In an emergency, you may have to consider the use of alternate water sources. Assuming you have a functional water filter, you want to extend it’s life for as long as possible, regardless of who manufactures your system, use the cleanest water available. If the water is dirty and contains sediment and plant debris, I would filter as much material out of the water as possible prior to running it through my filter. This will help extend the life of my filtration system. This can be done in many different improvised ways, some as simple as straining it thought a cloth. Homemade sand and rock filters can be used.
If you have access to underground water sources such as wells or springs these will normally be your best choice. When you go to collect water, avoid areas where high levels of salt may be found such as alkali pits or salt water. Cattle ponds and creeks where water runs off heavily fertilized fields should be avoided if possible. These may have high concentrations of nitrates. In the area where I live, you have to be a bit careful around old mines because of mercury contamination. If you can plan ahead, know as much as possible about your water sources ahead of time. It is always nice to know what is upstream.