Efficient metal removal from digested sludge and supernatant by nitrification-driven acidifying leaching and denitrification-driven alkalizing precipitation

The transfer of heavy metals from wastewater to sludge necessitates proper sludge management. This study proposed a two-step bioprocess, utilizing pH variations during nitrification (decreasing pH) and denitrification (increasing pH) to achieve heavy metals removal from digested sludge and supernatant, alongside simultaneous nitrogen removal in the sludge supernatant. In the first stage, digested sludge was acidified to a pH of approximately 2 through ammonia oxidation, facilitated by acid-tolerant ammonia-oxidizing bacteria (AOB). This step resulted in metal solubilization efficiencies of 84.1 ± 2.5% for Cu, 94.5 ± 2.3% for Zn, 83.9 ± 2.5% for Mg, and 72.3 ± 2.2% for Al. In the second stage, the metal ions and nitrogen-rich sludge supernatant derived from bioleaching were introduced into a hydrogen/carbon dioxide (H₂/CO₂)-based membrane biofilm reactor (MBfR). During this process, denitrification raised the pH to above 7, leading to nitrogen removal efficiencies exceeding 90% and the precipitation of over 90% of Cu, Zn, and Al. Long-term operation of the system revealed the presence of volatile fatty acids (VFAs) in MBfR, which functioned as secondary electron donors alongside hydrogen, as confirmed by in situ batch assays. Furthermore, microbial analysis identified denitrifiers (e.g., Comamonas, Denitratisoma) and fermenters (e.g., Sporomusa) within the biofilm, substantiating the biological processes underpinning the treatment. Overall, this two-step biological treatment effectively removed metals from sludge without reliance on external acids or alkalis, which promotes sustainable waste management practices.