Abstract
Basic understanding of formation of aerobic granular sludge (AGS) has mainly been derived from lab-scale systems with simple influents containing only highly diffusible volatile fatty acids (VFA) as organic substrate. This study compares start-up of AGS systems fed by different synthetic and municipal wastewaters (WW), characterised by increasing complexity in terms of non-diffusible organic substrate. Four AGS reactors were started with the same inoculum activated sludge and operated for one year. The development of AGS, settling characteristics, nutrient and substrate removal performance as well as microbial community composition were monitored. Our results indicate that the higher the content of diffusible organic substrate in the WW, the faster the formation of AGS. The presence of non-diffusible organic substrate in the influent WW led to the formation of small granules and to the presence of 20–40% (% of total suspended solids) of flocs in the AGS. When AGS was fed with complex influent WW, the classical phosphorus and glycogen accumulating organisms (PAO, GAO) were outcompeted by their fermentative equivalents. Substrate and nutrient removal was observed in all reactors, despite the difference in physical and settling properties of the AGS, but the levels of P and N removal depended on the influent carbon composition. Mechanistically, our results indicate that increased levels of non-diffusible organic substrate in the influent lower the potential for microbial growth deep inside the granules. Additionally, non-diffusible organic substrates give a competitive advantage to the main opponents of AGS formation – ordinary heterotrophic organisms (OHO). Both of these mechanisms are suspected to limit AGS formation. The presented study has relevant implications for both practice and research. Start-up duration of AGS systems treating high complexity WW were one order of magnitude higher than a typical lab-scale system treating VFA-rich synthetic WW, and biomass as flocs persisted as a significant fraction. Finally, the complex synthetic influent WW – composed of VFA, soluble fermentable and particulate substrate - tested here seems to be a more adequate surrogate of real municipal WW for laboratory studies than 100%-VFA WW.
Highlights
Aerobic granular sludge (AGS) systems have been developedAGS systems are often hampered by the wastewater composition (Guimara~es et al, 2018; de Kreuk and VAN Loosdrecht, 2006; Guimar~aes et al, 2017)
We propose that the presence of Zoogloea in R1 is an indirect consequence of the excellent settling properties of the AGS, which led to preferential flow and bypass of soluble chemical oxygen demand (COD) into the aerobic phase (Supplementary Information S15)
High fractions of diffusible organic substrates result in fast granulation and excellent settleability of AGS, whereas presence of non-diffusible XB in the influent hampers granulation, reduces settleability, and results in the presence of substantial fractions (20e40%) of flocs
Summary
Aerobic granular sludge (AGS) systems have been developedAGS systems are often hampered by the wastewater composition (Guimara~es et al, 2018; de Kreuk and VAN Loosdrecht, 2006; Guimar~aes et al, 2017). Lab-scale sequencing batch reactors (SBRs) have been extensively used to develop our fundamental understanding of AGS systems (Weissbrodt et al, 2013; He et al, 2016b; de Kreuk and VAN Loosdrecht, 2004). Those studies were mainly conducted using high concentrations of volatile fatty acids (VFA) (e.g., acetate and/or propionate) and phosphorus. If the growth of PAO and GAO is crucial for the formation of aerobic granules during treatment of VFA-rich influent, it is intuitive that granulation might be hampered during treatment of municipal WW containing high particulate organic substrate (XB) and low VFA fractions
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