The use of water in professional carpet cleaning is well tested and proven to be largely efficacious, but this substance is very rarely used in liquid form at the end of the day. At the end of the day, most carpet cleaners will heat the water up so that it turns into steam before they pump it into your carpet fibers, since the combination of high temperature and moisture will serve you well in terms of filth removal and mold remediation.
However, suffice it to say that unless you are using distilled pure water for carpet steam cleaning, chances are that you might see some white residue on your rug. If you think that hiring a professional might stop this from happening, you should know that this isn’t true at all due to the reason that the water still might have some sediment or minerals in it. The white powder that you see is basically the minerals that are left over after the water has dried, and they usually shouldn’t be prevalent enough to be visible so if you see them lying on your rug this suggests that your water is a bit too hard and low in quality.
Thankfully, it’s really not hard to remove white powder from your Persian rug after professional cleaners have their way with it. All you really need to do is vacuum it up, since it is basically just loose powder that will not require anything else whatsoever. You can also use a stiff bristled brush to make some headway here, although this might require a lot more physical exertion so using a vacuum cleaner is usually more convenient.