Microalgal–bacterial granule-based wastewater treatment: Effects of COD/N ratio on particle characteristics and nutrient removal

Abstract

Microalgal–bacterial granules (MBG) have attracted increasing attention for wastewater treatment due to their high pollutant removal efficiency and low energy demand. This study evaluated the influence of COD/N ratios on MBG properties and treatment performance using four sequencing batch photobioreactors operated at the COD/N ratios of 5, 10, 15, and 20. Although the MLVSS/MLSS ratio remained stable, granule morphology and settling exhibited clear shifts with varying COD/N ratios. The most compact and fast-settling granules were obtained at COD/N = 10, while the lower ratios generated smaller granules. The COD removal rate was stable at 15–17 mg COD/g VSS·d, while the highest ammonium and phosphorus removal rates peaked at COD/N = 5 (1.16 mg NH₄⁺–N/g VSS·d) and COD/N = 20 (0.15 mg TP/g VSS·d), respectively. These results indicate that the COD/N ratio and nutrient loading are critical operational parameters influencing both the physical characteristics of MBG and their nutrient removal performance.