Underground engineering

Today I drove to Bekkelaget waste water treatment works in Oslo to collect the effluent from an anaerobic digester. This contains the microbes that I'll need to produce methane from the paper mill waste water. I was a bit sceptical as I had found it on google satellite, however I could not see any of the pools and tanks associated with waste water treatment. When I arrived, I was shown to the anaerobic digester, and to my amazement, this was deep underground! Almost the entire plant was underground, the only exception was the methane upgrading and storage tanks... presumably because of the explosion risk. Blip is looking across the many sand filters