In an extreme emergency the public water supply could be severely compromised. I spoke with a victim and rescue worker who lived through Hurricane Katrina and they both said that one major issue was the lack of water. The water supply was severely compromised by dead animals, chemicals, raw sewage, etc. This type of severely water contamination requires a much higher level of water treatment.
Normal Filters are NOT Sufficient
In these severe cases you will need serious water filtration and, perhaps, multiple water treatments. Most water filters remove contaminants down to 0.1 – 0.3 microns. This level of filtration is fine for normal situations but completely inadequate if the water supply is severely fouled. If the water supply is compromised then use a 0.02 micron water filter. You can start with a more affordable filter and upgrade later. In addition you may want to boil or chemically treat the filtered water just to be extra safe.
If you do not have a 0.02 filter then you must treat the filtered water with chemicals, boiling, or UV. Use your filter to remove the majority of the contaminants down to 0.1 – 0.3 microns. This will remove most harmful contaminants with the exception of ultra-small microorganisms (viruses, etc.). To kill these microorganisms you can boil the water, treat it with chemicals, or using a UV purifier.
Chemicals & Heavy Metals
Most portables water filters and purifiers remove debris and biological contaminants. These filters will NOT remove dissolved contaminants such as salt, heavy metals, pesticides, etc. Carbon purifiers can absorb some of the contaminants but they require sufficient dwell time and the carbon must be replaced when it becomes saturated. Most filters and purifiers will NOT protect you from water contaminated by high chemical concentrations or heavy metals. Avoid water around mining tailing ponds or agricultural operations.
Removing chemicals and heavy metals from drinking water requires a much higher level of water purification. To remove these contaminants you would need a reverse osmosis system or a water distiller. With reverse osmosis pressure is used to try to push the contaminated water through a very small membrane. This membrane is so small that only water molecules pass through and the contaminants are left behind. With a water distiller the water is heated to the point where it evaporates and the water vapor is then cooled and collected as pure water. The water temperature is critical for distillation. If the water is too hot then the contaminants can evaporate with the water and they will NOT be removed. A distiller needs a highly accurate thermostat in order to work.