What kind of filter do I need for well water?
Humus, or tannin, describes a quite large range of compounds. Unfortunately, this means it can be difficult to treat - any given treatment method may or may not work well for your particular water.

On the cheap end, activated carbon filters can sometimes be sufficiently effective at removing tannins.

The most effective methods are reverse osmosis and anion exchange resins (similar to water softeners), although these methods are also fairly expensive and high maintenance.

For your concerns about microorganisms, the most practical, effective treatment is chlorination. Chlorination can also break down tannins, if given sufficient time and adequate concentrations. After treating the water, the remaining chlorine can be removed with an activated charcoal filter.