Alum sludge (AlS) refers to the inevitable by-product generated during the drinking water purification process, where Al-salt is used as a coagulant in the water industry. It has long been treated as “waste”, while landfill is its major final disposal destination. In fact, AlS is an underutilized material with huge potential for beneficial reuse as a raw material in various wastewater treatment processes. In the last two decades, intensive studies have been conducted worldwide to explore the “science” and practical application of AlS. This paper focuses on the recent developments in the use of AlS that show its strong potential for reuse in wastewater treatment processes. In particular, the review covers the key “science” of the nature and mechanisms of AlS, revealing why AlS has the potential to be a value-added material. In addition, the future focus of research towards the widespread application of AlS as a raw material/product in commercial markets is suggested, which expands the scope for AlS research and development.

Aerobic denitrifying microorganisms, with their strong environmental adaptability, low dissolved oxygen concentration requirements, rapid growth rate, and high nitrogen removal efficiency, significantly compensate for the shortcomings of traditional aerobic chemolithoautotrophic nitrification and anaerobic heterotrophic denitrification models. The introduction of aerobic denitrifiers can effectively enhance the removal of nitrate nitrogen. However, directly inoculating aerobic denitrifiers into wastewater leads to issues such as easy loss of bacterial cells and difficulty in forming a dominant flora, thus preventing the long-term maintenance of their enhancing effect on denitrification performance. To address this problem, microbial immobilization technology has been introduced into the remediation process of nitrogen-polluted water bodies. This technology can maintain a high biomass concentration, provide a stable breeding ground for microorganisms, and effectively prevent the rapid loss of microorganisms. This article systematically reviews the current status of the isolation of aerobic denitrifying bacteria, key enzymes, and genes, as well as the application progress of aerobic denitrifying bacteria and their immobilization technology, aiming to provide solid theoretical support for the practical application of aerobic denitrification technology and promote its further development in the field of nitrogen pollution control.

With the growing problem of global water pollution, nitrogen pollution has become a key factor affecting aquatic ecosystems and human health. The biological sponge iron system (BSIS) has gained attention as a research hotspot due to its efficient denitrification capability. This study focused on the changes in microbial community structure and the relative abundance and interrelationships of nitrogen cycle-related functional bacteria at different operational stages of the BSIS with a sponge iron (SFe) dosage of 90 g/L. The results showed that as the operation time of the reactor extended, the relative abundance of denitrifying genera such as Saccharimonadales, Arenimonas, and Acinetobacter significantly increased, while the relative abundance of Proteobacteria showed a trend of initial increase followed by a decrease. The relative abundance of nitrifying bacteria exhibited a more complex variation, whereas the abundance of denitrifying bacteria showed a continuous upward trend. In addition, there were complex interrelationships among different denitrifying bacteria, such as a positive correlation between Saccharimonadales and Acetobacteraceae, and a negative correlation between Rhodothermus and Pseudoxanthomonas. This study not only revealed the changes in the relative abundance and interrelationships of microbial communities and nitrogen cycle-related functional bacteria over time with an SFe dosage of 90 g/L, but also provided a new perspective for understanding the intrinsic mechanism of enhanced biological denitrification by sponge iron. These findings are of great significance for optimizing the operating parameters of the BSIS, improving denitrification efficiency, and promoting the practical application of this technology in the field of environmental engineering.

This study investigates the enhancement of nitrogen removal performance in an intermittent biological sponge iron system (BSIS) through the immobilization of aerobic denitrifying bacteria. The aim is to improve the efficiency of simultaneous nitrification and denitrification (SND) in the BSIS by optimizing the microbial community involved in nitrogen conversion. The immobilization technique not only stabilizes the microbial activity and abundance of aerobic denitrifying bacteria, but also promotes a more efficient denitrification process. The optimal material ratio of polyvinyl alcohol–sodium alginate gel beads was determined as 10 g/100 mL PVA, 4 g/100 mL SA, 2 g/100 mL CaCl₂, and 2 g/100 mL of bacterial suspension, achieving a maximum NO₃⁻-N removal rate of 91.73%. A response surface model (RSM), established for the operational conditions, (shaker speed, temperature, and pH) showed a high fitting degree (R2 = 0.9960) and predicted the optimal conditions for maximum NO₃⁻-N removal as 109.24 rpm, 23.6 °C, and pH 7.9. Compared to R1 (47.82%), R3 achieved a higher average total nitrogen (TN) removal rate of 95.49%, following the addition of immobilized aerobic denitrifying bacteria to the BSIS.