If you were to be asked to filter a sample of water from a river in a rural area, will your filtered water be 100% safe to drink?
If not, how would you go about ensuring that the filtered water is safe for human consumption?
No, your filtered water will not be 100% safe to drink. I doubt it anyway, but without knowing exactly what kind of filter you are using, I'm sticking with "no." Even if you were using a high end backpacking water filter, their filters fall between .2 and .02 microns. That's small, but there are a lot of little disease causing agents out there that are smaller. Those kinds of filters are good to protect you from most bacteria and protozoa, but not viruses. Some filters make use of UV light in order to further purify the water, but even the manufacturer says that it kills up to 99.99% of bacteria, protozoa, and viruses. In all likelihood, the water is perfectly safe to drink. I've used those filters on hiking trips, and I was fine. But your question says "100% safe to drink." A filter simply won't get you to 100% safe.
Like I said, the water is more than likely safe for human consumption with the water filter. But if you don't trust it, you could use chemical agents that further purify the water and are okay to drink. Iodine tablets, chlorine bleach, chlorine dioxide tablets, and/or calcium hypochlorate. The downside to using these is that you have to be careful to not overdo it and make yourself sick from the chemicals. Plus, they mess with the taste.
The last two methods of purification involve heat. You could distill the water, which makes incredibly clean water, but is energy intensive. The other is to simply boil the water for about 5 minutes. That's probably your best bet if energy for a heat source is not a problem.
Visual observations are not always reliable assessments of the content of substances. Therefore, you can do a number of short experiments to ensure safe water.
First, take some of the water purification pills and drop it in a container filled with the water. Take pH paper and dip it into the water sample. Read the level acidity in the sample. Remember, on the pH Scale water is normally 7. If the water falls below or above that level, there are some impurities.
To further purify the water, take the desired amount of water and bring the water to a boil. The impurities will most likely dissipate. Once again, take a water tablet or pH paper to gain a reading on the pH of the water. In addition, many science stores have water contamination measuring kits as well. They may be expensive, but it will give you a better level of confidence that the water is safe.