Formation, structure and function of aerobic granular sludge

Abstract

Aerobic granular sludge cultivation was investigated using two laboratory scale sequencing batch reactors (SBR). The SBR were operated in parallel using different kinds of readily biodegradable substrate. Aerobic granules could mainly be cultivated within 15 and 25 days. Granulation was characterised by two main stages. As initial selection step biomass with improved settling properties was concentrated in the reactor using high volumetric exchange ratio combined with fast fill and short settling. Filamentous microorganisms formed a structural backbone inside the first sludge aggregates. Secondly, the small and compact activated sludge aggregates - first granules - were initiated and grew further. The cyclic process with static fill and a balance between growth rate and detachment processes resulted in rather spherical granules. For all cultivated aerobic granules, generally, a fast and complete settling was observed. The granules’ settling was related to their aggregate size. The settling velocity could be described by modified Stokes’ law. The nutrient removal capacity was characterised by efficient COD removal combined with simultaneous nitrification / denitrification. Alternating anaerobic/aerobic conditions induced high and stable EBPR activity. In comparison to flocculent activated sludge, a similar specific surface area and related adsorption properties were determined whereas shear sensitivity was slightly lower. Using molecular-biological methods, especially fluorescence-in-situ-hybridisation (FISH) and the full cycle 16S rDNA approach, it was possible to determine the structurally involved filamentous bacteria, Sphaerotilus natans. In steady state conditions most of the population consisted of betaproteobacteria (approx. 89 %) and gammaproteobacteria (approx. 5 %), both characterised by a relatively limited diversity of species. Typical floc forming bacteria (Zooglea ramigera) were able to grow and dominate the microbial structure. Different microsensors were used to examine single granules harvested from the reactor. Granular structure was characterised by active surface layers which had a depth of around 0-500µm. Microorganisms in deeper layers were clearly less active. In the inner core diffusion limitation occurred. Additional anoxic and anaerobic microniches existed and channels and voids could be detected.