Energetic Valorization of Cereal and Exhausted Coffee Wastes Through Anaerobic Co-digestion With Pig Slurry

Abstract

In the past years, there has been steady growth in work relating to improve resource efficiency through waste minimization and bioenergy recovery to mitigate climate change. Agro-food industries produce large amounts of bio-waste, challenging innovative energetic valorization strategies in the framework of circular economy principles. Anaerobic digestion (AD) technology is an interesting route to stabilize organic matter and produce biogas as a renewable energy source. This study involves continuous co-digestion of pig slurry (PS), cereal and exhausted coffee wastes (CECW) performed in a continuously stirred tank reactor, with a hydraulic retention time (HRT) of 16 days under at mesophilic conditions (36.9 ± 0.3°C). The experimental trials, were designed to include different cereal and exhausted coffee liquor (CECL) shares in the feeding mixture, corresponding to different PS to CECL ratios (PS:CECL), respectively: 100:0 (T0), 90:10 (T1), 80:20 (T2), and 70:30 (T3), in terms of percentage of inlet feeding rate (v:v). The results obtained for the feeding rate (70:30) yield to the highest specific methane production (SMP = 341 ml.gVS−1) led to a 3.5-fold improvement in comparison with the reference scenario. The synergetic effect between the microbial consortia of PS and the high carbon to nitrogen ratio (C/N) of CECL explain the improvements achieved. The maximum soluble chemical oxygen demand (SCOD) reduction (84.0%) due to the high content and soluble chemical oxygen demand to total chemical oxygen demand ratio (SCOD/TCOD) corroborate the results achieved. The digester stability, evaluated by specific energetic loading rate, was below the limit (0.4 d−1). Results from ANOVA showed a significant effect of CECL on the resulting GPR and SMP values. Additionally, Tukey's “Honest Significant Difference” method, confirmed statistically significant differences between the trials T3-T0, T3-T1, T3-T2, and T2-T0. Thus, co-digestion of PS and of CECL seems to be a promising approach for bioenergy recovery and promoting biowastes circularity.

Understanding Anaerobic Co-digestion of Organic Wastes through Meta-Analysis

“…Out of 326 studies, 65 were obtained after screening, yielding 432 relevant AcoD data sets for meta-analysis and modeling, grouped according to their mono-digestion control, including the: (1) AM compared to AcoD of ruminant manure AM(RM):LB, − pig manure AM(PM):LB, chicken manure AM(CM):LB, AM:FW, ,− AM:slaughterhouse waste (SL), , AM:desugared molasses (MO), and AM:algae (AL); (2) LB mono-digestion compared to LB:AM(RM), ,,, LB:AM(PM), ,− LB:AM(CM), ,,, LB:FW, , LB:MO, and LB:SS; (3) SS mono-digestion to SS:fats, oils, grease (FOG), − SS:FW, − SS:AL, − SS:LB, , SS:high-carbon liquid substrate (L-C), and SS:glycerin (Gly); and (4) FW mono-digestion compared to FW:AM, ,, FW:LB, ,,− FW:SS, ,,,,, FW:high-nitrogen liquid substrate (L-N); as well as additional data sets ,…”