There are multiple levels of filtration. As long as the water has been purified properly, filtration at this point would mostly be to make the water more attractive. Since most of us are not used to, drinking water with, leaves, algae, dirt, etcetera. So, at least a minimal amount of filtration is recommended. Since, while you can ingest/digest the aforementioned, most of us would prefer not to.
My first path was to go with a Pelican combination whole house filter and salt free water softener. My plumber and others said whole house filter was the way to go for pure water and clear ice, not so much. Even Pelican suggested RO was not needed with the whole house filter. While I’m pleased with the Pelican system, the water is indeed soft and clean throughout my home, my faucets don’t ... full review
Assuming you can get a fire going, and have a metal container. After filtering as many of the particulates as possible. Fill your container with water, place over the fire, bring to a rapid boil, then allow to cool (drinking hot water can induce vomiting). Boiling will kill the harmful bacteria in the water, as they cannot withstand the temperature.
In addition to desalination, reverse osmosis is a more economical operation for concentrating food liquids (such as fruit juices) than conventional heat-treatment processes. Research has been done on concentration of orange juice and tomato juice. Its advantages include a lower operating cost and the ability to avoid heat-treatment processes, which makes it suitable for heat-sensitive substances such as the protein and enzymes found in most food products.
Upland lakes and reservoirs: Typically located in the headwaters of river systems, upland reservoirs are usually sited above any human habitation and may be surrounded by a protective zone to restrict the opportunities for contamination. Bacteria and pathogen levels are usually low, but some bacteria, protozoa or algae will be present. Where uplands are forested or peaty, humic acids can colour the water. Many upland sources have low pH which require adjustment.

The membranes used for reverse osmosis have a dense layer in the polymer matrix—either the skin of an asymmetric membrane or an interfacially polymerized layer within a thin-film-composite membrane—where the separation occurs. In most cases, the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high-concentration side of the membrane, usually 2–17 bar (30–250 psi) for fresh and brackish water, and 40–82 bar (600–1200 psi) for seawater, which has around 27 bar (390 psi)[8] natural osmotic pressure that must be overcome. This process is best known for its use in desalination (removing the salt and other minerals from sea water to produce fresh water), but since the early 1970s, it has also been used to purify fresh water for medical, industrial and domestic applications.

Accidental shootings are an obvious hazard of hunting, but guess what's just as bad: trees. "A tree stand hung 20 feet in the air should be treated like a loaded gun, because it is every bit as dangerous," says Marilyn Bentz, executive director of the National Bow hunter Educational Foundation. Most tree-stand accidents occur while a hunter is climbing, she says.
A reverse osmosis system is typically installed under the sink, but you can install it where your water enters the house, so all your water is filtered for contaminants. RO filter cartridges provide the most effective filtration of any water purifiers. The membrane and filters remove up to 99 percent of contaminants such as arsenic, lead, ammonia and chlorine, as well as toxic fluoride, sodium, nitrates and heavy metals. The 6 stage RO filters provide a deep filtering process, leaving you reverse osmosis water, free of sediments and toxins. RO water is perfect for drinking, cooking and making ice.
Chlorine is effective against bacteria and most viruses. Norovirus, an intestinal disease that causes diarrhea, is particularly resistant to chlorine and will require the water to sit twice as long instead of the standard 30 minutes before consumption. Giardia, a parasite with a protective coating, will survive in chlorine treated water for 45 minutes before its safe to drink.
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The other half of the tag team is to eliminate pollutants. The best way to do this is with a homemade carbon filter. This uses the same technology as Brita filters. Carbon is a chemically active substance, with a tendency to bind to most anything. At a microscopic level, charcoal is a heavily pitted and striated material, which vastly increases its real surface area. The result is that when water slowly runs over charcoal, pollutants find themselves glued to the charcoal surface. An improvised filter can be made out of ground-up charcoal, a strainer and a funnel. Bear Gryllis made a purification drinking straw out of little more than a reed and some charcoal bits for the Discovery Channel's "Man vs. Wild." It's a simple technique, but it is highly effective.
Bromine and iodine can also be used as disinfectants. However, chlorine in water is over three times more effective as a disinfectant against Escherichia coli than an equivalent concentration of bromine, and over six times more effective than an equivalent concentration of iodine.[16] Iodine is commonly used for portable water purification, and bromine is common as a swimming pool disinfectant.
That brings us to filtration by manufactured filters. These devices allow us to go into microfiltration and ultrafiltration. By simply running the water through these porous ceramic filters we can effectively remove bacteria and viruses depending on the quality of the filter and the pore size. See chart above. This is where high quality filters such as the Katadyn Combi Filter can filter down to 0.2-micron level capturing Giardia, Crypto, bacteria and most viruses. Some filters are chemically impregnated to ensure complete removal of bacteria. The information below will give more detail.
Boiling: Bringing water to its boiling point (about 100 °C or 212 F at sea level), is the oldest and most effective way since it eliminates most microbes causing intestine related diseases,[21] but it cannot remove chemical toxins or impurities.[22] For human health, complete sterilization of water is not required, since the heat resistant microbes are not intestine affecting.[21] The traditional advice of boiling water for ten minutes is mainly for additional safety, since microbes start getting eliminated at temperatures greater than 60 °C (140 °F). Though the boiling point decreases with increasing altitude, it is not enough to affect the disinfecting process.[21][23] In areas where the water is "hard" (that is, containing significant dissolved calcium salts), boiling decomposes the bicarbonate ions, resulting in partial precipitation as calcium carbonate. This is the "fur" that builds up on kettle elements, etc., in hard water areas. With the exception of calcium, boiling does not remove solutes of higher boiling point than water and in fact increases their concentration (due to some water being lost as vapour). Boiling does not leave a residual disinfectant in the water. Therefore, water that is boiled and then stored for any length of time may acquire new pathogens.
The addition of inorganic coagulants such as aluminum sulfate (or alum) or iron (III) salts such as iron(III) chloride cause several simultaneous chemical and physical interactions on and among the particles. Within seconds, negative charges on the particles are neutralized by inorganic coagulants. Also within seconds, metal hydroxide precipitates of the iron and aluminium ions begin to form. These precipitates combine into larger particles under natural processes such as Brownian motion and through induced mixing which is sometimes referred to as flocculation. Amorphous metal hydroxides are known as "floc". Large, amorphous aluminum and iron (III) hydroxides adsorb and enmesh particles in suspension and facilitate the removal of particles by subsequent processes of sedimentation and filtration.[6]:8.2–8.3
Most reverse osmosis systems require you to do a bit of under-sink installation and drill a hole for a separate dispenser, but you can also opt for a countertop model that saves your cabinet space and won’t require any drilling. The APEC Portable Countertop Reverse Osmosis Water Filter System can be set up quickly and easily with no permanent installation necessary.
Use a commercial water filter. A commercial water filter is the easiest and most effective way to filter sediment, pathogens, metals, and other pollutants from water. These filters contain special materials like charcoal, carbon, ceramic, sand, and cloth that are specially designed to filter out dangerous pollutants.[7] There are many different types of filters you can use, including:

As with any other filter type water purification method, careful attention has to be taken to pathogen/virus and chemicals size. During hurricane Katrina a lot of the water was contaminated with petroleum based chemicals from flooded cars. What is removed from the water is dependent on the filter pore size. However, it is difficult to beat the lightweight option that water purification straws and bottles provide for most situations.