Ke Zhou

Mixotrophic denitrification processes have a great potential in nitrogen removal in biological wastewater treatment processes. However, so far, few studies have focused on the mixotrophic denitrification system using Fe(II) as an exclusively assisted electron donors and the underlying mechanisms in such a process remain unclear. Furthermore, the mechanisms by which microorganisms cover carbon, nitrogen, phosphorus and iron in an iron-assisted mixotrophic system remain unrevealed. In this work, we explore the feasibility of using Fe(II) as an assisted electron donor for enhancing simultaneous nitrogen and phosphorus removal via long-term reactor operation and batch tests. The results show that Fe(II) could provide electrons for efficient nitrate reduction and that biological reactions played a predominant role in these systems. In these systems Thermomonas, a strain of nitrate-reduction Fe(II)-oxidation bacterium, was enriched and accounted for a maximum abundance of 60.2%. These findings indicate a great potential of the Fe(II)-assisted mixotrophic denitrification system for practical use as an efficient simultaneous nitrogen and phosphorus removal process.

Phosphorus (P) recovery from wastewater can be completed by iron-involved autotrophic denitrification via forming Fe(III)-P precipitates and/or adsorbing P onto Fe(III) oxyhydroxides. However, so far, most studies focused on the final P-containing products, while the P-capturing pathways in such a process remain unclear. In this work, autotrophic iron-dependent denitrification (AIDD) was used as a typical anoxic iron-involved P-capturing biosystem to investigate the main P recovery pathways. The AIDD biosystem showed a relatively stable capability of capturing P coupled with nitrate reduction. Direct formation of amorphous Fe(II)-P precipitates after the phosphate was fed, followed by microbially driven oxidation into Fe(III)-P minerals, was found to be the primary pathway for the P capture. In addition, adsorption of phosphate onto the formed iron oxyhydroxides also contributed to the P recovery. This work provides better understanding about recovering P in AIDD and iron-involved denitrification and highlights the important roles of iron oxidizers in the iron-related biological wastewater treatment processes.

Iron-dependent autotrophic denitrification (IDAD) has garnered increasing interests as an efficient method for removing nitrogen from wastewater with a low carbon to nitrogen ratio. However, an inevitable deterioration of IDAD performance casts a shadow over its further development. In this work, the hidden cause for such a deterioration is uncovered, and a viable solution to this problem is provided. Batch test results reveal that the aggregation of microbial cells and iron-bearing minerals induced a cumulative and reversible inhibition on the activity of IDAD sludge. Extracellular polymeric substances were found to play a glue-like role in the cell-iron mineral aggregates, where microbial cells were caged, and their metabolisms were suppressed. Adopting low-intensity ultrasound treatment efficiently restored the IDAD activity by disintegrating such aggregates rather than stimulating the microbial metabolism. Moreover, the ultrasonication-assisted IDAD bioreactor exhibited an advantageous nitrogen removal efficiency (with a maximum enhancement of 72.3%) and operational stability compared to the control one, demonstrating a feasible strategy to achieve long-term stability of the IDAD process. Overall, this work provides a better understanding about the mechanism for the performance deterioration and a simple approach to maintain the stability of IDAD.