I know you do not want to use anything to soapy, since suds are hard to get out of carpets and when the carpet dries the dirt will stick to the dry suds and will be dirtier than before!
You might want to read the directions so you don't damage the machine.
My husband was a professional carpet cleaner and he said make sure the water is very hot, you can boil the water and then add 1 cup of white vinegar if it is a big machine use a little more vinegar. The hotter the water the cleaner the carpet will be.
You can pre treat bad stains, but make sure you do not use something that leaves a sudsy residue. My husband won't even let spot treaters in the house because in the end they attract more dirt! LOL!
Vinegar is both a disinfectant and a deodorizer