Aerobic Granules: Microbial Landscape and Architecture, Stages, and Practical Implications

Abstract

For the successful application of aerobic granules in wastewater treatment, granules containing an appropriate microbial assembly able to remove contaminants should be retained and propagated within the reactor. To manipulate and/or optimize this process, a good understanding of the formation and dynamic architecture of the granules is desirable. Models of granules often assume a spherical shape with an outer layer and an inner core, but limited information is available regarding the extent of deviations from such assumptions. We report on new imaging approaches to gain detailed insights into the structural characteristics of aerobic granules. Our approach stained all components of the granule to obtain a high quality contrast in the images; hence limitations due to thresholding in the image analysis were overcome. A three-dimensional reconstruction of the granular structure was obtained that revealed the mesoscopic impression of the cavernlike interior of the structure, showing channels and dead-end paths in detail. In "old" granules, large cavities allowed for the irrigation and growth of dense microbial colonies along the path of the channels. Hence, in some areas, paradoxically higher biomass content was observed in the inner part of the granule compared to the outer part. Microbial clusters "rooting" from the interior of the mature granule structure indicate that granules mainly grow via biomass outgrowth and not by aggregation of small particles. We identify and discuss phenomena contributing to the life cycle of aerobic granules. With our approach, volumetric tetrahedral grids are generated that may be used to validate complex models of granule formation. G ranules are compact, quasispherical aggregates formed by self-immobilized, mixed microbial communities embedded in a matrix of extracellular bio-polymeric substances (1). The phenomenon of granule formation in wastewater treatment systems was first described in anaerobic bioreactors (2, 3). The technological success of anaerobic granules in the treatment of highly concentrated industrial wastewaters (4) has been crucial to stimulating research in developing their aerobic counterparts. Previous studies have shown that in sequencing batch reactors and under certain operational conditions, which include high shear stress and short settling times (5), activated sludge can form aerobic granules (5, 6). Compared to flocs, granules can accommodate five times the biomass concentration (7) and settle 10 times faster (8, 9), and these characteristics translate into small-footprint wastewater treatment plants. Therefore, the use of aerobic granular sludge-based systems stands as a promising alternative technology to conventional activated sludge treatment systems (10). To guarantee the successful application of the aerobic granular sludge technology, granules should be physically stable and should contain an appropriate microbial assembly able to remove the desired contaminants. Moreover, in the long term, this assembly should be retained and should propagate wit...

Analysis of bacterial, fungal and archaeal populations from a municipal wastewater treatment plant developing an innovative aerobic granular sludge process

“…As fungi are aerobic species their association with aerobic environment provided by the flocs is not surprising. The lower association of fungi in granules supports the idea that granules provide anaerobic environmental conditions in the sludge (Cydzik-Kwiatkowska and Wojnowska-Baryła 2015;Gonzalez-Gil and Holliger 2014;Weber et al 2009a, b). Therefore, our study supports a notion the sludge structure and composition is a primary factor in the biodiversity and richness of the AGS.…”

Microbial interactions and community assembly at microscales

“…Besides the case of nutrient particles in aquatic environments, other types of particle structures can be found in a variety of other environments as well. These include (but are not limited to): colonic crypts in the human gut, trichomes and other surface structures on leaves, granules in activated sludge bioreactors, dental plaque, and many others [29••,40–50]. Altogether, these particles represent discrete community units that can be individually sampled from the environment.…”

“…By modulating the abundance of particle-degrading bacteria and their exposure to particle surfaces, local interactions on POM can thus control the rates of particle degradation and biomass production, and consequently, the rates of carbon remineralization in the water column. Similar processes are likely to control the assembly of microscale biofilms in many other environments, including those in the oral microbiome or in granular sludge in waste water treatment reactors [50]. …”

Accelerated aerobic granulation using alternating feed loadings: Alginate-like exopolysaccharides

“…Aerobic granules have a layered structure with spatial distributions of polysaccharides, proteins, lipids and different functional strains (Adav et al, 2010). Gonzalez-Gil and Holliger (2014) applied imaging approaches to investigate the detailed http://dx.doi.org/10.1016/j.biortech.2014.08.092 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved. structure of aerobic granules, indicating that granules mainly grow via biomass outgrowth and not by small particle aggregation.…”