure builds up in your clean water container. This setup is wonderful for small groups because you can filter up to 1 1/2 gallons at a time.
The filter can be backflushed by blowing a tube full of filtered water back through the filter. I also blow into the clean end of the system when packing up to remove as much water as possible from the filter. The prefilter wrapped around the Katadyn filter is easily removed, rinsed in clear water and replaced. More detailed instructions for the filter cartridge, including recommended replacement intervals.
Dry weight of my setup is 6.6 oz. (includes the Platy filter connector but not the cord). I need to weigh it wet, since that's how it will be for most of a trip, and will add that information later along with photographs.