Fate and distribution of nutrients and heavy metals during hydrothermal carbonization of sewage sludge with implication to land application

Abstract

Hydrothermal carbonization has been considered effective to reduce sewage sludge volume and provide an opportunity to generate valuable byproducts for a wide range of potential applications. The fate and distribution of nutrients during hydrothermal carbonization of sewage sludge, along with that of heavy metals, is very important for realizing nutrient recovery and reuse by both hydrochar and liquid phase directly as soil amendments and/or organic fertilizer. In this study, we systematically investigated the migration and transformation of nutrients and heavy metals using chemical extractions and reaction severity (Ruyter model) methods at a temperature range of 120–300 °C for 30–180 min. With increasing reaction severity, carbon and nitrogen efficiency of hydrochar showed a liner and exponential decrease respectively, while phosphorus accumulated positively in hydrochar. Nitrogen species in hydrochar are mostly nitrogen-containing aromatic heterocycles whereas in liquid phase are predominantly ammonia-N and organic-N. The organic phosphorus in sewage sludge was transformed to inorganic species but non-apatite phosphorus showed an exponential reduction in hydrochar with increasing reaction severity, together with the decrease of orthophosphate in liquid phase. As for heavy metals, hydrothermal carbonization promoted decrease in direct bioavailable fractions (except arsenic) and increase in stable fraction (except cadmium and arsenic) in hydrochar as reaction severity elevated. Since hydrothermal effects led to redistribution of nutrients speciation and immobilization of heavy metals, non-farm land application of both hydrochar and liquid phase seems to be possible and unrestricted. This study provides fundamental knowledges for the construction of sludge management strategies to better nutrients recycling and reclamation.