AD has several advantages such as waste reduction in combination with energy production, however, certain hurdles exist. One such problem is ammonia inhibition which can occur during digestion of animal manure or highly proteinaceous wastes e.g. from slaughterhouses (Chen et al., 2008 and Serna-Maza et al., 2014). In aqueous solution ammonia nitrogen is present in two forms, ammonium ions and free ammonia (FA), depending on pH and temperature. FA is believed to be the main cause of inhibition since it is freely membrane-permeable and may diffuse passively into the cell, causing proton imbalance, and/or potassium deficiency (Gallert et al., 1998). The inhibitory concentrations of total ammonia nitrogen reported are in the range of 0.76–4 g N/l. Only few publications report the corresponding FAN levels but generally it is believed that concentrations above 100 mg/l cause process inhibition (Ortner et al., 2014).