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Poultry litter (PL) was pre-treated in order to reduce its nitrogen content and to increase the C/N ratio. The pre-treatment consisted of a first anaerobiosis phase of about 60 days in order to accumulate ammonia nitrogen, followed by an ammonia stripping phase by heating the substrate at 80 °C for 24 h. The digestion was performed with PL and pre-treated PL (TPL) after ammonia stripping as mono-substrate under four total solids loads, i.e. 5%, 10%, 15% and 20%. The TPL after ammonia stripping displayed lower ammonia (62–73%) and VFA (41–65%) concentrations compared to digesters with raw PL, while bio-methane yield increased about 8–124%. Bio-methane yields in the series with TPL after ammonia stripping were about 193, 196, 215 and 147 L CH 4 /kg COD , based on the COD added, for 5%, 10%, 15% and 20% TS load, respectively. The results indicate that lowering nitrogen content using the suggested process improves bio-methane yields significantly. [ABSTRACT FROM AUTHOR]