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Bioleaching is a successful and cost-effective strategy with which to remove heavy metals from contaminated sludge. Most of the current studies on bioleaching are concentrated on municipal sludge, and bioleaching of tannery sludge has not been studied extensively. The present study investigated the bioleaching of chromium (Cr) from tannery sludge using the mixture of iron-oxidizing bacteria (Acidithiobacillus ferrooxidans) and sulfur-oxidizing bacteria (Acidithiobacillus thiooxidans) in batch flasks. Changes in the pH, ORP, Fe2+ and SO4 2- concentrations and the efficiency of bioleaching of Cr from tannery sludge were analyzed. The results showed that the bioleaching efficiency of co-inoculation of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans was better than any of them. The co-inoculation with a mixture of iron-oxidizing and sulfur-oxidizing bacteria together with the addition of Fe2+ and elemental sulfur accelerated acid production and an increase in the oxidation-reduction potential originating from the bio-oxidation of Fe2+ and elemental sulfur. The bioleaching efficiency of co-inoculation of Cr increased as the pH of sludge decreased, reaching a maximum removal of 97.1%. The residual Cr concentration which is 531 mg/kg in bioleached sludge is below the thresholds for agriculture, according to China National Standard GB4284-2018 (<1000 mg/kg dry weight) for pollutants in sludges from agricultural use. The bioleaching efficiency was distinctly correlated with pH or ORP. The data presented showed that the bioleaching efficiency was negatively correlated (R² = 0.9864) with pH and positively (R² = 0.9821) with ORP, which implied the existence of a substantial indirect leaching mechanism responsible for Cr solubilization. [ABSTRACT FROM AUTHOR]

A novel process for sewage sludge bioleaching by mixed Thiobacilli (Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans) using a 12-stage, 180 L working volume continuous plug-flow bioreactor, is presented. The objective of the present study was to assess the impact of some parameters on the sludge dewaterability and to improve the sludge dewaterability by optimization of these parameters. The parameters examined were sludge moisture content, nutrients dosage, aeration rate, and the number of reactors. The order of the influence of these factors on sludge dewaterability was found to be sludge moisture content>nutrients dosage> aeration rate> number of reactors. The optimized conditions were: sludge moisture content, 98.0%, nutrients dosage, 9 g/L, aeration rate, 8 m³/h, and 10 reactors. Confirmation experiments conducted under optimum conditions demonstrate the sludge dewaterability to be remarkably improved. After 2 days of bioleaching, the moisture of bioleached sludge cake was reduced to below 60%. [ABSTRACT FROM AUTHOR]

Bioleaching is a promising technology for removal of metals from sludge and improvement of its dewaterability. Most of the previous studies of bioleaching were focused on removal of metals; bioleaching in cold environments has not been studied extensively. In this study, Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans were acclimated at 15 °C and co-inoculated to explore the optimal conditions for improvement of sludge dewaterability and removal of metals by the sequencing batch reactors. The data show after 6 days of bioleaching at 15 °C, 89.6% of Zn, 72.8% of Cu and 39.4% of Pb were removed and the specific resistance to filtration (SRF) was reduced to ~12%. In addition, the best conditions for bioleaching are an initial pH of 6, a 15% (v/v) inoculum concentration, and A. thiooxidans and A. ferrooxidans mixed in a ratio of 4:1. We found that bioleaching of heavy metals is closely related to final pH, while the sludge SRF is dominated by other factors. Bioleaching can be completed in 6 days, and the sludge dewaterability and removal of metals at 15 °C meet the requirements of most sewage treatment plants. [ABSTRACT FROM AUTHOR]