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 N ₂ 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.