Hydroxyapatite (HAP) formation in acetate-driven partial denitrification process: Enhancing sludge granulation and...

Current wastewater treatment systems often focus on nitrogen removal through partial denitrification and anammox processes. These methods, while effective for nitrogen, do not address phosphorus removal, a major contributor to water eutrophication. Existing literature has shown that phosphorus remains a persistent pollutant in treated effluents. Researchers have explored various strategies to enhance phosphorus removal, but none have combined it with partial denitrification. This gap motivated the investigation into whether hydroxyapatite formation could be induced in partial denitrification systems. Prior studies have identified the potential of calcium-based precipitation for phosphorus removal, but its integration with PD processes is novel. No prior work had resolved how to couple PD with HAP formation for simultaneous nitrogen and phosphorus removal. This uncertainty drove the need for a new approach that could address both pollutants in a single system. The study aimed to bridge this knowledge gap by testing the feasibility of HAP formation in PD systems. The novelty of this approach lies in its potential to enhance both sludge properties and phosphorus recovery.

The study aimed to explore the feasibility of inducing hydroxyapatite (HAP) precipitation in partial denitrification (PD) systems to achieve simultaneous phosphorus removal and sludge granulation. The specific problem addressed is the lack of an integrated solution for removing both nitrogen and phosphorus in wastewater treatment. The motivation stems from the environmental impact of phosphorus discharge into water bodies. The researchers sought to determine whether adding calcium could trigger HAP formation in PD systems. They also aimed to evaluate the effect of calcium concentration on phosphorus removal efficiency. Another objective was to assess the impact of HAP formation on sludge properties, such as granulation and settleability. The study aimed to provide a robust and efficient method for wastewater treatment that could be applied in real-world settings. The findings could contribute to the development of more sustainable and cost-effective wastewater treatment technologies.

The study involved operating three sequencing batch reactors (SBRs) under partial denitrification conditions. Each reactor received a different concentration of external calcium: 30, 60, and 120 mg/L. The reactors were monitored for phosphorus removal efficiency and sludge characteristics. The researchers measured granule size and settleability using standard sludge evaluation techniques. Confocal laser scanning microscopy was used to analyze the internal structure of the granules. X-ray diffraction (XRD) was employed to confirm the formation of hydroxyapatite (HAP). Elemental composition analysis was conducted to determine the presence of phosphorus and calcium in the granules. The study combined experimental and analytical methods to evaluate the effectiveness of the PD-HAP coupled process.

The highest phosphorus removal efficiency was observed in the reactor with 120 mg/L calcium, achieving 80% removal. The granule size in this reactor reached 906.1 μm during the stable period. This was significantly larger than the granules in the reactors with lower calcium concentrations. The sludge from the high calcium reactor exhibited excellent settleability, with an SVI5 of 20 mL/g MLSS. The MLVSS/MLSS ratio decreased, indicating increased inorganic content in the granules. Confocal laser scanning microscopy confirmed the accumulation of inorganic material within the granules. Elemental analysis showed high levels of phosphorus and calcium in the PD granules. X-ray diffraction results provided evidence of hydroxyapatite formation in the granules.

The study demonstrated that inducing hydroxyapatite (HAP) precipitation in partial denitrification (PD) systems can lead to effective phosphorus removal. The results suggest that calcium addition enhances both phosphorus removal and sludge granulation. The formation of HAP was confirmed through X-ray diffraction analysis. The study also showed that higher calcium concentrations improve granule size and settleability. The decrease in MLVSS/MLSS ratio indicated increased inorganic content in the granules. The findings support the potential of PD-HAP coupled processes for wastewater treatment. The researchers propose that this method could be a robust and efficient solution for nitrite production and phosphorus recovery. The study highlights the importance of integrating phosphorus removal with nitrogen removal in wastewater treatment systems.