I have worked on a couple of projects where ventilation of primary clarifiers was required.
There are a couple of low cover designs that I am aware of. One consists of an HDPE sheet with suitable supports to cover the clarifier and scraper mechanisms. The other consists of pie shaped fibreglass sections supported by radial guide bars across the clarifer. Both of these are reasonably expensive, but should pay themselves off quickly compared to the size of scrubber required for a building.
In both my projects, the clients required buildings to be constructed over the clarifiers in preference to low level covers. The clients were prepared to trade off the high air extraction rates you describe for ease of maintenance.
The main problems with the low level covers perceived by our clients were:
1. Difficulty of access to the clarifers for observation and regular maintenance; and
2. The need to replace the rotating perimeter drive bridges with fixed bridge centre drives. To my knowledge, the low level covers are only suitable for centre drive clarifiers as the covers interfere with the scraper on a perimeter drive. If your clarifiers are large, I suspect they will be perimeter drive and may require significant modification to allow the covers to be installed.
The 12 changes per hour sounds about right for personnel entry, however the scrubber required for low level covers will be very much smaller. The scrubbed volume would be much less than a building, and you could reduce the extraction rate to 4 changes per hour as nobody will go in on a regular basis.
Another option is to cover the effluent weirs only as I suspect this is where most of the odour will be evolved.
I suggest you do a bit of a search on the internet for suppliers of the clarifier covers in your area.
There is a company here in Australia called Geomembrane Technology that makes the plastic covers and I think some of the mining equipment suppliers (eg.Outokumpu) make the fibreglass covers. One plant I know of that has a number of the fibreglass covers is the Ulu Pandan WWTP in Singapore but I don't know where they got their covers.
Good luck.
Aussie Mike