There are problems concerning dry AD since it is not yet a mature technology unlike wet
AD (Abbasi-Guendouz, 2012). An important problem according to Karthikeyan &
Visvanathan (2013) is that better biogas yields with dry fermentation processes require
more inoculum, especially for batch reactors. Difficulties in mixing the substrate and
microbes and removing of the produced gas from solids create problems in dry AD
processes (Luostarinen et al. 2011a). Also too dry conditions lower process efficiency.
Abbassi-Guendouz (2012) noticed that methane production decreased as solids content
rose. There seems to be inhibition at over 35 % TS which could be due to higher
concentrations of inhibiting agents (Abbasi-Guendouz 2012) but too low water content
also slows down cell growth (Deublein & Steinhauser 2011). Abbassi-Guendouz (2012)
also noticed that methanogens were less abundant in dry reactors compared to wet reactors.
There seems to be a threshold of 30 % TS for efficient and stable methane production
(Abbassi-Guendouz (2012). Also mixing may be difficult and degassing and even supply
of nutrients is worse in a badly mixed reactor whereas local accumulation of inhibitors is
possible (Deublein & Steinhauser 2011).