The small filter in the first is a sanitization filter that can be discarded. They are single use.
You can operate the system, on tap water, but it is always better to pretreat it in order to get much large capacity out of the filters. For instance, pretreating the water with reverse osmosis can extend the life of the filters 15X which in turn reduces your cost by as much. Where you may get 100 gallons on tap water feed, you could expect as much as 1500 gallons of reverse osmosis feed. By adding a small deionizer in the Reverse osmosis system or by feeding the system with distilled water, you can get 10's of thousands of gallons out of the same filter set.
Here are some economical Reverse Osmosis systems you can look at :
Great for Lab Water Pretreatment
or Great Drinking Water
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