Rapid reformation of larger aerobic granular sludge in an internal-circulation membrane bioreactor after long-term operation: Effect of short-time aeration
Graphical abstract
Introduction
Aerobic granular sludge (AGS), containing aerobic surface and anaerobic interior simultaneously, is a large microbial aggregate with compact structure that allows simultaneous removal of carbon, nitrogen and phosphorus (Zhang et al., 2016). In the late of last century, AGS was observed and successfully cultivated in sequence bioreactors (SBRs) (Beun et al., 1999, Morgenroth et al., 1997), which aroused a great interest in using AGS as an alternative process of the conventional activated sludge (CAS) to treat both municipal and industrial wastewater. Up to date, an SBR is verified as the most effective way to cultivate AGS, and even a full-scale wastewater treatment plant (WWTP) with this approach has been built and operated successfully (Pronk et al., 2015). The merits of AGS, including rich biodiversity, high settling velocity, enhanced microbial functions, resilience to toxicity and good performance in simultaneous removing multiple contaminants, have been verified by different studies, which showed a promising aspect to replace CAS as the next generation technique for treating wastewater (van Loosdrecht and Brdjanovic, 2014).
SBR, though so far is regarded as the most commonly-used way to cultivate AGS, actually need very complex conditions to obtain mature AGS, such as a suitable applied organic loading rate, the presence of a feast-famine regime. Based on these basic conditions, the hydrodynamic selection of an upward flow subsequently washes the light floc sludge out, and keeps the dense mature AGS remaining and accumulating within the reactors. In this meaning, the successful cultivation of AGS in an SBR mainly relies on two essential factors, one is the occurrence of primary core granules, and the other is the hydrodynamic selection to wash out the light floc sludge from the reactor. Additionally, in SBRs, Verawaty et al. (2012) also found that the crushed granules acted as nuclei for floc sludge to attach, which accelerated the granule formation, and Long et al. (2014) rapidly cultured irregular and pale yellow granular sludge by inoculating a certain ratio of mature granules (about 25%) in the start-up stage. However, in SBRs, when filamentous bacteria overgrow, the settleability of biomass becomes worse, which causes most of sludge being washed out and the mature AGS disappearing, thus, the overgrowth of filamentous bacteria must be inhibited in an SBR (Liu and Liu, 2006), and the operation under an alkaline condition is an effective way to restrict the overgrowth of filamentous bacteria. Even though most reports about mature AGS were from SBRs (Nancharaiah and Kiran Kumar Reddy, 2018), the application in large-scale WWTPs was still too difficult due to the height of SBRs was a crucially restrictive factor (Corsino et al., 2016), therefore, conceiving a continuous-flow reactor might be a promising solution (Juang et al., 2010) to solve this problem.
In a previous investigation, an internal-circulation membrane bioreactor (IC-MBR) with continuous-flow was successfully built, and in which, the well-defined AGS was found to be self-cultivated directly (Chen et al., 2017). In this bioreactor, three factors, including the total retention of sludge particles by membrane modules, the occurrence of filamentous bacteria and internal circulation, were verified to be very essential in forming AGS. Such a finding was very interesting, and showed a novel cultivation mechanism that never reported before, in which, filamentous bacteria exhibited a special role in forming mature AGS, and was quite different with that in SBRs. This bioreactor was continuously operated for more than one year, and showed excellent performance in removing organic pollutants, total nitrogen (TN) and total phosphorous (TP). All the previously reported mechanisms about forming AGS almost focused on the initial stage of forming AGS from floc sludge (Chen et al., 2017). However, a practical bioreactor is generally operated for a long time, and the mature AGS within it may experience a series of continuous processes, including formation, growth, maturity, aging and breaking up. Actually, such a phenomenon was observed in a previous report (Wu et al., 2018), which indicated that, after long-term operation, the characteristics of biomass in an AGS bioreactor were quite different from that at the start-up stage. In such a situation, how does a sudden external disturbance influence on the performance of a long-term operating MBR? This issue is very essential to keep the stability of the whole system. Therefore, in the present investigation, an experiment was designed to explore the effect of short-time aeration on the reformation of larger AGS with the purpose to provide useful references for the practical application in the future.
Section snippets
Experimental procedure
A previously reported IC-MBR (Wu et al., 2018) was used in the present investigation after adjusting the composition of influent water. Prior to the investigation, it has been continuously operated for 383 days, which showed excellent performance in producing high-quality effluent. This work continued the operation of this bioreactor, and the observation and data collection started from the 384th day (the first day of the present investigation). The whole experiment period lasted for another
Variation of DO value and the growth of filamentous bacteria
The DO value at both zones and the temperature of the bioreactor was automatically measured by the installed DO probes, and the results are shown in Fig. 1.
As shown in Fig. 1, the temperature gradually decreased from 28.8 to 16.6 °C, then increased to 23.1 °C, which indicated it ranged in a normal fluctuating scope. The DO value in the mixing zone was very stable to be kept constantly at 0 mg/L, but on day 28, short-time aeration was applied on the biomass, which caused the DO value in this
Conclusions
The microbial community in an IC-MBR experienced a continuous succession with a gradual and slow change in its structure after long-term operation, and the average size of AGS within the bioreactor reached a balance at around 200 μm with an even distribution. Interestingly, after short-time aeration to the biomass, the growth of some filamentous bacteria (Saccharibacteria) was greatly promoted, and the surface of AGS tended to be more hydrophobic, which obviously changed the microbial community
Acknowledgement
This work was financially supported by the National Natural Science Foundation of China [No. 21476050].
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Co-first authors contributed equally to this work.