Unlocking higher methane yields and digestate nitrogen availability in soil through thermal treatment of feedstocks in a two-step anaerobic digestion

Abstract Background There is an increasing interest in using lignocellulosic feedstocks for biogas production. Treatment of these feedstocks prior to anaerobic digestion (AD) can enhance their accessibility to microorganisms involved in the process. To improve the digestion of recalcitrant feedstocks and boost biogas yields, many biogas plants now employ two-step AD systems, extending substrate residence times. However, the combined effect of feedstock treatment and two-step AD on methane yield and fertiliser value of digestates are underexplored. This study, therefore, evaluated the effectiveness of thermal treatment (TT) of pre-digested agricultural feedstocks before a secondary AD step on the carbon (C) and nitrogen (N) dynamics of digestates following application to soil. It also investigated the effects of TT on methane yields. Pre-digested feedstock (PDF) was treated at three different temperatures (70 °C, 120 °C and 180 °C) for 60 min, followed by parallel secondary AD steps using lab-scale continuous stirred-tank reactors (CSTR) and a batch test. Thermally treated feedstocks with and without a secondary AD step were applied to soil to study C and N dynamics and turnover for 2 months. Results TT at 180 °C increased ultimate CH 4 yields by 7.2%; however, it decreased the net mineral N release in soil from 42 to 34% (of N input). Adding a secondary AD step increased the net mineral N release in soil from an average of 39% to 47% (of N input), with the effect of TT levelling off. Moreover, the secondary AD step significantly reduced C mineralisation rates from an average of 37% to 26% (of C applied). Conclusions Overall, TT at 120–180 °C can improve biogas yields of recalcitrant feedstocks, but it may lead to the formation of refractory nitrogen compounds resistant to further degradation during AD, potentially resulting in a lower N fertiliser value of digestates. Graphical Abstract