Hydrothermal Carbonization of Soybean Milk Residue (Okara) for Nutrient and Energy Recovery

Turning agricultural waste and by-products into benefits has always been an environmental and economical goal, especially in soymilk and tofu industries. This study explores the nutrients and energy recovery from soybean milk residue (okara) utilizing hydrothermal carbonization (HTC). The research focused on: i) The optimization of the HTC process; ii) The humic acid and nutrient recovery from HTC liquor; and iii) The effects of HTC conditions on the fuel properties of hydrochars. We found that, at optimal conditions, 90.5% phosphorus and 70.8% nitrogen in the pristine okara were extracted into HTC solution. These led to the recovery of 82% humid acid and 99.9% phosphorus from HTC solution, respectively. Additionally, nitrogen in HTC liquor was recovered as (NH4)2SO4 solution of 1026.7 mg N/L. The attained hydrochar exhibited a higher heating value (HHV) (23.04 MJ/kg), equivalent to or greater than hydrochars derived from the raw okara (15.03 MJ/kg) and other agro-wastes (16.20 - 22.30 MJ/kg). This study proves that acid-supported HTC is a promising method for the simultaneous recovery of nutrients and energy from okara, opening the way for its successful valorization.