There are two very important things that Marlow witnesses in the Outer Station to represent European waste. The first is the uselessness of the work being performed by slave labor:
A heavy and dull detonation shook the ground, a puff of smoke came out of the cliff, and that was all. No change appeared on the face of the rock. They were building a railway. The cliff was not in the way or anything; but this objectless blasting was all the work going on.
Instead of focusing on the actual building of the railroad, the workforce is trying to level a mountain for no discernible purpose. Marlow can't see any reason for the mountain to be destroyed, but that is what they are doing, and so they are wasting manpower and supplies to no useful end goal. This is also represented in the useless hole that Marlow sees, dug for no obvious reason except to keep the men working.
When Marlow arrives at the Station, he meets the Accountant, who is described as follows:
I saw a high starched collar, white cuffs, a light alpaca jacket, snowy trousers, a clean necktie, and varnished boots. No hat. Hair parted, brushed, oiled, under a green-lined parasol held in a big white hand.
(Conrad, Heart of Darkness, eNotes eText)
To keep himself sane, the Accountant spends a lot of time on his personal appearance, trying to be an example of European civilization in the wilderness. Although he is a competent man, his affectations are of no larger purpose, and he even spends a great deal of time teaching an uncooperative native woman to help. While these personal hygienic rituals are useful in keeping the Accountant focused on his job, they don't help anyone else, and have no other purpose. However, Marlow is impressed, because he sees the Accountant's determination to remain civilized.
When Marlow arrives at the Outer Station, he sees complete chaos. There is rusted and overturned machinary. People are blowing up the side of a cave for no obvious reason. Natives are put to work digging holes without any purpose. This is his first view of the European influence on Africa.
At the inner station, Marlow needs rivets to fix his boat that has been sunk. Whenever someone comes in with supplies, they never have any rivets, yet Marlow saw tons of rivets at the Outer Station. Additionally, when there is a fire, Marlow witnesses someone trying to put out that fire by loading a bucket with water. The only problem is the bucket is full of holes, so how useful is that? The brickmaker doesn't make any bricks at all.
Everything Marlow views seems completely unneccesary and absurd. This is the picture of the European enterprise. It is inefficiant and pointless.