Ping Zheng

Anaerobic ammonium oxidation (anammox) is an energy-efficient and low-carbon process for nitrogen removal, but its wider application exposes anammox biomass to more complex contaminant mixtures. Existing reviews have examined contaminant inhibition and microbial responses, while their links to environmental risks have received less integrated attention. This review focuses on how representative chemical contaminants and particles affect anammox performance and microbial responses and how these changes may lead to environmental risks beyond nitrogen removal. Responses depend on contaminant type, concentration, exposure pattern, and biomass form (e.g., granules and biofilms). Common responses include contaminant retention in extracellular polymeric substances (EPS) and biomass, changes in granule or biofilm structure, cellular and redox stress, disruption of core anammox metabolism, and shifts in microbial interactions. These changes may lead to ammonium or nitrite leakage, risks related to antibiotic resistance genes and antibiotic-resistant bacteria, potential N₂O emissions, and contaminant accumulation in solids. Recovery of nitrogen removal alone does not confirm recovery of microbial activity or a reduction in retained contaminant burdens. We therefore propose evaluating contaminant-stressed anammox systems across liquid, gas, and solid phases, using a source-exposure-response-risk-control framework for monitoring, risk control, and recovery. This framework supports the risk assessment and long-term management of anammox-based treatment.

The presence of various heavy metals may influence the transformation of many organics by anaerobes under anaerobic conditions. Sometimes, the coexistence of both heavy metals and volatile fatty acids (VFAs) may cause an inhibitory impact on microbial species involved in such biotransformation. These toxic mixtures create disturbances in the bioreactors by inhibiting the ability of the microorganism to degrade the waste. To keep the anaerobic process horizontal, it is essential to know the concentrations of the heavy metals and VFAs during the wastewater treatment operations, otherwise, it would result in halting various operations. Other methods are time-consuming. However, the Microtox analysis provides quick and reliable information about the toxic concentrations of the mixtures by calculating the IC50 15 min luminosity inhibition through Photobacterium phosphoreum T3S. The current research work aimed to assess the impacts of Pb, Cd, Cu, and VFAs on luminescent anaerobic microbial species P. phosphoreum under laboratory conditions. The individual and combined acute toxicity assays were conducted following standard protocols. The results revealed that the combined treatments of individual heavy metals along VFAs caused partly additive toxic effects. The effects of two heavy metals along VFAs resulted in the synergistic type of toxic effects. The application of tri-mixtures of metals along VFAs also caused synergistic effects. The current data would be quite handy to conclude P. phosphoreum as a potential biosensor for metals and volatile fatty acids toxicity and to optimize various bioreactors against different heavy metals and VFA shocks by taking timely corrective measures.

In this study, a microbial fuel cell (MFC) that can achieve simultaneous anode anaerobic ammonium oxidation (anammox) and electricity generation (anode anammox MFC) by high-effective anammox bacteria fed with purely inorganic nitrogen media was constructed. As the influent concentrations of ammonium (NH₄⁺-N) and nitrite (NO₂⁻-N) gradually increased from 25 to 250 mg/L and 33–330 mg/L, the removal efficiencies of NH₄⁺-N, NO₂⁻-N and TN were over 90%, 90% and 80%, respectively, and the maximum volumetric nitrogen removal rate reached 3.01 ± 0.27 kgN/(m³·d). The maximum voltage and maximum power density were 225.48 ± 10.71 mV and 1308.23 ± 40.38 mW/m³, respectively. Substrate inhibition took place at high nitrogen concentrations (NH₄⁺-N = 300 mg/L, NO₂⁻-N = 396 mg/L). Electricity production performance significantly depended upon the nitrogen removal rate under different nitrogen concentrations. The reported low coulombic efficiency (CE, 4.09–5.99%) may be due to severe anodic polarization. The anode charge transfer resistance accounted for about 90% of the anode resistance. The anode process was the bottleneck for energy recovery and should be further optimized in anode anammox MFCs. The high nitrogen removal efficiency with certain electricity recovery potential in the MFCs suggested that anode anammox MFCs may be used in energy sustainable nitrogen-containing wastewater treatment.

Shewanella sp. strain F1, isolated from a lab-scale Fe(II)−dependent anaerobic denitrifying reactor, could reduce nitrate by oxidizing Fe(II). Its nitrate reduction rate and Fe(II) oxidation rate were 0.48 mg/(L·h) and 5.05 mg/(L·h) at OD₆₀₀ of 0.4786 with a five-fold diluent. Shewanella sp. was popular in Fe(III) reduction. Fewer studies about its ability for Fe(II) oxidation are available. The low pH was determined to be the switch for Shewanella sp. strain F1 to perform Fe(III) reduction or Fe(II) oxidation. Even under a low pH, the produced Fe(III) precipitated around cells from iron encrustation. By observation of the morphologies of strain F1, two corresponding microbial mechanisms were proposed. One was named Cyc 2−based Fe(II)-dependent denitrification, in which Fe(II) was oxidized by Cyc 2 in the outer cell membrane, and the produced Fe(III) precipitated on the cell wall surface to form tiled iron encrustation. The other was named Cyc 1−based Fe(II)−dependent denitrification, in which Fe(II) was oxidized on the existing iron precipitation on the cell wall surface to form towery iron encrustation, and the electron was transported to Cyc 1 in the periplasm. The efficiency determination and mechanism investigation of strain F1 will promote the development of autotrophic denitrification technology and meet the requirement of a low−carbon policy.