Your first example is ungrammatical, although in a very minor way which will pass entirely unnoticed in speech or casual writing. The author uses the wrong preposition to complete the comparative construction.
Americans are sending more than twice as much trash to landfills
than as the government estimates.
Than is proper with more, and I suspect the author confused himself by using more and then losing track of his sentence:
Americans are sending more trash to landfills okthan the government estimates.
But twice as much can be used by itself with entire propriety.
The government estimates that Americans send X million tons of trash to landfills every year. In fact, they send twice as much.
The Cambridge Grammar of the English Language calls this a fused-head determiner: the quantifier/determiner twice as much is permitted to “fuse” with its head nominal if the nominal can be identified from the context. You see the same thing with many sorts of determiner:
Those neckties are red.
These neckties These are blue.
Some frogs are green.
Other frogs Others are brown.