With gravity water filters water is placed in container above the filter and the filtered water drips into a clean container below the filter. Gravity filtering is slower but requires very little effort. Simply fill the upper container with unfiltered water, wait, and then use the water in the lower container. These filters come in kits that includes the filter, bucket connectors, a spade bit to drill the hole in the bucket, and a syringe to backflush the filter. You must supply a bucket and add the connectors to it (very easy to do).
The Squeeze can be used as either a personal filter or as a gravity filter. It comes in two kits: (1) Personal Kit or (2) All-in-One Kit. The Personal Kit includes the filter, backflush syringe, bladder, and straw. The All-in-One Kit includes the Personal Kit plus the bucket mounting hardware and spade bit. The Squeeze filters down to 0.1 microns which is sufficient for most water. Some microorganisms in extremely tainted water require more than a 0.1 micron filter. When filtering extremely tainted water first filter the water using the Squeeze and then do a chemical or UV
purification. The All-in-One Kit retails for about $50 and should filter about 100,000 gallons.
The Sawyer Point ZeroTWO is my filter of choice. It filters down to 0.02 microns which should filter all bacteria and viruses. This is a larger filter than the Squeeze and cannot be used on water bottles or bladders. It comes with a faucet adapter and can be cleaned by backflushing. Very few water filters remove contaminants down to the 0.02 micron level. This filter kit retails for around $140 and it should filter upward of 1,000,000 gallons.
The LifeStraw Mission is a gravity purifier that uses an advanced ultrafiltration membrane with a 0.02 micron (20nm) pore size to remove virtually all virus, bacteria, and protozoa. It will filter up to 4,750 gallons (18,000 liters) of water with a flow-rate of 9-12 liters/hour.
Click here for more details on 0.02 micron water filters.