There are several methods but two of the basic methods are the wet and dry methods. The term dry cleaning is bit misleading since all methods use some amount of water. And there absolutely no similarity to the dry cleaning method that is used to clean clothing.
1. One of the more popular so-called dry methods used is referred to as dry-chem or encapsulation. The processes goes something like this: There is a cleaning product that is sprayed onto the carpet and is then wiped up with an absorbent pad or cotton bonnet using a rotary type floor machine. (I explain this as, a person soaping up then just wiping off with a towel, no rinsing with water! They will get some of the dirt off but will be left with a residue). This method is actually surface or maintenance method. The down side is it does leave a residue. The benefit is that it does dry in 1- 3 hours.
2. The most popular cleaning method is known as Steam Cleaning or Hot water extraction.
There are basically 2 types:
A: The portable machine that plugs into your household current (one like you would rent a supermarket, not very good because it is very limited in its power and can leave the carpet very wet taking several days for the carpet to dry).
B: A much more preferred method is the powerful truck-mounted hot water extraction unit, which we use. This is a large powerful unit mounted in a truck or van. The process goes something like this: Here again a cleaner is applied to the carpet but it is then rinsed out with very hot soft water (you see once water is heated up to 120 degrees every 15-17 degree temperature rise doubles the waters power to clean, the water itself can become a very aggressive cleaner). The rinsing temperature can be adjusted to between 120 to 240 degrees depending on the type of carpet and soiling conditions that are found. This is the method we use and is the method that is recommended by Shaw Carpet Ind., the largest carpet manufacturer in the world. Scotchgard as well as all the other large manufactures also recommend this, in fact they require it, to honor their warranties. Not to mention the IICRC (Institute for Inspection, Cleaning and Restoration Certification) which we are members of.