The influence of wastewater (WW) composition and the bioaggregates types (floccular vs. aerobic granular sludge - AGS) on the content, physical-chemical, hydrogel and rheological properties of Alginate-Like Exopolymers (ALE) was studied. Results showed that ALE are a complex mixture of proteins, humic acids and polysaccharides. Overall, rather similar ALE content and composition was observed for the different types of sludge. Only the AGS fed with acetate and propionate yielded significantly larger amount of ALE (261 ± 33 mg VS_ALE/g VS_sludge, +49%) and of uronic sugars in ALE (254 ± 32 mg_{glucuronic acid}/g VS_ALE, +62%) than bioaggregates fed with no/very little volatile fatty acids. Mannuronic acids are involved in the cohesion of the hydrogels. ALE hydrogels elasticity changed significantly with the type/origin of the bioaggregates. ALE hydrogels elasticity from AGS was always higher than from flocs when fed with real WW. Hence, different types of sludge impact the properties of the recovered ALE.

Simultaneous nitrification-denitrification (SND) is, in theory, a key advantage of aerobic granular sludge systems over conventional activated sludge systems. But practical experience and literature suggests that SND and thus total nitrogen removal are limited during treatment of municipal wastewater using AGS systems. This study thus aims at quantifying the extent and understanding the mechanisms of SND during treatment of municipal wastewater with aerobic granular sludge (AGS) systems. Experiments (long-term and batch-tests) as well as mathematical modelling were performed. Our experimental results demonstrate that SND is significantly limited during treatment of low-strength municipal wastewater with AGS systems (14-39%), while almost full SND is observed when treating synthetic influent containing only diffusible substrate (90%). Our simulations demonstrate that the main mechanisms behind limited SND are (1) the dynamics of anoxic zone formation inside the granule, (2) the diffusibility and availability of electron-donors in those zones and (3) the aeration mode. The development of anoxic zones is driven by the utilisation of oxygen in the upper layers of the granule leading to transport limitations of oxygen inside the granule; this effect is closely linked to granule size and wastewater composition. Development of anoxic zones during the aerobic phase is limited for small granules at constant aeration at bulk dissolved oxygen (DO) concentration of 2 mgO₂ L^-1, and anoxic zones only develop during a brief period of the aerated phase for large granules. Modelling results further indicate that a large fraction of electron-donors are actually utilised in aerobic rather than anoxic redox zones - in the bulk or at the granule surface. Thus, full SND cannot be achieved with AGS treating low strength municipal wastewater if a constant DO is maintained during the aeration phase. Optimised aeration strategies are therefore required. 2-step and alternating aeration are tested successfully using mathematical modelling and increase TN removal to 40-79%, without compromising nitrification, and by shifting electron-donor utilisation towards anoxic redox conditions.

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.

Die ARA Sihltal wird heute ohne Vorklärung betrieben, soll jedoch aufgrund der zukünftigen Bevölkerungsentwicklung und zur energetischen Optimierung mit einer Vorklärung erweitert werden. Wegen der sehr engen Platzverhältnisse kann eine maschinelle Vorklärung (Trommelsieb) einfacher realisiert werden als eine konventionelle Vorklärung. Die Pilotversuche bestätigen, dass das Trommelsieb aus betrieblicher Sicht und bezüglich Abscheideleistung eine gleichwertige Vorklärung bietet.

Les boues granulaires aérobies offrent plusieurs avantages pour le traitement biologique des eaux usées municipales par rapport au procédé à boues activées conventionnels. Cependant, malgré un nombre croissant d’applications industrielles et municipales, notre recul sur les boues granulaires aérobies reste limité. L’EPFL et l’Eawag ont évalué au cours des dernières années les mécanismes, performances et limites des systèmes de boues granulaires aérobies. Dans cet article, nous résumons notre compréhension des facteurs influençant les propriétés des boues granulaires aérobies et fournissons une appréciation globale de cette nouvelle technologie.

This study aimed at evaluating the formation of aerobic granular sludge (AGS) for the treatment of real and low-strength municipal wastewater using a column sequencing batch reactor (SBR) operated in fill-draw mode (constant volume). The focus was on understanding how the wastewater upflow velocity (V_WW) applied during the anaerobic feeding influenced the sludge properties and in turn the substrate conversion. Two different strategies were tested: (1) washing-out the flocs by imposing high wastewater upflow velocities (between 5.9 and 16 m h⁻¹) during the anaerobic feeding (Approach #1) and (2) selective utilization of organic carbon during the anaerobic feeding (1 m h⁻¹) combined with a selective sludge withdrawal (Approach #2). A column SBR of 190 L was operated in constant volume during 1500 days and fed with real and low-strength municipal wastewater. The formation of AGS with SVI₃₀ of around 80 mL g_TSS⁻¹ was observed either at very low (1 m h⁻¹) or at high V_WW (16 m h⁻¹). At 16 m h⁻¹ the AGS was mainly composed of large and round granules (d > 0.63 mm) with a fluffy surface, while at 1 m h⁻¹ the sludge was dominated by small granules (0.25 < d < 0.63 mm). The AGS contained a significant fraction of flocs during the whole operational period. A considerable and continuous washout of biomass occurred at V_WW higher than 5.9 m h⁻¹ (Approach #1) due to the lower settling velocity of the AGS fed with municipal wastewater. The low sludge retention observed at V_WW higher than 5.9 m h⁻¹ deteriorated the substrate conversion and in turn the effluent quality. High solid concentrations (and thus solid retention time) were maintained during Approach #2 (V_WW of 1 m h⁻¹), which resulted in an excellent effluent quality. The study demonstrated that the formation of AGS is possible during the treatment of real and low-strength municipal wastewater in a SBR operated at constant volume. Low wastewater upflow velocities should be applied during the anaerobic feeding phase in order to ensure enough biomass retention and efficient substrate removal.

The effect of extracellular polymeric substances (EPS) on the meso-scale physical structure and hydraulic resistance of membrane biofilms during gravity driven membrane (GDM) filtration was investigated. Biofilms were developed on the surface of ultrafiltration membranes during dead-end filtration at ultra-low pressure (70 mbar). Biofilm EPS composition (total protein, polysaccharide and eDNA) was manipulated by growing biofilms under contrasting nutrient conditions. Nutrient conditions consisted of (i) a nutrient enriched condition with a nutrient ratio of 100:30:10 (C: N: P), (ii) a phosphorus limitation (C: N: P ratio: 100:30:0), and (iii) a nitrogen limitation (C: N: P ratio: 100:0:10). The structure of the biofilm was characterised at meso-scale using Optical Coherence Tomography (OCT). Biofilm composition was analysed with respect to total organic carbon, total cellular mass and extracellular concentrations of proteins, polysaccharides, and eDNA. 2D-confocal Raman mapping was used to characterise the functional group composition and micro-scale distribution of the biofilms EPS. Our study reveals that the composition of the EPS matrix can determine the meso-scale physical structure of membrane biofilms and in turn its hydraulic resistance. Biofilms grown under P limiting conditions were characterised by dense and homogeneous physical structures with high concentrations of polysaccharides and eDNA. Biofilm grown under nutrient enriched or N limiting conditions were characterised by heterogeneous physical structures with lower concentrations of polysaccharides and eDNA. For P limiting biofilms, 2D-confocal Raman microscopy revealed a homogeneous spatial distribution of anionic functional groups in homogeneous biofilm structures with higher polysaccharide and eDNA concentrations. This study links EPS composition, physical structure and hydraulic resistance of membrane biofilms, with practical relevance for the hydraulic performances of GDM ultrafiltration.