Ash-extracts from plant residues can provide sufficient buffering alkalinity and trace elements required to prevent operation instability to guarantee optimum methane yield during anaerobic digestion of agricultural residues

Abstract

In this study, continuously stirred tank reactors (CSTR) operated at mesophilic temperatures were used to study the effectiveness of low-cost plant ash-extracts from plant residues in providing sufficient buffering alkalinity and trace elements required to prevent operational instability during an AD process to optimize methane productivity. The mean specific methane production (SMP) obtained from the unsupplemented CSTR was 79.3 N mL CH4/gVS fed, while supplemented reactors were 258.1 and 297.3 N mL CH4/gVS fed, respectively, corresponding to methane contents of 24.6 %, 56.7 % and 57.0 %, respectively. The unsupplemented CSTR failed when VFA accumulation occurred with acetic acid ranging between 2262–8917 mg/L, propionate-to-acetate (P: A) ratio 1:2.1–1:3.2 and a FOS: TAC ratio 0.2–1.4. Supplementation of the failed CSTR with ash-extracts increased the buffering potential of the reactors by decreasing VFA concentration leading to positive signs of recovery and a gradual increase in biogas production (p < 0.005). The soluble trace elements content in the ash-extracts and strong buffering alkalinity stabilized AD processes, improved SMP and facilitated the recovery of the failing CSTR. Thus, ash-extract supplements can increase the profitability of the AD process. The production and sale of ash can also support economic activities by creating a new business opportunity as the pathway to impact.