This is a bioreactor for anaerobically digesting sewage-laced wastewater. In it, anaerobic bacteria produce a mix of methane and carbon dioxide; as much as 90% of the chemical energy in the wastewater can be converted to methane, which is typically exhausted continuously and collected for use as a fuel or for a reagent for other industrial chemical reactions.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.