Product review (5 stars): Additive not needed as often as Venta says
The water treatment addiditve is effective at promoting greater water adhesion to the rotating discs; which in turn produces increased water evaporation and a higher level of humidity in the room where the product is used. The chemicals also supposedly inhibit the formation of calcium deposits.

Initially, I used our Venta Airwasher with no water treatment additive. The instructions' FAQs indicate that this is fine. No calcium deposits were detected while using no water treatment. This is likely due to the fact that we have very low mineral content in our water. However, humidity output was disappointing. After adding one of the small bottles of water treatment that came with the unit there was a great increase in the amount of humidity produced. (Note: it was still less humidity than produced by a traditional steam or agitator style humidifier. But those humidifiers do not clean floating dust particles from the air, which the Venta does quite effectively.)

The water treatment additive chemicals are expensive and after substantial testing, I have concluded that you do not have to keep adding fresh bottles of additive. Here's how I clean our Venta Airwasher while also preserving the chemicals for repeated uses. It's really quite simple: 1. Fill the Venta up as you usually do. 2. Remove the rotating disc assembly. 3. Spray and clean the disc assembly. 4. Pour the water from the lower unit through a coffee filter into another container to capture all of the dirt and dust that has accumulated. 5. Throw the coffee filter away. 6. Wipe out the lower reservoir. 7. Pour the water (that still contains the chemical additives) back into the Venta reservoir. 6. Reinstall the rotating disc assembly. Return the top fan unit and you're ready to go.

This procedure will save you from wastefully throwing away the completely functional water treatment additive every time you clean the Venta; every 14 to 18 days. I recommend only using the additive twice a year after conducting a full citric acid cleaning as specified in the instructions.