Minimizing tannery sludge in landfilling through a mixed microbial culture approach: Effect of oxidizing pretreatment, temperature and hydraulic retention time on process performances and chromium fate.

Exploiting tannery sludge as potential source of short-chain fatty acids (SCFAs) and biogas offers a promising solution for a new management of this waste, which is currently landfilled due to chromium presence. This study assessed the optimal hydrogen peroxide (H 2 O 2) dosage, temperature (T) and hydraulic retention time (HRT) through hydrolysis and acidification tests to maximize SCFAs production and to recover biogas from the solid-rich fermentation residue. A dosage of 0.4 g H 2 O 2 /g TS and mesophilic T (40 °C) were the most suitable conditions to promote sludge biodegradability and SCFAs production in batch tests. Afterwards, among the two investigated HRT (4 and 8 days) in the semi-continuous processes, longer HRT increased SCFAs concentration and acidification yield up to 16.0 g COD SCFAs /L and 0.32 g COD SCFA /g VS 0 , respectively. Chromium was initially released in the liquid phase, but completely removed in less than 5 HRTs, allowing the utilization of fermentation liquid without environmental-safety issues. Finally, the solid-rich residue showed its potential as biogas source, having a specific gas production (SGP) of 0.48 m3/kg VS. According to the mass balance assessment, more than 50% of sludge can be saved from landfill, with potential economic benefit close to 2,700,000 €/y compared to the current management practice. [Display omitted] • An increase of 11–13% of tannery sludge biodegradability (k h) with 0.4 g H 2 O 2 /g TS. • Higher HRT (8 d) boosted the mesophilic SCFAs yield compared to shorter one (4 d). • Acetic acid accounted for more than 60% of the total SCFAs (COD basis). • Fermentation broth had no environmental issues due to the Cr precipitation. • The mass balance assessment showed a reduction of > 50% of this sludge to landfill. [ABSTRACT FROM AUTHOR]