Daniela: My water in drawn at sunset from a spring outside Jerusalem. I usually take two 1.5 liter bottles, but I have never used more than one. About the stuff becoming faster, there is actually a ruckus being raised by somebody in a Jerusalem Torah think tank, that 18 minutes is a mistaken measure, and the time is much longer. Indeed, you can mix flour and water and just watch it, and the classical halachic signs of chametz do not show up for a really really long time. As I understand the chametz process as a bio-chemical process, I don’t think the dust on the floor will speed things up, as it absorbs moisture, and salt definitely hampers yeast activity and it is the native yeasts that are critical to the hametz process.
The bitul concept is similar to the hefker concept. You can renounce ownership of what you do own but not of what you do not own. Similarly, I can declare the dough remnant ownerless and meaningless only while it is still not chametz and thus forbidden and not mine to make such stipulations about. And one can make these stipulations to start and stop as one likes, or in this case, the governing rule is set in place prior to its need, just as long as it is not yet chametz. In my case of a micro op, my cleanup is done, averagely at the 7 minute mark, at 11 minuteas I can be preparing the next batch. Also, what falls to the floor, provided it is tiny, the more the dirt, the more it is actually self batel. That the ants might eat what I miss sweeping is not a concern. And the mevutal stuff that we pick up and dispose of , is just because of the virtue of, and need for cleanliness.