Biofuel and toxicity characteristics of hydrochar derived from sewage sludge

Hydrothermal carbonization has been proposed as an effective method to improve the dewaterability of sewage sludge. The solid product of this process, hydrochar, is most commonly considered for energy production. However, its potential application as a fertilizer or soil improver has also been suggested. For this reason, it is necessary to determine both the combustion and toxic properties of hydrochar and its aqueous extracts prior to use. In this study, the hydrothermal carbonization of aerobically digested sewage sludge was investigated at 210 °C for a period of 2 h. The proximate and ultimate analyses, the higher heating value and thermal analysis of hydrochars were determined. The toxicity tests were assessed for aqueous extracts of hydrochar on the inhibition of crustaceans, bacteria and macrophytes. Regarding hydrochar, the inhibition of mono- and dicotyledonous plants was also determined. The results indicated that aqueous extracts of hydrochar were toxic to crustaceans and bacteria but did not inhibit macrophytes. Additionally, a stimulating effect on the above-ground parts of Hordeum vulgare and Sinapis alba was observed. In contrast, the direct application of hydrochar inhibited the growth of the tested plants.