Study on synchronous immobilization technology of heavy metals and hydrolyzed nitrogen during pyrolysis of sewage sludge

Abstract

The difficulty to simultaneously passivate heavy metals and enrich nutrients content in sewage sludge limits its application as soil nutrients. This paper concentrated on the synchronous immobilization effect of heavy metals and hydrolyzed nitrogen during pyrolysis of sewage sludge. Results showed that the forms of heavy metals transformed from bioavailable forms (F1 + F2) to stable forms (F3 + F4) and the leaching concentration reduced after low-temperature pyrolysis. However, hydrolyzed nitrogen would inevitably lose during pyrolysis treatment. With the addition of CaO, more hydrolyzed nitrogen could be retained in biochar by forming more stable nitrogen-containing compounds. The preservation efficiency of hydrolyzed nitrogen in biochar was over 50% when pyrolysis at 300 ºC with CaO. In addition, the addition of CaO would further reduce the heavy metals toxicity. In this study, the synchronous immobilization of heavy metals and hydrolyzed nitrogen in biochar was realized by adding CaO during low-temperature pyrolysis, contributing to achieving the resource utilization of sewage sludge as N-fertilizer to land use.