Sustainable bio-hydrothermal sequencing treatment for asbestos-cement wastes

[Display omitted] •A Eternit sample was detoxified by a combined biological and hydrothermal treatment.•The proposed treatment train led to the production of 0.5 molH2/L and 1.5 molCH4/L.•Chrysotile destruction was achieved after 8 h hydrothermal treatments at 80 °C.•The acid type used during the hydrothermal phase influenced the anaerobic digestion.•Phosphorous-based fertilizers were found in the sludge deriving from the treatment. In this paper, the treatment of asbestos-cement waste (ACW) has been attempted by a dark fermentation (DF) pre-treatment followed by hydrothermal and anaerobic digestion (AD) treatments. During DF, glucose, employed as a biodegradable substrate, was mainly converted to H2-rich biogas and organic acids (OAs). The latter caused the dissolution of the cement matrix and the partial structural collapse of chrysotile (white asbestos). To complete the chrysotile degradation, hydrothermal treatment of the DF effluents was performed under varying operating conditions (temperature, acid type, and load). After the addition of 5.0 g/L sulfuric acid, a temperature decrease, from 80 °C to 40 °C, slowed down the treatment. Similarly, at 100 °C, a decrease of sulfuric, lactic or malic acid load from 5.0 g/L to 1.0 g/L slowed down the process, regardless of acid type. The acid type did not affect the hydrothermal treatment but influenced the AD of the hydrothermal effluents. Indeed, when malic acid was used, the AD of the hydrothermally treated effluents resulted in the highest production of methane. At the end of the AD treatment, some magnesium ions derived from ACW dissolution participated in the crystallization of struvite, an ecofriendly phosphorous-based fertilizer.