<P><B>Abstract</B></P> <P>Water extraction of raw chicken manure elevated the carbon-to-nitrogen ratio 2.7-fold, i.e. from 7.48 to 19.81. The treated chicken manure (T-CM) thus became suitable for biogas fermentation as monosubstrate. Improved methane production was achieved in co-fermentations with either maize silage (24% more methane) or corn stover (19% more methane) relative to T-CM monosubstrate. The standardized biogas potential assay indicated that the methane yields varied with the organic loading rate between 160 and 250 mL CH<SUB>4</SUB>/g organic total solid (oTS). Co-fermentation with maize silage was sustainable in continuous anaerobic digestion for at least 4 months.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Water extraction of raw poultry manure removed a substantial portion of toxic nitrogen compounds. </LI> <LI> The treated solid fraction became amenable for AD as monosubstrate without acclimation. </LI> <LI> Co-digestion with either maize silage or corn stover facilitated AD. </LI> <LI> Sustainable biological methane production was achieved in continuous co-fermentation. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>