Hydrothermal carbonization (HTC) is a promising technology used for bioenergy conversion from bio-wastes such as sewage sludge, livestock manure, and food waste. To determine the optimum HTC reaction temperature in maximizing the gross energy recovery efficiency of poultry slaughterhouse sludge cake, a pilot-scale HTC reactor was designed and operated under reaction temperatures of 170, 180, 190, 200 and 22 °C. During the HTC reaction, the gross energy recovery efficiency was determined based on the calorific value of the HTC-biochar and ultimate methane potential of the HTC-hydrolysate. The poultry slaughterhouse sludge cake was assessed as a useful source for the bioenergy conversion with a high calorific value of approximately 27.7 MJ/kg. The calorific values of the HTC-biochar increased from 29.6 MJ/kg to 31.3 MJ/kg in accordance with the change in the reaction temperature from 170 °C to 220 °C. The ultimate methane potential of the HTC-hydrolysate was 0.222, 0.242, 0.237, 0.228 and 0.197 Nm³/kg-COD_added for the reaction temperatures of 170, 180, 190, 200 and 220 °C, respectively. The potential energy of feedstock was 4.541 MJ/kg. The total gross energy recovery (GER_total) was 4318 MJ/kg, of which the maximum value in the HTC reaction temperature was attained at 180 °C. Thus, the optimum temperature of the HTC reaction was 180 °C with a maximum GER_total efficiency of 95.1%.

Direct interspecies electron transfer (DIET), which does not involve mediation by electron carriers, is realized by the addition of conductive materials to an anaerobic digester, which then activates syntrophism between acetogenic and methanogenic microorganisms. This study aimed to investigate the effect of the addition of two conductive materials, zero-valent iron (ZVI) and magnetite, on the methane production and microbial consortium via DIET in the anaerobic digestion of food wastewater. The operation of a batch reactor for food wastewater without the addition of the conductive materials yielded a biochemical methane potential (B_u), maximum methane production rate (R_m), and lag phase time (λ) of 0.380 Nm³ kg⁻¹-VS_added, 15.73 mL day⁻¹, and 0.541 days, respectively. Upon the addition of 1.5% ZVI, B_u and R_m increased significantly to 0.434 Nm³ kg⁻¹-VS_added and 19.63 mL day⁻¹, respectively, and λ was shortened to 0.065 days. Simultaneously, Methanomicrobiales increased from 26.60% to 46.90% and Methanosarcinales decreased from 14.20% to 1.50% as the ZVI input increased from 0% to 1.50%. Magnetite, at an input concentration of 1.00%, significantly increased the B_u and R_m to 0.431 Nm³ kg⁻¹-VS_added and 18.44 mL day⁻¹, respectively. However, although magnetite improves the efficiency of methanogenesis via DIET, the effect thereof on the methanogen community remains unclear.

The amount of cattle manure generated accounts for over 40% of the livestock manure in South Korea. Most livestock manure is utilized as a fertilizer and a soil amendment. However, the soil nutrients have exceeded saturation in South Korea. Accordingly, cattle manure, including lignocellulosic biomass, was applied for solid fuel production in this study. The three different types of carbonization process, namely, hydrothermal carbonization, torrefaction, and carbonization (slow pyrolysis), were estimated for a comparison of the hydrochar, torrefied char, and biochar characteristics derived from cattle manure. The processes were performed at temperatures ranging from 190 to 450 °C. The evaluation of the hydrochar, torrefied char, and biochar produced by three processes was conducted by the proximate, ultimate, calorific value, fuel ratio, and energy yield, which were used for the analysis of fuel quality. Additionally, the ash properties, including silicon dioxide, chlorine, and base-to-acid ratio (B/A) on hydrochar, torrefied char, and biochar were investigated to predict ash deposition during combustion. These analyses are essential to stabilize the operation of the combustion chamber. The thermogravimetric analysis represented the upgraded quality of hydrochar, torrefied char, and biochar by three different carbonization processes.

