Membrane fouling in aerobic granular sludge (AGS)-membrane bioreactor (MBR): Effect of AGS size
Graphical abstract
Introduction
Aerobic granular sludge (AGS), produced during the biological wastewater treatment process with suitable culture condition, is a microbial aggregate of numerous self-immobilized functional microorganisms and has a diversified microbial communities and tightly compact structure (Juang et al., 2010, Liu et al., 2009, Pronk et al., 2015). Compared with the conventional activated sludges, AGS exhibited several advantages (Zhao et al., 2016), including abundant microbial biodiversity, great biomass concentration (up to 20 g TSS L−1), large relative density, the possibility to simultaneously degrade organic carbon and nutrients, low sludge yield, remarkable settling capability, and robust ability to withstand the high organic loading rate. (Lin et al., 2010, Wang et al., 2017, Zhao et al., 2016). These advantages made AGS bioreactor a promising wastewater treatment technology (Lotito et al., 2014).
The membrane bioreactor (MBR) is becoming widely adopted for the treatment and reclamation of both municipal and industrial wastewaters. This was due to its compact nature (Iorhemen et al., 2017, Mutamim et al., 2013), generation of high-quality (particulate-free) effluent (Ferrero et al., 2012, West et al., 2016), capability of withstanding high organic loading (Scholes et al., 2016, West et al., 2016), production of largely disinfected effluent (Meng and Liu, 2016, Scholes et al., 2016), and reduction in sludge generation (Fenu et al., 2010, Xue et al., 2016). However, membrane fouling remained the major drawback impeding the wider application of the MBR (Meng et al., 2009, Meng et al., 2017).
The combination of MBR with AGS would compose an AGS-MBR, which could utilize their advantages to improve the treatment efficiency and overcome their respective shortcomings. Li et al. (2012) found that the treatment performance of AGS-MBR, with lower COD and NH3-N concentration in the effluent, was more stable and better than that of the conventional MBR, because of its internal anaerobic, anoxic, and aerobic structure and the richer biological community. Besides, AGS-MBR also exhibited 50% higher membrane permeability and 10% better membrane cleaning efficiency than the conventional MBR, as AGS caused less membrane fouling than suspended sludge. Tay et al. (2007) reported that treatment efficiencies of AGS-MBR and submerged MBR were similar (exceeding 99% COD removal), but the permeability loss and TMP increment for AGS-MBR were very low and negligible. Another bench-scale study discovered that membrane flux of AGS-MBR was 2∼6 times than that of the conventional MBR Another bench-scale study discovered that final membrane flux (3.1 L h−1 m−2) of AGS-MBR was about 2 times than that (1.5 L h−1 m−2) of the conventional MBR (Li et al., 2005). Furthermore, for the long-term operation (100 day), AGS-MBR exhibited a low membrane fouling rate (below 0.1 kPa/day) (Yu et al., 2009) and low TMP (Xiang et al., 2010), even without any need for physical cleaning any more (Thanh et al., 2008, Xiang et al., 2010). Wang et al. (2013) found that MBR with AGS performed a long and stable operation of 61 days at high flux (20 L/(m2 h), which was much better than flocculent and bulking sludge. Xuan et al. (2010) reported that granular sludge could maintain the relatively independent shape and excellent porosity to enhance sludge filtering performance, due to its compact structure. Decreased C/N ratio worsened the granular quality, and exacerbated cake layer formation and pore blocking, thereby aggravating membrane fouling (Chen et al., 2017). Most investigations were dealing with research of AGS-MBR filtration performance, whereas research regarding its deep-seated membrane fouling mechanism was still not yet launched. AGSs have a wide size distribution. In membrane filtration process, the particle size significantly affected membrane fouling. Studying the effect of AGS size on membrane fouling was of significance for the regulation of AGS particle size and fouling control. AGS exhibits a favorable filtration characteristic and its size directly affects the structure and porosity of fouling layer. Nevertheless, to the best of our knowledge, the effect of AGS size on membrane fouling has not yet been conducted. Therefore, it is of great importance to quantitatively analyze the membrane fouling mechanism of AGS with various sizes.
To gain an insight into membrane fouling of AGS, there are still some problems that need to be addressed: 1) How the AGS size affects the membrane fouling? If there is a critical AGS size for the maximum or the minimum membrane fouling?; 2) Do EPS and SMP (soluble microbial product) affect AGS membrane fouling? What is the main membrane fouling mechanism?; 3) What is the interaction between AGS-foulants and shear rate on membrane? Can shear rate effectively reduce membrane fouling?
By considering these problems, the current study encompasses a detailed investigation into the fouling mechanisms of AGS with various size. The main research contents consist of 1) to clarify the fouling behavior at various AGS sizes; 2) to explore whether there is a critical AGS size for distinguishing the membrane fouling degree; 3) to describe the impact of EPS and SMP on membrane fouling; 4) to characterize the micromorphology of fouled membranes with various AGS sizes. The pivotal of this study is to analyze the membrane fouling mechanism of AGS-membrane filtration, especially for the effect of AGS size, as well is expected to facilitate the potential application in the future.
Section snippets
Experimental set-up and membranes
As showed in Fig. 1, the membrane module and bioreactor were independent. The main characteristic of AGS was shown in Table 1. In order to change the operation conditions (TMP, shear stress, and membrane type) repeatedly and rapidly, a small dead-end filtration device was employed as the membrane module. In this module, the membrane was located at the bottom of the cell. The built-in agitator could cause different shear stress on membrane with various rotating speeds. A constant pressure was
The determination of critical AGS size
The permeate fluxes and total fouling resistances were showed in Fig. 2. The AGSs with different particle sizes revealed various fluxes and fouling behaviors. Generally speaking, the higher size difference between foulants and membrane pores, the lower membrane fouling occurred. Interestingly, there was a critical AGS size for distinguishing membrane fouling. As shown in Fig. 2, above 1.2 mm, increasing AGS size, flux strengthened and fouling resistance reduced. Below 1 mm, lower AGS size
Conclusion
During the current study, the membrane fouling mechanism of AGS with various sizes, has been explored. The paper is noteworthy that there appeared a critical AGS size (1∼1.2 mm) for membrane fouling. Exceeding 1.2 mm, flux rose and fouling decreased with size, since the loose cake layer formed by larger AGS demonstrated a high permeability. Less than 1 mm, better flux and smaller fouling emerged at lower size, due to less EPS production. As for the critical size, the highest fouling was caused
Acknowledgement
The authors would like to acknowledge the financial support from the Guangdong Natural Science Foundation of China (No. 2017A030310540), and Guangdong Provincial Science and Technology Plan (No. 2016A050503041).
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