Study on co-hydrothermal treatment combined with pyrolysis of rice straw/ sewage sludge: Biochar properties and heavy metals behavior

Reasonable disposal of sewage sludge (SS) as a resource to avid secondary pollution is the major objective. In this study, co-hydrothermal treatment combined with pyrolysis (co-HTP) of rice straw (RS) and sewage sludge for production of biochar was evaluated. The biochar yield was evaluated, and the basic characteristics of biochar including fuel characteristics, porosity characteristics and crystalline characteristics, as well as the heavy metals behaviors were examined. The results indicated that biochar yield exhibited a decreasing trend with the increase of RS blending ratio, and there was a synergy between RS and SS during co-HTP process for the slightly higher biochar yield than that of the predicted values. The co-HTP process promoted the increase of aromaticity and stability of biochar, along with an obvious increase of specific surface area of 116.81 m2/g for H-7RS/3SS-Char. The results also indicated that co-HTP process decreased the total concentrations of heavy metals in biochar and had the promoting effect on heavy metals immobilization for reducing environmental risks due to the reduced percentage of heavy metals in bioavailable fractions.

Automated summary

This study evaluated the co-hydrothermal treatment combined with pyrolysis of rice straw and sewage sludge to produce biochar, aiming to reasonably dispose of sewage sludge as a resource and avoid secondary pollution. Results showed a synergy between rice straw and sewage sludge during the co-HTP process, leading to slightly higher biochar yield than predicted values. The co-HTP process also promoted the increase of aromaticity and stability of biochar, with a significant increase in specific surface area. Additionally, the process decreased the total concentrations of heavy metals in biochar and helped immobilize heavy metals to reduce environmental risks.