Mineral dissolution products drive anammox nitrogen removal via sludge aggregation

Anammox is an energy-efficient process for biological wastewater treatment, yet its performance is often limited by the environmental sensitivity of anammox bacteria. Sludge aggregation has been shown to enhance biomass retention by promoting sludge settleability. This study proposes that the dissolution of iron-bearing minerals (i.e., pyrite) promotes sludge aggregation and bacterial retention, and the anammox sludge in turn continuously stimulates further pyrite dissolution, creating a positive feedback loop that amplifies the enhancement effect. Experimental results demonstrated that supplementation with pyrite increased the nitrogen removal rate by 37.3%, with its dissolution accounting for 56.8% of this enhancement. To elucidate the interactions between anammox sludge and pyrite, comprehensive analyses were conducted on microorganisms, minerals, and extracellular polymeric substances (EPS). The findings revealed that anammox sludge continuously stimulated pyrite dissolution, as indicated by a decrease in pyrite's self-corrosion potential from 0.127 V to -0.076 V, along with an increase in its surface roughness from 2.02 nm to 5.54 nm after contact with sludge. Quartz crystal microbalance with dissipation (QCM-D) analysis further showed that pyrite dissolution facilitated sludge aggregation by enhancing EPS adhesion, evidenced by an increase in the ΔD/ΔF ratio from -0.2095 to -0.0897. These results highlight a synergistic dissolution-aggregation mechanism that offers a theoretical foundation for enhancing the stability and resilience of anammox systems treating wastewater through the application of iron-containing minerals.