Micro-scale observations of the structure of aerobic microbial granules used for the treatment of nutrient-rich industrial wastewater

Abstract

The structure and function of aerobic microbial granules from a lab-scale sequencing batch reactor treating nutrient-rich abattoir wastewater were investigated. These wastewater-fed granules were examined using a wide range of micro-scale techniques including light microscopy, scanning and transmission electron microscopy, fluorescent in situ hybridisation (FISH) combined with confocal laser scanning microscopy and oxygen and pH microsensors, in conjunction with a range of measurements in the bulk liquid phase. Interesting structural features were observed in these granules that have not been reported in synthetic-fed granules. The complex nature of abattoir wastewater was suggested to be responsible for accelerating the breaking process of large mature granules due to a rapid clogging of the granules pores and channels and for the very diverse microbial community observed displaying specific spatial distribution throughout the granules. More importantly, the dissolution at lower pH of mineral complexes associated to the granule matrix of extracellular polymeric substances might have caused the structural damages observed on the granules even though some pH buffer capacity was observed inside these granules. Ciliate protozoa were found to be very abundant on the surface of these wastewater-fed granules, which could potentially assist with reducing the high levels of suspended solids usually present in the aerobic granular sludge effluent. All these observations provide support to future studies on aerobic granular sludge treating real wastewater especially with regard to the granule structure and the mechanisms involved in their formation.

Aggregate Size and Architecture Determine Microbial Activity Balance for One-Stage Partial Nitritation and Anammox

“…Subsequent decay could give rise to formation of voids occupying 13 to 17% of the anoxic zone. In contrast to voids observed in aerobic granules (18), no connections were seen between a void and the outside of a granule.…”

“…In suspended biomass systems, settling properties determine the retention of biomass and are related to the microbial aggregate morphology (floc or granule) and size. Granules can be defined as compact and dense aggregates with an approximately spherical external appearance that do not coagulate under decreased hydrodynamic shear conditions and settle significantly faster than flocs (18). Toh and coworkers calculated a lower sludge volume index for aerobic granules than for aerobic flocs and also showed that there was an increase in the settling velocity with increasing granule size (35).…”

Dynamics of Microbial Community Structure of and Enhanced Biological Phosphorus Removal by Aerobic Granules Cultivated on Propionate or Acetate

“…treating the complex abattoir wastewater appeared highly dominated by Accumulibacter-like bacteria (approximately 80 to 90%), as revealed by FISH images of entire granule sections (31). In relative terms, it is possible that the intuitively expected high richness values due to more complex wastewaters may be counteracted by the strong physical and redox change selection pressure applied in the anaerobic-aerobic sequencing batch reactors.…”

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

“…Although this approach has increased our understanding of the process, models are often only validated against macroscale results with limited or no microscale observations (21,22). Despite the fact that the physical architecture of the granules is intrinsically related to mass transport, conversion processes and niches partitioning within the granules, few reports describe details on the microstructure of aerobic granules (23,24). In addition, an effective approach to stain all of the constituents of the aggregates is required because most previous research has focused on fluorescence methods to target specific bacterial types.…”