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Browsing by Author "An, Zhaohui"

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    Long-Term Stability of Nitrifying Granules in a Membrane Bioreactor without Hydraulic Selection Pressure
    An, Zhaohui; Zhang, Xueyao; Bott, Charles B.; Wang, Zhi-Wu (MDPI, 2021-06-10)
    To understand the long-term stability of nitrifying granules in a membrane bioreactor (GMBR), a membrane module was submerged in an airlift reactor to eliminate the hydraulic selection pressure that was believed to be the driving force of aerobic granulation. The long-term monitoring results showed that the structure of nitrifying granules could remain stable for 305 days in the GMBR without hydraulic selection pressure; however, the majority of the granule structure was actually inactive due to mass diffusion limitation. As a consequence, active biomass free of mass diffusion limitation only inhabited the top 60–80 µm layer of the nitrifying granules. There was a dynamic equilibrium between bioflocs and membrane, i.e., 25% of bioflocs attached on the membrane surface within the last nine days of the backwash cycle in synchronization with the emergence of a peak of soluble extracellular polymeric substances (sEPS), with a concentration of around 47 mg L−1. Backwash can eventually detach and return these bioflocs to the bulk solution. However, the rate of membrane fouling did not change with and without the biofloc attachment. In a certain sense, the GMBR investigated in this study functioned in a similar fashion as an integrated fixed-film activated sludge membrane bioreactor and thus defeated the original purpose of GMBR development. The mass diffusion problem and sEPS production should be key areas of focus in future GMBR research.
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    Mechanistic understanding of biogranulation for continuous flow wastewater treatment and organic waste valorization
    An, Zhaohui (Virginia Tech, 2022-04-20)
    Aerobic granular sludge has been regarded as a promising alternative to the conventional activated sludge which has been used for a century in that granular sludge offers advantages in high biomass retention, fast sludge-water separation, and small footprint requirement. However, this technology has been rarely applied in continuous flow reactors (CFRs) which are the most common type of bioreactors used in water resource recovery facilities across the world. Hence, the overarching goal of this study is to provide advanced understanding of biogranulation mechanism to enable industrial application of this technology. The lack of long-term stability study in CFRs has restricted its full-scale acceptability. The high settling velocity-based selection pressure has been regarded as the ultimate driving force towards biogranulation in sequential batch reactors (SBRs). In this study, this physical selection pressure was firstly weakened and then eliminated in CFRs to investigate its role in maintaining the long-term structural stability of aerobic granules. Given the fact that implementing settling velocity-based selection pressure only can cultivate biogranules in SBRs but not in completely stirred tank reactors (CSTRs), the essential role of feast/famine conditions was investigated. Seventeen sets of data collected from both literature and this study were analyzed to develop a general understanding of the granulation mechanisms. The outcome indicated that granulation is more sensitive to the feast/famine conditions than to the settling velocity-based selection pressure. The theory was tested in a CFR with 10-CSTR chambers connected in series to provide feast/famine conditions followed by a physical selector separating the slow-settling bioflocs and fast-settling biogranules into feast and famine zones, respectively. Along with successful biogranulation, the startup performance interruption problem inherent in SBRs was also resolved in this innovative design because the sludge loss due to physical washout selection was mitigated by returning bioflocs to the famine zone. Then, a cost-effective engineering strategy was put forward to promote the full-scale application of this advanced technology. With this generalized biogranulation theory, pure culture biogranules with desired functions for high value-added bioproducts were also investigated and achieved for the first time in this study, which paves a new avenue to harnessing granulation technology for intensifying waste valorization bioprocesses. 
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    Mechanistic understanding of the NOB suppression by free ammonia inhibition in continuous flow aerobic granulation bioreactors
    Kent, Timothy R.; Sun, Yewei; An, Zhaohui; Bott, Charles B.; Wang, Zhi-Wu (2019-10)
    A partial nitritation continuous flow reactor (CFR) was operated for eight months demonstrating that partial nitritation granular sludge can remain stable under continuous flow conditions. The ammonia oxidizing bacteria (AOB)-to-nitrite oxidizing bacteria (NOB) activity ratios were determined for a series of granule sizes to understand the impact of mass diffusion limitation on the free ammonia (FA) inhibition of NOB. When dissolved oxygen (DO) limitation is the only mechanism for NOB suppression, the AOB:NOB ratio was usually found to increase with the granule size. However, the trend is reversed when FA has an inhibitory effect on NOB, as was observed in this study. The decrease in AOB:NOB ratio indicates that smaller granules, e.g. diameter < 150 mu m are preferred for nitrite accumulation when high FA concentration is present, as in the partial nitritation process. The trend was further verified by observing the increase in the apparent inhibition coefficient as granule size increased. Indeed, this study for the first time quantified the effect of diffusion limitation on the apparent inhibition coefficient of NOB in aerobic granules. A mathematical model was then utilized to interpret the observed suppression of NOB and predicted that NOB suppression was only complete at the granule surface. The NOB that did survive in larger granules was forced to dwell within the granule interior, where the AOB growth declines due to DO diffusion limitation. This means FA inhibition can be taken advantage of as an effective means for NOB suppression in small granules or thin biofilms. Further, both FA inhibition and DO limitation were found to be required for the suppression of NOB in mainstream aerobic granules.