I do not think you are doing anything wrong! Base on the image provided, it looks like you have a great endgame factory, that has been going full blast, and it will take a long while to reduce the pollution. Some question come to mind. How many scrubbers do you have. What are the pollution settings on the map? How are the scrubbers arrayed? When scrubbers are put into the game, and pollution values are in the thousands, it will take a while for the pollution to be cleared out.
On one of my test maps, which had pollution values in the 50k range in the middle of the map, it took about 8 hours for visual difference. Best way to describe pollution in the game is that it is a wave, and once the wave starts going out, it will continue expanding, even when the there is no pollution being produced. Then after a while, when there is a pollution vacuum around the scrubbers, that wave states to implode, but that can take hours, depending on the how busy the map is. This part functions like a real nuclear explosion in slow motion, with a back-blast.
The technique I used to verify that they were working, was to display the pollution values on. On the default settings on a PC this can be done by pressing the F4 key. This will bring up a menu on the left side if the screen. Then scroll down till you see "show-pollution-values". to the right you will see, a check box and check it. Press F4 to turn that menu system off. Then zone out, in like in the image, and you will see pollution values for each chunk. Once done checking, values, the same procedure can be done in reverse, to turn off pollution values on the map, overplayed on the red pollution cloud.
The game renders pollution levels in a few shades of red, but beyond a certain value, it is all deep red. The numbers for the pollution are displayed as follows, 999.9/-9.99./m The first number shows how much pollution is in that chunk. The second number, usually negative lists what the maps natural absorption of pollution is in that chunk. Since this is an entity, it will not be listed for either pollution production or reduction on that display.
I was doing the same technique for testing that you described. The first map I did it on, had pollution values around 40-50k per chunk. On that map it took a long time to clean it up, like about 8 hours for a visual difference.
A second technique to verify that it is or is not working, is to open up the production statistics window, and click on the pollution tab. The scrubbers should be listed there, and it will display how much pollution your factory is producing and how much is the map is cleaning up, how much pollution biters are absorbing, as well as how much pollution is being scrubbed.
What I found worked well, was to place 4 pollution scrubbers per chunk, with a total of 12-16 pollution scrubbers around each mine or oil field. Within the base I would place 4 per chunk in a line in the middle of the base. Likewise, each outpost would also have a few scrubbers set up, but that depends on the design of the outpost. If I had an abundance of wood, then I would place the have charcoal scrubbers in hot spots, since each one removes 4 times as much.
Bottom line, in order for the pollution cloud to shrink, more pollution needs to be removed off the map then is being produced. Also if scrubbers are operating where there is no pollution, that will not lower the pollution values.
FYI, here is a simple design I use for laying down 4 pollution scrubbers per chunk (less power). I initialize that by placing only 10 (not more, not less) Prepared Water Filters into the assembly machine:
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Hiladdar
