Mining organic wastes by integrating hydrothermal carbonization and anaerobic digestion

The increasing solid waste by-products pose a great environmental risk. Wet organic wastes are the most problematic among the diverse solid waste types. Although they are generally putrescible, they often produce odorous gas emission, pathogen infection, and leachate discharge, among other problems during storage, transportation and disposal. Additionally, their high-value utilization and safe disposal are often limited by high moisture content. Due to the negative environmental impacts of solid wastes, several treatment strategies have been proposed. These strategies include biological processes such as anaerobic digestion and composting. Although they are promising approaches for treating wet organic wastes, composting products are always contaminated with organic pollutants and heavy metals. Likewise, the low methane yield during anaerobic digestion demands further treatment of the digestive, which is costly. Therefore, developing advanced and economically feasible high moisture content organic waste treatment methods is highly desirable.

Hydrothermal carbonization (HTC) is considered the most appropriate thermochemical process for the effective treatment of wet organic wastes. It involves passing biomass through a series of chemical reactions like dehydration, hydrolysis and condensation polymerization at different temperatures. At lower temperatures (<180 °C), HTC is normally utilized as a pretreatment method to enhance the biomass biodegradability for subsequent biological processes. At higher temperatures (180 – 350 °C), HTC operations involve more severe reactions to facilitate the conversion of high-water content biomass to coal-like hydrochar comparable to low-grade coal. Based on the biorefinery concept, using the aqueous product from HTC is instrumental in enhancing its technical and economic feasibility because the aqueous product contains high organics content that could serve as feedstock for HTC based biological processes. However, the anaerobic digestion of high-temperature HTC products remains largely underexplored.

Herein, Kongyun Zhu, Qiutong Liu, Chao Dang, Professor Aimin Li, and led by Professor Lei Zhang from Dalian University of Technology integrated HTC and anaerobically digested sludge as the feedstock for biorefinery-oriented full utilization of wet organic wastes – an emerging technology with promising capabilities. The process started by hydrothermally carbonizing the anaerobically digested sludge at different temperatures and characterizing the resulting products. Using mixed HTC products, several anaerobic digestion experiments were performed. The authors mainly focused on identifying the inhibitory substances, evaluating the biopotential of the aqueous and mixed products and finally exploring the available different process intensification strategies. The work is currently published in the journal, Bioresource Technology.

The research team findings showed that high HTC temperatures of 240 °C resulted in the accumulation of high Short-chain fatty acid (SCFAs) and a low yield of methane of about 60 ± 5 mL/g COD. Based on the GC-MS analysis, it was observed that the inhibitory contents pyrazines, ketones and pyridines in the aqueous solution increased from 13.14%, 0.55% and 0.4% at 180 °C to 23.34%, 5.13% and 2.89% at 240 °C, respectively. When the aqueous products obtained at high-temperature HTC (240 °C) were pretreated with carbonaceous materials, a significant improvement in the methane yield was reported. This suggested that the addition of carbonaceous materials as supplements plays a critical role in removing the inhibitory effects, thus improving the anaerobic digestion performance.

In summary, the authors investigated valorization of HTC products by anaerobic digestion with the aim of identifying the inhibitors and examining the biomethanization performance and process intensification capabilities. Besides the identified inhibitory compounds at 240 °C like pyrazines, pyrrolidines, piperidines and phenols, both solid and aqueous fractions also displayed strong inhibitory effects. The anerobic digestion performance could be improved by adding carbonaceous materials supplements. In a statement to Advances in Engineering, Professor Lei Zhang stated that their findings will advance the application of HTC based biorefinery of wet organic wastes.