How volumetric exchange ratio and carbon availability contribute to enhance granular sludge stability in a fill/draw mode SBR treating domestic wastewater?

Abstract

This work evaluated the granulation process and the stability of aerobic granular sludge in a pilot-scale reactor (1 m³), during a long-term operation (> 650 days) treating real low-strength domestic wastewater. Two operational strategies, distinguished by volumetric exchange ratio (VER: 50 and 75 %), were tested enabling higher organic loading rates. Different topics involving wastewater biological treatment was evaluated from physical-chemical and kinetic parameters to molecular biology techniques. Since process variability can only be described in statistical terms, multivariate statistical process control was applied to verify and integrate the results found. Successful granulation in terms of settling properties (Sludge volumetric index - SVI30 < 70 mL gTSS L−1 and SVI30/SVI5 ∼ 0.70) and granules fraction (>80 %) were achieved when applying VER = 75 %. Multivariate analysis supported that an increment in VER from 50 to 75 % is an important strategy for AGS stability, especially during rainy periods where the carbon availability is lower (OLR < 0.4 kgSCOD m-3 d−1). Carbon compounds were mainly removed during anaerobic feeding phase, however, the presence of particulate matter led to hydrolysis in the aerated phase. Nitrification was observed in both strategies. Nitrospira spp. was predominant under lower VER, whereas Nitrosomonas spp. overcame increasing VER. Although partial denitrification and low phosphorus removal were achieved under both strategies, these factors could be target by controlling the excess of DO in aeration phase and improving the availability of readily organic substrate in feeding phase.