BACKGROUNDA large quantity of heavy metal contaminated wastewater sludge is produced during the treatment of printed circuit board (PCB) manufacturing wastewater. The PCB wastewater sludge containing high concentrations of heavy metals, such as Cu and Zn, increases the potential for metal recycling. Although bioleaching is indicated as a promising technology to remove metals from electronic scrap and waste PCBs, its application for metal recovery from PCB wastewater sludge is still very limited. The purpose of this study was to develop a thermophilic bioleaching process operated in a sequencing batch reactor (SBR) to recover heavy metals from PCB wastewater sludge. RESULTSThe results show that an increase in sludge solid content from 0.5 to 5% (w/v) decreased the rate of pH reduction during the bioleaching process. It was also found that the efficiency and rate of metal solubilization decreased with increasing sludge solid content. At a sludge solid content of 0.5% (w/v), after four repeated feed/decant cycles with 10days per cycle (40days) of SBR operation, the maximum efficiencies of metal solubilization in the treated sludge were 65% and 100% for Cu and Zn, respectively. CONCLUSIONThe solubilization efficiency of heavy metals was lower during SBR operation with a longer 20-daycycle time. Therefore, a shorter 10-daycycle time for SBR operation is preferred for the thermophilic bioleaching of PCB wastewater sludge. In the treated sludge, the potential mobility and environmental risks of heavy metals were decreased significantly after the thermophilic bioleaching process. (c) 2013 Society of Chemical Industry