A method and an apparatus for processing organic material, the method employing at least two reactors and including the steps of combining carbon dioxide or carbon-dioxide containing gas and ammonia or ammonia-containing material in a first reactor to form a buffer compound/buffer compounds, then feeding the buffer compound or buffer compounds formed in the first reactor into a second reactor, and performing bioconversion on organic material in the second reactor. In that case, the carbon dioxide of the mixed carbon dioxide gas reacts with the ammonia, forming a buffer compound, such as ammonium bicarbonate and/or ammonium carbonate.