ANAEROBIC CO-DIGESTION OF THERMO-CHEMICALLY PRETREATED WHEAT STRAW WITH FOOD WASTE AND COW MANURE FOR ENHANCED BIO-METHANE PRODUCTION

Abstract

Agro-waste (wheat straw, WS) is considered the most effective substrate for energy recovery through anaerobic digestion (AD). However, the complex lignocellulosic structure obstructs their biotransformation and is a rate-limiting step of the AD process. Therefore, a comprehensive analysis of the pretreatment technologies, biodegradation (BD), energy balance, model fitting, and pretreatment effects on AD of WS is investigated. The physical pretreatments improve the biodegradation and kinetics of wheat straw but require more energy. In contrast, chemical pretreatment is effective for rapid reaction rates but is uneconomical due to high chemical costs. The chemical pretreatment induces cellulose loss up to 59% and lignin up to 69%, while the bio-degradability is as high as 88.3%. Biological pretreatment is mild and environmentally friendly but requires much time. Thermo-chemical pretreatments are associated with recalcitrant formation, and hence, an integrated bio-refinery approach is suggested to apply circular bio-economy. The cost for the supply of straw and digestate, cost of the agents, revenue from digestate, bio-fuel, heat, and carbon to nitrogen (C/N) ratio are essential factors in AD of lignocellulosic biomass. The FTIR, TGA-DTG, XRD, and SEM technologies can validate the efficiency of the pretreatment method, such as modifications in lignocellulosic functional groups, mass loss, crystal feature, surface topography, and morphology of lignocellulosic biomass. In 1st phase, four batch assays were performed to optimize AD parameters to ensure the synergic effects of co-digestion and find out the best inoculum-to-substrate ratio (ISR), C/N, and total solid (TS) percentage in sequence. The anaerobic co-digestion (AcoD) of three feedstock,WS, food waste (FW), and cattle manure (CM), achieved 20% higher biogas yield (416 mL/gVS added) over mono-digestion and 21% volatile solids (VS) removal. The ISR of 2 leads to the highest biogas yield (431 mL/gVS added) and VS removal (30.3%) over other ISRs (0.5, 1.0, 2.5) studied. The lower ISR (<2) tended to have lower pH due to insufficient anaerobes inside the digester. The C/N 35 (organic carbon/TKN) with ISR 2 yielded 17.4% higher biogas (443.5 mL/gVS added) than mono-digestion and 36.6% VS removal, and was the highest among the C/N ratios studied. The volatile fatty acid (VFA), alkalinity, and pH in the C/N 35 assay were more stable than other C/N assays. In the fourth batch assay, varying TS% (5, 7.5, 10, 12.5) were used with optimized ISR (2) and C/N (35). Higher TS% (10 and 12.5) had some lag phase but later achieved higher biogas production. The 12.5% TS assay achieved an 80% higher biogas yield (679 mL/gVS added) over mono-digestion, i.e., the highest among the TS% studied, with 48% VS removal. In conclusion, the co-digestion of mixed feedstock with ISR 2, C/N 35 (WS:FW:CM; 3:1:1), and TS 12.5% could degrade almost half of the substrate available for biodegradation. Modified Gompertz model (MGM), first-order model (FOM), transference model (TRM), and logistic model (LOM) were used for kinetic study and curve fitting of experimental data. The estimated specific rate constants for the optimized batch assays were 0.08, 0.12, 0.083, and 0.084. The data fit all the models well, with the coefficient of discrimination (R2) ranging from 0.882 to 0.999. Further biodegradation may require pretreatment of the recalcitrant WS. The recalcitrant lignocellulosic fraction in agro-waste obstructs its biotransformation and is a rate-limiting process step. Therefore, the effects of hydrothermal and thermal-alkaline pretreatment on the AcoD of WS were studied in 2nd phase. The hydrothermal pretreatment of WS revealed that 60 min reaction time at different temperatures (100-175 °C) was the best pretreatment time to achieve the highest substrate solubilization among different reaction time applied (30, 60, 90, 120 mins). The hydrothermal pretreatment degraded the hemicellulose predominantly by 53.4%. The furan derivatives, i.e., furfural and 5-hydroxyl-methyl-furfural (5-HMF), were formed during hydrothermal pretreatment of WS at 175 °C. The optimized pretreatment time was employed for thermal-alkali pretreatment at variable temperatures and NaOH doses. Thermal-alkali pretreatment at 125 °C-7% NaOH shows the highest (34%) biogas yield of 662 mL/gVS, followed