Abstract
Aerobic granular sludge is attractive for high-rate biological wastewater treatment. Biomass wash-out conditions stimulate the formation of aerobic granules. Deteriorated performances in biomass settling and nutrient removal during start-up have however often been reported. The effect of wash-out dynamics was investigated on bacterial selection, biomass settling behavior, and metabolic activities during the formation of early-stage granules from activated sludge of two wastewater treatment plants (WWTP) over start-up periods of maximum 60 days. Five bubble-column sequencing batch reactors were operated with feast-famine regimes consisting of rapid pulse or slow anaerobic feeding followed by aerobic starvation. Slow-settling fluffy granules were formed when an insufficient superficial air velocity (SAV; 1.8 cm s−1) was applied, when the inoculation sludge was taken from a WWTP removing organic matter only, or when reactors were operated at 30°C. Fast-settling dense granules were obtained with 4.0 cm s−1 SAV, or when the inoculation sludge was taken from a WWTP removing all nutrients biologically. However, only carbon was aerobically removed during start-up. Fluffy granules and dense granules were displaying distinct predominant phylotypes, namely filamentous Burkholderiales affiliates and Zoogloea relatives, respectively. The latter were predominant in dense granules independently from the feeding regime. A combination of insufficient solid retention time and of leakage of acetate into the aeration phase during intensive biomass wash-out was the cause for the proliferation of Zoogloea spp. in dense granules, and for the deterioration of BNR performances. It is however not certain that Zoogloea-like organisms are essential in granule formation. Optimal operation conditions should be elucidated for maintaining a balance between organisms with granulation propensity and nutrient removing organisms in order to form granules with BNR activities in short start-up periods.
Highlights
Aerobic granular sludge (AGS) wastewater treatment processes are attractive for intensive and high-rate biological nutrient removal (BNR) and secondary clarification in single sequencing batch reactors (SBR; de Bruin et al, 2004; Giesen et al, 2012)
The present study aimed to investigate the bacterial community dynamics during the formation of early-stage aerobic granules (0–60 days) in bubble-column SBRs operated under conditions selecting for a fast-settling biomass
The fluffy granules obtained in R1 with pulse feeding (6 min) were predominantly composed of Burkholderiales affiliates related to the Sphaerotilus-Leptothrix group (OTU-208, 23–33%), and by Zoogloea spp. belonging to Rhodocyclales (OTU-195, 12–27%)
Summary
Aerobic granular sludge (AGS) wastewater treatment processes are attractive for intensive and high-rate biological nutrient removal (BNR) and secondary clarification in single sequencing batch reactors (SBR; de Bruin et al, 2004; Giesen et al, 2012). Stable dense and fast-settling aerobic granules with tailored metabolic activities for the removal of carbon, nitrogen, and phosphorus are desired for the operation of robust AGS wastewater treatment plants (WWTP). The formation of aerobic granules has been stimulated by reactor start-up conditions leading to the wash-out of flocculent biomass and selecting for a fast-settling biomass, namely with the combination of short settling times of 3–5 min and short hydraulic retention times (HRT) of 6 h (Beun et al, 1999). Granules have been successfully cultivated with feast-famine regimes involving pulse feeding (3–5 min) followed by prolonged aeration (3–4 h; Morgenroth et al, 1997; Beun et al, 1999; Tay et al, 2002), or anaerobic feeding (1 h) followed by aerobic starvation (2 h; de Kreuk et al, 2005). High upflow aeration induce high shear and compaction forces at the surface of granules (Zima et al, 2007), and stimulate the production of exopolymeric substances (EPS) as well as hydrophobic www.frontiersin.org
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