Objective: The purpose of this study was to analyze the effect of the hydrothermal pretreat-ment of anaerobic digestion sludge cake (ADSC) of cattle manure on the solubilization of organic matter and the methane yield to improve the anaerobic digestion efficiency of cattle manure collected from the sawdust pens of cattle.Methods: Anaerobic digestion sludge cake of cattle manure was thermally pretreated at 160°C, 180°C, 200°C, and 220°C by a hydrothermal pressure reactor, and the biochemical methane potential of ADSC hydrolysate was analyzed. Methane yield recovered by the hydrothermal pretreatment of ADCS was estimated based on mass balance.Results: The chemical oxygen demand solubilization degree (CODs) of the hydrothermal hydrolysate increased to 63.56%, 67.13%, 70.07%, and 66.14% at the hydrothermal reaction temperatures of 160°C, 180°C, 200°C, and 220°C, respectively. Considering the volatile solids content obtained after the hydrothermal pretreatment, the methane of 10.2 Nm3/ton-ADSC was recovered from ADSC of 1.0 ton, and methane yields of ADSC hydrolysate increased to 15.6, 18.0, 17.4, and 17.2 Nm3/ton-ADSC.Conclusion: Therefore, the optimal hydrothermal reaction temperature that yielded the maximum methane yield was 180°C based on mass balance, and the methane yield from cattle manure containing sawdust was improved by the hydrothermal pretreatment of ADSC.

This study was conducted in order to examine the impact of magnetite (Fe₃O₄), a conductive material capable of promoting direct interspecies electron transfer (DIET) among microorganisms, on the efficiency of anaerobic digestion in a plug flow reactor (PFR) using food wastewater (FW) as the substrate. The effects of recovering and replenishing magnetite discharged along with the digestate during continuous operation of the PFR were also evaluated. A PFR with a total volume of 17 L was utilized as the reactor for anaerobic digestion. The inoculum was obtained from Icheon Biogas Research Facility, which operated with a mixture of pig slurry and FW in a 7:3 (w/w) ratio. FW was used as the substrate (volatile solids (VS) content of 85,865 mg-VS/L). The PFR was set for operation at 39 °C, and after a stabilization period of approximately 82 days, the hydraulic retention time (HRT) was set at 40 days. The study was conducted in three stages: stage 1 (83~122 days), stage 2 (123~162 days), and stage 3 (163~202 days). For the maintenance of an organic loading rate of 2.12 kg-VS/m³/d, 0.3 L/d of substrate was added every 24 h, and analysis of an equal amount of discharged digestate was performed. The experimental treatments included a control without the addition of magnetite after the stabilization period, treatment (T1) with addition of magnetite (20 mM in digestate) and subsequent recovery and replenishment of magnetite on the discharge of digestate, and treatment (T2) with addition of magnetite (20 mM) without the replenishment of magnetite. Analytical parameters included the characteristics of the discharged digestate (pH, NH₄⁺-N, chemical oxygen demand (COD_Cr), total volatile fatty acids (TVFAs), and alkalinity), and methane production (M_p). During the period of operation of the PFR after the stabilization period, no significant differences in pH and NH₄⁺-N, based on the recovery and replenishment of magnetite, were observed, and a stably functioning PFR was observed. However, in stage 2, due to the increased degradation of organic matter caused by DIET, the COD_Cr of T1 and T2 decreased by 9.42% compared with the control. In stage 3, the magnetite content in the reactor in T2 decreased by a maximum of 9.42% compared to T1. In stage 3, the M_p for T2 was similar to that of the control, with a maximum discharge of magnetite of 3.06%, and the M_p decreased by 5.40% compared to T1. Regarding the ratio of methanogens in the community, the results of an analysis of the digestate from stage 3 showed an increase in the community of acetotrophic methanogens, specifically Methanosarcina. The findings of this study confirm that DIET was effectively promoted by maintaining the concentration of 20 mM magnetite in the PFR while using FW as a substrate.