This particular issue is huge and it would take a series of articles to cover on a preparedness/survival level.
But we happened to stumble on some interesting comparison tables while studying for the Household Water Treatment and Safe Storage online seminar we are following.
They are in pages 52 and 53 of the following document by W.H.O.
We should note that the Log Removal unit is described in the lower part of the second table.
A much easier way to grasp it is that a 1 LRV Unit shows that only 10% remain afte treatment. In turn a 2 LRV unit allows only 1 in a 100 to pass, a 3 LRV reduction allows 1 in a 1,000 to pass, a 4 LRV reduction allows 1 in a 10,000 to pass, etc.
Another useful table is the following. It describes the various methods of water treatment and covers the range of their effectiveness.
Last, and courtesy of the seminar main instructor Rick Johnston, the following table can be proven useful when choosing your filtering method.
For reference outdoor/hiking filters are in the Micro filtration range and at 0.1microns for the Sawyer filter, 0.2 for the Lifestraw filter, 0.2-0.3 for the Katadyn filters and 0.3 for the Aquamira filters.
Also do notice that as filtration gets smaller a larger water pressure is needed, so elevation of the water reservoir is always helpful.