Ammonium-driven nitrification and methane-driven denitrification achieve simultaneous nitrogen and metal removal in anaerobically digested sludge

As a major sink of heavy metals, wastewater sludge requires management to avoid environmental and health risks, while conventional treatments (e.g. chemical leaching and precipitation) remain dependent on chemical additives. Leveraging the alkalinity variations inherent to nitrification and denitrification, we present a fully biological, chemical-free strategy for solubilizing metals from sludge to leachate, and subsequent elimination in the leachate together with nitrogen. In the first stage, nitrification, driven primarily by acid-tolerant ammonia-oxidizing bacteria (AOB), oxidized ammonium, consuming sludge alkalinity and reducing sludge pH to ∼2.0, enabling efficient metals solubilization (Cu 85.6%, Zn 95.2%, Mn 85.2%, Al 73.5%). In the second stage, the acidic leachate underwent further treatment in a continuous methane-based membrane biofilm reactor (MBfR) enriched with nitrite/nitrate-dependent anaerobic methane-oxidizing (n-DAMO) microorganisms ('Ca. Methylomirabilis': 1.41% and 'Ca. Methanoperedens': 2.34%). This process removed > 98.0% of total nitrogen at a rate of 753.3 ± 31.1 mg N/(L d). Due to the alkalinity produced during denitrification, the pH of MBfR was automatically raised to ∼8.0, facilitating > 95.0% precipitation of solubilized metals. This work demonstrates a novel technical pathway for sludge management utilizing new nitrogen-cycling microorganisms and abundant nitrogen embedded in sludge, offering an environmentally sustainable pathway for integrated nitrogen and metal management in wastewater sludge.