If there's a need in soaking up the excessive liquidity they have a range of other instruments, namely repos and manipulating rates on excess reserves.
When US treasury bonds mature the treasury must issue more bonds to repay the bonds that are maturing. If the Fed does not buy some of those bonds then the effect would be the same as if it had sold the bonds in the open market.
Are you saying that 80% of the Treasury bonds sold by the U.S. are bought by the Fed, and if the Fed doesn't buy them, nobody else will? That can't be true!
That is not what I am saying.
What I am saying is that if the treasury sells bonds that are not purchased by the fed then they will need to be purchased by banks. If banks buy these bonds then they will have less money to lend to other banks, corporations, small businesses, people and the like