Co‐digestion of Grass Silage and Cow Manure in a CSTR by Re‐circulation of Alkali Treated Solids of the Digestate

Abstract

Three laboratory, continuously stirred tank reactors (CSTRs) co-digesting grass silage and cow manure (forming 30% and 70% of substrate volatile solids (VS), respectively) were operated to evaluate the effects of re-circulating an alkali-treated and untreated solid fraction of the digestate back to the reactors. The CSTRs were operated at an organic loading rate (OLR) of 2 kg VS m(-3) day(-1) and hydraulic retention time (HRT) of 20 days with a semi-continuous mode of feeding. The feasibility of co-digestion with substrate VS containing 30% VS of crop was reinforced, resulting in average specific methane yield of about 180-185 1 CH4 kg(-1) VS. Re-circulation of the solid fraction of digestate back to the reactors in both alkali-treated and untreated forms decreased the methane yield by 11% and 21%, respectively, and resulted in operational problems such as scum formation and accumulation of the reactor materials. Batch studies were conducted to evaluate (i) the methane potentials of the solid fraction of digestate, and whole digestate with alkali treatments ranging from 20-60 g NaOH kg(-1) VS of substrate, and (ii) methane potentials of the accumulated reactor materials as top, middle and bottom layers. The solid fraction of digestate treated with 20 g NaOH kg(-1) VS showed higher specific methane yield (340 l CH4 kg(-1) VS) than the higher range of alkali treatments. The bottom layers of the control reactor and the reactor fed with alkali-treated solids gave a higher specific methane yield (93 and 85 l CH4 kg(-1) VS, respectively), and all three layers of untreated solids gave similar methane potentials.

New opportunities for agricultural digestate valorization: current situation and perspectives

“…After ten minutes of ball-milling, a higher methane potential (321 NmL CH 4 kg -1 VS) than that of raw solid digestate (117 NmL CH 4 kg -1 VS), was obtained 72 . Mild thermal post-treatment (80 °C, 1-3 h) did not affect the methane potential of digestate 36,79 whereas thermal posttreatment at higher temperature (120 °C, 30 min) had contrasted effect in enhancing the methane potential of only two digestates out of the three investigated 80 82 . In this case, the control reactor was fed with a mixture of grass silage and cow manure (0.43:1 w/w, % VS) at an OLR of 2 kg VS m -3 day -1 and an HRT of 20 days.…”

Comparison of various post-treatments for recovering methane from agricultural digestate

“…For instance, Jagadabhi et al [37] found a low decrease of methane potentials after the application of alkaline post-treatment on whole digestate. Other authors reported a decrease of methane potentials of SS-DIG after thermal posttreatment performed at 80°C for 3 h. According to Kaparaju et al [23], this decrease was probably attributed to changes in the chemical composition of the solubilized compounds (SCOD, nitrogen).…”

Optimisation of digester performance with increasing organic loading rate for mono- and co-digestion of grass silage and dairy slurry

“…Another study carried out by Jagadabhi et al (2008) looked at the co-digestion of grass silage and cow manure at 30% and 70% of the substrates' VS content with an emphasis on the recirculation of alkali-treated and untreated solid fractions of digestate to the reactors. Once more, a 30% share of grass (VS basis) was recommended in co-digestion, achieving methane yields in the range of 180-185 L CH 4 kg À1 VS, with recirculation of solids offering no enhancement to the process.…”