The effect of particulate organic substrate on the formation, composition and performance of aerobic granular sludge

Abstract

Today, aerobic granular sludge (AGS) technology is an established alternative to conventional activated sludge for the biological treatment of municipal wastewater (WW). But despite countless studies and full-scale applications of AGS, the effect of particulate organic substrate (XB) - the major constituent of organic substrate in municipal WW - on AGS start-up, performance, stability and microbial community is not well understood. Therefore this PhD thesis evaluated physical retention and microbial turnover pathways of XB during AGS sequencing batch reactor (SBR) operation, as well as the influence of XB on formation, process stability, settling performance, nutrient removal and microbial community composition of AGS systems. Physical retention of XB during AGS-SBR operation was identified as a 2-step process. Firstly, XB sedimented and accumulated in the bottom of the settled sludge bed and was retained through surface filtration by the emerging filter-cake. Thus, attachment to biomass was quite limited. Secondly, XB then was preferentially attaching to the flocs during fully-mixed conditions. Results from mathematical modelling of XB hydrolysis, conversion and turnover resembled and expanded those prior findings. Flocs played a major role in aerobic XB utilisation via aerobic oxidation by ordinary heterotrophic organisms (OHO). Synergies between flocs and granules were observed, where flocs diverted aerobic XB oxidation from the granules and thus supported anaerobic-feast aerobic-famine conditions for the granules, despite the minor mass fraction of flocs. Long-term operation of AGS systems fed with different WW additionally reinforced the results from physical XB retention experiments and mathematical modelling. Complex WW composed of low volatile fatty acids (VFA) and high XB fractions led to the formation of small granules and 10-40$\%$ ($\%$ of total suspended solids) of flocs as well as to increased start-up time, decreased nutrient removal and settling performance. The microbial community of AGS treating WW composed of XB was characterised by a high abundance of fermenting bacteria, like fermentative glycogen and phosphorus accumulating organisms (fGAO, fPAO, respectively). Simultaneous nitrification-denitrification (SND) was quite limited when AGS was fed with municipal WW containing XB. The main factors influencing SND in those AGS systems were identified to be the dynamic of anoxic formation and decay inside the granule and the availability of organic substrate in the anoxic granule layers. In addition, the PhD thesis could significantly contribute towards practical understanding of AGS for the treatment of municipal WW. Most importantly, AGS was distinguished as hybrid biofilm system, whereby biofilm (granules) and suspended growth (flocs) coexisted in synergy. Optimised aeration strategies such as intermittent aeration were identified to increase SND and total nitrogen removal by AGS systems fed with municipal WW significantly. Overall, AGS proved to be a simple, efficient and stable process for the treatment of municipal WW. However, expectations towards settleability, start-up duration and nutrient removal performance must be lowered if AGS is applied for the treatment of low-strength municipal WW containing XB.