Co-hydrothermal carbonization of sewage sludge and bamboo: hydrochar properties and risk assessment of heavy metals

The reduction of heavy metal (HM) risk while realizing energy utilization of sewage sludge (SS) remains an important challenge for SS treatment. The effects of pretreatment and additives (citric acid (CA), ferric chloride (FC), and fulvic acid (FA)) were explored based on co-hydrothermal carbonization (co-HTC) of SS and bamboo (BO). The results showed that the use of FC and FA pretreatment was beneficial in reducing the potential ecological risk of HMs and total concentration of Cu, Cr, and Ni. However, in terms of fuel properties, compared to pretreatment, additives were more beneficial in improving the fuel ratio and the high heating value (HHV). Specifically, among all samples, the maximum values of fuel ratio (0.49), HHV (12.85 MJ/kg), and combustibility index (8.00) were all achieved by employing different additives. It was worth noting that FC could not only improve fuel ratio but also obtain a low risk index (14.38) when used as an additive. Therefore, although pretreatment was more efficient in reducing the risk of HMs, additives could improve the fuel properties while alleviating the HMs risks to a certain extent. These findings contributed to the optimization of the HTC process of SS.

Automated summary

This study investigated the effects of pretreatment and additives on the reduction of heavy metal risk and improvement of fuel properties in sewage sludge and bamboo through co-hydrothermal carbonization. The results showed that additives were more beneficial in improving fuel properties, while pretreatment was more efficient in reducing heavy metal risks.