Sequencing batch reactor (SBR) as optimal method for production of granular activated sludge (GAS)--fluid dynamic investigations

Bacterial Selection during the Formation of Early-Stage Aerobic Granules in Wastewater Treatment Systems Operated Under Wash-Out Dynamics

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.

Effects of increase modes of shear force on granule disruption in upflow anaerobic reactors

Sludge washout is listed among the top practical problems of the high rate upflow anaerobic reactors. This study investigated quantitatively two sludge washout processes operated under different hydrodynamic shear increase modes with the intervals of 1 and 10 days respectively. The results reveal that the sludge washout accompanying with large-scale granule disruption could lead to performance failure with heavy sludge loss ratio of about 46.1% at sludge loss rate about 0.35 gVSS L(-1) d(-1) during the process with shear increase interval of 1 day, while the highest sludge loss rate was only 0.12 gVSS L(-1) d(-1) during the process with 10-day interval. The intensified shear conditions could weaken the granules through inhibiting the extracellular polymers production and bioactivity. As consequences, an outbreak of large-scale granule disruption would raise and then significantly accelerate the sludge washout. Since long interval could provide the granules the opportunity to recover from these negative effects to some extent, the shear increase strategy of long interval over 10 days is favorably recommended to operate full-scale reactors during the start-up and shock load periods. The pioneer use of the micro particle image velocimetry in this study offers the possibility to discover the real hydrodynamic conditions around granules at microscale for the first time and reveals that the shear force exerts directly on the granular surface as a mechanical disruption force and big granules undergo high disruption force. The granule disruption is a result of the competition between the granule and the ambient hydrodynamic shear conditions rather than a process with shear force as a sole dominant factor. These could facilitate the understanding of the complicated interactions between the hydrodynamics and reactor performance and favor then a better control of the full-scale reactors.