Valorization of Okara using Hydrothermal Treatment in combination with Anaerobic Digestion for Resource Recovery

  Global industrialization and population growth have increased the demand for energy and food, leading to a rise in food-processing by-products. Okara (soybean residue) is one of the most abundant by-products, with an annual production exceeding 14 million tons worldwide. Despite its high organic content, okara is often discarded due to its high moisture content and limited processing options. Converting okara into energy or high-value materials has become a significant challenge in light of resource scarcity and circular economy principles. This study proposes an efficient conversion pathway that combines hydrothermal treatment (HT-carbonization/liquefaction) with anaerobic digestion (AD) to transform okara into usable resources without energy-intensive drying.

  The hydrothermal process converts okara into bio-crude oil, hydrochar, and nutrient-rich aqueous phase, while simultaneously breaking down its high cellulose and protein content under high temperature and pressure. This process facilitates the conversion of large molecules into smaller molecules, which enhances methane production during subsequent anaerobic digestion and improves energy recovery efficiency.

  Experiments were conducted in a 500 cm³ high-temperature, high-pressure stainless-steel reactor equipped with a mechanical stirrer. Okara and deionized water were added at solid-to-liquid ratios of 1:10 and 1:5. After nitrogen purging, the reactor was pressurized to 2 MPa. The reaction temperature ranged from 200 to 300°C, with reaction times between 60 and 360 minutes. Results showed that the highest yields of hydrochar (38%) and bio-crude oil (31%) were achieved at 200°C and 120 minutes. Yields decreased when temperatures exceeded 230–250°C or reaction times were prolonged, as more carbon shifted to the aqueous and gaseous phases. At 300°C with short reaction times, moderate yields of hydrochar (20–23%) and bio-crude oil (25–26%) were obtained, indicating that high temperatures with limited exposure promote oil formation without excessive degradation.

  The remaining nutrient-rich liquid phase was subjected to co-digestion with sludge, maintaining a 1:1 liquid-to-sludge ratio based on volatile solids content. Gas was collected daily, and the bottles were opened weekly for gas, solid, and liquid analysis, as well as biodegradability assessments. Anaerobic co-digestion further enhanced methane production, significantly improving energy recovery. The liquid phase provided biodegradable organic matter, which was broken down by anaerobic microorganisms, resulting in increased methane yield and improved overall energy recovery efficiency.

  Additionally, a life cycle assessment (LCA) was conducted to evaluate the environmental impacts of the hydrothermal treatment combined with anaerobic digestion. From number of LCA paper results, it was shown that this integrated process had a lower environmental impact and higher resource efficiency compared to traditional waste management methods, such as landfilling, incineration, or direct AD.

  In conclusion, this integrated system presents a viable waste-to-resource pathway. The hydrochar produced can be used as a soil conditioner, and the bio-crude oil as a fuel. Methane can be utilized for power generation, while the remaining digestate can be applied as liquid fertilizer. The combination of hydrothermal treatment with anaerobic digestion, alongside the incorporation of life cycle assessment, highlights a promising circular economy strategy that reduces carbon emissions, fosters energy production, enhances resource recovery, and supports the sustainable use of agricultural by-products.