Process simulation of co-HTC of sewage sludge and food waste digestates and supercritical water gasification of aqueous effluent integrated with biogas plants

The objective of this article is to investigate the integration of a digestate treatment with a biogas plant processing sewage sludge and food waste via process simulations: co-HTC of mixed digestates and supercritical water gasification (SCWG) of the aqueous effluent. The optimum co-HTC conditions are selected based on the energetic yields, comparing relative equipment sizes besides the hydrochar product. The selected conditions are 200 °C, 30 % solid load, and 1-h residence time for the mixing ratios in scope: energetic yields of 3.58–3.59 MJ/kg reactor inlet. These conditions result in more than 60 % K, P, and N recovery on hydrochar. SCWG of the aqueous effluent provides complete mineral recovery in the solid form and surplus energy production through syngas while causing some nitrogen loss as N2 gas. Although the co-HTC data is calculated from individual HTC results, the synergetic effect on the energetic yield does not affect the selection of optimum conditions as investigated through co-HTC of sewage sludge and food waste (the origins of the digestates). Consequently, biogas plants can evolve into multi-product biorefineries through the proposed integration. Meanwhile, this study can guide future co-HTC experiments of food waste and sewage sludge digestates and reduce the required runs.