Digestate liquor recycle in minimal nutrients-supplemented anaerobic digestion of wheat straw

• A novel BioReactor Simulator was used in anaerobic digestion of wheat straw.
• Methane was generated efficiently from a minimal nutrient supplemented straw.
• Digestate liquor recycle elicited higher methane yield and process stability.
• Inoculating potential of the digestate correlated with process performance.
• Digestate liquor recycle may reduce process water and digestate disposal costs.

Anaerobic digestion (AD) of minimal nutrient-supplemented wheat straw and digestate liquor recycle was evaluated in semi-continuous processes using a novel BioReactor Simulator developed for easy accurate online normalised-gas measurement. Three scenarios (i) no recycle (NR), (ii) recycle of soluble nutrient (RSN), and (iii) recycle of nutrient and microbes (RNM) were investigated in order to evaluate their respective efficiencies. Although mono-digestion of lignocellulosic biomasses are often performed with very long solid retention times (SRT), the present study demonstrated an efficient process operating with a 30-day SRT and an organic loading rate of 4 g VS/L d. The best methane yield was 303 mLCH4/g VS achieved in the RSN process showing a 21% improvement as compared to the NR process. The methanogenic potential of the digestates from the RSN and RNM processes was comparable to fresh inoculum indicating efficient processes. The RNM and RSN processes showed superior process stability evidenced by minimal volatile fatty acid accumulation (˂0.5 g/L). As compared to the RNM process, RSN demonstrated the best performance. The improved process performance was probably due to higher nutrient and microbial concentration in the digestate-recycled processes. This study confirms the feasibility of digestate recycle in AD as an appropriate technology for treating nutrient-deficient substrates.