Abstract
This study focused on optimizing hydrothermal carbonization (HTC) of nutrient-rich dairy manure digestate using response surface methodology based on a face-centered central composite design (FCCD-RSM), aiming to identify optimal process conditions and develop predictive models that simultaneously maximize hydrochar yield and nutrient recovery. FCCD-RSM was employed to systematically evaluate the impact of HTC reaction temperature (180–240 °C) and reaction time (20–90min) on hydrochar yield, phosphorus content in hydrochar, and total ammonia-nitrogen (TAN) concentration in the HTC liquid. Results showed that the phosphorus content in hydrochar and the TAN concentration in the HTC liquid increased with increasing HTC temperature and reaction time, whereas hydrochar yield decreased. Based on experimental data, quadratic polynomial models were developed. Using RSM with equal weighting assigned to the three responses, the optimized conditions were determined to be 216.5 °C and 84.1min, resulting in a hydrochar yield of 79.03 ± 2.61%, phosphorus content in hydrochar of 1.76 ± 0.04%, and TAN concentration in the HTC liquid of 891.67 ± 16.67mg/L. These values were close to the predicted values and fell within the 95% confidence intervals. Therefore, the optimized HTC parameters can simultaneously and effectively produce phosphorus-rich hydrochar and TAN-rich liquid, providing insights into a more sustainable approach to circular waste management and nutrient recycling on dairy farms.