To have perfect water, once per year. If you buy the parts, you can change them yourself, which is simple. You can also engage a maintenance service so you won’t have to worry about anything, for 186€ per annum.

• Unpressurised osmosis. The filtered water is deposited in a tank you can access. These are the fastest, most modern and safest units, and are therefore the best quality.
• Pressurised osmosis. The filtered water is deposited in a hermetically sealed tank: this makes them less secure and they also reject the most water (per litre of filtered water).
• Direct osmosis. As these have no tank, the filtered water comes out of the tap directly. They are slower and more expensive to maintain, as the parts do not last long.

Of course not.
Not all membranes filter equally well.
Not all osmosis systems add the same types of minerals, or in the same amounts, into the water.
Not all of them disinfect the water.
Not all have the same safety systems.
Not all of them last as long.
Et cetera, et cetera, et cetera.

Many. But not all of them filter the same particles, nor with the same level of safety, and nor do they provide the water your body needs.

• Active carbon. Effective for eliminating chlorine, which is responsible for unpleasant smells and tastes. Not much more. It’s as simple as it is economical. But if you’re not going to do the cleaning and maintenance properly, it’s better to drink from the tap.
• Reverse osmosis. This includes active carbon, among other things. This is effective for eliminating ABSOLUTELY everything other than H2O from the water. These always include postfilters which add the minerals our body needs back into the filtered water.

A water filter is anything, of any size and any technology, which holds back undesired substances present in the water.