1. IntroductionAnaerobic digestion (AD) is a biological process in which organic fractions are converted to methane and carbon dioxide by different groups of bacteria and archaea. Because of its relatively high-energy density (LCV – 23 MJ/Nm3) compared to lignocellulosic feedstocks and ease of use, biogas has a variety of applications, including cooking, district heating, fuel, electricity generation and energy storage (Swedish Gas Technology Centre, 2012). Biogas has been used in di- verse scales from household levels between 1 and 6 m3 to industrial levels of > 10,000 m3 (Deublein and Steinhauser, 2011). Historically, biogas production was visualized as a byproduct from WWTP (waste- water treatment plant) and only in the last few decades, the market for biogas from solid substrates has grown. The reason for this rapid growth is the recognition of two-way benefits of biogas production, i.e. re- duction in a waste fraction, simultaneously generating energy and va- luable fertilizers.