Keyword searching (generally) works best for finding articles in Web of Science.
If you get too many results, try limiting to a controlled vocabulary term - there is a facet on the left side of the screen.
The same tricks that help you find books in the catalog will help you find articles in Engineering Village:
(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"