Optimizing Co-hydrothermal carbonization of municipal sludge and enteromorpha prolifera for enhanced hydrochar yield and adsorption capacity using response surface methodology

Co-hydrothermal carbonization (HTC) is considered as an effective way for harmless disposal and resource recovery of municipal sludge (MS) and enteromorpha prolifera (EP). In this work, the co-HTC of MS and EP were carried out under various reaction conditions (temperatures of 160–280 °C, residence time of 30–150 min, and MS percentages of 0–100 wt%). Response surface analysis was used to define the interactions of process parameters, and regression models were built to optimize the reaction conditions. The results showed that the most significant factors influencing hydrochar yield and adsorption capacity followed the order of MS percentage > temperature > time. Hydrochar yield increased with MS percentage increasing. Increasing temperature was detrimental to the production of hydrochar, but improved its adsorption capacity. The optimum condition was 250 °C, 120 min, and 33 wt% of MS percentage. Under this condition, the predicted yield and adsorption capacity of hydrochar reached up to 46.12 % and 30.02 mg·g⁻¹, respectively. Characterization results identified that the degree of condensation and aromatization of hydrochar increased compared with that of feedstock, the types of functional groups remained largely unchanged, while their relative abundances varied. The positive synergy facilitated the formation of O-containing and N-containing functional groups. This work employed response surface methodology to determine the optimal parameters and analyzed the mechanism of synergistic functional group formation. The findings are of great significance for the resource recovery and high-value utilization of MS and EP.