We find in Matthew 3:11 that the verb, βαπτιζω (3/77) is used twice. But there are two different uses one is first person and the other is third person.  So we have another person who will be baptizing.  

Who is baptizing? Well John identifies himself as one who was baptizing with water, but John also identifies that there is someone coming after him who will baptize with the Holy Spirit and Fire. 

What is the difference between these baptisms? Well John's baptism is one that is done out of repentance, but the baptism of this new person involves the Holy Spirit, 3rd Person of the Trinity, and fire.

When is this new baptism going to take place?  We are not yet sure, it is pointed to in the future. 

Where will this person be baptizing?  Not yet specified.

Why will this new baptism be done? Well in v12 John connects it with the judgment and the fire that John has warned about so far.  That this will also involve his judgment against the world. So even though John's baptism is for repentance the Baptism of John has yet to see its fulfillment.  

How will we know this new person? We will have to wait and see.  

Now the new Baptism that comes if it involves the Holy Spirit, 3rd Person of the Trinity, then we cannot say that it is the work of man. It has to be the work of the God.