by 646 mL/gVS biogas yield at 150 °C-1% NaOH assay (31% higher) over control. Although, the 125 °C-7% NaOH assay achieved the highest biogas yield, the 150 °C-1% NaOH assay was found more feasible considering the cost of a 6% higher chemical used in the earlier assay. The thermal-alkali pretreatment was observed to reduce the formation of recalcitrant compounds (HMF, Furfural) and increase the buffering capacity of the slurry over hydrothermal pretreatment. Principal component analysis (PCA) of the various pretreatment and AD operational parameters was carried out to study their in-depth correlation. Moreover, a kinetic study of the experimental data was performed to observe the biodegradation trend and compare it with the MGM and FOM. In 3rd phase, the thermal-acid (100-175 °C, 0.5-2% H2SO4 v/v) pretreatments of WS were performed to assess the acid mediated thermal pretreatment effects on WS solubilization, recalcitrant formation, lignocellulosic composition, and improvement in methane yield and VS removal. The 60-min pretreatment time was considered for thermal-acid pretreatment at varying temperatures of 100-175 °C and acid dosage (0.5, 1, 1.5, 2% H2SO4 v/v). The furfural and 5-HMF were generated at all studied thermal-acid pretreatment conditions owing to hemicellulose solubilization. The AcoD of hydrothermally and thermal-acid pretreated WS was performed with FW and CM in a batch assay. The hydrothermally pretreated WS showed 4-14% higher methane production, while the thermal-acid pretreated WS had 29-44% less methane production than control (untreated WS). High concentrations of furfural, 5-HMF, VFA, and NH4-N were the main parameters that negatively affected the methane production in batch assays that used thermal-acid pretreated WS. The kinetic analysis of the assays revealed that methane production was affected by furfural, 5-HMF, temperature, and acid dosing. Therefore, the calculated values through MGM and LOM deviated from the experimental values. Anaerobic digestion of agro-wastes in batch scale is well studied; however, their study in semi-continuous stirring tank reactor (sCSTR) to reflect the full scale is limited. Moreover, the life cycle assessment (LCA) and techno-economic assessment (TEA) of different scenarios, including biogas upgrading and energy co-generation via combined heat and power technology, are essential to compare their environmental impacts and cost-effectiveness. In 4th phase, AcoD of untreated, hydrothermally (150 °C, 60 min) and thermal-alkaline pretreated WS (150 °C, 1% NaOH, 60 min) with FW and CM (WS:FW:CM; 3:1:1) under different organic loading rates (OLR) of 1.25-3.75 gVS/L/d were studied in sCSTR for 340 days. The highest methane yield (302 mL/gVS/d, 30% more than control), VS removal (53.3%, 34% more than control) and better digester stability were achieved for thermal-alkaline pretreatment at an OLR of 1.67, and 45 days HRT. Subsequently, the 45-day HRT of all three digesters for the two scenarios (biogas upgrading and energy co-generation) were chosen for LCA and TEA. Energy generation scenarios of the three digesters, especially hydrothermally pretreated WS, exhibited excellent environmental performance across global warming potential (GWP) and fossil resource scarcity (FRS) due to the substitution effect achieved by generating energy. Biogas upgrading systems (BioCNG) showed higher capital investment and annual operating costs than energy generation systems (CHP) due to the employment of compressors in the process and operational expenditure (OPEX). However, it may lead to a more favorable economic performance than energy generation systems. In particular, hydrothermally pretreated WS appears to be the most lucrative AD system, with a payback period (PBP) of 5.8 years and an internal return rate (IRR) of 24%. Conversely, the net present value (NPV) values for energy generation systems (CHP) are negative, indicating that these plants are less economically attractive in comparison with biogas upgrading systems (BioCNG). This study comprehensively investigated the optimization of AD processing parameters, pretreatment parameters, OLR, HRT, LCA, and TEA for the AcoD of WS with FW and CM. The result of this study will be helpful for stakeholders and decision-makers. However, AcoD of WS with other feedstock, integrated bio-refinery and circular bio-economy, digestate quality and usage, process up-scaling, and LCA and TEA of pilot-scale studies are the recommendations for future works.