I live in Norway, and have well water at my cabin. In winter the water is fresh tasting and clear, while during warm periods in summer it gets more humus and its taste gets worse. I assume there may be harmful microorganisms in it as well as the visible humus. What kind of filter do I need to get rid of both the humus and any microorganisms?
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.
I use a 5-stage osmotic purification for all my drinking water. It's awesome. Absolutely pure water. You can either get a trickle system which slowly fills up a container, or an expansion tank which stores a lightly pressurized reservoir. My tank lets me fill up four(4) 1-liter glass bottles in about 3-4 minutes. I put them in the refrigerator. Since I store the water in glass bottles there is no plastic (=chemicals/odors) involved. Osmotically purified water can be used in most situations where you need distilled water.
For tap water (washing) you can use an ordinary carbon filter. Use a large one so you get good water pressure through it. Carbon filters don't sanitize or purify water, they just get rid of large particles.
This APEC system is what I use: