Keyword searching (generally) works best for finding articles on a topic
If you get too many results, try limiting to a particular subject - usually there is a list of subject links on one side of the screen.
The same tricks to help you find books in the catalog, will help you find articles
(co-digestion or codigestion)
leachate and recirculation
produc* (finds produce, produces, product, production, etc.)
Put it all together:
(co-digestion or codigestion) and produc* and biogas and "methane yield"