Fluctuation and Re-Establishment of Aerobic Granules Properties during the Long-Term Operation Period with Low-Strength and Low C/N Ratio Wastewater

Long-term structure stability of aerobic granules is critical to maintaining stable wastewater treatment performance. In this study, granulation and long-term sludge-treating synthetic with a low chemical oxygen demand nitrogen (COD/N) ratio 4:1 COD concentration 400 mg/L in anoxic-oxic conditions were investigated for over 300 days. Inoculated suspended sludge gradually transformed into granules-dominant on day 80. Due the improved volume index after 30 min settling (SVI30), mixed liquor solids (MLSS) reached 5.2 g/L 140. Without any external intervention or disturbance, started disintegrate from 140, causing increase SVI decrease biomass until 210, average size reduced 243 µm. From granular be re-established by re-granulation, granule increased 500 µm 302. During these disintegration re-granulation periods, there was no obvious difference terms removal nitrification, but microbial species found more diverse Thauera Sphingomonas dominant. Although intervention, food microorganisms (F/M) varied significantly due changes caused strong selective pressure change sludge-settling ability reactor. F/M ratios should controlled between 0.3 1.0 gCOD/gSS·d maintain minimize fluctuation properties under used study. able re-establish itself disintegration, controlling certain range would benefit stability. The findings study are significant deepen understanding low-strength and, thus, provide guidance granules.